Magnetic recording medium

[0002] The present invention relates to a coating type high capacity magnetic recording medium capable of high density recording. More specifically, the present invention relates to a coating type high capacity magnetic recording medium for high density recording which comprises a magnetic layer on a substantially nonmagnetic lower layer wherein the uppermost magnetic layer contains a ferromagnetic metal powder.

[0003] In the field of a magnetic disc, a 2 MB MF-2HD floppy disc using Co-modified iron oxide has been generally loaded in a personal computer. However, along with the increase in the amount of data to be dealt with, the capacity thereof has become insufficient and the increase of the capacity of the floppy disc has been demanded.

[0004] In the field of a magnetic tape, with the prevalence of an office computer, such as a minicomputer, a personal computer and a work station, a magnetic tape for recording computer data as external storage medium (a so-called backup tape) has been vigorously studied. For the realization of the magnetic tape for such a use, the improvement of recording capacity has been strongly demanded conjointly with the miniaturization of a computer and the increase of information processing ability (e.g., information throughput).

[0005] Magnetic layers comprising an iron oxide, a Co-modified iron oxide, CrO₂, a ferromagnetic metal powder, or a hexagonal ferrite powder dispersed in a binder, which are coated on a nonmagnetic support, have been conventionally widely used in magnetic recording media. Ferromagnetic metal powders and hexagonal ferrite powders among these have been known to have excellent high density recording characteristics.

[0006] In the case of a disc, as high capacity discs using ferromagnetic metal powders which are excellent in high density recording characteristics, there are 10 MB MF-2TD and 21 MB MF-2SD, and as high capacity discs using hexagonal ferrite, there are 4 MB MF-2ED and 21 MB Floptical, however, any of these are not satisfactory with respect to capacities and properties. As is the circumstance, various attempts have been made to improve high density recording characteristics. Examples thereof are described below.

[0007] For improving characteristics of a disc-like magnetic recording medium, JP-A-64-84418 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) proposes the use of a vinyl chloride resin having an acidic group, an epoxy group and a hydroxyl group, JP-B-3-12374 (the term “JP-B” as used herein means an “examined Japanese patent publication”) proposes the use of a metal powder having a coercive force Hc of 1,000 Oe or more and a specific surface area of from 25 to 70 m²/g, and JP-B-6-28106 proposes to regulate the specific surface area and magnetic susceptibility of magnetic powders and contain an abrasive.

[0008] For improving the durability of a disc-like magnetic recording medium, JP-B-7-85304 proposes use of a fatty acid ester having an unsaturated fatty acid ester and an ether bond, JP-B-7-70045 proposes the use of a fatty acid ester having a branched fatty acid ester and an ether bond, JP-A-54-124716 proposes the use of a-nonmagnetic powder having a Mohs' hardness of 6 or more and a higher fatty acid ester, JP-B-7-89407 proposes to regulate the volume and the surface roughness of voids containing a lubricant to from 0.005 to 0.025 μm, JP-A-61-294637 proposes the use of a fatty acid ester having a low melting point and a fatty acid ester having a high melting point, JP-B-7-36216 proposes use an abrasive having a particle size of from ¼ to ¾ of the magnetic layer thickness and a fatty acid ester having a low melting point, and JP-A-3-203018 proposes use of a metallic magnetic powder containing Al and a chromium oxide.

[0009] As the constitution of a disc-like magnetic recording medium having a nonmagnetic lower layer and an intermediate layer, JP-A-3-120613 proposes the constitution comprising an electrically conductive layer and a magnetic layer containing a metal powder, JP-A-6-290446 proposes the constitution comprising a magnetic layer having a thickness of 1 μm or less and a nonmagnetic layer, JP-A-62-159337 proposes the constitution comprising an intermediate layer comprising a carbon and a magnetic layer containing a lubricant, and JP-A-5-290358 proposes the constitution comprising a nonmagnetic layer in which the carbon size is regulated.

[0010] On the other hand, a disc-like magnetic recording medium comprising a thin magnetic layer and a functional nonmagnetic layer has been developed in recent years and floppy discs of the class with the capacity of 100 MB are now on the market. As floppy discs-showing these characteristics, JP-A-5-109061 proposes the constitution comprising a magnetic layer having Hc of 1,400 Oe or more and a thickness of 0.5 μm or less and a nonmagnetic layer containing electrically conductive particles, JP-A-5-197946 proposes the constitution comprising abrasives having particle sizes larger than the thickness of the magnetic layer, JP-A-5-290354 proposes the constitution comprising a magnetic layer having the thickness of 0.5 μm or less and the fluctuation of the thickness of within ±15%, in which the surface electric resistance is regulated, and JP-A-6-68453 proposes the constitution in which two kinds of abrasives having different particle sizes are contained and the amount of the abrasives on the surface is regulated.

[0011] Further, in the field of a tape-like magnetic recording medium, with the prevalence of an office computer, such as a minicomputer and a personal computer, a magnetic tape for recording computer data as external storage medium (a so-called backup tape) has been vigorously studied. For the realization of the magnetic tape for such use, the improvement of recording capacity has been strongly demanded conjointly with the miniaturization of a computer and the increase of information processing capability. In addition, the use in various environmental conditions due to widening of use environments of magnetic tapes (in particular, under fluctuating temperature/humidity conditions), reliability on data storage, and reliability on performance, such as stable recording/readout of data in multiple running due to repeating use at high speed, have been increasingly demanded.

[0012] Magnetic tapes which are used in digital signal recording systems vary according to each system, for example, magnetic tapes corresponding to a so-called DLT type, 3480, 3490, 3590, QIC, a D8 type and a DDS type are known. In every system, the magnetic tape comprises, on one surface side of a nonmagnetic support, a magnetic layer of a single layer structure having a comparatively thick layer thickness, e.g., from 2.0 to 3.0 μm, containing a ferromagnetic powder, a binder and an abrasive, and a back coating layer provided on the surface side of the support opposite to the side having the magnetic layer for purposes of preventing winding disarrangement and maintaining good running durability. However, in general, in a magnetic layer of a single layer structure having a comparatively thick layer thickness as described above, there is a problem of thickness loss which generates the reduction of output.

[0013] For the improvement of the reduction of reproduction output due to thickness loss, thinning of a magnetic layer has been known. For example, JP-A-5-182178 discloses a magnetic recording medium comprising a nonmagnetic support having thereon a lower nonmagnetic layer containing an inorganic powder dispersed in a binder and an upper magnetic layer containing a ferromagnetic powder dispersed in a binder and having a thickness of 1.0 μm or less, which is coated on the lower nonmagnetic layer while the nonmagnetic layer is still wet.

[0014] However, with the rapid trend of the increase of the capacity and density of disc-like and tape-like magnetic recording media, it has become difficult to obtain satisfactory characteristics even with these techniques. It has also become difficult to make the increase of the capacity and density compatible with durability. The present inventors have proposed in JP-A-63-103423 a magnetic recording medium comprising a magnetic layer containing a ferromagnetic metal powder containing from 1 to 6 wt % of aluminum in terms of the weight of metals, and a binder containing a resin having a polar group, wherein the ferromagnetic metal powder is invested with higher hardness than the hardness of conventional similar ferromagnetic metal powders without reducing magnetic characteristics when the ferromagnetic metal powder contains an aluminum component in a specific range of amount. Further, the present inventors have proposed that a magnetic layer comprising a ferromagnetic metal powder and an ordinary abrasive dispersed in a binder containing a resin having a polar group possesses both high running durability and electromagnetic characteristics. The magnetic medium has certainly exhibited excellent durability but when it is applied to a magnetic disc of high capacity and high density at high speed rotation of 1,800 rpm or more, there is still room for improvement of durability and electromagnetic characteristics.

[0015] An object of the present invention is to provide a magnetic recording medium which is markedly improved in electromagnetic characteristics, in particular, high density recording characteristics, and which unites high durability, in particular, an error rate in high density recording region is conspicuously improved. Specifically, an object of the present invention is to provide a high capacity magnetic recording medium, in particular, a disc-like magnetic recording medium having a high recording capacity of from 0.17 to 2 G bit, preferably from 0.2 to 2 G bit, and particularly preferably from 0.35 to 2 G bit.

[0016] As a result of earnest studies to provide a magnetic recording medium which is excellent in electromagnetic characteristics and durability, in particular, markedly improved in an error rate in a high density recording region, the present inventors have found that high capacity, high density recording characteristics and excellent durability of the object of the present invention can be obtained by the magnetic recording medium having the constitution described below, thus the present invention has been attained.

[0017] That is, the present invention can be attained by a magnetic recording medium which comprises a support having thereon a magnetic layer comprising a ferromagnetic metal powder dispersed in a binder, which is a magnetic recording medium for recording signals of from 0.17 to 2 G bit/inch²of areal recording density, wherein the coercive force of the magnetic layer is 1,800 Oe or more, the ferromagnetic metal powder comprises Fe and Co, and the atomic ratio of Al/(Fe+Co) is from 3.0 to 15.4%, or the present invention can be attained by a magnetic recording medium which comprises a support having thereon a magnetic layer comprising a ferromagnetic metal powder dispersed in a binder, which is a magnetic recording medium for recording signals of from 0.17 to 2 G bit/inch²of areal recording density, wherein the coercive force of the magnetic layer is 1,800 Oe or more, the ferromagnetic metal powder comprises Fe and Co, and the atomic ratio of the sum total of rare earth elements/(Fe+Co) is from 0.5 to 9.0%.

[0018] Preferred embodiments of the present invention are described below.

[0019] (1) In the above magnetic recording medium, a substantially nonmagnetic lower layer is provided between the support and the magnetic layer.

[0020] (2) The above magnetic recording medium is a disc-like magnetic recording medium.

[0021] (3) In the above magnetic recording medium, the magnetic layer has a dry thickness of from 0.05 to 0.30 μm and φm of from 10.0×10⁻³to 1.0×10⁻³emu/cm².

[0022] (4) In the above magnetic recording medium, the ferromagnetic metal powder comprises Fe and Co, and the atomic ratio of the sum total of rare earth elements/Al is from 0.05 to 1.2.

[0023] (5) In the above magnetic recording medium, the ferromagnetic metal powder comprises Fe and Co, and the atomic ratio of the sum total of rare earth elements/Al is from 0.1 to 0.6.

[0024] (6) In the above magnetic recording medium, the ferromagnetic metal powder comprises Fe and Co, the atomic ratio of the sum total of rare earth elements/(Fe+Co) is from 1.0 to 6.0%, and the atomic ratio of the sum total of rare earth elements/Al is from 0.1 to 0.6.

[0025] (7) In the above magnetic recording medium, the ferromagnetic metal powder has the atomic ratio of the sum total of rare earth elements/(Fe+Co) of from 1.0 to 8.0%, and the atomic ratio of Mg/(Fe+Co) of from 0.05 to 3.0%.

[0026] (8) In the above magnetic recording medium, the ferromagnetic metal powder has an acicular ratio of from 3.0 to 9.0.

[0027] (9) In the above magnetic recording medium, the ferromagnetic metal powder has a long axis length of from 0.04 to 0.12 μm and a crystallite size of from 80 to 180 Å.

[0028] (10) In the above magnetic recording medium, the rare earth element is Y or Nd.

[0029] The object of the present invention can be achieved by the above-described construction. Preferably the magnetic layer has a dry thickness of from 0.05 to 0.25 μm, φm of from 8.0×10⁻³to 1.0×10⁻³emu/cm², and the magnetic recording medium of the present invention is a magnetic recording medium for recording signals of from 0.17 to 2 G bit/inch²of areal recording density. The present inventors have found that the magnetic recording medium having high capacity, excellent high density characteristics and excellent durability, in which, in particular, the error rate in high density recording region has been markedly improved, which could not be obtained by conventional techniques, could be obtained by adopting the constitution of the present invention.

[0030] In the magnetic recording medium of the present invention, the ferromagnetic metal powder comprises at least Fe and Co.

[0031] The magnetic layer after storage has a fluctuation of Hc (ΔHc) of preferably from −5.0% to +10.0%, more preferably from −3.0% to +8.0%, most preferably from −1.0% to +6.0%. Further, the magnetic layer after storage has decrease of φm (i.e., Δφm) of preferably within 10%, more preferably within 6%.

[0032] The term “a substantially nonmagnetic lower layer” means a lower layer which may have a magnetic property but not too much for participating in recording and hereinafter referred to simply as “a lower layer” or “a nonmagnetic layer”. When a magnetic powder is contained in the lower layer, the content is preferably less than ½ of the content of the inorganic powder.

[0033] Areal recording density is a value obtained by multiplying linear recording density by track density.

[0034] φm is the amount of magnetic moment (emu/cm²) which can be directly measured from the magnetic layer per unit area of one side using a vibrating sample magnetometer (VSM, a product of Toei Kogyo Co., Ltd.) at Hm 10 KOe, which is equal to the value obtained by multiplying magnetic flux density (Bm) (unit G=4 πemu/cm³) by the thickness (cm). Accordingly, the unit of φm is represented by emu/cm²or G·cm.

[0035] Linear recording density means a bit number of signals recorded per 1 inch in the recording direction.

[0036] These linear recording density, track density and areal recording density are values determined by each system.

[0037] That is, the present inventors have elaborated some means in an attempt to improve the magnetic layer thickness, the coercive force (Hc) and the central plane average surface roughness as to the linear recording density, and the optimization of the φm as to the track density for the improvement of the areal recording density.

[0038] More preferred embodiments of the present invention are described below.

[0039] As for the magnetic recording medium as a whole, (1) the magnetic layer has a central plane average surface roughness of 5.0 nm or less, preferably 4.0 nm or less, and more preferably 3.0 nm or less, measured by 3D-MIRAU method; (2) the magnetic layer has a coercive force of preferably 1,800 Oe or more, more preferably 2,000 Oe or more, and most preferably 2,100 Oe or more and the ferromagnetic metal powder has preferably a long axis length of 0.12 μm or less, more preferably 0.10 μm or less (particularly lower limit is 0.04 μm); (3) the magnetic recording medium is a magnetic recording medium for recording signals of from 0.17 to 2 G bit/inch², preferably from 0.20 to 2 G bit/inch², and more preferably from 0.35 to 2 G bit/inch², of areal recording density: (4) the magnetic recording medium is a magnetic recording medium for the system of a high transfer rate of 1.0 MB/sec. or more, more preferably 2.0 MB/sec. or more; (5) the magnetic recording medium is a magnetic recording medium for the high capacity floppy disc system of disc rotation speed of preferably 1,800 rpm or more, more preferably 2,000 rpm or more, and most preferably 3,000 rpm or more; (6) the magnetic recording medium is a magnetic recording medium which has realized subordination transposition capable of recording/reproduction with the conventional 3.5 inch type floppy discs; (7) the magnetic recording medium is a magnetic recording medium for a high capacity floppy disc system adopting a dual discrete gap head having both a narrow gap for high density recording and a broad gap for the conventional 3.5 inch type floppy discs; (8) the magnetic recording medium is a magnetic recording medium for the high capacity floppy disc system adopting a head which floats by disc rotation; and (9) the magnetic recording medium is a magnetic recording medium for the high capacity floppy disc system adopting a head which floats by disc rotation and, at the same time, a linear type voice coil motor as a driving motor of the head. As for the improvement of the magnetic powder, (1) the ferromagnetic metal powder comprises Fe as a main component, has an average long axis length of from 0.12 μm or less, and an acicular ratio of from 4.0 to 9.0; and (2) the ferromagnetic metal powder comprises Fe as a main component, has an average long axis length of from 0.10 μm or less, and a crystallite size of from 80 to 180 Å. As for the improvement of the support, (1) the support has a central plane average surface roughness of preferably 5 nm or less, more preferably 4.0 nm or less; (2) the support has a thermal shrinkage factor of 0.5% or less both at 100° C. for 30 minutes and at 80° C. for 30 minutes in every direction of in-plane of the support; and (3) the support has a temperature expansion coefficient of from 10⁻⁴to 10⁻⁸/° C. in every direction of in-plane of the support. As for the improvement of the lubricant, (1) the lower layer and/or the magnetic layer contain(s) at least three kinds in total of a fatty acid and/or a fatty acid ester; (2) the fatty acid and the fatty acid ester have the same fatty acid residues with each other; (3) the fatty acid contains at least a saturated fatty acid and the fatty acid ester contains at least a saturated fatty acid ester or an unsaturated fatty acid ester; (4) the fatty acid ester contains a monoester and a diester; (5) the fatty acid ester contains a saturated fatty acid ester and an unsaturated fatty acid ester; and (6) the surface of the magnetic layer has a C/Fe peak ratio of from 5 to 100 when the surface is measured by the Auger electron spectroscopy. As for the improvement of the nonmagnetic powder for the lower layer, (1) the lower layer contains a carbon black having a particle size of from 5 nm to 80 nm and the magnetic layer contains a carbon black having a particle size of from 5 nm to 300 nm; (2) the lower layer contains a carbon black having an average particle size of from 5 nm to 80 nm a carbon black having an average particle size of 80 nm or more; (3) the lower layer and the magnetic layer each contains a carbon black having an average particle size of from 5 nm to 80 nm; (4) the lower layer contains an acicular inorganic powder having an average long axis length of preferably 0.20 μm or less, more preferably 0.10 μm or less, and an acicular ratio of preferably from 3.0 to 9.0, more preferably from 4.0 or 9.0; (5) the lower layer contains an acicular inorganic powder and the magnetic layer contains an acicular ferromagnetic metal powder, and the average long axis length of the acicular inorganic powder is from 1.1 to 3.0 times of the average long axis length of the acicular ferromagnetic metal powder; and (6) the lower layer and/or the magnetic layer contain(s) a phosphorus compound and the lower layer contains an acicular or spherical inorganic powder. As for the improvement of the abrasive for the magnetic layer, (1) the magnetic layer contains at least an abrasive having an average particle size of from 0.01 to 0.30 μm; (2) the magnetic layer contains at least a diamond powder having an average particle size of 0.2 μm or less, preferably from 0.01 to 1.0 μm; (3) the magnetic layer contains two kinds of abrasives having a Mohs' hardness of 9 or more; (4) the magnetic layer preferably contains an α-alumina and a diamond powder; and (5) the magnetic layer preferably contains at least an abrasive in an amount of 15.0 wt % or less based on the weight of the magnetic metal powder. As for the improvement of the binder, (1) the lower layer and/or the magnetic layer contain(s) at least a polyurethane having a glass transition temperature of preferably from 0° C. to 100° C., more preferably from 30° C. to 100° C.; and (2) the lower layer and/or the magnetic layer contains at least a polyurethane having a breaking stress of from 0.05 to 10 kg/mm².

[0040] The present inventors have found that a magnetic recording medium, in particular, a disc-like magnetic recording medium, in the recording capacity system of areal recording density of from 0.17 to 2 G bit/inch², having excellent high density characteristics and excellent durability, in particular, markedly improved error rate in high density recording region, which could not be obtained by conventional techniques, could be obtained by adopting the above constitution of the present invention.

[0041] A magnetic recording medium, in particular, a disc-like magnetic recording medium, having high density characteristics and high durability in the recording capacity system of areal recording density of from 0.17 to 2 G bit/inch², preferably from 0.2 to 2 G bit/inch², more preferably from 0.35 to 2 G bit/inch², which could never be achieved by any coating type magnetic recording media known in the world, can be obtained as a result of organically combining and synthesizing these points as shown below.

[0042] The points aimed at in the present invention include (1) high Hc and hyper-smoothing, (2) ensuring of durability by the improvement of a composite lubricant, a binder with high durability and a ferromagnetic powder, and the use of an abrasive with high hardness (3) ultra-thinning of the magnetic layer and the reduction of fluctuation in the interface between the magnetic layer and the lower layer, (4) the increase of packing density of powders (a ferromagnetic powder and a nonmagnetic powder), (5) ultra-fine granulation of powders (a ferromagnetic powder and a nonmagnetic powder), (6) stabilization of head touch, (7) dimensional stability and servomechanism, (8) improvement of thermal shrinkage factors of the magnetic layer and the support, and (9) the functions of a lubricant at high temperature and low temperature, and the present invention has been achieved by combining and synthesizing these points.

[0043] High Hc and hyper-smoothing are described in the first place. Hc of the magnetic layer can be increased to 1,800 Oe or more, preferably 2,000 Oe or more, and more preferably 2,100 Oe or more by using a ferromagnetic powder with high Hc, thereby high capacity and high density can be obtained. With respect to hyper-smoothing, a smooth magnetic layer can be obtained by making the central plane average surface roughness 5.0 nm or less, preferably 4.0 nm or less, and employing ATOMM® structure. High capacity and high density can be attained by the central plane average surface roughness of preferably 4.0 nm or less. Subsequently, the above item (2) ensuring of durability by the improvement of a composite lubricant, a binder with high durability and a ferromagnetic powder, and the use of an abrasive with high hardness is described. With respect to a composite lubricant, fundamental concepts for the enhancement of lubrication capability are shown below.

[0044] (1) A plurality of lubricants having different functions and capabilities are used in combination.

[0045] (2) A plurality of lubricants having similar functions and capabilities are used in combination.

[0046] Due to the above item (1), a variety of functions and capabilities can be attained under various conditions. Further, due to the above item (2), affinity and compatibility of lubricants with each other can be ensured and good functions of lubricants can be exhibited.

[0047] Examples of combinations of a plurality of lubricants having different functions and capabilities as in the above item (1) are shown below.

[0048] 1) A Lubricant having a fluid lubrication function and a lubricant having a boundary lubrication function are used in combination.

[0049] 2) A polar lubricant and a nonpolar lubricant are used in combination.

[0050] 3) A liquid lubricant and a solid lubricant are used in combination.

[0051] 4) Lubricants having different polarities, in particular, a fatty acid and/or a fatty acid ester, are used in combination. For example, a fatty acid monoester and a fatty acid diester are used in combination.

[0052] 5) Lubricants having different melting points and different boiling points, in particular, a fatty acid and/or a fatty acid ester, are used in combination.

[0053] 6) Lubricants which differ in lengths of carbon atom number, in particular, a fatty acid and/or a fatty acid ester, are used in combination.

[0054] 7) A straight chain lubricant and a branched chain lubricant, in particular, a fatty acid and/or a fatty acid ester, are used in combination. For example, a straight chain fatty acid ester and a branched fatty acid ester are used in combination.

[0055] 8) A lubricant having saturated carbon chain and a lubricant having unsaturated carbon chain, in particular, a fatty acid and/or a fatty acid ester, are used in combination. For example, a saturated fatty acid ester and unsaturated fatty acid ester are used in combination.

[0056] 9) Lubricants having different affinities with a binder are used in combination.

[0057] 10) Lubricants having different affinities with an inorganic powder are used in combination.

[0058] By the combined use of lubricants as in the above item (1), a variety of functions and capabilities can be attained under various conditions.

[0059] Examples of combinations of a plurality of lubricants having similar functions and capabilities as in the above item (2) are shown below.

[0060] 1) Fatty acid residues of a fatty acid and a fatty acid ester are made the same with each other.

[0061] 2) Fatty acid esters having the same fatty acid residues with each other and/or having the same alcohol residues with each other are used in combination.

[0062] 3) Two or more saturated fatty acid are used in combination.

