JPS5853495B2 - magnetic recording medium - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a magnetic recording medium using cobalt-containing iron oxide magnetic powder as a recording element.

Cobalt-containing iron oxide magnetic powder is a conventionally widely used γ
-It has a higher coercive force than cobalt-free iron oxide magnetic powder such as Fe2O3, and when used in magnetic recording media, it has various advantages such as high density recording and high sensitivity in high frequency range. ing.

Various types of such cobalt-containing iron oxide magnetic powders have been proposed so far, and a typical example is one in which cobalt atoms are dissolved in the crystal lattice of iron oxide.

However, with iron oxide powder containing cobalt as a solid solution,
The solid solution of cobalt atoms causes very poor thermal stability, and when this is used as a recording element of a magnetic recording medium, there is a drawback that transfer or heating demagnetization occurs.

In order to overcome these drawbacks, the present applicant proposed the method described in Japanese Patent Publication No. 49-49475, namely:
A ferromagnetic iron oxide powder is used as a nucleus crystal, and this is dispersed in a mixed solution of an iron salt or a metal salt solution containing an iron salt and a cobalt salt, and an alkaline solution having a concentration such that at least cobalt hydroxide precipitates. proposed a cobalt-containing iron oxide magnetic powder made by epitaxially growing ferrite metal on the core crystals through an oxidation reaction using an oxidizing gas.

However, since the cobalt-containing iron oxide magnetic powder obtained as described above is oxidized by oxidizing gas during the manufacturing process, divalent iron is present in the cobalt-containing iron oxide layer grown on the nuclear crystals. Even if Fe3O4, which has a high content of divalent iron, is used as a nucleus crystal, the resulting cobalt-containing iron oxide powder will have a large surface electrical resistance, and therefore it cannot be used as a recording element. The magnetic recording medium used has a large surface electrical resistance, which tends to increase the amount of charge on the magnetic recording medium during recording and reproduction.

In order to prevent this charging, it may be possible to add a conductive material such as carbon black to the magnetic layer, but increasing the amount of such non-magnetic material is not desirable due to the characteristics of the magnetic recording medium. .

An object of the present invention is to provide a magnetic recording medium using a cobalt-containing iron oxide magnetic powder that eliminates the above-mentioned drawbacks, has excellent thermal stability such as transfer characteristics, and has good electrical conductivity.

The magnetic recording medium of the present invention has an inner part of the powder made of ferromagnetic iron oxide and a surface layer of an iron oxide layer containing divalent iron and cobalt, and a powder containing divalent iron and trivalent iron throughout the powder. A cobalt-containing iron oxide magnetic powder whose ratio (Fe2/Fe3+) is in the range of 0.10 to 0.13 and a higher divalent iron content on the powder surface than inside the powder, and this powder. It is characterized by containing a binder that disperses and binds.

In order to produce the above cobalt-containing iron oxide magnetic powder used in this invention, for example, ferromagnetic iron oxide powder is
A cobalt salt and a ferrous salt are dispersed in a metal salt solution, and an aqueous solution containing an alkali in an amount equivalent to or more than the metal salt contained in the metal salt solution is added to the solution. The ferrous iron oxide powder is treated under conditions where the ferrous ions in the solution are hardly oxidized by air while maintaining the temperature, so that the surface of the ferromagnetic iron oxide powder is oxidized mainly containing divalent iron and cobalt. Forms an iron layer.

Since the cobalt-containing iron oxide powder of the present invention obtained in this manner contains a large amount of divalent iron on the particle surface, γ-Fe 203, which is generally considered to have low electrical conductivity, was used as a starting material. Even if the particles as a whole contain the same amount of divalent iron, they have superior electrical conductivity compared to other particles, and when used as a recording element of a magnetic recording medium, a magnetic recording medium with extremely low surface resistance can be obtained. can be manufactured.

This surface electrical resistance depends on the content of divalent iron in the surface layer of the particles, and as the amount increases, the surface electrical resistance decreases.

In order to increase the content of divalent iron in the particle surface layer, for example, in the above method, the amount of ferrous salt used in the reaction may be increased.

As this ferrous salt, ferrous sulfate, ferrous chloride, ferrous nitrate, etc. are used, but if they are used in an amount of 0.1 mole or more based on the ferromagnetic iron oxide powder that is the starting material, the above-mentioned results can be achieved. A similar effect can be obtained.

When producing the cobalt-containing iron oxide magnetic powder of the present invention by the above method, the coercive force of the obtained ferromagnetic powder depends on the amount of cobalt salt such as cobalt sulfate and ferrous salt used in the reaction, the amount of alkali added, and There is a correlation with the processing temperature, and the coercive force increases as the amount of cobalt salt and ferrous salt increases, or as the amount of alkali added is greater than the equivalent of the metal salt in the reaction system, or as the processing temperature increases. do.

