Magnetic Recording Medium

This invention relates to magnetic recording media, and more particularly, to magnetic recording media having a magnetic layer in the form of a continuous thin film formed by the so-called oblique incidence evaporation process.

Among magnetic recording media for use in video, audio and other applications, active research and development works have been made on magnetic recording media, usually magnetic tapes having a magnetic layer in the form of a continuous thin film because of the compactness of a roll of tape.

The preferred magnetic layers for such continuous film type media are deposited films of Co, Co--Ni, Co--O, Co--Ni--O and similar systems formed by the so-called oblique incidence evaporation process in which cobalt and optional elements are evaporated and directed at a given angle with respect to the normal to the substrate because such evaporated films exhibit superior characteristics. The evaporated magnetic layers, however, have many disadvantages including high dynamic friction, low film strength, rough head contact, and low runnability (that is, the durability of tape which travels in frictional contact with rigid members in a video machine) resulting in reduced output after repeated travels. In the case of video application, the evaporated magnetic layers exhibit a short life time when operated in the still mode (to be simply referred to as "still life", hereinafter). In addition, there occur many dropouts in such layers.

Under these circumstances, a variety of topcoats have heretofore been proposed which cover obliquely evaporated magnetic layers. For example, Japanese Patent Application Kokai Nos. 53-88704 and 55-93533 disclose such topcoats formed from linear chain saturated fatty acids and esters thereof by evaporation and coating techniques. These prior art topcoats, however, are yet insufficient with respect to some of the above requirements, for example, runnability, still life, and dropout.

It is, therefore, an object of the present invention to provide a magnetic recording medium having a continuous thin magnetic layer covered with a novel topcoat which has improved characteristics including low dynamic friction, high strength and lubricity, smooth head contact, increased runnability, prolonged still life, and minimized dropouts.

According to the present invention, there is provided a magnetic recording medium comprising a substrate, a continuous thin magnetic layer on the substrate, and a topcoat formed over the magnetic layer, characterized in that the topcoat consists of a compound having the formula: ##STR2## where R is selected from alkyl radicals having 10 to 22 carbon atoms, and

Y and Y' are independently selected from the group consisting of hydrogen atom, alkyl radicals and monovalent cations, and Y and Y' may together represent a divalent cation.

The above and other objects, features, and advantages of the present invention will be more fully understood by reading the following description when taken in connection with the accompanying drawing, in which:

FIG. 1 is a partially cut-away, perspective view of the magnetic recording medium of the present invention.

Referring to FIG. 1, there is illustrated a magnetic recording medium generally designated at 10 according to the present invention. The magnetic recording medium 10 includes a substrate 11, a magnetic layer 12 formed thereon, and a topcoat 13. These elements will be described in more detail hereinafter.

The magnetic recording medium of the present invention has a magnetic layer on a substrate. The magnetic layer is of continuous thin film type coextending over the substrate and is generally composed solely of or based on cobalt or nickel. In the preferred embodiment of the present invention, the magnetic layer may contain cobalt (Co) as an essential base. It may preferably consist essentially of cobalt; cobalt and nickel; cobalt and oxygen; or cobalt, nickel, and oxygen. That is, the magnetic layer may consist essentially of cobalt alone or a mixture of cobalt with nickel and/or oxygen.

Where the layer consists essentially of cobalt and nickel, the weight ratio of Co/Ni may preferably be at least 1.5.

The magnetic layer may further contain oxygen in addition to cobalt or cobalt and nickel. The presence of oxygen contributes to further improvements in electromagnetic characteristics and runnability. In this case, the atomic ratio of O/Co (when nickel free) or O/(Co+Ni) is preferably not more than 0.45, more preferably from 0.15 to 0.4.

More preferred results are obtained when the magnetic layer contains chromium in addition to cobalt; cobalt and nickel; cobalt and oxygen; or cobalt, nickel, and oxygen. The presence of chromium contributes to further improvements in electromagnetic characteristics, output level, signal-to-noise (S/N) ratio, and film strength. In this case, the weight ratio of Cr/Co (when nickel free) or Cr/(Co+Ni) is preferably in the range of 0.001 to 0.1, more preferably 0.005 to 0.05.