[0063] 4) Saturated fatty acids are respectively used in the fatty acid residue parts of a fatty acid and a fatty acid ester.

[0064] 5) Unsaturated fatty acids are respectively used in the fatty acid residue parts of a fatty acid and a fatty acid ester.

[0065] 6) Three or more fatty acid esters alone are used in combination.

[0066] 7) Fatty acid parts of a fatty acid and a fatty acid amide are made the same with each other.

[0067] By the combined use of lubricants as in the above item (2), affinity and compatibility of lubricants with each other can be ensured and good functions of lubricants can be exhibited.

[0068] By the combined use of lubricants as in the above items (1) and (2), not only a variety of functions and capabilities can be attained under various conditions but also affinity and compatibility of lubricants with each other can be ensured and good functions of lubricants can be exhibited.

[0069] The binder with high durability is described below. By the incorporation of a resin having a polar group, in particular, a polyurethane resin, having high dispersibility, high glass transition temperature and high breaking stress, durability of the binder can be improved. It is preferred for the polyurethane resin to have 2 or more OH groups, more preferably 3 or more, and most preferably 4 or more, at the terminal of the molecule because the reactivity with polyisocyanate, which is a polyfunctional curing agent, becomes high and the coated film of three dimensional network can be formed after curing. With respect to the improvement of the ferromagnetic powder, durability can be improved by increasing the Al component which can heighten the hardness of the ferromagnetic powder. Ensuring of durability by using an abrasive having high hardness is described. Higher durability can be obtained by the combined use of a diamond powder having a Mohs' hardness of 10 with conventionally used abrasives having a Mohs' hardness of 9 or so, e.g., an α-alumina. Next, ultra-thinning of the magnetic layer and the reduction of fluctuation in the interface between the magnetic layer and the lower layer in the above item (3) is described. By reducing the thickness of the magnetic layer to preferably from 0.05 to 0.30 μm, more preferably from 0.05 to 0.25 μm, and reducing the fluctuation in the interface between the magnetic layer and the lower layer, uniform, smooth and thin magnetic layer can be obtained, thereby higher capacity and higher density of the magnetic recording medium can be attained. The increase of the packing density of powders (a ferromagnetic powder and a nonmagnetic powder) in the above item (4) is described. By high packing density of a fine ferromagnetic powder, specifically, a fine ferromagnetic metal powder, preferably having an average particle length of 0.15 μm or less, more preferably 0.12 μm or less, and most preferably 0.10 μm or less, high φm can be obtained thereby higher capacity and higher density of the magnetic recording medium can be attained. Durability can be improved by high packing density of an inorganic powder. Ultra-fine granulation of powders (a ferromagnetic powder and a nonmagnetic powder) in the above item (5) is described. By the use of a fine ferromagnetic powder, specifically, a fine ferromagnetic metal powder, having an average long axis length of preferably 0.15 μm or less, more preferably 0.12 μm or less, and most preferably 0.10 μm or less, in particular, with the case of a ferromagnetic metal powder, by the ultra-fine granulation of the powder such as an average long axis length of 0.10 μm or less, an acicular ratio of from 4.0 to 9.0, and a crystallite size of from 80 Å to 180 Å, and with the case of a nonmagnetic powder in the lower layer, if it is an acicular powder, by the ultra-fine granulation of the powder such as an average long axis length of preferably 0.20 μm or less, more preferably 0.10 μm or less, higher packing density and hyper-smoothing of the magnetic layer can be attained, thereby higher capacity and higher density of the magnetic recording medium can be attained. Stabilization of head touch in the above item (6) is described. Stabilization of head touch can be contrived by an appropriate strength, flexibility and smoothness of the magnetic recording medium as a whole, thereby higher capacity and higher density of the magnetic recording medium can be attained stably even at high speed running and high rotation rate. Dimensional stability and servomechanism in the above item (7) is described. For example, when the support has a thermal shrinkage factor of 0.5% or less both at 100° C. for 30 minutes and at 80° C. for 30 minutes in every direction of in-plane of the support, and a temperature expansion coefficient of from 10⁻⁴to 10⁻⁸/° C. in every direction of in-plane of the support, dimensional stability of the support can be obtained, thereby higher capacity and higher density of the magnetic recording medium can be attained stably even at high speed running and high rotation rate, and improvement of thermal shrinkage factors of the magnetic layer and the support in the above item (8) can also achieved. With respect to the functions of a lubricant at high temperature and low temperature in the above item (9), desired lubricating functions at both high temperature and low temperature can be obtained by selecting and combining various lubricants described above based on specific concepts.

[0070] In the field of personal computers where the tendency of multimedia has been increasingly progressed, high capacity recording media have attracted public attentions in place of conventional floppy discs and, for example, ZIP disc, has been on sale from IOMEGA CORP., U.S.A. This is a recording medium comprising a lower layer and a magnetic thin layer developed by the present inventors using ATOMM® (Advanced Super Thin Layer & High Output Metal Media Technology), and products of 3.7 inches with the recording capacity of 100 MB or more are on the market. The capacity of from 100 to 120 MB discs is almost equal to the capacity of MO (3.5 inches), i.e., one disc has the capacity of recording newspaper articles of from seven to eight month period. A transfer rate indicating write/readout time of data is 2 MB or more per a second, which is equal to a hard disc, and the working speed is 20 times of conventional floppy discs and more than 2 times of the MO, therefore, extremely advantageous. In addition, as this recording medium comprising a lower layer and a magnetic thin layer is the same coating type medium as floppy discs used at present, mass production is feasible, accordingly inexpensive as compared with hard discs and the MO.

[0071] As a result of eager investigations based on the knowledge on these media, the present inventors have achieved the present invention of a magnetic recording medium, in particular, a disc-like magnetic recording medium, in the recording capacity system of areal recording density of from 0.17 to 2 G bit/inch², preferably from 0.2 to 2 G bit/inch², more preferably from 0.35 to 2 G bit/inch², and φm of preferably from 10.0×10⁻³to 1.0×10⁻³emu/cm², particularly preferably from 8.0×10⁻³to 1.0×10⁻³emu/cm², which has markedly big recording capacity as compared with the above ZIP disc and the MO (3.5 inches). This recording medium also has high capacity, high density characteristics and excellent durability which could never be achieved by any products known in the world and, in particular, the error rate in high density recording region is noticeably improved, and this is the invention applicable to a magnetic tape, e.g., a computer tape.

[0072] The magnetic recording medium of the present invention comprises an ultra-thin magnetic layer containing a magnetic powder of ultra-fine particles excellent in high output and high dispersibility, and a lower layer containing a spherical or acicular inorganic powder, and by thus reducing the thickness of the magnetic layer, a magnetic force offset in the magnetic layer can be reduced, the output in a high frequency region can be markedly increased, further, overwriting characteristics can be improved. By the improvement of a magnetic head, the effect of the ultra-thin magnetic layer can be further exhibited by the combined use with a narrow gap head and digital recording characteristics can be improved.

[0073] The upper magnetic layer is a thin layer having a thickness of preferably from 0.05 to 0.30 μm, more preferably from 0.05 to 0.25 μm, so as to match the performance required from the magnetic recording system and the magnetic head of high density recording. Such a uniform and ultra-thin magnetic layer is attained by high dispersion and high packing density realized by the combined use of a fine magnetic powder and nonmagnetic powder with a dispersant and a high dispersible binder. The magnetic powders used are preferably magnetic powders capable of achieving high output, excellent in high dispersibility and high randomizing property for inducing suitabilities of high capacity floppy discs and computer tapes as far as possible. That is, high output and high durability can be attained by the use of ferromagnetic metal powders of extremely fine particles which are capable of achieving high output, in particular, having an average long axis length of preferably 0.15 μm or less, more preferably 0.12 μm or less, and particularly preferably 0.10 μm or less, a crystallite size of from 80 to 180 Å, and containing a large amount of Co, and further Al, Si, Y and Nd components. For the realization of a high transfer rate, running stability and durability during high speed rotation can be ensured making use of a three dimensional network binder system suitable for an ultra-thin magnetic layer. A composite lubricant capable of maintaining the effect thereof during use under various temperature and humidity conditions and in high rotation use can be incorporated into upper and lower layers and, further, with making the lower layer have a role of the tank of the lubricant so as to be able to always supply an appropriate amount of the lubricant to the upper magnetic layer to thereby heighten the durability of the upper magnetic layer to improve the reliance. Cushioning effect of the lower layer can bring about good head touch and stable running property.

[0074] A high transfer rate is required in a high capacity recording system, e.g., a transfer rate of 1.4 MB/sec. in Zip and a maximum transfer rate of 3.6 MB/sec. in HiFD®. For that sake, it is necessary that the rotation speed of a magnetic disc for a high capacity recording system should be taken up one or more places as compared with conventional FD systems. Specifically, the rotation speed of a magnetic disc is preferably 1,800 rpm or more, more preferably 2,000 rpm or more, and most preferably 3,000 rpm or more. For example, the rotation speed of a magnetic disc is 2,968 rpm in Zip and 3,600 rpm in HiFD®. In the other systems, it is estimated that the rotation speed of a magnetic disc is 5,400 rpm and the transfer rate is 7.5 MB/sec when a recording capacity is 650 MB (0.65 GB). Recording track density is improved with the increase of capacity/density of magnetic recording. In general, a servo recording area is provided on a medium to ensure traceability of a magnetic head against a recording track. In the magnetic recording medium according to the present invention, a base whose dimensional stability is isotropically heightened is preferably used as the support base, thereby further stabilization of the traceability is devised. The smoothness of the magnetic layer can be further improved by using a hyper-smooth base.

[0075] The increment of density of magnetic recording of a disc-like magnetic recording medium requires the improvement of linear recording density and track density. Characteristics of a support are important factors for the improvement of track density. The dimensional stability of a support base, in particular, isotropy, is considered in the recording medium according to the present invention. Servo recording is an indispensable technique in recording/reproduction of high track density, but the improvement can be contrived from the medium side by making the support base isotropic as far as possible.

[0076] Advantages of changing the magnetic layer of the present invention from a monolayer (i.e., a single layer) to the ATOMM® structure are thought to be as follows.

[0077] (1) Improvement of electromagnetic characteristics by the thin layer structure of the magnetic layer;

[0078] (2) Improvement of durability by stable supply of lubricants;

[0079] (3) High output by smoothing the upper magnetic layer; and

[0080] (4) Easiness of imparting required functions by functional separation of the magnetic layer.

[0081] These functions cannot be sufficiently attained only by making the magnetic layer a multilayer structure. For constituting a multilayer structure, a successive multilayer system comprising successively constituting the layers is generally used. In this system, the lower layer is coated, cured or dried, then the upper magnetic layer is coated in the same way, cured, and surface-treated. In the case of a floppy disc (FD), as different from a magnetic tape, the same treatments are conducted on both surface sides. After a coating step, a disc undergoes a slitting step, a punching step, a shell incorporation step, and a certifying step, thus a final product is completed. Simultaneous coating or successive coating of coating the upper magnetic layer while the lower layer is still wet is preferred in view of the productivity.

[0082] Electromagnetic characteristics can be widely improved by the thin layer structure of the magnetic layer as follows.

[0083] (1) Improvement of the output in a high frequency region by the improvement of characteristics of recording demagnetization;

[0084] (2) Improvement of overwriting characteristics; and

[0085] (3) Security of window margin.

[0086] Durability is an important factor for a magnetic recording disc. In particular, for realizing a high transfer rate, it is necessary that the rotation speed of a magnetic disc should be taken up one or more places as compared with conventional FD systems, and security of the durability of a magnetic disc is an important problem when the magnetic disc is sliding with a magnetic head and parts in a cartridge at a high speed. For improving durability of a disc, there are means such as a binder formulation to increase the film strength of a disc per se, and a lubricant formulation to maintain a sliding property of a disc with a magnetic head. In the magnetic recording medium according to the present invention, a three dimensional network binder system which has shown actual results in conventional FD systems is used in the binder formulation by being modified.

[0087] In the present invention, lubricants are used in combination of a plurality of kinds respectively exhibiting superior effects in various temperature and humidity conditions under which they are used, and each lubricant exhibits its function in different temperature (low temperature, room temperature, high temperature) and humidity (low humidity, high humidity) atmospheres, thereby totally stable lubrication effect can be maintained.

[0088] By using two layer structure, the durability of the upper magnetic layer can be heightened with making the lower layer have a role of the tank of a lubricant capable of always supplying an appropriate amount of a lubricant to the upper magnetic layer. There is a limit on the amount of a lubricant which can be contained in the ultra-thin magnetic layer. Simple reduction of the thickness of the magnetic layer causes the reduction of the absolute amount of a lubricant, and it follows that running durability is deteriorated. In this case, it was difficult to well balance the thickness of the magnetic layer with the amount of the lubricant. The improvement of electromagnetic characteristics could be compatible with the improvement of durability by imparting different functions to the upper layer and the lower layer and making up for each other. This functional separation was particularly effective in a system where a medium was slid on a magnetic head at a high speed.

[0089] In addition to the maintaining function of a lubricant, a controlling function of surface electrical resistance can be imparted to the lower layer. For controlling electrical resistance, in general, a solid electrically conductive material such as a carbon black is added to a magnetic layer in many cases. Such a material not only restricts the increase of the packing density of magnetic powders but also influences the surface roughness of the magnetic layer as the thickness of the magnetic layer becomes thinner. Incorporation of electrically conductive materials in the lower layer can eliminate these defects.

[0090] With the progress of multimedia in society, needs for image recording have been increased more and more not only in the industry but also in general homes. The high capacity magnetic recording medium according to the present invention has capabilities capable of sufficiently responding to demands such as function/cost as a medium for recording images, as well as data such as letters and figures. The high capacity magnetic recording medium according to the present invention is based on the coating type magnetic recording medium which has shown actual results and ensures reliability for a long period of time and is excellent in cost performance.

[0091] The present invention has been attained for the first time by heaping up the above various factors, and making them worked synergistically and organically. The thus-obtained magnetic recording medium by adopting or rejecting and combining every technique has capability applicable to, e.g., HiFD®, which has been developed by joint development by Fuji Photo Film Co., Ltd. with Sony Corp. HiFD® has been developed to meet the demand for a new data recording system of high performance having high capacity and high data transfer rate with the rapid development of information processing capability of personal computers in recent years and sharp increase of throughput to be dealt with. On the other hand, 3.5 inch type floppy discs of the present have prevailed worldwide as easily usable recording media. HiFD® has been developed as a new system which can read out and reuse accumulated massive data using these discs even after this. HiFD® for 3.5 inch type floppy disc is a high capacity floppy disc system of the next generation which has high capacity of 200 MB, high transfer rate of 3.6 MB/sec, and has realized subordination transposition capable of recording/reproduction with 3.5 inch type floppy discs of the present. High capacity of 200 MB of HiFD® has been realized by an ultra-thin layer coating type metal disc newly developed and by the adoption of a dual discrete gap head having both a narrow gap for high density recording and a broad gap for a 3.5 inch type floppy disc of the present, which can easily process data file of huge volume such as an image and a sound. HiFD® has realized a high transfer rate of a maximum of 3.6 MB/sec. as compared with a transfer rate of 0.06 MB/sec. of conventional 3.5 inch type floppy discs (2HD) due to high linear recording density and high speed disc rotation such as 3,600 rpm, which is high speed processing of about 60 times as compared with conventional systems. Further, by the adoption of a floatation type dual discrete gap head and, at the same time, by the use of a linear type voice coil motor as a driving motor of the head, HiFD® has achieved high speed random access of about 3 to 4 times as compared with conventional 3.5 inch type floppy disc drives. The floatation type dual discrete gap head, similar to a hard disc, floats by disc rotation and does not contact with the disc during recording/reproduction leading to long lifetime and high reliability. In addition, by the dual discrete gap head, subordination transposition capable of recording/reproduction with 3.5 inch type floppy discs of the present has been realized. Further, abrasion of a disc can be reduced by the integration of a new mechanism capable of soft head loading, and high reliability can be attained by the loading of an error correcting function. The magnetic recording medium according to the present invention has been developed to be applicable to such a high capacity floppy disc system of the next generation which has high capacity of 200 MB, high transfer rate of 3.6 MB/sec, and has realized subordination transposition capable of recording/reproduction with 3.5 inch type floppy discs of the present.

[0092] Magnetic Layer

[0093] The lower layer and the ultrathin magnetic layer of the magnetic recording medium according to the present invention may be provided on either one side of the support or may be provided on both sides. The upper layer may be coated while the lower layer coated is still wet (W/W coating) or may be coated after the lower layer coated is dried (W/D coating). Simultaneous or successive wet on wet coating is preferred in view of the productivity but in the case of a disc-like medium, wet on dry coating can be sufficiently used. In the multilayer construction according to the present invention, as the upper layer and the lower layer can be formed simultaneously or successively (with W/W coating), a surface treatment step, e.g., a calendering step, can be utilized effectively and surface roughness of the upper magnetic layer can be improved even the layer is an ultrathin layer. The coercive force (Hc) of the magnetic layer is essential to be 1,800 Oe or more, and the maximum magnetic flux density (Bm) of the magnetic metal powder is preferably from 2,000 to 5,000 G.

[0094] Ferromagnetic Powder

[0095] Ferromagnetic powders which can be used in the present invention are preferably ferromagnetic alloy powders containing α-Fe as a main component.

[0096] Ferromagnetic metal powders which can be used in the present invention preferably comprise Fe and Co as main components and also contain Al, Mg and a rare earth element.

[0097] The ferromagnetic metal powder of the present invention is composed of Fe and Co as main components, and the atomic ratio of Al/(Fe+Co) is from 3.0 to 15.4%, preferably from 4.5 to 15.0%, and more preferably from 6.0 to 12.0%. The atomic ratio of the sum total of rare earth elements/(Fe+Co) is from 0.5 to 9.0%, preferably from 1.0 to 8.0%, and more preferably from 1.0 to 6.0%. The atomic ratio of Mg/(Fe+Co) is preferably from 0.05 to 3.0%, more preferably from 0.1 to 2.5%, and particularly preferably from 0.1 to 2.0%.

[0098] The proportion of Co to Fe is necessary to be from 3 to 50 atomic %, preferably from 5 to 45 atomic %, more preferably from 10 to 45 atomic %, and particularly preferably from 20 to 35 atomic %. The ferromagnetic metal powder comprises Fe and Co, and the atomic ratio of the sum total of rare earth elements/Al is preferably from 0.05 to 1.20, more preferably from 0.10 to 1.0, and particularly preferably from 0.1 to 0.6. The elements largely exist on surface of the powder.

[0099] When the ferromagnetic metal powder used in the magnetic layer of the present invention contains Fe, Co, Al, Mg and rare earth elements within the above ranged, excellent high density recording characteristics, excellent running durability and excellent weather resistance can be obtained. By virtue of the incorporation of the above essential components in the above-described respective ranges, the ferromagnetic metal powder is imparted with narrow and uniform particle size distribution, appropriate hardness and excellent dispersibility, although particle sizes are fine, as a result, excellent running durability and excellent weather resistance can be obtained. That is, if the atomic ratio of Al/(Fe+Co) is less than 4.5%, running durability is deteriorated, while when it is greater than 15.4%, high density recording characteristics are deteriorated. If the atomic ratio of the sum total of rare earth elements/(Fe+Co) is less than 0.5%, it is difficult to obtain excellent high density recording characteristics, and if it is greater than 9.0%, excellent running durability can hardly be obtained. When the atomic ratio of Mg/(Fe+Co) is less than 0.05%, the compatibility of excellent high density recording characteristics and excellent running durability becomes difficult, and when it is greater than 3.0%, excellent high density recording characteristics, excellent running durability and excellent weather resistance are liable to be incompatible.

[0100] Examples of the rare earth elements which can be used in the ferromagnetic metal fine powder of the present invention include Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. Among these, Y is preferred.

[0101] The content of Y is preferably from 40 to 100 atomic %, more preferably from 55 to 100 atomic %, based on the sum total of rare earth elements.

[0102] The preparation method of the ferromagnetic metal fine powder is not particularly limited but the method as described in JP-A-8-279137 is exemplified.

[0103] Concretely, goethite is first formed from an aqueous solution of Fe salt, or an aqueous solution of Fe salt and Co salt, and then to the obtained suspension of goethite are added and mixed an aqueous solution of a Co-containing compound, a rare earth element compound, an Al-containing compound, an Mg-containing compound or a compound of elements described below to prepare the goethite suspension.

[0104] The thus-obtained goethite suspension is granulated, dried, reduced and then gradually oxidized to obtain the ferromagnetic metal fine powder of the present invention.

[0105] Further, the following methods other than the above method for preparing the ferromagnetic metal fine powder is exemplified.

[0106] For example, the monodispersed hematite particle, or the goethited hematite particle, if necessary, is treated with a Co-containing compound, a rare earth element compound, an Al-containing compound or an Mg-containing compound, and then reduced to prepare the ferromagnetic metal fine powder of the present invention.

[0107] Now, in the step of forming the goethite, a part of Al compound may be added.

[0108] The ferromagnetic metal fine powder of the present invention can be prepared by well-known processes, such as a method comprising reducing a composite organic acid salt (mainly an oxalate) with a reducing gas (e.g., hydrogen); a method comprising pyrolysis of a metal carbonyl compound; a method comprising adding to an aqueous solution of a ferromagnetic metal a reducing agent (e.g., sodium boronhydride, hypophosphite or hydrazine) to conduct reduction; and a method comprising evaporating a metal in a low pressure inert gas to obtain a fine powder.

[0109] Now, in order to satisfy the composition of the ferromagnetic metal fine powder and further the desired electromagnetic characteristics according to the present invention, the kind and amount of the compound added, the dehydration condition, the reducing condition and the gradual oxidation condition may be properly changed. Also, in order to improve the electromagnetic characteristics, the ferromagnetic metal fine powder may be reduced again.

[0110] Examples of the gradual oxidation treatments include a method comprising immersing powders in an organic solvent and then drying; a method comprising immersing powders in an organic solvent and then charging an oxygen-containing gas to form oxide films on the surfaces thereof and drying; and a method comprising forming oxide films on the surfaces of powders by regulating partial pressure of an oxygen gas and an inert gas without using an organic solvent.

[0111] Among these treatments, the method comprising forming oxide films on the surfaces of powders by regulating partial pressure of an oxygen gas and an inert gas without using an organic solvent is preferred for the present invention.

[0112] These ferromagnetic powders may contain, in addition to the prescribed atoms, the following atoms, e.g., Si, S, Ca, Ti, V, Cr, Cu, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg, Pb, Bi, P, Mn, Zn, Ni, Sr and B. These elements (i.e., atoms) are usually used in an amount of from 5×10⁻⁴to 1×10⁻¹atomic % based on Fe.