Therefore, by appropriately selecting these conditions, a cobalt-containing iron oxide magnetic powder having a desired coercive force can be obtained.

In terms of coercive force and thermal stability, the ratio of divalent iron to trivalent iron in the iron oxide is F.
It has been found that when iron oxide is in an intermediate oxidation state with e2/Fe3+ in the range of 0.05 to about 0.10, its coercive force becomes even higher and good results are obtained in terms of thermal stability. Therefore, for example, when producing the ferromagnetic powder of the present invention by the above method, iron oxide powder in such an intermediate oxidation state may be used as the starting material.

In order to obtain iron oxide powder in such an intermediate oxidation state, for example, a method is adopted in which γ-Fe203 powder is partially reduced in a reducing gas such as hydrogen gas until the Fe2/Fe3+ in the particles falls within the above range. good.

According to this method, it is possible to distribute a large amount of divalent iron on the particle surface, so if this is used as a starting material, a product with even lower surface electrical resistance can be obtained.

On the other hand, from the point of view of improving thermal stability to a particularly high degree, it can be said that particles whose insides are made of γ-Fe2O3 have better transfer characteristics, so they may be selected as appropriate depending on the purpose.

As stated above, 1. The cobalt-containing iron oxide magnetic powder used in this invention has an iron oxide layer mainly composed of divalent iron and cobalt formed on the surface of ferromagnetic iron oxide. Compared to iron oxide magnetic powder, which is a solid solution of iron oxide, it has much better thermal stability such as transfer characteristics and thermal demagnetization, and it also has excellent electrical conductivity because the particle surface contains a large amount of divalent iron. When this is used as a recording element of a magnetic recording medium, a magnetic recording medium for high-density recording with less transfer and heating demagnetization and low surface electrical resistance can be obtained.

Next, the present invention will be specifically explained with reference to Examples.

Example 1 Particle diameter: 0.3μ, axial ratio (major axis/minor axis): approximately 10. Coercive force (
Acicular γ-Fe203 with a saturation magnetization (hereinafter referred to as σ) of 74 emu/g and 330 oersted (hereinafter referred to as Hc).
The powder was reduced in a hydrogen stream at a temperature of 230°C for 3 hours to obtain Hc = 370 oersted, σs = 78
Iron oxide magnetic powder with emu/g and Fe2/Fe''=0.10 was obtained.

3 kg of the magnetic powder obtained above was placed in aqueous solution 101 in which 2.4 moles of ferrous sulfate and 1.2 moles of cobalt sulfate were dissolved.
was added and thoroughly dispersed, and then 21.6 mol of N was added.
An aqueous solution 101 containing dissolved aOH was added.

The temperature of this dispersion was raised to 80° C., and stirring was continued at this temperature for 6 hours while preventing air from entering as much as possible.

Next, the dispersion was filtered to remove the magnetic powder, which was thoroughly washed with water and dried.

The cobalt-containing iron oxide magnetic powder thus obtained has an Hc of 640 oersted and a σS of 79.5 emu/
It was g.

Also, as a result of chemical analysis, Fe2:/Fe3+ is 0.13
As a result of atomic absorption spectrometry, it contained 2.35 at.% of cobalt.

Next, a magnetic paint having the following composition was prepared using the above cobalt-containing iron oxide magnetic powder.

Cobalt-containing iron oxide magnetic powder 75 parts by weight Vinyl chloride-vinyl acetate copolymer 25 Dioctyl phthalate 5 Toluene
100 〃Methyl isobutyl ketone 100
This magnetic paint was applied onto a polyester film with a thickness of 12 μm to a dry thickness of about 6 μm, dried, and then cut into a predetermined width to produce a magnetic tape.

Example 2 3 kg of the same slanted γ-Fe203 powder used in Example 1 was added to an aqueous solution 101 in which 3.8 moles of ferrous sulfate and 1.5 moles of cobalt sulfate were dissolved, and the mixture was thoroughly stirred and dispersed. Thereafter, an aqueous solution 101 in which 31.8 mol of NaOH was dissolved was added.

The temperature of this dispersion was raised to 100° C., and stirring was continued at this temperature for 6 hours while preventing the incorporation of air as much as possible.

Next, the magnetic powder was separated, washed with water, and then dried.

The cobalt-containing iron oxide magnetic powder thus obtained has an Hc of 640 oersted and a σS of 77.8 emu/
It was g.

In addition, as a result of chemical analysis, Fe”/Fe3+ is 0.101
As a result of atomic absorption spectrometry, it contained 2.95 at.% of cobalt.

A magnetic tape was manufactured in the same manner as in Example 1 using this magnetic powder.