The magnetic layer may further contain trace elements, particularly transition metal elements, for example, Fe, Mn, V, Zr, Nb, Ta, Mo, W, Ti, Cu, Zn, etc.

The magnetic layer preferably consists of a coalescence of particles of columnar structure oriented oblique to the normal to the substrate because of enhanced electromagnetic characteristics. More specifically, particles of columnar structure are preferably oriented at an angle of 20 to 60 degrees with respect to the normal to the major surface of the substrate. Each columnar particle generally extends throughout the thickness of the magnetic layer and has a diameter of the order of 50 to 500 angstroms. Cobalt and optional metals such as nickel and chromium form the columnar structure particles themselves while oxygen, when added, is generally present on the surface of each columnar structure particle essentially in the form of oxides. The magnetic layer generally has a thickness of 0.05 to 0.5 microns.

The magnetic layer may be formed on the substrate either directly or via an undercoat layer. Further, the magnetic layer is generally formed as a single layer, but in some cases, it may be made up from a plurality of laminated sub-layers with or without an intermediate layer interposed therebetween.

The magnetic layer is generally formed by the so-called oblique incidence evaporation process. The oblique incidence evaporation process may be any of well-known techniques preferably using an electron beam gun while the minimum incident angle with respect to the normal to the substrate is preferably 30 degrees. Evaporation conditions and post-treatments are well known in the art and any suitable ones may be selected therefrom. One effective post-treatment is a treatment for incorporating oxygen into the magnetic layer, which is also well known in the art. For further information about this evaporation process, reference should be made to D. E. Speliotis et al., "Hard magnetic films of iron, cobalt and nickel", J. Appl. Phys., 36, 3, 972 (1965) and Y. Maezawa et al., "Metal thin film video tape by vacuum deposition", IERE Conference Proceedings 54 (The Fourth International Conference on Video and Data Recording, The University of Southampton, Hampshire, England, 20-23 April, 1982), pp. 1-9.

On the magnetic layer is formed a topcoat consisting of a phosphate having the formula: ##STR3## where R is selected from alkyl radicals having 10 to 22 carbon atoms, and

Y and Y' are independently selected from the group consisting of hydrogen atom, alkyl radicals and monovalent cations, and Y and Y' may together represent a divalent cation.

The alkyl radicals represented by R are preferably straight chain, unsubstituted alkyl radicals having 10 to 22 carbon atoms. If the alkyl radicals of R contain less than 10 carbon atoms, the resulting topcoat and hence, magnetic recording medium exhibits increased dynamic friction as well as reduced runnability and still life, and increased dropouts all outside the critical level. On the other hand, if the alkyl radicals of R contain more than 22 carbon atoms, dropouts increase beyond the critical level. R is preferably selected from straight chain, unsubstituted alkyl radicals having 12 to 22 carbon atoms because further improvements in dynamic friction, runnability, and dropout are expectable.

Y and Y' may be the same or different radicals and are independently selected from hydrogen atom, alkyl radicals and monovalent cations. Alternatively, Y and Y' may together represent a divalent cation.

When Y is an alkyl radical, or both Y and Y' are alkyl radicals, the total number of carbon atoms contained in R and Y (that is, dialkyl phosphates) or in R, Y and Y' (that is, trialkyl phosphates) is preferably 44 or less because more dropouts will otherwise occur. More preferably, in the case of dialkyl phosphates, the total number of carbon atoms contained in R and Y is 40 or less, especially in the range of from 20 to 40. In the case of trialkyl phosphates, the total number of carbon atoms contained in R, Y and Y' is 44 or less, especially in the range of from 20 to 44.

The alkyl radicals represented by R, Y, and Y' are preferably unsubstituted, straight chain ones although they may be substituted with a halogen, hydroxyl, cyano, alkyl, aryl, alkoxy, aryloxy, alkoxycarbonyl, aryloxycarbonyl, acyl, acylamino, carbamoyl, alkylsulfonylamino, sulfamoyl, sulfonyl radical or the like, or they may be branched ones.