[0113] The concrete examples in which Co, Al and rare earth elements are added to the ferromagnetic metal fine powders in addition to Fe are described in JP-A-6-215360, JP-A-7-210856, JP-A-8-185624 and JP-A-8-279142. These ferromagnetic powders may be previously treated with the later described dispersant, lubricant, surfactant, and antistatic agent before dispersion. Specific examples thereof are disclosed in JP-B-44-14090, JP-B-45-18372, JP-B-47-22062, JP-B-47-22513, JP-B-46-28466, JP-B-46-38755, JP-B-47-4286, JP-B-47-12422, JP-B-47-17284, JP-B-47-18509, JP-B-47-18573, JP-B-39-10307, JP-B-46-39639, U.S. Pat. Nos. 3,026,215, 3,031,341, 3,100,194, 3,242,005, and 3,389,014. Specific examples of ferromagnetic metal powders containing Mg are disclosed in JP-B-1-51042, JP-B-8-31366, JP-A-63-222404 and JP-A-5-54371. Specific examples of ferromagnetic metal powders containing Fe, Co, Al, Mg and rare earth elements are disclosed in JP-A-9-27117 and JP-A-9-35247.

[0114] Ferromagnetic powders which can be preferably used in the magnetic layer according to the present invention have a specific surface area (S_BET) as measured by the BET method of from 40 to 80 m²/g, preferably from 45 to 70 m²/g. When S_BETis less than 40 m²/g, noise increases and when more than 80 m²/g, good surface property is obtained with difficulty, which is not preferred. Ferromagnetic powders which can be preferably used in the magnetic layer according to the present invention have a crystallite size of generally from 80 to 180 Å, preferably from 100 to 180 Å, and more preferably from 110 to 175 Å. The length of the long axis of ferromagnetic powders is generally from 0.01 to 0.25 μm, preferably from 0.03 to 0.15 μm, and more preferably from 0.03 to 0.12 μm. Ferromagnetic powders preferably have an acicular ratio of from 3.0 to 15.0, more preferably from 3.0 to 12.0, and most preferably from 3.0 to 9.0. Ferromagnetic metal powders have a saturation magnetization (σ_s) of generally from 100 to 180 emu/g, preferably from 110 to 170 emu/g, and more preferably from 125 to 160 emu/g. Ferromagnetic metal powders have a coercive force (Hc) of preferably from 1,700 to 3,500 Oe, and more preferably from 1,800 to 3,000 Oe. Surfaces of ferromagnetic metal powders are preferably covered with a dense oxide film. Ferromagnetic metal powders have a coercive force of preferably from 2,000 to 4,000 Oe, and more preferably from 2,100 to 3,500 Oe.

[0115] Ferromagnetic metal powders preferably have a water content of from 0.01 to 2%. The water content of ferromagnetic metal powders is preferably optimized by selecting the kinds of binders. The pH of ferromagnetic metal powders is preferably optimized by the combination with the binder to be used. The pH range is from 4 to 12, preferably from 6 to 10. Ferromagnetic metal powders may be surface-treated with Al, Si, P or oxides thereof, if necessary. The amount thereof is from 0.1 to 20%, preferably from 0.1 to 10%, based on the ferromagnetic metal powders. Adsorption of a lubricant, e.g., fatty acid, becomes 100 mg/m²or less by conducting a surface treatment, which is, therefore, preferred. Soluble inorganic ions (e.g., Na, Ca, Fe, Ni, Sr, etc.) are sometimes contained in ferromagnetic metal powders. It is preferred substantially not to contain such soluble inorganic ions but the properties of ferromagnetic metal powders are not particularly affected if the content is 200 ppm or less. Ferromagnetic metal powders for use in the present invention preferably have less voids and the value thereof is 20% by volume or less, more preferably 5% by volume or less. The shape of ferromagnetic metal powders is not particularly limited, and any shape such as an acicular shape, an ellipsoidal shape or a spindle shape may be used so long as it satisfies the above-described properties as to particle sizes. Switching Field Distribution (SFD) of a ferromagnetic metal powder itself is preferably small, preferably 1.0 or less. It is necessary to make Hc distribution of ferromagnetic metal powders narrow. When the SFD is 1.0 or less, electromagnetic characteristics are excellent, high output can be obtained, reversal of magnetization becomes sharp and peak shift is less, therefore, suitable for high density digital magnetic recording. For achieving small Hc distribution, making particle size distribution of goethite in ferromagnetic metal powders good and preventing sintering are effective methods.

[0116] Nonmagnetic Layer

[0117] The lower layer is described in detail below. Inorganic powders contained in the lower layer of the present invention are nonmagnetic powders. They can be selected from the following inorganic compounds such as metal oxide, metal carbonate, metal sulfate, metal nitride, metal carbide, metal sulfide, etc. Examples of inorganic compounds are selected from the following compounds and they can be used alone or in combination, e.g., α-alumina having an alpha-conversion rate of 90% or more, β-alumina, γ-alumina, θ-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, hematite, goethite, corundum, silicon nitride, titanium carbide, titanium oxide, silicon dioxide, stannic oxide, magnesium oxide, tungsten oxide, zirconium oxide, boron nitride, zinc oxide, calcium carbonate, calcium sulfate, barium sulfate, and molybdenum disulfide. Of these compounds, particularly preferred are titanium dioxide, zinc oxide, iron oxide and barium sulfate because they have small particle size distribution and various means for imparting functions, and more preferred are titanium dioxide and α-iron oxide. These nonmagnetic powders preferably have an average particle size of from 0.005 to 2 μm. If desired, a plurality of nonmagnetic powders each having a different average particle size may be combined, or a single nonmagnetic powder having a broad particle size distribution may be employed so as to attain the same effect as such a combination. A particularly preferred particle size of nonmagnetic powders is from 0.01 to 0.2 μm. In particular, when the nonmagnetic powder is a granular metal oxide, the average particle size thereof is preferably 0.08 μm or less, and when it is an acicular metal oxide, the average long axis length thereof is preferably 0.3 μm or less, more preferably 0.2 μm or less, and particularly preferably 0.10 μm or less. Nonmagnetic powders for use in the present invention have an acicular ratio of preferably from 3 to 12, more preferably from 4 to 9; a tap density of from 0.05 to 2 g/ml, preferably from 0.2 to 1.5 g/ml; a water content of from 0.1 to 5 wt %, preferably from 0.2 to 3 wt %, and more preferably from 0.3 to 1.5 wt %; a pH value of from 2 to 11, particularly preferably between 5.5 and 10; a specific surface area (S_BET) of from 1 to 100 m²/g, preferably from 5 to 80 m²/g, and more preferably from 10 to 70 m²/g; a crystallite size of from 0.004 to 1 μm, and more preferably from 0.04 to 0.1 μm; an oil absorption amount using DBP (dibutyl phthalate) of from 5 to 100 ml/100 g, preferably from 10 to 80 ml/100 g, and more preferably from 20 to 60 ml/100 g; and a specific gravity of from 1 to 12, preferably from 3 to 6. The shape of nonmagnetic powders may be any of acicular, spherical, polyhedral, or tabular shape. Nonmagnetic powders preferably have a Mohs' hardness of from 4 to 10. The SA (stearic acid) absorption amount of nonmagnetic powders is from 1 to 20 μmol/m², preferably from 2 to 15 μmol/m², and more preferably from 3 to 8 μmol/m². The pH thereof is preferably between 3 and 6. The surfaces of these nonmagnetic powders are preferably covered with Al₂O₃, SiO₂, TiO₂, ZrO₂, SnO₂, Sb₂O₃, ZnO or Y₂O₃. Preferred in the point of dispersibility are Al₂O₃, SiO₂, TiO₂and ZrO₂, and more preferred are Al₂O₃, SiO₂and ZrO₂. They can be used in combination or alone. A method in which the surface treatment may be performed by coprecipitation, alternatively, surface treatment of particles may be previously performed to be covered with alumina in the first place, then the alumina-covered surface is covered with silica, or vice versa, according to purposes. The surface-covering layer may be porous layer, if necessary, but a homogeneous and dense surface is generally preferred.

[0118] Specific examples of nonmagnetic powders for use in the lower layer according to the present invention include HIT-100 (average particle size: 0.11 μm), and ZA-G1 (manufactured by Sumitomo Chemical Co., Ltd.) as alumina, Nanotite (average particle size: 0.06 μm) (manufactured by Showa Denko Co., Ltd.), α-hematite DPN-250, DPN-250BX (average long axis length: 0.16 μm, average short axis length: 0.02 μm, axis ratio: 7.45), DPN-245, DPN-270BX, DPN-550BX, DPN-550RX (average long axis length: 0.15 μm, average short axis length: 0.02 μm, axis ratio: 7.5), and DPN-650RX (manufactured by Toda Kogyo Co., Ltd.), α-hematite α-40 (manufactured by Titan Kogyo Co., Ltd.), α-hematite E270, E271, E300 and E303 (manufactured by Ishihara Sangyo Kaisha Ltd.) as iron oxide, TTO-51B (average particle size: from 0.01 to 0.03 μm), TTO-55A (average particle size: from 0.03 to 0.05 μm), TTO-55B (average particle size: from 0.03 to 0.05 μm), TTO-55C (average particle size: from 0.03 to 0.05 μm), TTO-55S (average particle size: from 0.03 to 0.05 μm), TTO-55D (average particle size: from 0.03 to 0.05 μm), and SN-100 (manufactured by Ishihara Sangyo Kaisha Ltd.), titanium oxide STT-4D (average particle size: 0.013 μm), STT-30D (average particle size: 0.09 μm), STT-30 (average particle size: 0.12 μm), STT-65C (average particle size: 0.12 μm) (manufactured by Titan Kogyo Co., Ltd.), titanium oxide MT-100S (average particle size: 0.015 μm, MT-100T (average particle size: 0.015 μm), MT-150W (average particle size: 0.015 μm), MT-500B (average particle size: 0.035 μm), MT-600B (average particle size: 0.050 μm), MT-100F, and MT-500HD (manufactured by Teika Co., Ltd.) as titanium oxide, FINEX-25 (average particle size: 0.5 μm) (manufactured by Sakai Chemical Industry Co., Ltd.) as zinc oxide, BF-1 (average particle size: 0.05 μm, BF-10 (average particle size: 0.06 μm), BF-20 (average particle size: 0.03 μm), and ST-M (manufactured by Sakai Chemical Industry Co., Ltd.) as barium sulfate, DEFIC-Y and DEFIC-R (manufactured by Dowa Mining Co., Ltd.), AS2BM and TiO₂P25 (manufactured by Nippon Aerosil Co., Ltd.), and 100A, 500A and calcined products thereof (manufactured by Ube Industries, Ltd.). Particularly preferred nonmagnetic powders are titanium dioxide and α-iron oxide.

[0119] Preparation of α-iron oxide (hematite) is performed as follows. α-Fe₂O₃powders are obtained from acicular goethite particles as precursor particles. Acicular goethite particles can be produced by any of the following methods.

[0120] (1) A method in which an aqueous alkali hydroxide solution is added to an aqueous ferrous salt solution in equivalent or more amount to thereby prepare a suspension having pH of 11 or more containing ferrous hydroxide colloid, then an oxygen-containing gas is introduced to the suspension obtained at 80° C. or less to form acicular goethite particles by the oxidation reaction of ferrous ions;

[0121] (2) A method in which an aqueous ferrous salt solution is reacted with an aqueous alkali carbonate solution to thereby prepare a suspension containing FeCO₃, then an oxygen-containing gas is introduced to the suspension obtained to form spindle-like goethite particles by the oxidation reaction of ferrous ions;

[0122] (3) A method in which an aqueous alkali hydroxide solution or an aqueous alkali carbonate solution is added to an aqueous ferrous salt solution in the amount of less than equivalent, thereby an aqueous ferrous salt solution containing ferrous hydroxide colloid is prepared, then an oxygen-containing gas is introduced to the aqueous ferrous salt solution obtained to form acicular goethite nucleus particles by the oxidation reaction of ferrous ions, thereafter an aqueous alkali hydroxide solution is added to the aqueous ferrous salt solution containing the acicular goethite nucleus particles in the amount of equivalent or more based on Fe²⁺ in the aqueous ferrous salt solution, then again an oxygen-containing gas is introduced to the aqueous ferrous salt solution to grow the acicular goethite nucleus particles; and

[0123] (4) A method in which an aqueous alkali hydroxide solution or an aqueous alkali carbonate solution is added to an aqueous ferrous salt solution in the amount of less than equivalent, thereby an aqueous ferrous salt solution containing ferrous hydroxide colloid is prepared, then an oxygen-containing gas is introduced to the aqueous ferrous salt solution obtained to form acicular goethite nucleus particles by the oxidation reaction of ferrous ions, thereafter the acicular goethite nucleus particles are grown in an acidic or neutral region.

[0124] Further, different kinds of elements such as Ni, Zn, P or Si, which are generally added to the reaction solution during the goethite particle-forming reaction for improving the properties of the powder, may be added. Acicular α-Fe₂O₃particles can be obtained by dehydrating acicular goethite particles, which are precursor particles, in the range of 200 to 500° C. and further, if necessary, annealing the particles by heat treatment at 350 to 800° C. A sintering inhibitor such as P, Si, B, Zr or Sb may be adhered to the surface of acicular goethite particles to be dehydrated or annealed. The reason why annealing by heat treatment at 350 to 800° C. is conducted is because it is preferred to fill the voids which have occurred on the surface of acicular α-Fe₂O₃particles obtained by the dehydration by melting the extreme surface of particles to obtain smooth surfaces.

[0125] The α-Fe₂O₃powder for use in the present invention can be obtained by dispersing acicular α-Fe₂O₃particles obtained by dehydration or annealing in an aqueous solution to make a suspension, adding Al compounds to the suspension obtained and adjusting the pH, covering the surface of acicular α-Fe₂O₃particles with the above Al compounds, filtering, washing, drying, pulverizing and, if necessary, performing other treatments such as deaeration, compaction and the like. Aluminum salt such as aluminum acetate, aluminum sulfate, aluminum chloride, and aluminum nitride, and aluminic acid alkali salt such as sodium aluminate can be used as the aluminum compound to be used. In this case, the addition amount of the Al compound is from 0.01 to 50% by weight in terms of Al based on the α-Fe₂O₃powder. If the content is less than 0.01% by weight, dispersion in the binder resin is insufficient and if it exceeds 50% by weight, Al compounds suspending around surfaces of particles unfavorably interact with each other. The nonmagnetic powder for use in the lower layer according to the present invention may be covered with one or two or more selected from the group consisting of P, Ti, Mn, Ni, Zn, Zr, Sn and Sb, as well as Si compounds, together with Al compounds. The content of these compounds used together with Al compounds is each from 0.01 to 50% by weight based on the α-Fe₂O₃powder. If the content is less than 0.01% by weight, the improvement of dispersibility by the addition can hardly be obtained, and if it exceeds 50% by weight, Al compounds suspending around surfaces of particles unfavorably interact with each other.

[0126] The producing method of titanium dioxide is as follows. The producing method of titanium dioxide mainly comprises a sulfuric acid process and a chlorine process. A sulfuric acid process comprises digesting raw ores of ilmenite with sulfuric acid and extracting Ti and Fe as sulfate. Iron sulfate is removed by crystallization-separation, the resulting titanyl sulfate solution is purified by filtration, water-containing titanium oxide is precipitated by thermal hydrolysis, the precipitated product is filtrated and washed, impurities are removed by washing, then a particle size-adjusting agent is added and calcined at 80 to 1,000° C., thereby crude titanium oxide is obtained. A rutile type and an anatase type are separated by the kind of nucleating agent added at hydrolysis. This crude titanium oxide is pulverized, graded, and surface treated. In a chlorine process, natural rutile and synthetic rutile are used as raw ores. Ores are chlorinated in a high temperature reduction state, Ti becomes TiCl₄and Fe becomes FeCl₂, and the iron oxide solidified by cooling is separated from the liquid TiCl₄. The crude TiCl₄obtained is purified by fraction, then a nucleating agent is added thereto and reacted with oxygen instantaneously at 1,000° C. or more, thereby crude titanium oxide is obtained. The finishing method for imparting to the crude titanium oxide formed in the oxidation decomposition process the property of pigment is the same as in the sulfuric acid process.

[0127] After the above titanium oxide material is dry ground, water and a dispersant are added thereto, grains are wet ground, and coarse grains are classified by means of a centrifugal separator. Subsequently, a fine grain slurry is put in a surface treatment bath and surface covering with metal hydroxide is conducted here. In the first place, a predetermined amount of an aqueous solution of salts such as Al, Si, Ti, Zr, Sb, Sn, Zn is added to the tank, acid or alkali is added to neutralize the solution, and surfaces of titanium oxide particles are covered with the hydroxide produced. The water-soluble salts by-produced are removed by decantation, filtration and washing, the pH of the slurry is adjusted finally and filtrated, and washed with pure water. The washed cake is dried using a spray drier or a band drier. The dried cake is finally ground by jet milling, thereby the final product is obtained.

[0128] Besides the water system, it is also possible to perform surface treatment by introducing AlCl₃and SiCl₄vapor to the titanium oxide powder, then water vapor is flowed to conduct surface treatment with Al and Si.

[0129] By the incorporation of carbon blacks into the lower layer, a desired micro Vickers' hardness can be obtained in addition to the well-known effects of reducing surface electrical resistance (Rs) and light transmittance. Further, it is also possible to obtain the effect of stocking a lubricant by the incorporation of carbon blacks into the lower layer. Furnace blacks for rubbers, thermal blacks for rubbers, carbon blacks for coloring, acetylene blacks, etc. can be used therefor. Carbon blacks used in the lower layer should optimize the following characteristics by the desired effects and sometimes more effects can be obtained by the combined use.

[0130] Carbon blacks for use in the lower layer according to the present invention have a specific surface area (S_BET) of from 100 to 500 m²/g, preferably from 150 to 400 m²/g, a DBP oil absorption of from 20 to 400 ml/100 g, preferably from 30 to 400 ml/100 g, an average particle size of from 5 to 80 nm, preferably from 10 to 50 nm, and more preferably from 10 to 40 nm, and a small amount of carbon blacks having an average particle size of larger than 80 nm may be contained in the lower layer. Carbon blacks for use in the lower layer have pH of from 2 to 10, a water content of from 0.1 to 10%, and a tap density of from 0.1 to 1 g/ml. Specific examples of carbon blacks for use in the present invention include BLACKPEARLES 2000 (average particle size: 15 nm), 1400 (average particle size: 13 nm), 1300 (average particle size: 13 nm), 1100 (average particle size: 14 nm), 1000, 900 (average particle size: 15 nm), 800, 880 and 700, L (average particle size: 24 nm), VULCAN XC-72 (average particle size: 30 nm), and P (average particle size: 19 nm) (manufactured by Cabot Co., Ltd.), #3050B, #3150B, #3250B (average particle size: 30 nm), #3750B, #3950B (average particle size: 16 nm), #950 (average particle size: 16 nm), #650B, #970B, #850B (average particle size: 18 nm), MA-600 (average particle size: 18 nm), MA-230, #4000 and #4010 (manufactured by Mitsubishi Kasei Corp.), CONDUCTEX SC (average particle size: 17 nm), SC-U (average particle size: 20 nm), 975 (average particle size: 20 nm), RAVEN 8800 (average particle size: 13 nm), 8000 (average particle size: 13 nm), 7000 (average particle size: 14 nm), 5750 (average particle size: 17 nm), 5250 (average particle size: 19 nm), 5000 (average particle size: 12 nm), 3500 (average particle size: 16 nm), 2100 (average particle size: 17 nm), 2000 (average particle size: 18 nm), 1800 (average particle size: 18 nm), 1500 (average particle size: 18 nm), 1255 (average particle size: 23 nm), 1250 (average particle size: 21 nm), and 1035 (average particle size: 27 nm) (manufactured by Columbia Carbon Co., Ltd.), Ketjen Black EC (average particle size: 30 nm) (manufactured by Akzo Co., Ltd.), and #80 (average particle size: 20 nm), #70 (average particle size: 27 nm), #60 (average particle size: 49 nm), #55 (average particle size: 68 nm), and Asahi Thermal (average particle size: 72 nm) (manufactured by Asahi Carbon Co., Ltd.). Carbon blacks having an average particle size of larger than 80 nm which may be used in the lower layer include #50 (average particle size: 94 nm) and #35 (average particle size: 82 nm) (manufactured by Asahi Carbon Co., Ltd.). Carbon blacks for use in the present invention may previously be surface-treated with a dispersant, may be grafted with a resin, or a part of the surface thereof may be graphitized before use. Carbon blacks may be previously dispersed in a binder before addition to the coating solution. Carbon blacks can be used within the range not exceeding 50% by weight based on the above inorganic powders and not exceeding 40% by weight based on the total weight of the nonmagnetic layer. These carbon blacks can be used alone or in combination. Regarding carbon blacks for use in the present invention, for example, the disclosure in Handbook of Carbon Blacks(edited by Carbon Black Association of Japan) may be referred to.

[0131] Organic powders can be used in the lower layer according to the purpose. Examples of such organic powders include an acryl styrene resin powder, a benzoguanamine resin powder, a melamine resin powder, and a phthalocyanine pigment. In addition, at least one of a polyolefin resin powder, a polyester resin powder, a polyamide resin powder, a polyimide resin powder, and a polyethylene fluoride resin powder can also be used. The producing methods thereof are disclosed in JP-A-62-18564 and JP-A-60-255827.

[0132] Binder

[0133] Binder resins, lubricants, dispersants, additives, solvents, dispersing methods, etc., used for the magnetic layer described below can be used in the lower layer and the backing layer. In particular, with respect to the amounts and the kinds of binder resins, and the amounts and the kinds of additives and dispersants, well-known prior art techniques regarding the magnetic layer can be applied in the lower layer according to the present invention.

[0134] Conventionally well-known thermoplastic resins, thermosetting resins, reactive resins and mixtures of these resins are used as a binder in the present invention. Thermoplastic resins having a glass transition temperature of from −100 to 150° C., a number average molecular weight of from 1,000 to 200,000, preferably from 10,000 to 100,000, and a polymerization degree of about 50 to 1,000 can be used in the present invention.

[0135] Examples thereof include polymers or copolymers containing as a constituting unit the following compounds, such as vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylate, vinylidene chloride, acrylonitrile, methacrylic acid, methacrylate, styrene, butadiene, ethylene, vinyl butyral, vinyl acetal and vinyl ether; polyurethane resins and various rubber resins. Examples of thermosetting resins and reactive resins usable in the present invention include phenol resins, epoxy resins, curable type polyurethane resins, urea resins, melamine resins, alkyd resins, acrylic reactive resins, formaldehyde resins, silicone resins, epoxy-polyamide resins, mixtures of polyester resins and isocyanate prepolymers, mixtures of polyesterpolyol and polyisocyanate, and mixtures of polyurethane and polyisocyanate. Details on these resins are described in Plastic Handbook, published by Asakura Shoten. It is also possible to use well-known electron beam curable type resins in each layer. Examples of these resins and producing methods are disclosed in detail in JP-A-62-256219. These resins can be used alone or in combination. Examples of preferred combinations include at least one selected from vinyl chloride resins, vinyl chloride-vinyl acetate copolymers, vinyl chloride-vinyl acetate-vinyl alcohol copolymers, and vinyl chloride-vinyl acetate-maleic anhydride copolymers with polyurethane resins, or combinations of these resins with polyisocyanate.

