Magnetic Recording Powder Having Low Curie Temperature and High Saturation Magnetization

1. Field of the Invention

The present invention relates to a magnetic recording powder having a low Curie temperature and a high saturation magnetization which powder is suitable for use in forming a magnetic recording layer for a magnetic card, magnetic tape, magnetic disk or like magnetic storage medium.

2. Description of the Relevant Art

In general, as magnetic recording powders having a low Curie temperature, magnetic recording powders having a saturation magnetization (.sigma.s) of 8 to 10 emu/g, a Curie temperature (Tc) of about 150.degree. C. and a coercive force (iHc) of 5 kOe or greater have frequently been used as disclosed in, for example, Japanese Unexamined Patent Publication No. 3-124003.

The development of high density magnetic recording in recent years has been remarkable. Accordingly, a magnetic recording powder used to form a magnetic recording layer is required to have a higher saturation magnetization. However, the conventionally used magnetic recording powders cannot meet such demand because each has a low saturation magnetization of 8 to 10 emu/g.

Accordingly, the present inventors carried out research in order to develop a magnetic recording powder having a low Curie temperature (Tc) of about 150.degree. C., a coercive force (iHc) of 5 kOe or greater, and a high saturation magnetization (.sigma.s) and obtained the following results from such research which are hereinafter described.

When a magnetic recording powder is made of a composite oxide having a hexagonal ferrite type crystal structure and containing:

a) 14 to 20 atomic % (hereinafter, "%") of strontium oxide (hereinafter, "SrO") and/or barium oxide (hereinafter, "BaO"),

b) 15 to 40 atomic % of chromium oxide (hereinafter, Cr.sub.2 O.sub.3),

c) 2 to 15 atomic % of one or more of copper oxide (hereinafter, "CuO") , zinc oxide (hereinafter, "ZnO") and magnesium oxide (hereinafter "MgO"),

d) 2 to 15 atomic % of one or more of titanium oxide, zirconium oxide and tin oxide (hereinafter, "TiO" "ZrO.sub.2 " and "SnO.sub.2 ", respectively) and the remainder being substantially iron oxide (hereinafter, "Fe.sub.2 O.sub.3 ") and unavoidable impurities, the magnetic recording powder has a high saturation magnetization of 15 emu/g or higher with a Curie temperature of 155.degree. C. or lower and a coercive force of 5 kOe or greater. Such a magnetic recording powder allows for higher density recording.

This invention is developed based on the above research results. Hereinafter, a description is provided on why the composition of the magnetic recording powder according to the invention is defined as set forth above.

(a) SrO, BaO

These components are essential for forming a hexagonal ferrite type crystal structure which is necessary to attain magnetic characteristics required for the magnetic recording powder. The range of the content of such components is determined to be 14 to 20% because the saturation magnetization (.sigma.s) is reduced both when their content is less than 14% and when it is more than 20%. Preferably, the content range of such 5 components is 16 to 18%.

(b) CR.sub.2 O.sub.3

CR.sub.2 O.sub.3 acts to lower the Curie temperature of the magnetic recording powder. The range of the content of CR.sub.2 O.sub.3 is determined to be 15 to 40% because the Curie temperature cannot be lowered to a desired level when the CR.sub.2 O.sub.3 content is less than 15%, and because the saturation magnetization (.sigma.s) is reduced when the CR.sub.2 O.sub.3 content is more than 40%. Preferably, the range of the Cr.sub.2 O.sub.3 content is 20 to 35%; and it is still more preferable to have such CR.sub.2 O.sub.3 content to be 25 to 35%.

If zinc oxide is the only member of component c), above, and titanium oxide is the only member of component b), above, the amount of chromium oxide is preferably 15 to 30 atomic %, yet still more preferably 20 to 28 atomic %.