Comparative Example 1 The same acicular γ-F e 203 powder used in Example 1 was heat-treated at 300° C. for 3 hours in a hydrogen stream.

Chemical analysis of this reduced iron oxide powder revealed that Fe2/Fe3+ was 0.350.

This magnetic powder was used in the same manner as in Example 1 except that 31 air per minute was blown into the mixed solution in which the magnetic powder was dispersed to cause an oxidation reaction and to form a surface layer. Iron magnetic powder was obtained.

The magnetic powder thus obtained had an Hc of 590 oersted and a σS of 86. It was Oemu/g.

Also, as a result of chemical analysis, Fe2/Fe3+ is 0.340
As a result of atomic absorption spectrometry, it contained 3.73 at.% of cobalt.

A magnetic tape was manufactured in the same manner as in Example 1 using this magnetic powder.

Comparative Example 2 In Comparative Example 2, a cobalt-containing iron oxide magnetic powder in which cobalt atoms were uniformly dissolved in iron oxide powder was produced by the following method.

The same acicular γ-Fe203 powder 3- as used in Example 1 was added to an aqueous solution of No. 61 in which o, s mol (252 g) of cobalt sulfate and 1.5 kg of sodium citrate were dissolved, and the mixture was thoroughly stirred and dispersed. After that, the pH of this dispersion was adjusted to 9.5 with a 2N aqueous sodium hydroxide solution.

Next, this dispersion was placed in an autoclave and heated to 200
A hydrothermal reaction was carried out at ℃ for 3 hours.

After the reaction was completed, the resulting precipitate was washed with water and dried to obtain acicular magnetic powder.

The thus obtained cobalt-containing iron oxide magnetic powder has an Hc of 600 oersted and a σS of 78.5 ern.
It was u/g.

Also, as a result of chemical analysis, Fe2/Fe3+ is 0.110
As a result of atomic absorption spectrometry, it contained 1.57 at.% of cobalt.

A magnetic tape was manufactured in the same manner as in Example 1 using this magnetic powder.

Comparative Example 3 The iron oxide magnetic powder with Fe2/Fe"+ of 0.350 obtained in Comparative Example 1 was used as a nucleus crystal, and was treated in the same manner as in Example 1 to obtain cobalt-containing iron oxide magnetic powder.

The Hc of this powder is 620 oersted, and the σ5 is 87 em.
u/g1Fe"/);'e"+ was 0.380, and the cobalt content was 2.35 at.%.

A magnetic tape was manufactured in the same manner as in Example 1 using this magnetic powder.

The cobalt-containing iron oxide powders of each of the above examples and comparative examples were immersed in a 1N hydrochloric acid solution to gradually dissolve from the particle surface, and the magnetic powder was taken out at regular intervals to measure its coercive force and cobalt content. As a result, in the magnetic powder of Comparative Example 2, the coercive force and cobalt content hardly changed even as the dissolution progressed, whereas in Examples 1 and 2 and Comparative Examples 1 and 3, as the amount of dissolution increased, A significant decrease in coercive force and cobalt content was observed.

This is because in Comparative Example 2, cobalt is uniformly dissolved in the iron oxide crystal lattice, whereas in Examples 1 and 2 and Comparative Examples 1 and 3, an iron oxide layer containing cobalt is formed on the particle surface. It shows that

Next, the surface electrical resistance and transfer characteristics of each magnetic tape were measured, and the results shown in the following table were obtained.

As is clear from the above results, the magnetic tape using the magnetic powder of the present invention has excellent transfer characteristics and has a very low surface electrical resistance.

This is because, although the magnetic powder of the present invention has a relatively small value of Fe2/Fe3+, a large amount of divalent iron exists on the particle surface.

In the magnetic powder of Comparative Example 1, the value of Fe2/Fe3+ is larger than in any of the examples of this invention, but in the magnetic powder of Comparative Example 1, there is almost no divalent iron on the particle surface.
It is thought that the surface electrical resistance of the magnetic tape using this material is increased.

In addition, in the magnetic powder of Comparative Example 3, good results were obtained in terms of surface electrical resistance of the magnetic tape due to the presence of a large amount of divalent iron on the particle surface, but the value of Fe2/Fe3+ was lower than that of the comparative example. As in case 1, it is too high and the transfer characteristics cannot be satisfied.

Claims (1)

[Claims] 1 The inside of the powder is made of ferromagnetic iron oxide and has an iron oxide layer containing divalent iron and cobalt as a surface layer, and the ratio of divalent iron to trivalent iron of the entire powder (Fe 2 /
This powder is dispersed and bound with a cobalt-containing iron oxide magnetic powder whose F e 3 ”) is in the range of 0.10 to 0.13 and has a higher divalent iron content on the surface of the powder than inside the powder. A magnetic recording medium containing a binder.