[0136] As polyurethane resins, those having well-known structures can be used, e.g., polyester polyurethane, polyether polyurethane, polyether polyester polyurethane, polycarbonate polyurethane, polyester polycarbonate polyurethane, polycaprolactone polyurethane, etc. Preferably, at least one polar group selected from the following groups is introduced into the above binders by copolymerization or addition reaction for the purpose of further improving the dispersibility and the durability, e.g., —COOM, —SO₃M, —OSO₃M, —P═O(OM)₂, —O—P═O(OM)₂(wherein M represents a hydrogen atom, or an alkali metal salt group), —NR², —N⁺R₃(R represents a hydrocarbon group), an epoxy group, —SH, or —CN. The content of the polar group is from 10⁻¹to 10⁻⁸mol/g, preferably from 10⁻²to 10⁻⁶mol/g. It is preferred that polyurethane resins have at least one OH group at each terminal of polyurethane molecule, i.e., two or more in total, in addition to the above polar groups. As OH groups form three dimensional network structure by crosslinking with the polyisocyanate curing agent, they are preferably contained in the molecule as many as possible. In particular, as the reactivity with the curing agent is high, OH groups are preferably present at terminals of the molecule. It is preferred for polyurethane to have 3 or more OH groups, particularly preferably 4 or more OH groups, at terminals of the molecule. When polyurethane is used in the present invention, the polyurethane has a glass transition temperature of from −50 to 150° C., preferably from 0 to 100° C., breaking extension of from 100 to 2,000%, breaking stress of from 0.05 to 10 kg/mm², and a yielding point of from 0.05 to 10 kg/mm². Due to these physical properties, coated film exhibiting good mechanical properties at high rotation rate of preferably 1,800 rpm or more, more preferably 3,000 rpm or more, can be obtained.

[0137] Specific examples of binders for use in the present invention include VAGH, VYHH, VMCH, VAGF, VAGD, VROH, VYES, VYNC, VMCC, XYHL, XYSG, PKHH, PKHJ, PKHC, and PKFE (manufactured by Union Carbide Co., Ltd.), MPR-TA, MPR-TA5, MPR-TAL, MPR-TSN, MPR-TMF, MPR-TS, MPR-TM, and MPR-TAO (manufactured by Nisshin Chemical Industry Co., Ltd.), 1000W, DX80, DX81, DX82, DX83, and 100FD (manufactured by Electro Chemical Industry Co., Ltd.), MR-104, MR-105, MR-110, MR-100, MR-555, and 400X-110A (manufactured by Nippon Zeon Co., Ltd.) as vinyl chloride copolymers; Nippollan N2301, N2302, and N2304 (manufactured by Nippon Polyurethane Co., Ltd.), Pandex T-5105, T-R3080, T-5201, Burnock D-400, D-210-80, Crisvon 6109 and 7209 (manufactured by Dainippon Chemicals and Ink.), Vylon UR8200, UR8300, UR8700, RV530, and RV280 (manufactured by Toyobo Co., Ltd.), polycarbonate polyurethane, Daipheramine 4020, 5020, 5100, 5300, 9020, 9022, and 7020 (manufactured by Dainichi Seika K.K.), polyurethane, MX5004 (manufactured by Mitsubishi Kasei Corp.), polyurethane, Sunprene SP-150 (manufactured by Sanyo Chemical Industries Co. Ltd.), polyurethane, Salan F310 and F210 (manufactured by Asahi Chemical Industry Co., Ltd.) as polyurethane resins, etc.

[0138] The amount of the binder for use in the nonmagnetic layer and the magnetic layer according to the present invention is from 5 to 50 wt %, preferably from 10 to 30 wt %, based on the amount of the nonmagnetic powder or the magnetic powder. When vinyl chloride resins are used, the amount thereof is from 5 to 30 wt %, when polyurethane resins are used, the amount of the polyurethane resin is from 2 to 20 wt %, and it is preferred polyisocyanate is used in an amount of from 2 to 20 wt % in combination with these resins. However, for instance, when head corrosion is caused by a slight amount of chlorine due to dechlorination, it is possible to use polyurethane alone or a combination of polyurethane and isocyanate alone.

[0139] The magnetic recording medium according to the present invention may comprise two or more layers. Accordingly, the amount of the binder, the amounts of vinyl chloride resins, polyurethane resins, polyisocyanate or other resins contained in the binder, the molecular weight of each resin constituting the magnetic layer, the amount of polar groups, or the above-described physical properties of resins can of course be varied in the nonmagnetic layer and the magnetic layer, according to necessity. These factors should be rather optimized in respective layers. Well-known techniques with respect to multilayer magnetic layers can be used in the present invention. For example, when the amount of the binder is varied in each layer, it is effective to increase the amount of the binder contained in the magnetic layer to reduce scratches on the surface of the magnetic layer. For improving the head touch against the head, it is effective to increase the amount of the binder in the nonmagnetic layer to impart flexibility.

[0140] Examples of the polyisocyanates which can be used in the present invention include isocyanates, e.g., tolylenediisocyanate, 4,4′-diphenylmethanediisocyanate, hexamethylenediisocyanate, xylylenediisocyanate, naphthylene-1,5-diisocyanate, o-toluidinediisocyanate, isophoronediisocyanate, and triphenylmethanetriisocyanate; reaction products of these isocyanates with polyalcohols; and polyisocyanates formed by condensation reaction of isocyanates. These polyisocyanates are commercially available under the trade names of Coronate L, Coronate HL, Coronate 2030, Coronate 2031, Millionate MR, and Millionate MTL (manufactured by Nippon Polyurethane Co., Ltd.), Takenate D-102, Takenate D-110N, Takenate D-200, and Takenate D-202 (manufactured by Takeda Chemical Industries, Ltd.), and Desmodur L, Desmodur IL, Desmodur N, and Desmodur HL (manufactured by Sumitomo Bayer Co., Ltd.). These may be used alone or in combinations of two or more thereof, taking advantage of a difference in curing reactivity in each layer.

[0141] Carbon Black, Abrasive

[0142] Examples of carbon blacks for use in the magnetic layer according to the present invention include furnace blacks for rubbers, thermal blacks for rubbers, carbon blacks for coloring, acetylene blacks, etc. Carbon blacks for use in the magnetic layer of the present invention preferably have a specific surface area (S_BET) of from 5 to 500 m²/g, a DBP oil absorption of from 10 to 400 ml/100 g, an average particle size of from 5 to 300 nm, pH of from 2 to 10, a water content of from 0.1 to 10%, and a tap density of from 0.1 to 1 g/ml. Specific examples of carbon blacks for use in the magnetic layer of the present invention include BLACKPEARLES 2000 (average particle size: 15 nm), 1300 (average particle size: 13 nm), 1000 (average particle size: 16 nm), 900 (average particle size: 15 nm), 905, 800 (average particle size: 17 nm), and 700 (average particle size: 18 nm), VULCAN XC-72 (average particle size: 30 nm), and STERLING FT (average particle size: 180 nm) (manufactured by Cabot Co., Ltd.), #80 (average particle size: 20 nm), #60 (average particle size: 49 nm), #55 (average particle size: 68 nm), #50 (average particle size: 94 nm), and #35 (average particle size: 94 nm) (manufactured by Asahi Carbon Co., Ltd.), #2400B (average particle size: 15 nm), #2300 (average particle size: 15 nm), #900 (average particle size: 16 nm), #1000 (average particle size: 18 nm), #30 (average particle size: 30 nm), #40 (average particle size: 20 nm), and #10B (average particle size: 84 nm) (manufactured by Mitsubishi Kasei Corp.), CONDUCTEX SC (average particle size: 17 nm), RAVEN 150 (average particle size: 18 nm), 50 (average particle size: 21 nm), 40 (average particle size: 24 nm), and 15 (average particle size: 27 nm), RAVEN-MT-P (average particle size: 275 nm) and RAVEN-MT-P beads (average particle size: 330 nm) (manufactured by Columbia Carbon Co., Ltd.), Ketjen Black EC40 (average particle size: 30 nm) (manufactured by Akzo Co., Ltd.), and Thermal Black (average particle size: 270 nm) (manufactured by Cancarb Co., Ltd.). Carbon blacks for use in the present invention may previously be surface-treated with a dispersant, may be grafted with a resin, or a part of the surface thereof may be graphitized before use. Carbon blacks may be previously dispersed in a binder before addition to the magnetic coating solution. These carbon blacks may be used alone or in combination. Carbon blacks are preferably used in an amount of from 0.1 to 30 wt % based on the amount of the ferromagnetic powder. Carbon blacks can serve various functions such as preventing static charges, reducing a friction coefficient, imparting a light-shielding property and improving a film strength. Such functions vary depending upon the kind of carbon blacks to be used. Accordingly, it is of course possible in the present invention to select and determine the kinds, the amounts and the combinations of the carbon blacks to be added to the upper magnetic layer and the lower nonmagnetic layer, on the basis of the above mentioned various properties such as the particle size, the oil absorption amount, the electroconductivity and the pH value, or these should be rather optimized in respective layers. Regarding carbon blacks for use in the magnetic layer of the present invention, for example, the disclosure in Handbook of Carbon Blacks(edited by Carbon Black Association of Japan) can be referred to.

[0143] As the abrasive usable in the present invention, well-known materials essentially having a Mohs' hardness of 6 or more may be used alone or in combination. Examples of such abrasives include α-alumina having an alpha-conversion rate of 90% or more, β-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum, artificial diamond, silicon nitride, silicon carbide, titanium carbide, titanium oxide, silicon dioxide, and boron nitride. Composites composed of these abrasives (abrasives obtained by surface-treating with other abrasives) may also be used. Compounds or elements other than the main component are often contained in these abrasives, but the intended effect can be attained so far as the content of the main component is 90% or more. Abrasives essentially have a particle size of from 0.01 to 2 μm, preferably from 0.01 to 1 μm, more preferably from 0.01 to 0.5 μm, and particularly preferably from 0.01 to 0.3 μm, and, in particular, for improving electromagnetic characteristics, abrasives having narrow particle size distribution are preferred. For improving durability, a plurality of abrasives each having a different particle size may be combined according to necessity, or a single abrasive having a broad particle size distribution may be employed so as to attain the same effect as such a combination. Preferably, abrasives for use in the present invention have a tap density of from 0.3 to 2 g/ml, a water content of from 0.1 to 5%, a pH value of from 2 to 11 and a specific surface area (S_BET) of from 1 to 30 m²/g. The shape of the abrasives to be used in the present invention may be any of acicular, spherical and die-like shapes. Preferably, the abrasive has a shape partly with edges, because a high abrasive property is given. Specific examples of abrasives for use in the present invention include as examples of α-iron oxide, AKP-12 (average particle size: 0.50 μm), AKP-15 (average particle size: 0.45 μm), AKP-20 (average particle size: 0.39 μm), AKP-30 (average particle size: 0.23 μm), AKP-50 (average particle size: 0.16 μm), HIT-20, HIT-30, HIT-55 (average particle size: 0.20 μm), HIT-60, HIT-70 (average particle size: 0.15 μm), HIT-80, and HIT-100 (average particle size: 0.11 μm) (manufactured by Sumitomo Chemical Co., Ltd.), ERC-DBM (average particle size: 0.22 μm), HP-DBM (average particle size: 0.22 μm), and HPS-DBM (average particle size: 0.19 μm) (manufactured by Reynolds International Inc.), WA10000 (average particle size: 0.29 μm) (manufactured by Fujimi Kenma K.K.), UB20 (average particle size: 0.13 μm) (manufactured by Uemura Kogyo K. K.), G-5, as examples of chromium oxide, G-5 (average particle size: 0.32 μm), Kromex U2 (average particle size: 0.18 μm), and Kromex U1 (average particle size: 0.17 μm) (manufactured by Nippon Chemical Industrial Co., Ltd.), as examples of α-iron oxide, TF100 (average particle size: 0.14 μm) and TF140 (average particle size: 0.17 μm) (manufactured by Toda Kogyo Co., Ltd.), as examples of silicon carbide, β-Random Ultrafine (average particle size: 0.16 μm) (manufactured by Ibiden Co., Ltd.), and as examples of silicon dioxide, B-3 (average particle size: 0.17 μm) (manufactured by Showa Mining Co., Ltd.). These abrasives may also be added to a nonmagnetic layer, if necessary. By incorporating abrasives into a nonmagnetic layer, it is possible to control the surface shape or prevent abrasives from protruding. Particle sizes and amounts of abrasives to be added to a magnetic layer and a nonmagnetic layer should be selected independently at optimal values.

[0144] In the case of high capacity floppy discs of rotation rate of 1,800 rpm or more, in particular 3,000 rpm or more, it is preferred to use a diamond powder as an abrasive.

[0145] Diamond powders which can be used in the present invention are essential to have the average particle size of 2 μm or less, preferably from 0.01 to 1.0 μm, more preferably from 0.05 to 0.8 μm, and most preferably from 0.05 to 0.3 μm. When the average particle size is less than 0.01 μm, the effect of improving durability is liable to lower as compared to the addition amount, while when it is larger than 2.0 μm, noise is liable to increase even though durability is improved, which is not suitable for achieving the object of the present invention.

[0146] In the present invention, the maximum size of each diamond particle is taken as a particle size, and the average value of determined values of 500 particles by random sampling by means of an electron microscope is taken as an average particle size.

[0147] The addition amount of a diamond powder in the present invention is from 0.01 to 5 wt %, preferably from 0.03 to 3.00 wt %, based on the weight of the ferromagnetic powder. If the addition amount is less than 0.01 wt %, durability is obtained with difficulty and if it exceeds 5 wt %, the effect of noise reduction by means of the addition of a diamond powder is reduced.

[0148] The addition amount and the average particle size of a diamond powder are regulated within the above ranges from the viewpoint of noise and durability, but the addition amount thereof is preferably as small as possible in view of noise. It is preferred in the magnetic recording medium of the present invention to appropriately select the amount and the average particle size suitable for magnetic recording devices from the above ranges.

[0149] Further, with respect to the particle size distribution of a diamond powder, it is preferred that the number of particles having the particle size of 200% or more of the average particle size accounts for 5% or less of the entire number of diamond particles, and the number of particles having the particle size of 50% or less of the average particle size accounts for 20% or less of the entire number of diamond particles. The maximum value of the particle size of the diamond powder for use in the present invention is about 3.00 μm, preferably about 2.00 μm, and the minimum value is about 0.01 μm, preferably about 0.02 μm.

[0150] Particle size distribution is found by counting numbers of respective sizes based on the average particle size at the time of particle size determination.

[0151] Particle size distribution of a diamond powder also influences durability and noise of the magnetic medium. If the particle size distribution is broader than the above-described range, the effect corresponding to the average particle size set up in the present invention deviates as described above, i.e., if many particles have too large particle sizes, noise is increased and the head is scratched. While when there exist many particles having too small particle sizes, abrasive effect is insufficient. Further, a diamond powder having extremely narrow particle size distribution is expensive, therefore, the above-described range is economically advantageous as well.

[0152] Diamond powders can be used in combination with conventionally used abrasives, e.g., an alumina abrasive, in the present invention. Better effects on durability and SN ratio are obtained when a small amount of a diamond powder alone is used but, for economical reason, etc., an alumina abrasive can be used in combination with a diamond powder in an amount of preferably from 1 to 30 wt %, more preferably from 3 to 25 wt %, based on the magnetic powder. In this case, addition amount of abrasives can be considerably reduced due to the addition of a diamond powder as compared with the amount necessary to ensure durability with alumina alone, which is preferred in view of the security of durability and the reduction of noise.

[0153] Producing methods of a diamond powder of a micrometer size include (1) a static high pressure method, (2) an explosion method, and (3) a vapor phase method. In a static high pressure method (1), a crystal having a particle size of several 10 μm or more is prepared in the first place, and the resulting crystal is pulverized to obtain a diamond powder of a sub-micrometer size. In an explosion method (2), extra high pressure is generated by shock wave by means of the explosion of an explosive, and a black smoke is converted to a diamond by making use of the generated extra high pressure. The diamond produced by this method is a polycrystalline diamond which is said to have a primary particle of something between 20 Å and 50 Å. In a vapor phase method (3), a gaseous compound containing a carbon such as a hydrocarbon is charged into a closed container under the condition of normal pressure or less with a hydrogen gas, a high temperature zone is formed therein by plasma, etc., and the starting compound is decomposed to precipitate a diamond on a substrate, e.g., Si or Mo.

[0154] Specific examples of diamond powders include LS600F, LS600T, LS600F coated products (coated products coated with 30% or 56% nickel), LS-NPM and BN2600 (manufactured by LANDS SUPERABRASIVES, CO.), which are preferred as diamond powders with arbitrary particle sizes of from 0 to 100 μm are available. Besides the above, IRM 0-1/4 (average particle size: 0.12 μm), and IRM 0-1 (average particle size: 0.60 μm) (manufactured by Tomei Diamond Industrial Co., Ltd.) can be used.

[0155] Additive

[0156] As additives which can be used in the magnetic layer and the nonmagnetic layer of the present invention, those having a lubrication effect, an antistatic effect, a dispersing effect and a plasticizing effect may be used, and by the combined use of additives, comprehensive improvement of capacities can be contrived. As additives having a lubricating effect, lubricants giving remarkable action on adhesion caused by the friction of surfaces of materials with each other are used. Lubricants are classified into two types. Lubricants used for a magnetic recording medium cannot be judged completely whether they show fluid lubrication or boundary lubrication, but according to general concepts they are classified into higher fatty acid esters, liquid paraffins and silicon derivatives showing fluid lubrication and long chain fatty acids, fluorine surfactants and fluorine-containing high polymers showing boundary lubrication. In a coating type magnetic recording medium, lubricants exist in a state dispersed in a binder or in a state partly adsorbed onto the surface of a ferromagnetic powder, and they migrate to the surface of a magnetic layer. The speed of migration depends on whether the compatibility of the binder and the lubricant is good or bad. The speed of migration is slow when the compatibility of the binder and the lubricant is good and the migration speed is fast when the compatibility is bad. As one idea as to good or bad of the compatibility, there is a means of comparison of dissolution parameters of both. A nonpolar lubricant is effective for fluid lubrication and a polar lubricant is effective for boundary lubrication. In the present invention, at least three in total of these higher fatty acid ester showing fluid lubrication and long chain fatty acid showing boundary lubrication having respectively different characteristics are preferably used in combination to obtain high capacity, high density and high durability. Solid lubricants can also be used in combination with these.

[0157] Examples of solid lubricants which can be used in the present invention include molybdenum disulfide, tungsten graphite disulfide, boron nitride, and graphite fluoride. Examples of long chain fatty acids showing boundary lubrication include monobasic fatty acids having from 10 to 24 carbon atoms (which may contain an unsaturated bond or which may be branched) and metal salts thereof (e.g., with Li, Na, K or Cu). Examples of fluorine surfactants and fluorine-containing high polymers include fluorine-containing silicons, fluorine-containing alcohols, fluorine-containing esters, fluorine-containing alkyl sulfates and alkali metal salts thereof. Examples of higher fatty acid esters showing fluid lubrication include mono-fatty acid esters, di-fatty acid esters or tri-fatty acid esters composed of a monobasic fatty acid having from 10 to 24 carbon atoms (which may contain an unsaturated bond or may be branched) and any one of mono-, di-, tri-, tetra-, penta- and hexa-alcohols having from 2 to 12 carbon atoms (which may contain an unsaturated bond or may be branched), and fatty acid esters of monoalkyl ethers of alkylene oxide polymers. In addition to the above, examples further include liquid paraffins, and as silicon derivatives, silicone oils such as dialkylpolysiloxane (the alkyl has from 1 to 5 carbon atoms), dialkoxypolysiloxane (the alkoxy has from 1 to 4 carbon atoms), monoalkyl-monoalkoxypolysiloxane (the alkyl has from 1 to 5 carbon atoms and the alkoxy has from 1 to 4 carbon atoms), phenylpolysiloxane, and fluoroalkylpolysiloxane (the alkyl has from 1 to 5 carbon atoms), silicons having a polar group, fatty acid-modified silicons and fluorine-containing silicons.

[0158] Examples of other lubricants which can be used in the present invention include alcohols such as mono-, di-, tri-, tetra-, penta- or hexa-alcohols having from 12 to 22 carbon atoms (which may contain an unsaturated bond or may be branched), alkoxy alcohols having from 12 to 22 carbon atoms, and fluorine-containing alcohols, polyethylene waxes, polyolefins such as polypropylenes, ethylene glycols, polyglycols such as polyethylene oxide waxes, alkyl phosphates and alkali metal salts thereof, alkyl sulfates and alkali metal salts thereof, polyphenyl ethers, fatty acid amides having from 8 to 22 carbon atoms, and aliphatic amines having from 8 to 22 carbon atoms.

[0159] Examples of additives having an antistatic effect, a dispersing effect and a plasticizing effect include phenylphosphonic acids, specifically PPA (manufactured by Nissan Chemical Industries, Ltd.), etc., α-naphthylphosphoric acids, phenylphosphoric acids, diphenylphosphoric acids, p-ethylbenzenephosphonic acids, phenylphosphinic acids, aminoquinones, various kinds of silane coupling agents, titanium coupling agents, fluorine-containing alkyl sulfates and alkali metal salts thereof.

[0160] A lubricant particularly preferably used in the present invention are fatty acids and fatty acid esters, in addition, other different lubricants and additives can be used in combination with them as well. Specific examples thereof are exemplified below. As fatty acid, examples of saturated fatty acids include caprylic acid (C₇H₁₅COOH, melting point: 16° C.), pelargonic acid (C₈H₁₇COOH, melting point: 15° C.), capric acid (C₉H₁₉COOH, melting point: 31.5° C.), undecylic acid (C₁₀H₂₁COOH, melting point: 28.6° C.), lauric acid (C₁₁H₂₃COOH, melting point: 44° C.), specifically NAA-122 (manufactured by Nippon Oils and Fats Co., Ltd.), tridecylic acid (C₁₂H₂₅COOH, melting point: 45.5° C.), myristic acid (C₁₃H₂₇COOH, melting point: 58° C.), specifically NAA-142 (manufactured by Nippon Oils and Fats Co., Ltd.), pentadecylic acid (C₁₄H₂₉COOH, melting point: 53 to 54° C.), palmitic acid (C₁₅H₃₁COOH, melting point: 63 to 64° C.), specifically NAA-160 (manufactured by Nippon Oils and Fats Co., Ltd.), heptadecylic acid (C₁₆H₃₃COOH, melting point: 60 to 61° C.), stearic acid (C₁₇H₃₅COOH, melting point: 71.5 to 72° C.), specifically NAA-173K (manufactured by Nippon Oils and Fats Co., Ltd.), nonadecanoic acid (C₁₈H₃₇COOH, melting point: 68.7° C.), arachic acid (C₁₉H₃₉COOH, melting point: 77° C.), and behenic acid (C₂₁H₄₃COOH, melting point: 81 to 82° C.). Examples of unsaturated fatty acids include oleic acid (C₁₇H₃₃COOH(cis), melting point: 16° C.), specifically oleic acid manufactured by Kanto Kagaku Co., Ltd., elaidic acid (C₁₇H₃₃COOH(trans), melting point: 44 to 45° C.), specifically elaidic acid manufactured by Wako Pure Chemical Industries Ltd., cetoleic acid (C₂₁H₄₁COOH, melting point: 33.7° C.), erucic acid (C₂₁H₄₁COOH, melting point: 33.4 to 34° C.), specifically erucic acid manufactured by Nippon Oils and Fats Co., Ltd., brassidic acid (C₂₁H₄₁COOH(trans), melting point: 61.5° C.), linoleic acid (C₁₇H₃₁COOH, boiling point: 228° C. (14 mm)), and linolenic acid (C₁₇H₂₉COOH, boiling point: 197° C. (4 mm)). Examples of branched saturated fatty acids include isostearic acid (CH₃CH(CH₃)(CH₂)₁₄COOH, melting point: 67.6 to 68.1° C.).