(c) CUO, ZnO and MgO

CuO, ZnO and MgO act to enhance the saturation magnetization (.sigma.s), while suppressing the Curie temperature from increasing. The range of the content of one or more of CuO, ZnO and MgO is determined to be 2 to 15% because a desired saturation magnetization cannot be obtained when the content is less than 2% and because the saturation magnetization (.sigma.s) and the coercive force (iHc) are reduced when the content is more than 15%. Preferably, the content range thereof is 6 to 11%.

(d) TiO.sub.2, ZrO.sub.2, SnO.sub.2

By coexisting with one or more of ZnO, CuO and MgO, such components act to further enhance the saturation magnetization (.sigma.s). The range of the content of such components is determined to be 2 to 15% because a desired saturation magnetization (.sigma.s) cannot be obtained when the content of such components is less than 2% and because the saturation magnetization (.sigma.s) and the coercive force (iHc) are reduced when the content of such components is more than 15%. Preferably, the content range thereof is 6 to 11%.

An especially preferred composition is: a) 14 to 20 atomic % of strontium oxide and/or barium oxide, b) 15 to 30 atomic % of chromium oxide, c) 2 to 15 atomic % of zinc oxide, d) 2 to 15 atomic % of titanium oxide, substantial remainder being iron oxide and unavoidable impurities. Also, here the preferred amount of strontium oxide and/or barium oxide is 16 to 18 atomic %, but the preferred amount of chromium oxide is 20 to 28 atomic %, while the preferred amount of zinc oxide is 6 to 11 atomic %, and the preferred amount of titanium oxide is 6 to 11 atomic %.

Another preferred composition is: a) 14 to 20 atomic of strontium oxide and/or barium oxide, b) 20 to 40 atomic % of chromium oxide, c) 2 to 15 atomic % of magnesium oxide, d) 2 to 15 atomic % of titanium oxide and/or zirconium oxide, substantial remainder being iron oxide and unavoidable impurities.

Also, here the preferred amount of strontium oxide and/or barium oxide is 16 to 18 atomic %, the preferred amount of chromium oxide is 25 to 35 atomic %, the preferred amount of magnesium oxide is 6 to 11 atomic % and the preferred amount of titanium oxide and/or zirconium oxide is 6 to 11 atomic %.

Still another preferred composition is: a) 14 to 20 atomic % of strontium oxide and/or barium oxide, b) 20 to 40 atomic % of chromium oxide, c) 2 to 15 atomic % of copper oxide, d) 2 to 15 atomic % of at least one member of the group consisting of titanium oxide, zirconium oxide and tin oxide, substantial remainder being iron oxide and unavoidable impurities.

Also, here the preferred amount of strontium oxide and/or barium oxide is 16 to 18 atomic % the preferred amount chromium oxide is 25 to 35 atomic % the preferred amount of copper oxide is 6 to 11 atomic %, and the preferred amount of at least one of titanium oxide, zirconium oxide and tin oxide is 6 to 11 atomic %.

The following examples show preferred embodiments, and are not intended to restrict the spirit and scope of this invention.

The materials powders that were prepared include BaCO.sub.3 powder, SrCO.sub.3 powder, Cr.sub.2 O.sub.3 powder, CuO powder, TiO.sub.2 powder, SnO.sub.2 powder and Fe.sub.2 O.sub.3 powder, each powder having a specified average particle diameter of 0.5 to 3.0 .mu.m. These powders were compounded so as to have compounding compositions, as shown in TABLE-1 to TABLE-3, and were kneaded by means of a kneader. Thereafter, the resulting kneaded material was pre-calcined at a temperature of 1150.degree. to 1250.degree. C. in the atmosphere for 6 hours. The obtained pre-calcined material was pulverized by means of an attritor, and calcined at a temperature of 1250.degree. to 1450.degree. C. in the atmosphere for 6 hours. The calcined material was further pulverized by means of the attritor to have an average particle diameter of as small as 0.4 .mu.m, and annealed at a temperature of 500.degree. to 1000.degree. C. in the atmosphere for 5 hours. In this manner, the magnetic recording powders 1 to 21 of this invention, and the comparative magnetic recording powders 1 to 14 having component compositions shown in TABLE-1 to TABLE-3 were produced. It is noted that one of the components of the comparative magnetic recording powders 1 to 14 falls outside the range according to the invention (and, as such, is marked with an asterisk (*) in TABLE-3 ).