Y and/or Y may also represent a monovalent cation, or they may together represent a divalent cation.

The monovalent cations include (1) alkali metal ions such as Li.sup.+, Na.sup.+, and K.sup.+ ; and (2) onium ions including ammonium ion (NH.sub.4.sup.+). Examples of the onium ions are primary to quaternary ammonium ions, primary to quaternary phosphonium ions, and primary to tertiary solfonium ions, which all may have an alkyl radical such as methyl, ethyl, n-butyl, isoamyl, n-dodecyl, n-octadecyl, etc. or an aryl radical such as phenyl, tolyl, naphthyl, etc.; the alkyl and aryl radicals being possibly substituted with a cyano, alkyl, aryl, alkoxy, aryloxy, alkoxycarbonyl, aryloxycarbonyl, acyl, acylamino, carbamoyl, alkylsulfonylamino, sulfamoyl, sulfonyl radical or the like. Also included are cyclic, secondary, tertiary and quaternary, N-hydrogen, N-alkyl and N,N-dialkyl ammonium ions which may have a 5- or 6-membered ring such as pyridine, imidazole, pyrrole, 2-pyrroline, pyrrolidine, piperidine, pyrazole, pyrazoline, imidazoline, etc.

In one preferred embodiment, either Y or Y' is or both Y and Y' are selected from ammonium ions having a C.sub.10-22 alkyl radical. These ammonium ions may preferably be primary or secondary ammonium ions. Where either Y or Y' is a primary ammonium, the alkyl radical may preferably contain 12 to 22 carbon atoms, especially 12 to 18 carbon atoms. Where one or both of Y and Y' are secondary ammonium ions, one of the alkyl radicals attached to the nitrogen atom of the secondary ammonium may preferably contain 12 to 22 carbon atoms, especially 12 to 18 carbon atoms, and the total number of carbon atoms contained in both the alkyl radicals is 13 to 40. These alkyl radicals of the ammonium ions may preferably of straight chain as well as unsubstituted although substituted alkyl radicals as exemplified above and branched alkyl radicals are within the scope of the present invention.

Examples of the divalent cations are Mg.sup.++, Ca.sup.++, Sn.sup.++, Ba.sup.++, etc., and bisammonium and bisphosphonium ions. Also included are divalent or trivalent or polyvalent ammonium cations having the general formula: B-[N.sup.+ (R').sub.3 ].sub.m where B represents a hydrocarbon residue, preferably an alkylene radical such as methylene and ethylene, R' represents hydrogen or a substituted or unsubstituted alkyl or aryl radical, and m is a whole number of 2 or 3 or more.

In some caes, Y and/or Y' can individually be a divalent or polyvalent cation which bonds with two or more phosphate residues, but usually, Y and/or Y' may be a monovalent cation.

The phosphate compounds of the above formula may be synthesized by any well-known process or they may be commercially available. The use of a mixture of two or more such compounds is also within the scope of the present invention.

Examples of the phosphate compounds represented by the above general formula are shown below for the purpose of illustration and not by way of limitation.

The topcoat consisting of such a phosphate ester or salt or a mixture thereof has a thickness ranging from a monomolecular layer to 0.3 microns, preferably from 0.005 to 0.1 microns.

The topcoat may preferably be formed by evaporation or sputtering process. Evaporation, typically by resistance heating, and sputtering, typically RF or DC sputtering may be carried out under well-known conditions, for example, at a melt temperature of 80.degree. to 100.degree. C. and an operating pressure of 1.times.10.sup.-3 to 1.times.10.sup.-1 Pa. Film preparation processes other than the evaporation and sputtering processes may also be employed, but such other processes, for instance, gravure coating and calender coating are very difficult to form a uniform film as thin as defined above.

The substrate which can be used in the medium of the invention is not particularly limited although flexible substrates, particularly substrates of polyester, polyimide, polypropylene and other resins are preferred, commonly in the form of a length of tape. The thickness of the substrate varies over a wide range and is preferably in the range of 5 to 20 microns. The back surface of the substrate which is opposite to the magnetic layer-bearing surface may preferably have a surface roughness or height irregularity of more than 0.05 microns (inclusive) as expressed in RMS (root mean square) value.