[0161] Examples of esters are described below. Examples of laurates include isocetyl laurate (C₁₁H₂₃COOCH₂CH(C₆H₁₃)C₈H₁₇), oleyl laurate (C₁₁H₂₃COOC₁₈H₃₅), and stearyl laurate (C₁₁H₂₃COOC₁₈H₃₇); examples of myristates include isopropyl myristate (C₁₃H₂₇COOCH(CH₃)₂), specifically Enujerubu IPM (manufactured by Shin-Nihon Rika Co., Ltd.), butyl myristate (C₁₃H₂₇COOC₄H₉), isobutyl myristate (C₁₃H₂₇COO-iso-C₄H₉), specifically Enujerubu IBM (manufactured by Shin-Nihon Rika Co., Ltd.), heptyl myristate (C₁₃H₂₇COOC₇H₁₅), octyl myristate (C₁₃H₂₇COOC₈H₁₇), isooctyl myristate (C₁₃H₂₇COOCH₂CH(C₂H₅)C₄H₉), and isocetyl myristate (C₁₃H₂₇COOCH₂CH(C₆H₁₃)C₈H₁₇).

[0162] Examples of palmitates include octyl palmitate (C₁₅H₃₁COOC₈H₁₇), decyl palmitate (C₁₅H₃₁COOC₁₀H₂₁), isooctyl palmitate (C₁₅H₃₁COOCH₂CH(C₂H₅)C₄H₉), isocetyl palmitate (C₁₅H₃₁COOCH₂CH(C₆H₁₃)C₈H₁₇), 2-octyldodecyl palmitate (C₁₅H₃₁COOCH₂CH(C₈H₁₇)C₁₂H₂₅), 2-hexyldodecyl palmitate (C₁₅H₃₁COOCH₂CH(C₆H₁₃)C₁₂H₂₅), and oleyl palmitate (C₁₅H₃₁COOC₁₈H₃₅).

[0163] Examples of stearates include propyl stearate (C₁₇H₃₅COOC₃H₇), isopropyl stearate (C₁₇H₃₅COOCH(CH₃)₂), butyl stearate (C₁₇H₃₅COOC₄H₉), specifically butyl stearate manufactured by Nippon Oils and Fats Co., Ltd., sec-butyl stearate (C₁₇H₃₅COOCH(CH₃)C₂H₅), tert-butyl stearate (C₁₇H₃₅COOC(CH₃)₃), amyl stearate (C₁₇H₃₅COOC₅H₁₁), isoamyl stearate (C₁₇H₃₅COOCH₂CH₂CH(CH₃)₂), hexyl stearate (C₁₇H₃₅COOC₆H₁₃), heptyl stearate (C₁₇H₃₅COOC₇H₁₅), specifically MYB-185 (manufactured by Matsumoto Yushi Co., Ltd.), octyl stearate (C₁₇H₃₅COOC₈H₁₇), specifically N-octyl stearate manufactured by Nippon Oils and Fats Co., Ltd., isooctyl stearate (C₁₇H₃₅COO-iso-C₈H₁₇), specifically FAL-123 (manufactured by Takemoto Yushi Co., Ltd.), decyl stearate (C₁₇H₃₅COOC₁₀H₂₁), isodecyl stearate (C₁₇H₃₅COO-iso-C₁₀H₂₁), dodecyl stearate (C₁₇H₃₅COOC₁₂H₂₅), isotridecyl stearate (C₁₇H₃₅COO-iso-C₁₃H₂₇), 2-ethylhexyl stearate (C₁₇H₃₅COOCH₂CH—(C₂H₅)C₄H₉), isohexadecyl stearate or isocetyl stearate (C₁₇H₃₅COO-iso-C₁₆H₃₃), specifically Enujerubu HDS (manufactured by Shin-Nihon Rika Co., Ltd.), isostearyl stearate (C₁₇H₃₅COO-iso-C₁₈H₃₇), and oleyl stearate (C₁₇H₃₅COOC₁₈H₃₇).

[0164] Examples of behenates include isotetracosyl behenate (C₂₁H₄₃COOCH₂CH (C₆H₁₃)C₁₂H₂₅), specifically Enujerubu DTB (manufactured by Shin-Nihon Rika Co., Ltd.).

[0165] Examples of glycol type esters include those disclosed in JP-A-59-227030 and JP-A-59-65931, e.g., butoxyethyl stearate (C₁₇H₃₅COOCH₂CH₂OC₄H₉), butoxyethyl oleate (C₁₇H₃₃COOCH₂CH₂OC₄H₉), diethylene glycol monobutyl ether stearate or butoxyethoxyethyl stearate (C₁₇H₃₅COO(CH₂CH₂O)₂—C₄H₉), tetraethylene glycol monobutyl ether stearate (C₁₇H₃₅COO(CH₂CH₂O)₄C₄H₉), diethylene glycol monophenyl ether stearate (C₁₇H₃₅COO(CH₂CH₂O)₂C₆H₆), and diethylene glycol mono-2-ethylhexyl ether stearate (C₁₇H₃₅COO(CH₂CH₂O)₂CH₂CH(C₂H₅)C₄H₉).

[0166] Examples of isostearates include isocetyl isostearate (iso-C₁₇H₃₅COOCH₂CH(C₆H₁₃)C₈H₁₇), specifically I.C.I.S. (manufactured by Higher Alcohol Co., Ltd.), oleyl isostearate (iso-C₁₇H₃₅COOC₁₈H₃₅), stearyl isostearate (iso-C₁₇H₃₅COOC₁₈H₃₇), isostearyl isostearate (iso-C₁₇H₃₅COO-iso-C₁₈H₃₇), and eicosenyl isostearate (iso-C₁₇H₃₅COOC₂₂H₄₃).

[0167] Examples of oleates include butyl oleate (C₁₇H₃₃COOC₄H₉), specifically Enujerubu BO (manufactured by Shin-Nihon Rika Co., Ltd.), oleyl oleate (C₁₇H₃₃COOC₁₈H₃₅), and ethylene glycol dioleyl (C₁₇H₃₃COOCH₂CH₂OCOC₁₇H₃₃).

[0168] Examples of erucates include oleyl erucic acid (C₂₁H₄₁COOC₁₈H₃₅).

[0169] Examples of diesters include dioleyl maleate (C₁₈H₃₅OCOCH═CHCOOC₁₈H₃₅), neopentyl glycol didecanoate (C₁₀H₂₁COOCH₂C(CH₃)₂CH₂OCOC₁₀H₂₁), ethylene glycol dilaurate (C₁₁H₂₃COOCH₂CH₂OCOC₁₁H₂₃), ethylene glycol dioleyl (C₁₇H₃₃COOCH₂CH₂OCOC₁₇H₃₃), 1,4-butanediol distearate (C₁₇H₃₅COO(CH₂)₄OCOC₁₇H₃₅), 1,4-butanediol dibehenate (C₂₁H₄₃COO(CH₂)₄OCOC₂₁H₄₃), 1,10-decanediol dioleyl (C₁₇H₃₃COO(CH₂)₁₀OCOC₁₇H₃₃), and 2-butene-1,4-diol cetoleyl (C₂₁H₄₁COOCH₂CH═CHCH₂OCOC₂₁H₄₁).

[0170] Examples of triesters include caprylic acid triglyceride (C₇H₁₅COOCH₂CH(OCOC₇H₁₅)CH₂OCOC₇H₁₅.

[0171] In addition to the above-described fatty acid esters and fatty acids, examples of additives which can be used include alcohols such as oleyl alcohol (C₁₈H₃₅OH), stearyl alcohol (C₁₈H₃₇OH), and lauryl alcohol (C₁₂H₂₅OH).

[0172] Examples of fatty acid amides include lauric acid amide (C₁₁H₂₃CONH₂), specifically lauric acid amide manufactured by Tokyo Kasei Co., Ltd., myristic acid amide (C₁₃H₂₇CONH₂), palmitic acid amide (C₁₅H₃₁CONH₂), oleic acid amide (cis-C₈H₁₇CH═CH(CH₂)₇CONH₂), specifically Armoslip CP-P (manufactured by Lion Akzo Co., Ltd.), erucic acid amide (cis-C₈H₁₇CH═CH(CH₂)₁₁CONH₂), specifically Armoslip E (manufactured by Lion Akzo Co., Ltd.), and stearic acid amide (C₁₇H₃₅CONH₂), specifically Armide HT (manufactured by Lion Akzo Co., Ltd.).

[0173] Examples of silicone compounds include TAV-3630, TA-3, and KF-69 (manufactured by Shin-Etsu Chemical Co., Ltd.).

[0174] Additionally, examples of other additives which may be used include nonionic surfactants such as alkylene oxides, glycerols, glycidols and alkylphenol-ethylene oxide adducts; cationic surfactants such as cyclic amines, ester amides, quaternary ammonium salts, hydantoin derivatives, heterocyclic compounds, phosphoniums and sulfoniums; anionic surfactants containing an acidic group such as carboxylic acid, sulfonic acid, phosphoric acid, sulfate groups or phosphate groups; and amphoteric surfactants such as amino acids, aminosulfonic acids, sulfates or phosphates of amino alcohols and alkylbetains. The details of these surfactants are described in Handbook of Surfactants(published by Sangyo Tosho Co., Ltd.). These lubricants and antistatic agents may not always be 100% pure and may contain impurities such as isomers, non-reacted materials, byproducts, decomposed products and oxides, in addition to the main component. However, the content of such impurities is preferably 30% or less, more preferably 10% or less.

[0175] Lubricants and surfactants for use in the present invention respectively have different physical functions. The kinds, amounts and proportions of combination generating synergistic effect of these lubricants should be determined optimally in accordance with the purpose. The nonmagnetic layer and the magnetic layer can separately contain different fatty acids each having a different melting point so as to prevent bleeding out of the fatty acids to the surface, or different esters each having a different boiling point, a different melting point or a different polarity so as to prevent bleeding out of the esters to the surface. Also, the amounts of surfactants are controlled so as to improve the coating stability, or the amount of the lubricant in the lower layer is made larger than the amount in the magnetic layer so as to improve the lubricating effect of the surface thereof. Examples are by no means limited thereto. In general, the total amount of the lubricants is from 0.1 to 50%, preferably from 2 to 25%, based on the amount of the magnetic powder or the nonmagnetic powder.

[0176] All or a part of the additives to be used in the present invention may be added to the magnetic coating solution or the nonmagnetic coating solution in any step of the preparation. For example, additives may be blended with a magnetic powder before the kneading step, may be added during the step of kneading a magnetic powder, a binder and a solvent, may be added during the dispersing step, may be added after the dispersing step, or may be added immediately before coating. According to the purpose, there is a case of capable of attaining the object by coating all or a part of the additives simultaneously with or successively after the coating of the magnetic layer. According to the purpose, lubricants may be coated on the surface of the magnetic layer after the calendering treatment or after the completion of slitting.

[0177] Layer Construction

[0178] The thickness of the support in the magnetic recording medium of the present invention is, for example, from 2 to 100 μm, preferably from 2 to 80 μm. Particularly, the thickness of the support for a computer tape is from 3.0 to 6.5 μm, preferably from 3.0 to 6.0 μm, more preferably from 4.0 to 5.5 μm.

[0179] An undercoating layer (or a subbing layer) may be provided between the support and the nonmagnetic or magnetic layer for adhesion improvement. The thickness of this undercoating layer is from 0.01 to 0.5 μm, preferably from 0.02 to 0.5 μm. The nonmagnetic layer and the magnetic layer of the magnetic recording medium according to the present invention may be provided on both surface sides of the support or may be provided on either one surface side. When the nonmagnetic layer and the magnetic layer are provided on only one surface side of the support, a back coating layer may be provided on the surface side of the support opposite to the side having the nonmagnetic layer and magnetic layer for the purpose of static charge prevention and curling correction. The thickness of this back coating layer is from 0.1 to 4 μm, preferably from 0.3 to 2.0 μm. Well-known undercoating layers and back coating layers can be used for this purpose.

[0180] The thickness of the magnetic layer of the magnetic recording medium of the present invention can be optimally selected according to the saturation magnetization amount of the head used, the head gap length, and the recording signal zone, and is generally from 0.05 to 0.30 μm, preferably from 0.05 to 0.25 μm, and more preferably from 0.05 to 0.20 μm. The magnetic layer may comprise two or more layers each having different magnetic characteristics and well-known multilayer magnetic layer structures can be applied to the present invention.

[0181] The thickness of the lower nonmagnetic layer of the medium according to the present invention is generally from 0.2 to 5.0 μm, preferably from 0.3 to 3.0 μm, and more preferably from 1.0 to 2.5 μm. The lower layer of the recording medium of the present invention exhibits the effect of the present invention so long as it is substantially a nonmagnetic layer even if, or intendedly, it contains a small amount of a magnetic powder as an impurity, which is as a matter of course regarded as essentially the same construction as in the present invention. The term “substantially a nonmagnetic layer” means that the residual magnetic flux density of the lower layer is 100 G or less and the coercive force of the lower layer is 100 Oe or less, preferably the residual magnetic flux density and the coercive force are zero. If the lower layer contains a magnetic powder, the content of the magnetic layer is less than ½ of the entire inorganic powder contained in the lower layer.

[0182] Back Coating Layer

[0183] In general, a magnetic tape for a computer data recording is decidedly required to have an excellent repeating-running property as compared with a video tape and an audio tape. For maintaining such a high running durability, it is preferred for the back coating layer to contain a carbon black and an inorganic powder.

[0184] Two kinds of carbon blacks respectively having different average particle sizes are preferably used in combination. In this case, a combined use of a fine carbon black having the average particle size of from 10 to 20 nm and a coarse carbon black having the average particle size of from 230 to 300 nm is preferred. In general, by the incorporation of a fine carbon black as above, the surface electrical resistance of the back coating layer and light transmittance can be set up at low values. There are many kinds of magnetic recording apparatuses making use of light transmittance of a tape and making it as signals of operation, therefore, the addition of fine carbon blacks are particularly effective in such a case. In addition, a fine carbon black is in general excellent in retention of a liquid lubricant and contributes to the reduction of a friction coefficient when a lubricant is used in combination. On the other hand, a coarse carbon black having a particle size of from 230 to 300 nm has a function as a solid lubricant and forms minute protrusions on the surface of a back coating layer to reduce contact area and contributes to the reduction of a friction coefficient. However, a coarse carbon black has a drawback such that particles are liable to drop out from the back coating layer due to the tape sliding during severe running leading to the increase of the error rate.

[0185] Specific examples of fine carbon blacks commercially available include RAVEN 2000B (average particle size: 18 nm) and RAVEN 1500B (average particle size: 17 nm) (manufactured by Columbia Carbon Co., Ltd.), BP800 (average particle size: 17 nm) (manufactured by Cabot Co., Ltd.), PRINTEX90 (average particle size: 14 nm), PRINTEX95 (average particle size: 15 nm), PRINTEX85 (average particle size: 16 nm), PRINTEX75 (average particle size: 17 nm) (manufactured by Degussa Co., Ltd.), and #3950 (average particle size: 16 nm) (manufactured by Mitsubishi Kasei Corp.).

[0186] Specific examples of coarse carbon blacks commercially available include THERMAL BLACK (average particle size: 270 nm) (manufactured by Cancarb Co., Ltd.) and RAVEN MTP (average particle size: 275 nm) (manufactured by Columbia Carbon Co., Ltd.).

[0187] When two kinds of carbon blacks respectively having different average particle sizes are used in combination in a back coating layer, the proportion of the contents (by weight) of a fine carbon black having a particle size of from 10 to 20 nm and a coarse carbon black having a particle size of from 230 to 300 nm is preferably the former/the latter of from 98/2 to 75/25, more preferably from 95/5 to 85/15.

[0188] The content of the carbon black in a back coating layer (the total amount when two kinds are used) is generally from 30 to 80 weight parts, preferably from 45 to 65 weight parts, based on 100 weight parts of the binder.

[0189] It is preferred to use two kinds of inorganic powders respectively having different hardness.

[0190] Specifically, a soft inorganic powder having a Mohs' hardness of from 3 to 4.5 and a hard inorganic powder having a Mohs' hardness of from 5 to 9 are preferably used in combination.

[0191] By the addition of a soft inorganic powder having a Mohs' hardness of from 3 to 4.5, a friction coefficient can be stabilized against repeating-running. Moreover, a sliding guide pole is not scratched off in hardness within this range. The average particle size of such a soft inorganic powder is preferably from 30 to 50 nm.

[0192] Examples of soft inorganic powders having a Mohs' hardness of from 3 to 4.5 include, e.g., calcium sulfate, calcium carbonate, calcium silicate, barium sulfate, magnesium carbonate, zinc carbonate and zinc oxide. They can be used alone or in combination of two or more. Of these, calcium carbonate is particularly preferred.

[0193] The content of the soft inorganic powder in a back coating layer is preferably from 10 to 140 weight parts, more preferably from 35 to 100 weight parts, based on 100 weight parts of the carbon black.

[0194] By the addition of a hard inorganic powder having a Mohs' hardness of from 5 to 9, the strength of the back coating layer is increased and running durability is improved. When such hard inorganic powders are used together with carbon blacks and the above-described soft inorganic powders, deterioration due to repeating sliding is reduced and strong back coating layer can be obtained. Appropriate abrasive capability is imparted to the back coating layer by the addition of the hard inorganic powder and the adhesion of scratched powders to a tape guide pole is reduced. In particular, when the hard inorganic powder is used in combination with a soft inorganic powder (in particular, calcium carbonate), sliding characteristics against a guide pole having a rough surface is improved and the stabilization of a friction coefficient of the back coating layer can also be brought about.

[0195] The average particle size of hard inorganic powders is preferably from 80 to 250 nm, more preferably from 100 to 210 nm.

[0196] Examples of hard inorganic powders having a Mohs' hardness of from 5 to 9 include, e.g., α-iron oxide, α-alumina, and chromium oxide (Cr₂O₃). These powders may be used alone or in combination. Of the above, α-iron oxide and α-alumina are preferred. The content of hard inorganic powders in the back coating layer is generally from 3 to 30 weight parts, preferably from 3 to 20 weight parts, based on 100 weight parts of the carbon black.

[0197] When the above soft inorganic powder and hard inorganic powder are used in combination in the back coating layer, it is preferred to use them selectively such that the difference of hardness between soft and hard inorganic powders is 2 or more, more preferably 2.5 or more, and particularly preferably 3 or more.

[0198] It is preferred that the above-described two kinds of inorganic powders respectively having different hardness and specific average particle sizes and the above-described two kinds of carbon blacks respectively having different specific average particle sizes are contained in the back coating layer. In particular, in this combination, calcium carbonate is preferably contained as a soft inorganic powder.

[0199] Lubricants may be contained in the back coating layer. Lubricants can be arbitrarily selected from among those which can be used in a magnetic layer or a nonmagnetic layer as described above. The content of lubricants added to the back coating layer is generally from 1 to 5 weight parts based on 100 weight parts of the binder.

[0200] Support

[0201] The support for use in the present invention preferably has a thermal shrinkage factor of 0.5% or less both at 100° C. for 30 minutes and at 80° C. for 30 minutes in every direction of in-plane of the support, more preferably 0.2% or less. Moreover, the above-described thermal shrinkage factors of the support at 100° C. for 30 minutes and at 80° C. for 30 minutes are preferably almost equal in every direction of in-plane of the support with difference of not more than 10%. The support is preferably a nonmagnetic support. As a nonmagnetic support for use in the present invention, well-known films such as polyesters (e.g., polyethylene terephthalate or polyethylene naphthalate), polyolefins, cellulose triacetate, polycarbonate, polyamide, polyimide, polyamideimide, polysulfone, polyaramide, aromatic polyamide, or polybenzoxazole can be used. Highly strong supports such as polyethylene naphthalate or polyamide are preferably used. If necessary, a lamination type support as disclosed in JP-A-3-224127 can be used to vary the surface roughnesses of the magnetic layer surface and the base surface. The support may be previously subjected to surface treatments, such as a corona discharge treatment, a plasma treatment, an adhesion assisting treatment, a heat treatment, and a dust removing treatment. Aluminum or glass substrate can also be used as a support in the present invention.

[0202] For attaining the object of the present invention, it is preferred to use the support having a central plane average surface roughness (SRa) of 5.0 nm or less, preferably 4.0 nm or less, more preferably 2.0 nm or less, measured by a surface roughness meter “TOPO-3D” (a product of WYKO Co., Ltd., U.S.A.) by MIRAU method. It is preferred that the support not only has a small central plane average surface roughness but also is free from coarse protrusions (having a height) of 0.5 μm or more. Surface roughness configuration is freely controlled by the size and the amount of fillers added to the support. Examples of such fillers include acryl-based organic powders, as well as oxides or carbonates of Ca, Si and Ti. The support for use in the present invention preferably has the maximum height (SRmax) of 1 μm or less, ten point average roughness (SRz) of 0.5 μm or less, central plane peak height (SRp) of 0.5 μm or less, central plane valley depth (SRv) of 0.5 μm or less, central plane area factor (SSr) of from 10% to 90%, and average wavelength (Sλa) of from 5 μm to 300 μm. For obtaining desired electromagnetic characteristics and durability, surface protrusion distribution of the support can be controlled arbitrarily by fillers, e.g., the number of protrusions having sizes of from 0.01 μm to 1 μm can be controlled each within the range of from 0 to 2,000 per 0.1 mm².

[0203] The F-5 value of the nonmagnetic support for use in the present invention is preferably from 5 to 50 kg/mm², a thermal shrinkage factor of the support at 100° C. for 30 minutes is preferably 3% or less, more preferably 1.5% or less, and particularly preferably 0.5% or less, and a thermal shrinkage factor at 80° C. for 30 minutes is preferably 1% or less, more preferably 0.5% or less, and particularly preferably 0.2% or less. The support has a breaking strength of from 5 to 100 kg/mm², an elastic modulus of from 100 to 2,000 kg/mm², a temperature expansion coefficient of from 10⁻⁴to 10⁻⁸/° C., preferably from 10⁻⁵to 10⁻⁶/° C., and a humidity expansion coefficient of 10⁻⁴/RH % or less, preferably 10⁻⁵/RH % or less. These thermal characteristics, dimensional characteristics and mechanical strength characteristics are preferably almost equal in every direction of in-plane of the support with difference of not more than 10%.