The saturation magnetization (.sigma.s), Curie temperature (Tc), and coercive force (iHc) were measured for the above magnetic recording powders. The measurement results are shown in TABLE-4 and TABLE-5. Further, the crystal structure of each of the inventive magnetic recording powders 1 to 21 was observed by means of an X-ray diffractometer.

From the results shown in TABLE-4 and TABLE-5, it is seen that each of the magnetic recording powders 1 to 21 of this invention has an exceedingly high saturation magnetization (.sigma.s) of 15 emu/g or higher, and excellent magnetic characteristics: a coercive force of 5 kOe or greater and a Curie temperature of 155.degree. C. or lower. On the contrary, as seen in the comparative magnetic recording powders 1 to 14, at least one of the characteristics (i.e., the saturation magnetization (.sigma.s), coercive force (iHc) and Curie temperature (Tc)) becomes poor when the content of any of the components (marked with an asterisk (*) in TABLE-3) falls outside the range according to the invention.

Moreover, each of the magnetic recording powders 1 to 21 of this invention had a hexagonal ferrite type crystal structure.

As set forth above, the magnetic recording powder according to the invention has a saturation magnetization (.sigma.s) of 15 emu/g or higher, which is considerably higher than the saturation magnetization (.sigma.s) of 8 to 10 emu/g of the conventional magnetic recording powders; and the Curie temperature (Tc) and coercive force (iHc) thereof are as good as those of the conventional magnetic recording powders. Thus, the use of this magnetic recording powder for a magnetic card, magnetic tape, magnetic disk or like magnetic storage medium results in excellent effects (such as, a resulting thinner magnetic storage medium and high density recording).

The material powders that were prepared include BaCO.sub.3 powder, SrCO.sub.3 powder, Cr.sub.2 O.sub.3 powder, ZnO powder, TiO.sub.2 powder and Fe.sub.2 O.sub.3 powder, each powder having a specified average particle diameter of 0.5 to 3.0 .mu.m. Such powders were compounded to have compounding compositions, as shown in TABLE-6 and TABLE-7, and kneaded by means of a kneader. Thereafter, the resulting kneaded material was pre-calcined at a temperature of 1150.degree. to 1250.degree. C. in the atmosphere for 6 hours. The obtained pre-calcined material was pulverized by means of an attritor, and calcined at a temperature of 1250.degree. to 1350.degree. C. in the atmosphere for 6 hours. The calcined material was further pulverized by means of the attritor in order to have an average particle diameter of as small as 0.4 .mu.m, and annealed at a temperature of 400.degree. to 800.degree. C. in the atmosphere for 5 hours. In this manner, the inventive magnetic recording powders 1 to 11 and the comparative magnetic recording powders 1 to 10 having component compositions as shown in TABLE-6 and TABLE-7 were produced. It is noted that one of the components of the comparative magnetic recording powders 1 to 10 falls outside the range according to the invention (and is marked with an asterisk (*) in TABLE-7).

The saturation magnetization (.sigma.s), Curie temperature (Tc) and coercive force (iHc) were measured for the above magnetic recording powders. The measurement results are shown in TABLE-8 and TABLE-9. Furthermore, the crystal structure of each of the inventive magnetic recording powders 1 to 11 was observed by means of an X-ray diffractometer.

From the results shown in TABLE-8 and TABLE-9, it is seen that each of the inventive magnetic recording powders 1 to 11 has an exceedingly high saturation magnetization (.sigma.s) of 15 emu/g or higher, and excellent magnetic characteristics: a coercive force of 5 kOe or greater and a Curie temperature of 155.degree. C. or lower. On the contrary, as seen in the comparative magnetic recording powders 1 to 10, at least one of the characteristics (i.e., the saturation magnetization (.sigma.s), coercive force (iHc) and Curie temperature (Tc)) becomes poor when the content of one of the components (marked with an asterisk (*) in TABLE-7) falls outside the range according to the invention.