The magnetic recording medium of the invention is useful in video, audio and other applications.

The magnetic recording medium of the invention, which is top coated with a phosphate having the above-defined formula, has a number of benefits, for example, critically reduced dynamic friction. In addition, film strength, lubricity, and head contact are greatly improved. As a result, the runnability and still life of the magnetic recording medium are substantially increased with minimized dropouts.

Examples of the present invention are presented below by way of illustration, but not by way of limitation.

Using a Co--Ni alloy having a Co/Ni weight ratio of 4/1 and a Co--Ni--Cr alloy having a Co/Ni/Cr weight ratio of 75/20/5, a magnetic layer of 0.2 .mu.m thick was formed on a polyethylene terephthalate (PET) film of 10 .mu.m thick by the oblique incidence evaporation process. The alloy-deposited films are designated Samples A0 and B0.

In the oblique incidence evaporation, the incident angle was 45 degrees with respect to the normal to the film or substrate, and the atmosphere contained argon and oxygen at partial pressures P.sub.Ar =2.times.10.sup.-2 Pa and P.sub.O.sbsb.2 =1.times.10.sup.-2 Pa.

The resulting magnetic layers had the same compositions as the corresponding alloys except that oxygen was introduced therein, and both had an atomic ratio of O/(Co+Ni) of B 0.2. The layers consisted of coalesced columnar structure particles grown at an angle of about 40.degree. with respect to the normal to the major surface of the substrate, extending throughout the layer thickness, and having a diameter of 0.01 .mu.m.

An Auger electron-spectroscopy analysis on the layer with ion milling showed that cobalt (Co) is less in proximity to the surface and oxygen (O) is chemically shifted and has a profile rich in proximity to the surface, indicating that oxygen is present bonded with the metals on the surface of columnar particles.

Next, topcoats of 0.01 .mu.m thick as shown in Table 1 were formed on the magnetic layers of Samples A0 and B0 using various organic compounds as the evaporation source in an atmosphere having a partial pressure of argon P.sub.Ar =4.times.10.sup.-3 Pa. The thus coated films are designated Samples A1 to A14 and B1 to B10.

The samples were tested for various properties. A commercially available VTR equipment was loaded with each sample in the form of video tape.

A sample tape was driven over 50 passes in the VRT equipment. The dynamic friction coefficient .mu. of the sample was measured at a temperature of 40.degree. C. and a relative humidity of 80% both at the initial and after 50 passes.

A sample tape was driven over 50 passes in the VTR equipment. The magnitude of signal output of the sample was measured at 4 MHz both at the initial and after 50 passes. The output reduction after 50 passes was expressed in dB.

The VTR equipment loaded with a sample tape was operated in the still mode. The still life time (expressed in minute) of the sample tape was measured until the still picture disappeared.

A sample tape was played in the reproduction mode. Dropouts were counted as the number per minute of output reductions by 16 dB or more.

The results are shown in Table 1.

The procedure of Example 1 was repeated except that onium salts of straight-chain unsubstituted alkyl phosphates were used to form topcoats. The resulting films are designated Samples C1 to C21 and D1 to D10.

The test results are shown in Table 2.

The procedure of Example 1 was repeated except that salts of straight-chain unsubstituted alkyl phosphate monoesters were used to form topcoats. The resulting films are designated Samples E1 to E19 and F1 to F12.

The test results are shown in Table 3.

The procedure of Example 1 was repeated except that ammonium salts of straight-chain unsubstituted alkyl phosphate monoesters were used to form topcoats. The resulting films are designated Samples G1 to G22 and H1 to H10.

The test results are shown in Table 4.

The procedure of Example 1 was repeated except that phosphate diesters and triesters were used to form topcoats. The resulting films are designated Samples J1 to J21 and K1 to K12.

The test results are shown in Table 5.

The data in Tables 1 to 5 reveal that the alkyl phosphate topcoat of the present invention exhibits improved performance over the prior art topcoats and that the number of carbon atoms in the alkyl is critical.