[0204] Producing Method

[0205] Processes of preparing the magnetic coating solution for use in the magnetic recording medium of the present invention comprises at least a kneading step, a dispersing step and, optionally, blending steps to be carried out before and/or after the kneading and dispersing steps. Any of these respective steps may be composed of two or more separate stages. Materials such as a magnetic powder, a nonmagnetic powder, a binder, a carbon black, an abrasive, an antistatic agent, a lubricant, a solvent, and the like for use in the present invention may be added at any step at any time. Each material may be added at two or more steps dividedly. For example, polyurethane can be added dividedly at a kneading step, a dispersing step, or a blending step for adjusting viscosity after dispersion. For achieving the object of the present invention, the above steps can be performed partly with conventionally well-known techniques. Powerful kneading machines such as an open kneader, a continuous kneader, a pressure kneader or an extruder are preferably used in a kneading step. When a kneader is used, all or a part of a binder (preferably 30% or more of the total binders) are kneading-treated in the range of from 15 parts to 500 parts per 100 parts of the magnetic powder or nonmagnetic powder together with a magnetic powder or a nonmagnetic powder. Details of these kneading are disclosed in JP-A-1-106338 and JP-A-1-79274. When dispersing a magnetic layer solution and a nonmagnetic layer solution, glass beads can be used but dispersing media having a high specific gravity is preferably used and zirconia beads, titania beads and steel beads are suitable for this purpose. Optimal particle size and packing density of these dispersing media should be selected. Well-known dispersing apparatuses can be used in the present invention.

[0206] The following methods are preferably used for coating the magnetic recording medium having a multilayer construction of the present invention. As the first method, the lower layer is coated by any of gravure coating, roll coating, blade coating, and extrusion coating apparatuses, which are ordinarily used in the coating of a magnetic coating solution, and the upper layer is coated while the lower layer is still wet by means of the support pressing type extrusion coating apparatus disclosed in JP-B-1-46186, JP-A-60-238179 and JP-A-2-265672. As the second method, the upper layer and the lower layer are coated almost simultaneously using the coating head equipped with two slits for feeding coating solution as disclosed in JP-A-63-88080, JP-A-2-17971 and JP-A-2-265672. And as the third method, the upper layer and the lower layer are coated almost simultaneously using the extrusion coating apparatus equipped with a backup roll as disclosed in JP-A-2-174965. For preventing the deterioration of the electromagnetic characteristics of the magnetic recording medium due to agglomeration of magnetic powders, it is preferred to impart shear to the coating solution in the coating head by the methods as described in JP-A-62-95174 and JP-A-1-236968. With respect to the viscosity of the coating solution, the range of the numeric values disclosed in JP-A-3-8471 is necessary to be satisfied. For realizing the constitution of the present invention, successive multilayer coating method in which the magnetic layer is coated on the lower layer after the lower layer has been coated and dried can of course be used without impairing the effect of the present invention. However, for reducing coating defects and improving quality, e.g., dropout, it is preferred to use the above simultaneous multilayer coating method.

[0207] In the case of a magnetic disc, isotropic orienting property can be sufficiently obtained in some cases without conducting orientation using orientating apparatus, but it is preferred to use well-known random orientation apparatuses, such as disposing cobalt magnets diagonally and alternately or applying an alternating current magnetic field using a solenoid. Isotropic orientation in a ferromagnetic metal fine powder is in general preferably in-plane two dimensional random orientation, but it may be three dimensional random orientation having vertical components. Further, it is possible to impart isotropic magnetic characteristics in the circumferential direction by vertical orientation using well-known methods, e.g., using different pole and counter position magnets. In particular, vertical orientation is preferred when the disc is used for high density recording. Circumferential orientation can be conducted using spin coating.

[0208] In the case of a magnetic tape, orientation is conducted in the machine direction using a cobalt magnet and a solenoid. In orientation, it is preferred that the drying position of the coated film can be controlled by controlling the temperature and the amount of drying air and coating rate. Coating rate is preferably from 20 to 1,000 m/min. and the temperature of drying air is preferably 60° C. or more. Preliminary drying can be performed appropriately before entering the magnet zone.

[0209] Use of heat resisting plastic rolls such as epoxy, polyimide, polyamide and polyimideamide, or metal rolls is effective for calendering treatment. Metal rolls are usable for the treatment particularly when magnetic layers are coated on both surface sides. Treatment temperature is preferably 50° C. or more, more preferably 100° C. or more. Line pressure is preferably 200 kg/cm or more, more preferably 300 kg/cm or more.

[0210] Physical Properties

[0211] Saturation magnetic flux density of the magnetic layer of the magnetic recording medium according to the present invention is from 2,000 to 5,000 G when a ferromagnetic metal powder is used. Coercive force (Hc) and (Hr) are from 1,800 to 5,000 Oe, preferably from 1,800 to 3,000 Oe. Coercive force distribution is preferably narrow, and SFD and SFDr are preferably 0.6 or less. Squareness ratio is from 0.55 to 0.67, preferably from 0.58 to 0.64, in the case of two dimensional random orientation, from 0.45 to 0.55 in the case of three dimensional random orientation, and in the case of vertical orientation, 0.6 or more, preferably 0.7 or more in the vertical direction, and when diamagnetical correction is conducted, 0.7 or more, preferably 0.8 or more. Orientation ratio of two dimensional random orientation and three dimensional random orientation is preferably 0.8 or more. In the case of two dimensional random orientation, squareness ratio, Br, Hc and Hr in the vertical direction are preferably from 0.1 to 0.5 times of those in the in-plane direction.

[0212] In the case of a magnetic tape, squareness ratio is 0.7 or more, preferably 0.8 or more.

[0213] The friction coefficient of the magnetic recording medium according to the present invention against a head at temperature of −10° C. to 40° C. and humidity of 0% to 95% is 0.5 or less, preferably 0.3 or less, the surface inherent resistivity of the magnetic surface is preferably from 10⁴to 10¹²Ω/sq, the charge potential is preferably from −500 V to +500 V, the elastic modulus at 0.5% elongation of the magnetic layer is preferably from 100 to 2,000 kg/mm²in every direction of in-plane, the breaking strength is preferably from 10 to 70 kg/mm², the elastic modulus of the magnetic recording medium is preferably from 100 to 1,500 kg/mm²in every direction of in-plane, the residual elongation is preferably 0.5% or less, and the thermal shrinkage factor at every temperature of 100° C. or less is preferably 1% or less, more preferably 0.5% or less, and most preferably 0.1% or less. The glass transition temperature of the magnetic layer (the maximum of elastic modulus loss by dynamic viscoelasticity measurement at 110 Hz) is preferably from 50° C. to 120° C., and that of the lower nonmagnetic layer is preferably from 0° C. to 100° C. The elastic modulus loss is preferably within the range of from 1×10⁶to 8×10⁹dyne/cm², and loss tangent is preferably 0.2 or less. If loss tangent is too large, adhesion failure is liable to occur. These thermal and mechanical characteristics are preferably almost equal in every direction of in-plane of the medium with difference of not more than 10%. The residual amount of the solvent in the magnetic layer is preferably 100 mg/m²or less, more preferably 10 mg/m²or less. The void ratio is preferably 30% by volume or less, more preferably 20% by volume or less, with both of the nonmagnetic layer and the magnetic layer. The void ratio is preferably smaller for obtaining high output but in some cases a specific value should be preferably secured depending on purposes. For example, in a disc-like medium which is repeatedly used, large void ratio contributes to good running durability in many cases.

[0214] The magnetic layer has a central plane surface roughness (Ra) of 5.0 nm or less, preferably 4.0 nm or less, more preferably 3.5 nm or less, and particularly preferably 3.3 nm or less, measured by a surface roughness meter “TOPO-3D” (a product of WYKO Co., Ltd., U.S.A.) by MIRAU method. The magnetic layer for use in the present invention preferably has the maximum height (SRmax) of 0.5 μm or less, ten point average roughness (SRz) of 0.3 μm or less, central plane peak height (SRp) of 0.3 μm or less, central plane valley depth (SRv) of 0.3 μm or less, central plane area factor (SSr) of from 20% to 80%, and average wavelength (Sλa) of from 5 μm to 300 μm. For obtaining desired electromagnetic characteristics and a friction coefficient, a number of surface protrusion of the magnetic layer having sizes (i.e., height) of from 0.01 μm to 1 μm can be controlled arbitrarily within the range of from 0 to 2,000 by controlling the surface property due to fillers in the support, the particle size and the amount of the magnetic powders added to the magnetic layer, or by the surface shape of rolls of calender treatment. The range of curling is preferably not within ±3 mm.

[0215] When the magnetic recording medium according to the present invention comprises a nonmagnetic layer and a magnetic layer, these physical properties in the nonmagnetic layer and the magnetic layer can be varied according to purposes. For example, the elastic modulus of the magnetic layer is made higher to improve running durability and at the same time the elastic modulus of the nonmagnetic layer is made lower than that of the magnetic layer to improve the head touching of the magnetic recording medium.

[0216] Preparation Method 1—(W/W)

[0217] Each of the above compositions of the coating solutions for the magnetic layer and the nonmagnetic layer was blended in a kneader, then dispersed with a sand mill. Polyisocyanate was added to each resulting dispersion solution, in an amount of 10 parts to the nonmagnetic layer coating solution, and 10 parts to the magnetic layer coating solution. Further, 40 parts of cyclohexanone was added to each solution. Each solution was filtered through a filter having an average pore diameter of 1 μm to obtain coating solutions for forming the nonmagnetic layer and the magnetic layer.

[0218] These coating solutions were simultaneously multilayer-coated on a polyethylene terephthalate support having a thickness of 62 μm and a central plane average surface roughness of 3 nm of the surface side on which a magnetic layer was to be coated. The nonmagnetic layer coating solution was coated in a dry thickness of 1.5 μm, immediately thereafter the magnetic layer coating solution was coated on the nonmagnetic layer so as to give the magnetic layer having the thickness of 0.15 μm. The coated layers were subjected to random orientation while the magnetic layer and the nonmagnetic layer were still wet by passing through an alternating current magnetic field generator having two magnetic field intensities of frequency of 50 Hz, magnetic field intensity of 250 Gauss and frequency of 50 Hz, magnetic field intensity of 120 Gauss. After drying, the coated layer was subjected to calendering treatment with calenders of 7 stages at 90° C. at line pressure of 300 kg/cm. The obtained web was punched to a disc of 3.7 inches, the disc was subjected to a surface treatment by abrasives, encased in 3.7 inch cartridge having a liner inside (a zip-disc cartridge manufactured by Iomega Co., Ltd., U.S.A.), and equipped the cartridge with prescribed mechanism parts to obtain a 3.7 inch floppy disc. A part of samples was subjected to machine direction orientation using Co magnets with the same pole and counter positions of 4,000 G before random orientation treatment.

[0219] In this case, it is preferred to increase the frequency and magnetic field intensity of the alternating current magnetic field generator so as to achieve finally sufficient random orientation, thereby 98% or more of orientation ratio can be obtained.

[0220] Preparation Method 1-2 (W/D)

[0221] Each of the above compositions of the coating solutions for the magnetic layer and the nonmagnetic layer was blended in a kneader, then dispersed with a sand mill. Polyisocyanate was added to each resulting dispersion solution, in an amount of 10 parts to the nonmagnetic layer coating solution, and 10 parts to the magnetic layer coating solution. Further, 40 parts of cyclohexanone was added to each solution. Each solution was filtered through a filter having an average pore diameter of 1 μm to obtain coating solutions for forming the nonmagnetic layer and the magnetic layer.

[0222] The nonmagnetic layer coating solution was coated in a dry thickness of 1.5 μm on a polyethylene terephthalate support having a thickness of 62 μm and a central plane average surface roughness of 3 nm of the surface side on which the magnetic layer was to be coated, dried, and subjected to calendering treatment. The magnetic layer coating solution was coated by blade coating on the nonmagnetic layer so as to give the magnetic layer having the thickness of 0.15 μm. The coated layers were subjected to random orientation by passing through an alternating current magnetic field generator having two magnetic field intensities of frequency of 50 Hz, magnetic field intensity of 250 Gauss and frequency of 50 Hz, magnetic field intensity of 120 Gauss. The procedure was carried out in the same manner as in Preparation Method 1—1 hereafter. Calendering of the nonmagnetic layer may be omitted.

[0223] Preparation Method 1-3 (Spin Coating)

[0224] Each of the above compositions of the coating solutions for the magnetic layer and the nonmagnetic layer was blended in a kneader, then dispersed with a sand mill. Polyisocyanate was added to each resulting dispersion solution, in an amount of 10 parts to the nonmagnetic layer coating solution, and 10 parts to the magnetic layer coating solution. Further, 40 parts of cyclohexanone was added to each solution. Each solution was filtered through a filter having an average pore diameter of 1 μm to obtain coating solutions for forming the nonmagnetic layer and the magnetic layer.

[0225] The nonmagnetic layer coating solution was coated in a dry thickness of 1.5 μm by spin coating on a polyethylene terephthalate support having a thickness of 62 μm and a central plane average surface roughness of 3 nm of the surface side on which a magnetic layer was to be coated and dried. The magnetic layer coating solution was coated by spin coating on the nonmagnetic layer so as to give the magnetic layer having the thickness of 0.15 μm. The coated layers were oriented using a Co magnets with the same pole and counter positions of 6,000 G in the circumferential direction and the surface of the magnetic layer was smoothed by batch system rolling treatment by which the same pressure as in Preparation Method 1—1 can be applied. The procedure was carried out in the same manner as in Preparation Method 1—1 hereafter. Also, the magnetic layer may be coated by spin coating on the nonmagnetic layer while the nonmagnetic layer coated by spin coating is still wet. By using the spin coating process, not only the amount of residual magnetization in the recording direction can be made large but also vertical magnetization components of ferromagnetic metal powders of short acicular ratio can be reduced and symmetric property of reproduced wave form can be improved.

[0226] Preparation Method 1-4 (Monolayer)

[0227] The coating solution for the magnetic layer prepared in Preparation Method 1—1 was coated on a polyethylene terephthalate support so as to reach the thickness of 0.15 μm. The procedure was carried out in the same manner as in Preparation Method 1—1 hereafter.

[0228] Various samples were prepared by combining the above-described each method arbitrarily as shown in Table 1—1. The results of evaluations are shown in Table 1—2.

[0229] Various samples of magnetic discs were prepared in the same manner as in Example 1-11, except that the components of the magnetic coating solution 1ML-2 (a ferromagnetic metal powder and an α-alumina (an abrasive)) were changed as shown in Tables 1-3 and 1-4. The results of evaluations are shown in Tables 1-5 and 1-6.

[0230] Methods of evaluations of each sample are as follows.

[0231] (1) Magnetic characteristics (Hc):

[0232] Magnetic characteristics were measured using a vibrating sample magnetometer (a product of Toei Kogyo Co., Ltd.) by Hm 10 KOe.

[0233] (2) Central plane average surface roughness (Ra):

[0234] Surface roughness (Ra) of the area of about 250 μm×250 μm, Rrms, peak-valley values were measured using “TOPO3D” (a product of WYKO, U.S.A.) by 3D-MIRAU method. The wavelength of measurement was about 650 nm and spherical compensation and cylindrical compensation were applied. Measurement was performed using a light interference type non-contact surface roughness meter.

[0235] (3) Areal recording density:

[0236] Areal recording density means a value obtained by multiplying linear recording density by track density.

[0237] (4) Linear recording density:

[0238] Linear recording density means a bit number of signals recorded per 1 inch in the recording direction.

[0239] (5) Track density:

[0240] Track density means a track number per 1 inch.

[0241] (6) φm:

[0242] φm is the amount of magnetization per unit area of a magnetic recording medium, which is represented by Bm (Gauss) multiplying thickness. This is the value obtained by using a vibrating sample magnetometer (a product of Toei Kogyo Co., Ltd.) at Hm 10 KOe, which can be directly measured.

[0243] (7) Error rate of disc:

[0244] The above signals of linear recording density were recorded on the disc by (2,7) RLL modulation system and error rate was measured.

[0245] (8) Thickness of magnetic layer:

[0246] The sample having a thickness of about 0.1 μm was cut out with a diamond cutter in the machine direction of the magnetic medium, observed with a transmission type electron microscope of from 10,000 to 100,000, preferably from 20,000 to 50,000 magnifications and photographed. The print size of the photograph was from A4 (i.e., 297×210 mm) to A5 (i.e., 210×148 mm) sizes. The present inventors paid attentions to the difference of the shapes of the ferromagnetic powders and the nonmagnetic powders of the magnetic layer and the nonmagnetic layer and rimmed the interface and the surface of the magnetic layer with black color by visual judgement. Thereafter, the distance of the rimmed lines was measured by the image processing apparatus “IBAS2” (manufactured by Zeiss Corp.). Measurement was conducted from 85 to 300 times when the length of the sample photograph was 21 cm. The average measured value at that time was taken as d, and the standard deviation of the measured value was taken as σ. d depended on the description in JP-A-5-298653 and σ was obtained by equation (2) in JP-A-5-298653. di means each measured value and n is from 85 to 300.

[0247] (9) Running durability:

[0248] A floppy disc drive (“ZIP100”, a product of IOMEGA CORP., U.S.A., rotation number: 2,968 rpm) was used. The head was fixed at the position of radius of 38 mm. Recording was conducted at recording density of 34 kfci, then reproduced the signals recorded and this was taken as 100%. The disc was run for 25 days (600 hours) under the following thermo-cycle condition, which being taken as one cycle. Output was monitored every 24 hours of running and the point when the initial reproduction output became 70% or less was taken as NG.

[0249] Thermo-Cycle Flow

[0250] 25° C., 50% RH, 1 hr→(temperature up, 2 hr)→60° C., 20% RH, 7 hr→(temperature down, 2 hr)→25° C., 50% RH, 1 hr→(temperature down, 2 hr)→5° C., 50% RH, 7 hr→(temperature up, 2 hr)→(this cycle was repeated).

[0251] (10) Liner wear:

[0252] A sample was run for 600 hours under the same condition as in the evaluation of running durability with the head being off, and after completion of running, cartridge case of the sample was opened and the surface of the magnetic layer was visually evaluated by the following criteria.

[0253] o: No defect was observed on the surface of the magnetic layer.

[0254] Δ: Fine scratches were generated on a part of the surface of the magnetic layer.

[0255] x: Fine scratches were generated on the entire surface of the magnetic layer.

[0256] (11) Characteristics of ferromagnetic metal powder:

[0257] Evaluation of the characteristics of the ferromagnetic metal powder was conducted according to the method disclosed in JP-A-8-279137.

[0258] (12) Weather resistance of magnetic layer:

[0259] The weather resistance of the magnetic layer was measured by the following measurement method after the magnetic recording medium was stored under the circumstance of 60° C. and 90% RH for 7 days.

[0260] The fluctuation of Hc (ΔHc) of the magnetic layer after storage was calculated by the following equation:

[0261] ΔHc(%)=100×(Hc after storage−Hc before storage)/Hc before storage

[0262] The fluctuation of Hc (ΔHc) after storage is preferably from −5.0% to +10.0%, more preferably from −3.0% to +8.0%, most preferably from −1.0% to +6.0%.

[0263] Also, decrease of φm (i.e., Δφm) was calculated by the following equation:

[0264] Δφm (%)=100×(φm before storage−φm after storage)/φm before storage

[0265] The φm's before storage and after storage were measured using a vibrating sample magnetometer (“VSM-5” produced by Toei Kogyo Co., Ltd.) under the condition of a time constant of 0.1 sec., a sweeping speed of 3 min./10 KOe and a magnetic field measured of 10 KOe.

[0266] The decrease of φm (i.e., Δφm) is preferably within 10%, more preferably within 6%.

[0267] In each of Examples 1-14 to 1-16, the disc in Example 1-11 was used and error rate was determined with varying linear recording density and track density.

[0268] From the results in Table 1-2, it can be seen that the error rate of the magnetic disc according to the present invention, in particular, in high density recording region is 1×10⁻⁵or less, which is conspicuously excellent.

[0269] In running durability of Table 1-6, 600 means 600 hours.

[0270] As is apparent from the results in Table 1-6, the magnetic disc according to the present invention is excellent in high density characteristics and also has excellent running durability.

[0271] The magnetic recording medium according to the present invention comprises a support having thereon a magnetic layer comprising a ferromagnetic metal powder dispersed in a binder, which is a magnetic disc for recording signals of from 0.17 to 2 G bit/inch²of areal recording density, wherein the coercive force of the magnetic layer is 1,800 Oe or more, the ferromagnetic metal powder is composed of Fe and Co as main components, and the atomic ratio of Al/(Fe+Co) is from 4.5 to 15.0%, the magnetic layer has a dry thickness of from 0.05 to 0.25 μm and φm of preferably from 8.0×10⁻³to 1.0×10⁻³emu/cm². It can be seen that the magnetic recording medium of the present invention having high capacity, excellent high density characteristics, in particular, markedly improved running durability, has been realized due to the above constitution of the present invention, which could never be obtained by conventional techniques.

[0272] Preparation Method 2-1 (W/W)

[0273] Each of the above compositions of the coating solutions for the magnetic layer and the nonmagnetic layer was blended in a kneader, then dispersed with a sand mill. Polyisocyanate was added to each resulting dispersion solution, in an amount of 10 parts to the nonmagnetic layer coating solution, and 10 parts to the magnetic layer coating solution. Further, 40 parts of cyclohexanone was added to each solution. Each solution was filtered through a filter having an average pore diameter of 1 μm to obtain coating solutions for forming the nonmagnetic layer and the magnetic layer.

[0274] These coating solutions were simultaneously multilayer-coated on a polyethylene terephthalate support having a thickness of 62 μm and a central plane average surface roughness of 3 nm of the surface side on which a magnetic layer was to be coated. The nonmagnetic layer coating solution was coated in a dry thickness of 1.5 μm, immediately thereafter the magnetic layer coating solution was coated on the nonmagnetic layer so as to give the magnetic layer having the thickness of 0.15 μm. The coated layers were subjected to random orientation while the magnetic layer and the nonmagnetic layer were still wet by passing through an alternating current magnetic field generator having two magnetic field intensities of frequency of 50 Hz, magnetic field intensity of 250 Gauss and frequency of 50 Hz, magnetic field intensity of 120 Gauss. After drying, the coated layer was subjected to calendering treatment with calenders of 7 stages at 90° C. at line pressure of 300 kg/cm. The obtained web was punched to a disc of 3.7 inches, the disc was subjected to a surface treatment by abrasives, encased in 3.7 inch cartridge having a liner inside (a zip-disc cartridge manufactured by Iomega Co., Ltd., U.S.A.), and equipped the cartridge with prescribed mechanism parts to obtain a 3.7 inch floppy disc. A part of samples was subjected to machine direction orientation using Co magnets with the same pole and counter positions of 4,000 G before random orientation treatment.

[0275] In this case, it is preferred to increase the frequency and magnetic field intensity of the alternating current magnetic field generator so as to achieve finally sufficient random orientation, thereby 98% or more of orientation ratio can be obtained.

[0276] Preparation Method 2—2 (W/D)

[0277] Each of the above compositions of the coating solutions for the magnetic layer and the nonmagnetic layer was blended in a kneader, then dispersed with a sand mill. Polyisocyanate was added to each resulting dispersion solution, in an amount of 10 parts to the nonmagnetic layer coating solution, and 10 parts to the magnetic layer coating solution. Further, 40 parts of cyclohexanone was added to each solution. Each solution was filtered through a filter having an average pore diameter of 1 μm to obtain coating solutions for forming the nonmagnetic layer and the magnetic layer.