Furthermore, each of the magnetic recording powders 1 to 11 of this invention had a hexagonal ferrite type crystal structure.

As discusses above, the magnetic recording powder according to this invention has a saturation magnetization (.sigma.s) of 15 emu/g or higher, which is considerably higher than the saturation magnetization (.sigma.s) of 8 to 10 emu/g of the conventional magnetic recording powders. Further, the Curie temperature (Tc) and coercive force (iHc) of the magnetic recording powders of this invention are as good as those of the conventional magnetic recording powders. Thus, the use of the magnetic recording powder for a magnetic card, magnetic tape, magnetic disk or like magnetic storage medium results in significant benefits; such as, a thinner magnetic storage medium and high density recording.

The material powders that were prepared include BaCO.sub.3 powder, SrCO.sub.3 powder, Cr.sub.2 O.sub.3 powder, MgO powder, TiO.sub.2 powder and Fe.sub.2 O.sub.3 powder, each powder having a specified average particle diameter of 0.5 to 3.0 .mu.m. These powders were compounded to have 15 compounding compositions as shown in TABLE-10 and TABLE-11 and kneaded by means of a kneader. Thereafter, the resulting kneaded material was pre-calcined at a temperature of 1150.degree. to 1250.degree. C. in the atmosphere for 6 hours. The obtained pre-calcined material was pulverized by means of an attritor, and calcined at a temperature of 1250.degree. to 1350.degree. C. in the atmosphere for 6 hours. The calcined material was further pulverized by means of the attritor in order to have an average particle diameter of as small as 0.4 .mu.m, and annealed at a temperature of 400.degree. to 800.degree. C. in the atmosphere for 5 hours. In this manner, the magnetic recording powders 1 to 13 of this invention, and the comparative magnetic recording powders 1 to 12 having component compositions as shown in TABLE-10 and TABLE-11 were produced. It is noted that one of the components of the comparative magnetic recording powders 1 to 12 falls outside the range according to the invention (and is marked with an asterisk (*) in TABLE-11).

The saturation magnetization (.sigma.s), Curie temperature (Tc) and coercive force (iHc) were measured for the above magnetic recording powders. The measurement results are shown in TABLE-12 and TABLE-13. Further, the crystal structure of each of the inventive magnetic recording powders 1 to 13 was observed by means of an X-ray diffractometer.

From the results shown in TABLE-12 and TABLE-13, it is seen that each of the inventive magnetic recording powders 1 to 13 has an exceedingly high saturation magnetization (.sigma.s) of 15 emu/g or higher, and excellent magnetic characteristics: a coercive force of 5 kOe or greater and a Curie temperature of 155.degree. C. or lower. On the contrary, as seen in the comparative magnetic recording powders 1 to 12, at least one of the characteristics (i.e., the saturation magnetization (.sigma.s), coercive force (iHc) and Curie temperature (Tc)) becomes poor when the content of one of the components (marked with an asterisk (*) in TABLE-11) falls outside the range according to the invention.

Further, each of the inventive magnetic recording powders 1 to 13 had a hexagonal ferrite type crystal structure.

As seen from the above, the magnetic recording powder according to this invention has a saturation magnetization (.sigma.s) of 15 emu/g or higher, which is considerably higher than the saturation magnetization (.sigma.s) of 8 to 10 emu/g of the conventional magnetic recording powders. The Curie temperature (Tc) and coercive force (iHc) of the magnetic recording powder of this invention are as good as those of the conventional magnetic recording powders. Thus, the use of this magnetic recording powder for a magnetic card, magnetic tape, magnetic disk or like magnetic storage medium brings about an excellent benefits and advantages, including a thinner magnetic storage medium and high density recording.

While the invention has been particularly shown and described in reference to preferred embodiments thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the spirit and scope of the invention.