[0278] The nonmagnetic layer coating solution was coated in a dry thickness of 1.5 μm on a polyethylene terephthalate support having a thickness of 62 μm and a central plane average surface roughness of 3 nm of the surface side on which the magnetic layer was to be coated, dried, and subjected to calendering treatment. The magnetic layer coating solution was coated by blade coating on the nonmagnetic layer so as to give the magnetic layer having the thickness of 0.15 μm. The coated layers were subjected to random orientation by passing through an alternating current magnetic field generator having two magnetic field intensities of frequency of 50 Hz, magnetic field intensity of 250 Gauss and frequency of 50 Hz, magnetic field intensity of 120 Gauss. The procedure was carried out in the same manner as in Preparation Method 2-1 hereafter. Calendering of the nonmagnetic layer may be omitted.

[0279] Preparation Method 2-3 (Spin Coating)

[0280] Each of the above compositions of the coating solutions for the magnetic layer and the nonmagnetic layer was blended in a kneader, then dispersed with a sand mill. Polyisocyanate was added to each resulting dispersion solution, in an amount of 10 parts to the nonmagnetic layer coating solution, and 10 parts to the magnetic layer coating solution. Further, 40 parts of cyclohexanone was added to each solution. Each solution was filtered through a filter having an average pore diameter of 1 μm to obtain coating solutions for forming the nonmagnetic layer and the magnetic layer.

[0281] The nonmagnetic layer coating solution was coated in a dry thickness of 1.5 μm by spin coating on a polyethylene terephthalate support having a thickness of 62 μm and a central plane average surface roughness of 3 nm of the surface side on which a magnetic layer was to be coated and dried. The magnetic layer coating solution was coated by spin coating on the nonmagnetic layer so as to give the magnetic layer having the thickness of 0.15 μm. The coated layers were oriented using a Co magnets with the same pole and counter positions of 6,000 G in the circumferential direction and the surface of the magnetic layer was smoothed by batch system rolling treatment by which the same pressure as in Preparation Method 2-1 can be applied. The procedure was carried out in the same manner as in Preparation Method 2-1 hereafter. Also, the magnetic layer may be coated by spin coating on the nonmagnetic layer while the nonmagnetic layer coated by spin coating is still wet. By using the spin coating process, not only the amount of residual magnetization in the recording direction can be made large but also vertical magnetization components of ferromagnetic metal powders of short acicular ratio can be reduced and symmetric property of reproduced wave form can be improved.

[0282] Preparation Method 2-4 (Monolayer)

[0283] The coating solution for the magnetic layer prepared in Preparation Method 2-1 was coated on a polyethylene terephthalate support so as to reach the thickness of 0.15 μm. The procedure was carried out in the same manner as in Preparation Method 2-1 hereafter.

[0284] Various samples were prepared by combining the above-described each method arbitrarily as shown in Table 2-1. The results of evaluations are shown in Table 2-2.

[0285] Various samples of magnetic discs were prepared in the same manner as in Example 2-11, except that the components of the magnetic coating solution 2ML-2 (a ferromagnetic metal powder) were changed as shown in Table 2-3 and the addition amount of α-alumina was changed to 8 parts. Further, in Table 2-3, the atomic ratio A of ferromagnetic metal powder 2M-9 shows atomic ratio of each component of Nd and Y to (Fe+Co), the atomic ratio A of 2M-9 is, therefore, 3.3%. The sum total of rare earth elements to calculate the atomic ratio A of other ferromagnetic metal powders is considered substantially a component of Y or Nd. The results of evaluations are shown in Tables 2-4 and 2-5.

[0286] Methods of evaluations of each sample are as follows.

[0287] (1) Magnetic characteristics (Hc):

[0288] Magnetic characteristics were measured using a vibrating sample magnetometer (a product of Toei Kogyo Co., Ltd.) by Hm 10 KOe.

[0289] (2) Central plane average surface roughness (Ra):

[0291] Surface roughness (Ra) of the area of about 250 μm×250 μm, Rrms, peak-valley values were measured using “TOPO3D” (a product of WYKO, U.S.A.) by 3D-MIRAU method. The wavelength of measurement was about 650 nm and spherical compensation and cylindrical compensation were applied. Measurement was performed using a light interference type non-contact surface roughness meter.

[0292] (3) Areal recording density:

[0293] Areal recording density means a value obtained by multiplying linear recording density by track density.

[0294] (4) Linear recording density:

[0295] Linear recording density means a bit number of signals recorded per 1 inch in the recording direction.

[0296] (5) Track density:

[0297] Track density means a track number per 1 inch.

[0298] (6) φm:

[0299] φm is the amount of magnetization per unit area of a magnetic recording medium, which is represented by Bm (Gauss) multiplying thickness. This is the value obtained by using a vibrating sample magnetometer (a product of Toei Kogyo Co., Ltd.) at Hm 10 KOe, which can be directly measured.

[0300] (7) Error rate of disc:

[0301] The above signals of linear recording density were recorded on the disc by (2,7) RLL modulation system and error rate was measured.

[0302] (8) Thickness of magnetic layer:

[0303] The sample having the thickness of about 0.1 μm was cut out with a diamond cutter in the machine direction of the magnetic medium, observed with a transmission type electron microscope of from 10,000 to 100,000, preferably from 20,000 to 50,000 magnifications and photographed. The print size of the photograph was from A4 (i.e., 297×210 mm) to A5 (i.e., 210×148 mm) sizes. The present inventors paid attentions to the difference of the shapes of the ferromagnetic powders and the nonmagnetic powders of the magnetic layer and the nonmagnetic layer and rimmed the interface and the surface of the magnetic layer with black color by visual judgement. Thereafter, the distance of the rimmed lines was measured by the image processing apparatus “IBAS2” (manufactured by Zeiss Corp.). Measurement was conducted from 85 to 300 times when the length of the sample photograph was 21 cm. The average measured value at that time was taken as d, and the standard deviation of the measured value was taken as σ. d depended on the description in JP-A-5-298653 and a was obtained by equation (2) in JP-A-5-298653. di means each measured value and n is from 85 to 300.

[0304] (9) Running durability:

[0305] A floppy disc drive (“ZIP100”, a product of IOMEGA CORP., U.S.A., rotation number: 2,968 rpm) was used. The head was fixed at the position of radius of 38 mm. Recording was conducted at recording density of 34 kfci, then reproduced the signals recorded and this was taken as 100%. The disc was run for 25 days (600 hours) under the following thermo-cycle condition, which being taken as one cycle. Output was monitored every 24 hours of running and the point when the initial reproduction output became 70% or less was taken as NG.

[0306] Thermo-Cycle Flow

[0307] 25° C., 50% RH, 1 hr→(temperature up, 2 hr)→60° C., 20% RH, 7 hr→(temperature down, 2 hr)→25° C., 50% RH, 1 hr→(temperature down, 2 hr)→5° C., 50% RH, 7 hr→(temperature up, 2 hr)→(this cycle was repeated).

[0308] (10) Liner wear:

[0309] A sample was run for 600 hours under the same condition as in the evaluation of running durability with the head being off, and after completion of running, cartridge case of the sample was opened and the surface of the magnetic layer was visually evaluated by the following criteria.

[0310] o: No defect was observed on the surface of the magnetic layer.

[0311] Δ: Fine scratches were generated on a part of the surface of the magnetic layer.

[0312] x: Fine scratches were generated on the entire surface of the magnetic layer.

[0313] (11) Characteristics of ferromagnetic metal powder:

[0314] Evaluation of the characteristics of the ferromagnetic metal powder was conducted according to the method disclosed in JP-A-8-279137.

[0315] In each of Examples 2-14 to 2-16, the disc in Example 2-11 was used and error rate was determined with varying linear recording density and track density.

[0316] From the results in Table 2—2, it can be seen that the error rate of the magnetic disc according to the present invention, in particular, in high density recording region is 1×10⁻⁵or less, which is conspicuously excellent.

[0317] In running durability of Table 2-5, 600 means 600 hours.

[0318] As is apparent from the results in Table 2-5, the magnetic disc according to the present invention is excellent in high density characteristics and also has excellent running durability.

[0319] The magnetic recording medium according to the present invention comprises a support having thereon a magnetic layer comprising a ferromagnetic metal powder dispersed in a binder, which is a magnetic disc for recording signals of from 0.17 to 2 G bit/inch², preferably from 0.2 to 2 G bit/inch², of areal recording density, wherein the magnetic layer has a dry thickness of from 0.05 to 0.25 μm, φm of preferably from 8.0×10⁻³to 1.0×10⁻³emu/cm², and the coercive force of 1,800 Oe or more, and the ferromagnetic metal powder is composed of Fe and Co as main components, the atomic ratio of the sum total of rare earth elements/(Fe+Co) is preferably from 1.0 to 6.0%, and the atomic ratio of the sum total of rare earth elements/Al is from 0.10 to 0.60. The magnetic recording medium of the present invention having excellent high density characteristics and, in particular, markedly improved running durability has been realized due to the above constitution of the present invention, which could never be obtained by conventional techniques.

[0320] Preparation Method 3-1 (W/W)

[0321] Each of the above compositions of the coating solutions for the magnetic layer and the lower nonmagnetic layer was blended in a kneader, then dispersed with a sand mill. Polyisocyanate was added to each resulting dispersion solution, in an amount of 10 parts to the lower nonmagnetic layer coating solution, and 10 parts to the magnetic layer coating solution. Further, 40 parts of cyclohexanone was added to each solution. Each solution was filtered through a filter having an average pore diameter of 1 μm to obtain coating solutions for forming the nonmagnetic layer and the magnetic layer.

[0322] These coating solutions were simultaneously multilayer-coated on a polyethylene terephthalate support having a thickness of 62 μm and a central plane average surface roughness of 3 nm of the surface side on which a magnetic layer was to be coated. The nonmagnetic layer coating solution was coated in a dry thickness of 1.5 μm, immediately thereafter the magnetic layer coating solution was coated on the nonmagnetic layer so as to give the magnetic layer having the thickness of 0.15 μm. The coated layers were subjected to random orientation while the magnetic layer and the nonmagnetic layer were still wet by passing through an alternating current magnetic field generator having two magnetic field intensities of frequency of 50 Hz, magnetic field intensity of 250 Gauss and frequency of 50 Hz, magnetic field intensity of 120 Gauss. After drying, the coated layer was subjected to calendering treatment with calenders of 7 stages at 90° C. at line pressure of 300 kg/cm. The obtained web was punched to a disc of 3.7 inches, the disc was subjected to a surface treatment by abrasives, encased in 3.7 inch cartridge having a liner inside (a zip-disc cartridge manufactured by Iomega Co., Ltd., U.S.A.), and equipped the cartridge with prescribed mechanism parts to obtain a 3.7 inch floppy disc. A part of samples was subjected to machine direction orientation using Co magnets with the same pole and counter positions of 4,000 G before random orientation treatment.

[0323] In this case, it is preferred to increase the frequency and magnetic field intensity of the alternating current magnetic field generator so as to achieve finally sufficient random orientation, thereby 98% or more of orientation ratio can be obtained.

[0324] Preparation Method 3-2 (W/D)

[0325] Each of the above compositions of the coating solutions for the magnetic layer and the nonmagnetic layer was blended in a kneader, then dispersed with a sand mill. Polyisocyanate was added to each resulting dispersion solution, in an amount of 10 parts to the nonmagnetic layer coating solution, and 10 parts to the magnetic layer coating solution. Further, 40 parts of cyclohexanone was added to each solution. Each solution was filtered through a filter having an average pore diameter of 1 μm to obtain coating solutions for forming the nonmagnetic layer and the magnetic layer.

[0326] The nonmagnetic layer coating solution was coated in a dry thickness of 1.5 μm on a polyethylene terephthalate support having a thickness of 62 μm and a central plane average surface roughness of 3 nm of the surface side on which the magnetic layer was to be coated, dried, and subjected to calendering treatment. The magnetic layer coating solution was coated by blade coating on the nonmagnetic layer so as to give the magnetic layer having the thickness of 0.15 μm. The coated layers were subjected to random orientation by passing through an alternating current magnetic field generator having two magnetic field intensities of frequency of 50 Hz, magnetic field intensity of 250 Gauss and frequency of 50 Hz, magnetic field intensity of 120 Gauss. The procedure was carried out in the same manner as in Preparation Method 3-1 hereafter. Calendering of the nonmagnetic layer may be omitted.

[0327] Preparation Method 3—3 (Spin Coating)

[0328] Each of the above compositions of the coating solutions for the magnetic layer and the nonmagnetic layer was blended in a kneader, then dispersed with a sand mill. Polyisocyanate was added to each resulting dispersion solution, in an amount of 10 parts to the nonmagnetic layer coating solution, and 10 parts to the magnetic layer coating solution. Further, 40 parts of cyclohexanone was added to each solution. Each solution was filtered through a filter having an average pore diameter of 1 μm to obtain coating solutions for forming the nonmagnetic layer and the magnetic layer.

[0329] The nonmagnetic layer coating solution was coated in a dry thickness of 1.5 μm by spin coating on a polyethylene terephthalate support having a thickness of 62 μm and a central plane average surface roughness of 3 nm of the surface side on which a magnetic layer was to be coated and dried. The magnetic layer coating solution was coated by spin coating on the nonmagnetic layer so as to give the magnetic layer having the thickness of 0.15 μm. The layers coated were oriented using a Co magnets with the same pole and counter positions of 6,000 G in the circumferential direction and the surface of the magnetic layer was smoothed by batch system rolling treatment by which the same pressure as in Preparation Method 3-1 can be applied. The procedure was carried out in the same manner as in Preparation Method 3-1 hereafter. Also, the magnetic layer may be coated by spin coating on the nonmagnetic layer while the nonmagnetic layer coated by spin coating is still wet. By using the spin coating process, not only the amount of residual magnetization in the recording direction can be made large but also vertical magnetization components of ferromagnetic metal powders of short acicular ratio can be reduced and symmetric property of reproduced wave form can be improved.

[0330] Preparation Method 3-4 (Monolayer)

[0331] The coating solution for the magnetic layer prepared in Preparation Method 3-1 was coated on a polyethylene terephthalate support so as to reach the thickness of 0.15 μm. The procedure was carried out in the same manner as in Preparation Method 3-1 hereafter.

[0332] Various samples were prepared by combining the above-described each method arbitrarily as shown in Table 3-1. The results of evaluations are shown in Table 3-2.

[0333] Various samples of magnetic recording media were prepared in the same manner as in Example 3-11, except that the components of the magnetic coating solution 3ML-2 (a ferromagnetic metal powder) were changed as shown in Table 3—3 and the addition amount of α-alumina was changed to 8 parts. Further, in Table 3—3, the atomic ratio B of ferromagnetic metal powder 3M-10 shows atomic ratio of each component of Nd and Y to (Fe+Co), the atomic ratio A of 3M-10 is, therefore, 6.3%. The sum total of rare earth elements to calculate the atomic ratio B of other ferromagnetic metal powders is considered substantially a component of Y or Sm. The results of evaluations are shown in Tables 3-4 and 3-5.

[0334] Methods of evaluations of each sample are as follows.

[0335] (1) Magnetic characteristics (Hc):

[0336] Magnetic characteristics were measured using a vibrating sample magnetometer (a product of Toei Kogyo Co., Ltd.) by Hm 10 KOe.

[0337] (2) Central plane average surface roughness (Ra):

[0338] Surface roughness (Ra) of the area of about 250 μm×250 μm was measured using “TOPO3D” (a product of WYKO, U.S.A.) by 3D-MIRAU method. The wavelength of measurement was about 650 nm and spherical compensation and cylindrical compensation were applied. Measurement was performed using a light interference type non-contact surface roughness meter.

[0339] (3) Areal recording density:

[0340] Areal recording density means a value obtained by multiplying linear recording density by track density.

[0341] (4) Linear recording density:

[0342] Linear recording density means a bit number of signals recorded per 1 inch in the recording direction.

[0343] (5) Track density:

[0344] Track density means a track number per 1 inch.

[0345] (6) φm:

[0346] φm is the amount of magnetization per unit area of a magnetic recording medium, which is represented by Bm (Gauss) multiplying thickness. This is the value obtained by using a vibrating sample magnetometer (a product of Toei Kogyo Co., Ltd.) at Hm 10 KOe, which can be directly measured.

[0347] (7) Error rate of disc:

[0348] The above signals of linear recording density were recorded on the disc by (2,7) RLL modulation system and error rate was measured.

[0349] (8) Thickness of magnetic layer:

[0350] The sample having the thickness of about 0.1 μm was cut out with a diamond cutter in the machine direction of the magnetic medium, observed with a transmission type electron microscope of from 10,000 to 100,000, preferably from 20,000 to 50,000 magnifications and photographed. The print size of the photograph was from A4 (i.e., 297×210 mm) to A5 (i.e., 210×148 mm) sizes. The present inventors paid attentions to the difference of the shapes of the ferromagnetic powders and the nonmagnetic powders of the magnetic layer and the nonmagnetic layer and rimmed the interface and the surface of the magnetic layer with black color by visual judgement. Thereafter, the distance of the rimmed lines was measured by the image processing apparatus “IBAS2” (manufactured by Zeiss Corp.). Measurement was conducted from 85 to 300 times when the length of the sample photograph was 21 cm. The average measured value at that time was taken as d, and the standard deviation of the measured value was taken as σ. d depended on the description in JP-A-5-298653 and σ was obtained by equation (2) in JP-A-5-298653. di means each measured value and n is from 85 to 300.

[0351] (9) Running durability:

[0352] A floppy disc drive (“ZIP100”, a product of IOMEGA CORP., U.S.A., rotation number: 2,968 rpm) was used. The head was fixed at the position of radius of 38 mm. Recording was conducted at recording density of 34 kfci, then reproduced the signals recorded and this was taken as 100%. The disc was run for 25 days (600 hours) under the following thermo-cycle condition, which being taken as one cycle. Output was monitored every 24 hours of running and the point when the initial reproduction output became 70% or less was taken as NG.

[0353] Thermo-Cycle Flow

[0354] 25° C., 50% RH, 1 hr→(temperature up, 2 hr)→60° C., 20% RH, 7 hr→(temperature down, 2 hr)→25° C., 50% RH, 1 hr→(temperature down, 2 hr)→5° C., 50% RH, 7 hr→(temperature up, 2 hr)→(this cycle was repeated).

[0355] (10) Liner wear:

[0356] A sample was run for 600 hours under the same condition as in the evaluation of running durability with the head being off, and after completion of running, cartridge case of the sample was opened and the surface of the magnetic layer was visually evaluated by the following criteria.

[0357] o: No defect was observed on the surface of the magnetic layer.

[0358] Δ: Fine scratches were generated on a part of the surface of the magnetic layer.

[0359] x: Fine scratches were generated on the entire surface of the magnetic layer.

[0360] (11) Characteristics of ferromagnetic metal powder:

[0361] Evaluation of the characteristics of the ferromagnetic metal powder was conducted according to the method disclosed in JP-A-8-279137.

[0362] (12) Starting torque:

[0363] Starting torque at the time of head-on in 3.5 inch floppy disc drive was determined using torque gauge model 300 ATG (a product of Tonichi Seisakusho Co., Ltd.). (1) The sample was stored at 23° C., 50% RH as it was (before storage), (2) the sample was stored at 60° C., 90% RH for 10 days, then allowed to stand at room temperature for 24 hours, and determined at 23° C., 50% RH (after storage), and starting torque was compared before and after storage.

[0364] o: No difference was observed in starting torque before and after storage

[0365] Δ: A little increase was observed in starting torque after storage

[0366] x: Considerable increase was observed in starting torque after storage

[0367] In each of Examples 3-14 to 3-16, the disc in Example 3-11 was used and error rate was determined with varying linear recording density and track density.

[0368] From the results in Table 3-2, it can be seen that the error rate of the magnetic medium according to the present invention, in particular, in high density recording region is 1×10⁻⁵or less, which is conspicuously excellent.

[0369] In running durability of Table 3-5, 600 means 600 hours.

[0370] As is apparent from the results in Table 3-5, the magnetic disc according to the present invention is excellent in high density characteristics and also has excellent running durability.

[0371] The magnetic recording medium according to the present invention comprises a support having thereon a magnetic layer comprising a ferromagnetic metal powder dispersed in a binder, which is a magnetic recording medium for recording signals of from 0.17 to 2 G bit/inch²of areal recording density, wherein the ferromagnetic metal powder is composed of Fe and Co as main components, the atomic ratio A of Al/(Fe+Co) is from 4.5 to 15.0%, the magnetic layer has a dry thickness of preferably from 0.05 to 0.25 μm, and φm of preferably from 1.0×10⁻³to 8.0×10⁻³emu/cm², and the atomic ratio B of the sum total of rare earth elements/(Fe+Co) is preferably from 1 to 8%, and the atomic ratio C of Mg/(Fe+Co) is preferably from 0.05 to 3.0%. The magnetic recording medium of the present invention having excellent high density characteristics and, in particular, markedly improved running durability has been realized due to the above constitution of the present invention, which could never be obtained by conventional techniques.

[0372] Preparation Method 4-1 (Disc: W/W)

[0373] Each of the above ten compositions of the coating solutions for the magnetic layer and the nonmagnetic layer was blended in a kneader, then dispersed with a sand mill. Polyisocyanate was added to each resulting dispersion solution, in an amount of 10 parts to the nonmagnetic layer coating solution, and 10 parts to the magnetic layer coating solution. Further, 40 parts of cyclohexanone was added to each solution. Each solution was filtered through a filter having an average pore diameter of 1 μm to obtain coating solutions for forming the nonmagnetic layer and the magnetic layer.

[0374] These coating solutions were simultaneously multilayer-coated on a polyethylene terephthalate support having a thickness of 62 μm and a central plane average surface roughness of 3 nm of the surface side on which a magnetic layer was to be coated. The nonmagnetic layer coating solution was coated in a dry thickness of 1.5 μm, immediately thereafter the magnetic layer coating solution was coated on the nonmagnetic layer so as to give the magnetic layer having the thickness of 0.15 μm. The coated layers were subjected to random orientation while the magnetic layer and the nonmagnetic layer were still wet by passing through an alternating current magnetic field generator having two magnetic field intensities of frequency of 50 Hz, magnetic field intensity of 250 Gauss and frequency of 50 Hz, magnetic field intensity of 120 Gauss. After drying, the coated layer was subjected to calendering treatment with calenders of 7 stages at 90° C. at line pressure of 300 kg/cm. The obtained web was punched to a disc of 3.7 inches, the disc was subjected to a surface treatment by abrasives, encased in 3.7 inch cartridge having a liner inside (a zip-disc cartridge manufactured by Iomega Co., Ltd., U.S.A.), and equipped the cartridge with prescribed mechanism parts to obtain a 3.7 inch floppy disc. A part of samples was subjected to machine direction orientation using Co magnets with the same pole and counter positions of 4,000 G before random orientation treatment.

[0375] In this case, it is preferred to increase the frequency and magnetic field intensity of the alternating current magnetic field generator so as to achieve finally sufficient random orientation, thereby 98% or more of orientation ratio can be obtained.

[0376] When barium ferrite magnetic powder is used, vertical orientation can be performed besides the above-described orientation. Further, if necessary, discs after being punched may be subjected to post treatments, e.g., a thermal treatment at high temperature (generally from 50 to 90° C.) to accelerate curing of coated layers, or a burnishing treatment with an abrasive tape to scrape surface protrusions.

[0377] Preparation Method 4-2 (Computer Tape: W/W)

[0378] Each of the above compositions of the coating solutions for the magnetic layer and the nonmagnetic layer were blended in a kneader, then dispersed with a sand mill. Polyisocyanate was added to each resulting dispersion solution, in an amount of 2.5 parts to the nonmagnetic layer coating solution, and 3 parts to the magnetic layer coating solution. Further, 40 parts of cyclohexanone was added to each solution. Each solution was filtered through a filter having an average pore diameter of 1 μm to obtain coating solutions for forming the nonmagnetic layer and the magnetic layer.

[0379] These coating solutions were simultaneously multilayer-coated on an aramide support (trade name: Mictron) having a thickness of 4.4 μm and a central plane average surface roughness of 2 nm of the surface side on which a magnetic layer was to be coated. The nonmagnetic layer coating solution was coated in a dry thickness of 1.7 μm, immediately thereafter the magnetic layer coating solution was coated on the nonmagnetic layer so as to give the magnetic layer having a thickness of 0.15 μm. The coated layers were oriented with a cobalt magnet having a magnetic force of 6,000 G and a solenoid having a magnetic force of 6,000 G while both layers were still wet. After drying, the coated layer was subjected to calendering treatment with calenders of 7 stages comprising metal rolls at 85° C. at a rate of 200 m/min. Subsequently, a backing layer (100 parts of a carbon black having an average particle size of 17 nm, 80 parts of calcium carbonate having an average particle size of 40 nm, and 5 parts of α-alumina having an average particle size of 0.2 μm were dispersed in a nitrocellulose resin, a polyurethane resin and a polyisocyanate) having a thickness of 0.5 μm was coated. The obtained web was slit to a width of 3.8 mm. The magnetic layer surface of the thus-produced tape was cleaned with a tape cleaning apparatus of a nonwoven fabric and a razor blade pressed against the surface of the tape, which was attached to a machine having delivery and winding-up movement of a slit product. The thus-obtained magnetic tape was incorporated in a cartridge for DDS.

[0380] Preparation Method 4-3 (Disc: W/D)

[0381] Each of the above ten compositions of the coating solutions for the magnetic layer and the nonmagnetic layer was blended in a kneader, then dispersed with a sand mill. Polyisocyanate was added to each resulting dispersion solution, in an amount of 10 parts to the nonmagnetic layer coating solution, and 10 parts to the magnetic layer coating solution. Further, 40 parts of cyclohexanone was added to each solution. Each solution was filtered through a filter having an average pore diameter of 1 μm to obtain coating solutions for forming the nonmagnetic layer and the magnetic layer.

[0382] The nonmagnetic layer coating solution was coated in a dry thickness of 1.5 μm on a polyethylene terephthalate support having a thickness of 62 μm and a central plane average surface roughness of 3 nm of the surface side on which the magnetic layer was to be coated, dried, and subjected to calendering treatment. The magnetic layer coating solution was coated by blade coating on the nonmagnetic layer so as to give the magnetic layer having the thickness of 0.15 μm. The coated layers were subjected to random orientation by passing through an alternating current magnetic field generator having two magnetic field intensities of frequency of 50 Hz, magnetic field intensity of 250 Gauss and frequency of 50 Hz, magnetic field intensity of 120 Gauss. The procedure was carried out in the same manner as in Preparation Method 4-1 hereafter. Calendering of the nonmagnetic layer may be omitted.

[0383] Preparation Method 4—4 (Computer Tape: W/D)

[0384] Each of the above compositions of the coating solutions for the magnetic layer and the nonmagnetic layer were blended in a kneader, then dispersed with a sand mill. Polyisocyanate was added to each resulting dispersion solution, in an amount of 2.5 parts to the nonmagnetic layer coating solution, and 3 parts to the magnetic layer coating solution. Further, 40 parts of cyclohexanone was added to each solution. Each solution was filtered through a filter having an average pore diameter of 1 μm to obtain coating solutions for forming the nonmagnetic layer and the magnetic layer.

[0385] The nonmagnetic layer coating solution was coated in a dry thickness of 1.7 μm on an aramide support (trade name: Mictron) having a thickness of 4.4 μm and a central plane average surface roughness of 2 nm of the surface side on which a magnetic layer was to be coated, dried, and subjected to calendering treatment. The magnetic layer coating solution was coated by blade coating on the nonmagnetic layer so as to give the magnetic layer having the thickness of 0.15 μm. The coated layers were oriented with a cobalt magnet having a magnetic force of 6,000 G and a solenoid having a magnetic force of 6,000 G. The procedure was carried out in the same manner as in Preparation Method 4-2 hereafter. Calendering of the nonmagnetic layer may be omitted.

[0386] Preparation Method 4-5 (Disc: Spin Coating)

[0387] Each of the above ten compositions of the coating solutions for the magnetic layer and the nonmagnetic layer were blended in a kneader, then dispersed with a sand mill. Polyisocyanate was added to each resulting dispersion solution, in an amount of 10 parts to the nonmagnetic layer coating solution, and 10 parts to the magnetic layer coating solution. Further, 40 parts of cyclohexanone was added to each solution. Each solution was filtered through a filter having an average pore diameter of 1 μm to obtain coating solutions for forming the nonmagnetic layer and the magnetic layer.

[0388] The nonmagnetic layer coating solution was coated in a dry thickness of 1.5 μm by spin coating on a polyethylene terephthalate support having a thickness of 62 μm and a central plane average surface roughness of 3 nm of the surface side on which a magnetic layer was to be coated and dried. The magnetic layer coating solution was coated by spin coating on the nonmagnetic layer so as to give the magnetic layer having the thickness of 0.15 μm. The coated layers were oriented using a Co magnets with the same pole and counter positions of 6,000 G in the circumferential direction and the surface of the layer was smoothed by batch system rolling treatment by which the same pressure as in Preparation Method 4-1 can be applied. The procedure was carried out in the same manner as in Preparation Method 4-1 hereafter. Also, the magnetic layer may be coated by spin coating on the nonmagnetic layer while the nonmagnetic layer coated by spin coating is still wet. By using the spin coating process, not only the amount of residual magnetization in the recording direction can be made large but also vertical magnetization components of ferromagnetic metal powders of short acicular ratio can be reduced and symmetric property of reproduced wave form can be improved.

[0389] The above compositions of the coating solution for the backing layer were blended in a continuous kneader, then dispersed with a sand mill. The resulting dispersion solution was filtered through a filter having an average pore diameter of 1 μm to obtain a coating solution for forming the backing layer.

[0390] With respect to samples obtained by combining the above-described each method arbitrarily as shown in Table 4-1, magnetic characteristics, central plane average surface roughness, areal recording density, etc., were determined.

[0391] (1) Magnetic characteristics (Hc):

[0392] Magnetic characteristics were measured using a vibrating sample magnetometer (a product of Toei Kogyo Co., Ltd.) by Hm 10 KOe.

[0393] (2) Central plane average surface roughness (Ra):

[0394] Surface roughness (Ra) of the area of about 250 μm×250 μm, Rrms, peak-valley values were measured using “TOPO3D” (a product of WYKO, U.S.A.) by 3D-MIRAU method. The wavelength of measurement was about 650 nm and spherical compensation and cylindrical compensation were applied. Measurement was performed using a light interference type non-contact surface roughness meter.

[0395] (3) Areal recording density:

[0396] Areal recording density means a value obtained by multiplying linear recording density by track density.

[0397] (4) Linear recording density:

[0398] Linear recording density means a bit number of signals recorded per 1 inch in the recording direction.

[0399] (5) Track density:

[0400] Track density means a track number per 1 inch.

[0401] (6) φm:

[0402] φm is the amount of magnetization per unit area of a magnetic recording medium, which is represented by Bm (Gauss) multiplying thickness. This is the value obtained by using a vibrating sample magnetometer (a product of Toei Kogyo Co., Ltd.) at Hm 10 KOe, which can be directly measured.

[0403] (7) Error rate of tape:

[0404] The above signals of linear recording density were recorded on the tape by 8-10 conversion PR1 equalization system and error rate of the tape was measured using a DDS drive.

[0405] (8) Error rate of disc:

[0406] The above signals of linear recording density were recorded on the disc by (2,7) RLL modulation system and error rate was measured.

[0407] (9) Thickness of magnetic layer:

[0408] The sample having the thickness of about 0.1 μm was cut out with a diamond cutter in the machine direction of the magnetic medium, observed with a transmission type electron microscope of from 10,000 to 100,000, preferably from 20,000 to 50,000 magnifications and photographed. The print size of the photograph was from A4 (i.e., 297×210 mm) to A5 (i.e., 210×148 mm) sizes. The present inventors paid attentions to the difference of the shapes of the ferromagnetic powders and the nonmagnetic powders of the magnetic layer and the nonmagnetic layer and rimmed the interface and the surface of the magnetic layer with black color by visual judgement. Thereafter, the distance of the rimmed lines was measured by the image processing apparatus “IBAS2” (manufactured by Zeiss Corp.). Measurement was conducted from 85 to 300 times when the length of the sample photograph was 21 cm. The average measured value at that time was taken as d, and the standard deviation of the measured value was taken as σ. d depended on the description in JP-A-5-298653 and a was obtained by equation (2) in JP-A-5-298653. di means each measured value and n is from 85 to 300.

[0409] (10) Running durability:

[0410] A floppy disc drive (“ZIP100”, a product of IOMEGA CORP., U.S.A., rotation number: 2,968 rpm) was used. The head was fixed at the position of radius of 38 mm. Recording was conducted at recording density of 34 kfci, then reproduced the signals recorded and this was taken as 100%. The disc was run for 1,500 hours under the following thermo-cycle condition, which being taken as one cycle. Output was monitored every 24 hours of running and the point when the initial reproduction output became 70% or less was taken as NG.

[0411] Thermo-Cycle Flow

[0412] 25° C., 50% RH, 1 hr→(temperature up, 2 hr)→60° C., 20% RH, 7 hr→(temperature down, 2 hr)→25° C., 50% RH, 1 hr→(temperature down, 2 hr)→5° C., 50% RH, 7 hr→(temperature up, 2 hr)→(this cycle was repeated).

[0413] (11) Liner wear:

[0414] A sample was run for 1,000 hours under the same condition as in the evaluation of running durability with the head being off, and after completion of running, cartridge case of the sample was opened and the surface of the magnetic layer was visually evaluated by the following criteria.

[0415] o: No defect was observed on the surface of the magnetic layer.

[0416] Δ: Fine scratches were generated on a part of the surface of the magnetic layer.

[0417] x: Fine scratches were generated on the entire surface of the magnetic layer.

[0418] (12) Liner adhesion:

[0419] A sample was run for 1,000 hours under the same condition as in the evaluation of running durability with the head being off, and after completion of running, cartridge case of the sample was opened and the surface of the magnetic layer was visually evaluated by the following criteria.

[0420] o: Liner was not adhered on the surface of the magnetic layer.

[0421] Δ: Liner was adhered on a part of the surface of the magnetic layer.

[0422] x: Liner was adhered on the entire surface of the magnetic layer.

[0423] (13) Starting torque:

[0424] Starting torque at the time of head-on in LS-102 drive (a product of Imation Co., Ltd.) was determined using torque gauge model 300 ATG (a product of Tonichi Seisakusho Co., Ltd.) (unit: g·cm).

[0425] (14) Measurement of C/Fe

[0426] C/Fe value was determined using Auger electron spectrometer PHI-660 type manufactured by Φ Co. Conditions of measurement were as follows.

[0427] Accelerating voltage of primary electron beam: 3 KV

[0428] Electric current of sample: 130 nA

[0429] Magnification: 250-fold

[0430] Inclination angle: 30°

[0431] The C/Fe ratio is obtained as the C/Fe peak by integrating the values obtained by the above conditions in the region of kinetic energy of 130 eV to 730 eV three times and finding the strengths of KLL peak of the carbon and LMM peak of the iron as differentials.

[0432] In each of Examples 4-18 to 4-20 and Reference Example 4-1, the disc in Example 4-13 was used and error rate was determined with varying linear recording density and track density.

[0433] As described above, the above signals of linear recording density were recorded on the tape by 8-10 conversion PR1 equalization system and error rate of the tape was measured using a DDS drive. In each of Examples 4-33, 4-34 and Reference Example 4-2, the tape in Example 4-24 was used and error rate was determined with varying linear recording density and track density.

[0434] From the results in Tables 4-2 and 4—4, it can be seen that the error rates of the magnetic recording media (i.e., disc and computer tape) according to the present invention, in particular, in high density recording region, are 1×10⁻⁵or less, which are conspicuously excellent as compared with conventional disc-like media.

[0435] The magnetic recording medium according to the present invention comprises a support having provided thereon in the order from the support a substantially nonmagnetic lower layer and a magnetic layer comprising a ferromagnetic metal powder dispersed in a binder, which is a magnetic recording medium for recording signals of from 0.17 to 2 G bit/inch², preferably from 0.20 to 2 G bit/inch², of areal recording density, wherein the coercive force of the magnetic layer is 1,800 Oe or more, the ferromagnetic metal powder has the average particle length of 0.12 μm or less, and the magnetic layer has a dry thickness of from 0.05 to 0.30 μm, preferably from 0.05 to 0.25 μm, and φm of from 10.0×10⁻³to 1.0×10⁻³emu/cm², preferably 8.0×10⁻³to 1.0×10⁻³emu/cm². The magnetic recording medium of the present invention having high capacity, excellent high density characteristics, high durability, and markedly improved error rate in high density recording region has been realized due to the above constitution of the present invention, which could never be obtained by conventional techniques.

[0436] Magnetic characteristics of the magnetic powders used in Example 5 are shown in Table 5-1.

[0437] Magnetic discs and magnetic tapes were produced using the magnetic powders shown in Table 5-1 below.

[0438] Preparation Method 5-1a (Disc)

[0439] Each of the above compositions of the coating solutions for the magnetic layer and the nonmagnetic layer was blended in a kneader, then the diamond powder shown in Table 5-2 was added to each solution (or not added) and dispersed with a sand mill. Subsequently, the prescribed amount of dispersed α-alumina HIT55 (SLH55, manufactured by Sumitomo Chemical Co., Ltd) shown in Table 5-2 was added (or not added), further, polyisocyanate was added to each resulting dispersion solution, in an amount of 10 parts to the nonmagnetic layer coating solution, and 10 parts to the magnetic layer coating solution. Further, 40 parts of cyclohexanone was added to each solution. Each solution was filtered through a filter having an average pore diameter of 1 μm to obtain coating solutions for forming the nonmagnetic layer and the magnetic layer.

[0440] These coating solutions were simultaneously multilayer-coated on a polyethylene terephthalate support having a thickness of 62 μm and a central plane average surface roughness of 3 nm of the surface side on which a magnetic layer was to be coated. The nonmagnetic layer coating solution was coated in a dry thickness of 1.5 μm, immediately thereafter the magnetic layer coating solution was coated on the nonmagnetic layer so as to give the magnetic layer having the thickness of 0.2 μm. The layer containing the ferromagnetic metal powder was subjected to random orientation while the magnetic layer and the nonmagnetic layer were still wet by passing through an alternating current magnetic field generator having two magnetic field intensities of frequency of 50 Hz, magnetic field intensity of 250 Gauss and frequency of 50 Hz, magnetic field intensity of 120 Gauss. After drying, the coated layer was subjected to calendering treatment with calenders of 7 stages at 90° C. at line pressure of 300 kg/cm. The obtained web was punched to a disc of 3.7 inches, the disc was subjected to a surface treatment by abrasives, thereby a magnetic recording disc was obtained.

[0441] Preparation Method 5-1b (Disc)

[0442] Magnetic recording discs were prepared in the same manner as in Preparation Method 5-1a, except that the polyethylene terephthalate support having a central plane average surface roughness of 7 nm was used.

[0443] Preparation Method 5-2 (Computer Tape)

[0444] Each of the above compositions of the coating solutions for the magnetic layer and the nonmagnetic layer was blended in a kneader, then the diamond powder shown in Table 5-4 was added to each solution (or not added), further, the prescribed amount of dispersed α-alumina HIT55 (SLH55, manufactured by Sumitomo Chemical Co., Ltd) shown in Table 5-4 was added (or not added) and dispersed with a sand mill. Subsequently, polyisocyanate was added to each resulting dispersion solution, in an amount of 2.5 parts to the nonmagnetic layer coating solution, and 3 parts to the magnetic layer coating solution. Further, 40 parts of cyclohexanone was added to each solution and each solution was filtered through a filter having an average pore diameter of 1 μm to obtain coating solutions for forming the nonmagnetic layer and the magnetic layer.

[0445] These coating solutions obtained were simultaneously multilayer-coated on an aramide support (trade name: Mictron) having a thickness of 4.4 μm and a central plane average surface roughness of 2 nm of the surface side on which a magnetic layer was to be coated. The nonmagnetic layer coating solution was coated in a dry thickness of 1.7 μm, immediately thereafter the magnetic layer coating solution was coated on the nonmagnetic layer so as to give the magnetic layer having a thickness of 0.15 μm. The coated layers were oriented with a cobalt magnet having a magnetic force of 6,000 G and a solenoid having a magnetic force of 6,000 G while both layers were still wet. After drying, the coated layer was subjected to calendering treatment with calenders of 7 stages comprising metal rolls at 85° C. at a rate of 200 m/min. Subsequently, a backing layer (100 parts of a carbon black having an average particle size of 17 nm, 80 parts of calcium carbonate having an average particle size of 40 nm, and 5 parts of α-alumina having an average particle size of 200 nm were dispersed in a nitrocellulose resin, a polyurethane resin and a polyisocyanate) having a thickness of 0.5 μm was coated. The obtained web was slit to a width of 3.8 mm. The magnetic layer surface of the thus-produced tape was cleaned with a tape cleaning apparatus of a nonwoven fabric and a razor blade pressed against the surface of the tape, which was attached to a machine having delivery and winding-up movement of a slit product. Thus, tape samples were obtained.

[0446] The above-obtained magnetic powders, magnetic discs and computer tapes were respectively evaluated as follows.

[0447] Measuring Method

[0448] (1) Magnetic characteristics (Hc, σ₂):

[0449] Magnetic characteristics were measured using a vibrating sample magnetometer (a product of Toei Kogyo Co., Ltd.) by Hm 10 KOe.

[0450] (2) Diamond powder (particle size distribution):

[0451] An appropriate amount of a diamond powder was photographed by an electron microscope. From the electron microphotographs, 500 particles were randomly sampled, and an average particle size φ, the proportion of the number of particles having the particle size of 200% or more of the average particle size φ accounting for in the entire number of diamond particles (ΔN200) (%), and the proportion of the number of particles having the particle size of 50% or less of the average particle size φ accounting for in the entire number of diamond particles (ΔN50) (%) were obtained by the above-described methods.

[0452] (3) Central plane average surface roughness (Ra):

[0453] Surface roughness (Ra) of the area of about 250 μm×250 μm was measured using “TOPO3D” (a product of WYKO, U.S.A.) by 3D-MIRAU method. The wavelength of measurement was about 650 nm and spherical compensation and cylindrical compensation were applied. Measurement was performed using a light interference type non-contact surface roughness meter.

[0454] (4) Electromagnetic characteristics of a disc:

[0455] Output/reproduction was measured using a disc tester manufactured by Kokusai Denshi Kogyo Co., Ltd. (the late Tokyo Engineering Co., Ltd.) and a SK606B type disc evaluation apparatus by a metal-in-gap head having a gap length of 0.3 μm. Recording was conducted at the position of radius of 24.6 mm by a recording wavelength of 90 KFCI, then reproduction output of a head amplifier was determined by an oscilloscope 633 type manufactured by Techtronics Co., Ltd.

[0456] S/N ratio: After DC erasure of the disc used for determining reproduction output, reproduction output (noise level) was measured by a TR4171 type spectrum analyzer (manufactured by Advantes Co., Ltd.).

[0457] −20log (noise/reproduction output) was taken as S/N ratio and represented as a relative value taking the S/N value of Sample No. 5-1 as 0 dB.

[0458] (5) Electromagnetic characteristics of a tape:

[0459] C/N ratio (tape): digital signals were recorded and reproduced with a recording head (MIG, gap length: 0.15 μm, 1.8 T) being attached to a drum tester. Relative speed of head-medium was 3 m/sec, recording wavelength was 0.35 μm, and modulated noise was determined.

[0460] (6) Durability:

[0461] (1) Durability of a magnetic disc:

[0462] A floppy disc drive (“ZIP100”, a product of IOMEGA CORP., U.S.A., rotation number: 2,968 rpm) was used. The head was fixed at the position of radius of 38 mm. The disc was run under the following thermo-cycle condition, which being taken as one cycle. The point when scratches were visually observed on the surface of the sample was taken as NG. The time of durability of Sample No. 5-1 was taken as 100%.

[0463] Thermo-Cycle Flow

[0464] 25° C., 50% RH, 1 hr→(temperature up, 2 hr)→60° C., 20% RH, 7 hr→(temperature down, 2 hr)→25° C., 50% RH, 1 hr→(temperature down, 2 hr)→5° C., 10% RH, 7 hr→(temperature up, 2 hr)→(this cycle was repeated).

[0465] (1) Durability of a computer tape:

[0466] Predetermined signals were recorded using a DDS drive. The disc was run at 50° C., 10% RH while monitoring reproduced signals, and the point when the initial reproduction output reached 70% was taken as NG. The durability of Sample No. 5-13 was taken as 100%.

[0467] The results of evaluations of magnetic discs and computer tapes are shown in Tables 5-3 and 5—5, respectively.

[0468] As is apparent from the results in Example 5, noise in electromagnetic characteristics can be improved while retaining durability by adding to the magnetic layer a diamond powder preferably having an average particle size of from 0.01 to 1.0 μm in an amount of preferably from 0.01 to 10% by weight based on the magnetic powder.

[0469] The magnetic recording medium according to the present invention comprises a support having thereon a magnetic layer comprising a ferromagnetic metal powder dispersed in a binder, which is a magnetic recording medium for recording signals of from 0.17 to 2 G bit/inch²of areal recording density, wherein the magnetic layer has a coercive force of 1,800 Oe or more, and the ferromagnetic metal powder is composed of Fe and Co as main components, the atomic ratio of Al/(Fe+Co) is from 3.5 to 15.4%; or the magnetic recording medium according to the present invention comprises a support having thereon a magnetic layer comprising a ferromagnetic metal powder dispersed in a binder, which is a magnetic recording medium for recording signals of from 0.17 to 2 G bit/inch²of areal recording density, wherein the magnetic layer has a coercive force of 1,800 Oe or more, the ferromagnetic metal powder is composed of Fe and Co as main components, the atomic ratio of the sum total of rare earth elements/(Fe+Co) is preferably from 0.5 to 9.0%, and more preferably the magnetic recording medium of the present invention is a magnetic recording medium for recording signals of from 0.20 to 2 G bit/inch²of areal recording density, wherein the magnetic recording medium has a dry thickness of preferably from 0.05 to 0.30 μm. The magnetic recording medium of the present invention having high capacity, excellent high density characteristics, high durability and markedly improved error rate in high density recording region has been realized due to the above constitution of the present invention, which could never be obtained by conventional techniques of coating type magnetic recording medium.

[0470] While the invention has been described in detail and with reference to specific examples thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.