JPS6012763B2 - Manufacturing method of ferromagnetic powder - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a ferromagnetic powder useful as a recording element of a magnetic recording medium, and an object of the present invention is to produce a cobalt-containing iron oxide magnetic powder that has a high coercive force and is stable over time. The purpose is to provide powder.

As the performance of magnetic recording media has improved, recording elements using iron oxide magnetic powder containing cobalt have been developed.

Cobalt-containing iron oxide magnetic powder has various advantages over the conventionally used cobalt-free iron oxide magnetic powder, such as higher coercive force, higher density recording, and higher sensitivity in the high frequency range. have. Various methods for producing such cobalt-containing iron oxide magnetic powder have been proposed so far.

As one of the most useful methods, the present inventors have
- Fe203 and iron oxide magnetic powder obtained by partially reducing it are used as nucleus crystals, and after this is dispersed in a cobalt salt or a solution containing a cobalt salt and an iron salt, an alkaline solution is added thereto, and cobalt is added on the nucleus crystals. We have found a method for producing cobalt-containing iron oxide magnetic powder in which a cobalt-containing iron oxide layer is formed. Cobalt-containing iron oxide magnetic powder has excellent magnetic properties,
Although it is extremely useful as a recording element for magnetic recording media, when this cobalt-containing iron oxide magnetic powder is heat-treated at a temperature of 60 to 200°C in an oxidizing atmosphere, its coercive force is further increased and the coercive force is It was found that a ferromagnetic powder that does not undergo any physical change can be obtained.

As a result of investigating the cause of this, we found that cobalt-containing iron oxide magnetic powder containing ferrous and ferric iron has a coercive force that changes depending on the ratio of ferrous to ferric iron, Fe20/Fe30, and
For those containing divalent iron atoms where 20/Fe30 is 0.10 or more and 0.3 or less, the above Fe20/Fe30 is 0.05 to 0.13, particularly preferably around 0.1. It has been found that the coercive force increases when the above-mentioned heat treatment is performed until .

The drawing shows the coercive force of cobalt-containing iron oxide magnetic powder and the ratio of ferrous to ferric iron in this magnetic powder, Fe20/Fe30,
In the figure, curve 1 shows the case of the magnetic powder before heat treatment, and curves 2, 3, and 4 show the case where Fe20/FeW is 0.1f, 0.2, 0. The case where magnetic powder No. 3 was heat-treated at about 100° C. by the method of the present invention is shown.

As is clear from this figure, Fe20/Fey is approximately 0.1
It can be seen that the coercive force is maximum when it is around 0, and the larger the value of Fe20/Fe30 in the raw magnetic powder, the larger the change in coercive force. Therefore, when a material with Fe20/Fe30 larger than about 0.10 is heated in an oxidizing atmosphere such as air, ferrous iron changes to ferric iron and Fe20/Fe30 is larger than about 0.10.
It is clear that the coercive force increases as Fe30 approaches a value of about 0.10. Cobalt-containing iron oxide magnetic powder containing ferrous iron has good electrical conductivity and excellent dispersibility in binders, so it is suitable as a recording element for magnetic recording media. ,usually,
It is produced by using reduced y-Fe2Q as a core crystal and forming a cobalt-containing iron oxide layer on the core crystal.

However, since it is difficult to control the reduction of y-Fe203, it is difficult to adjust the ratio of Fe20/Fe30 to about 0.1 as described above. On the other hand, Fe20/F
After reduction so that e30 contains ferrous iron of 0.10 or more and 0.3 or less, oxidation is performed to adjust the value of Fe20/Fe30 to decrease, the oxidation temperature and reaction time are It can be controlled relatively easily and reliably using methods such as the above, and an optimal magnetic powder** powder can be easily produced.
Therefore, in advance, y-Fe203 is excessively reduced, and then Fe20/F of the raw material cobalt-containing iron oxide magnetic powder is reduced.
If the oxidation temperature and reaction time are appropriately selected according to the value of e30, a magnetic powder containing ferrous iron with the optimum Fe20/Fe30 can be easily obtained. For example, if Fe20/Fe30 is a large value. In this case, a high oxidation temperature and a long reaction time are selected; on the other hand, when Fe20/Fe30 is small, a low oxidation temperature and a short reaction time are selected.

In particular, when the oxidation treatment is performed for a relatively long period of time, the coercive force of the magnetic powder irreversibly increases, resulting in a ferromagnetic powder that has a high coercive force and does not change over time.

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

Example 1 Acicular y-Fe203 powder (coercive force HC: 33
0 Oelsted, saturation magnetization.

s: 74 emu/evening, about 0.3 mt of grains, about 8 koji ratio) in a hydrogen stream at the heating temperature and time shown in the table below to produce Hc as shown in the table below. A magnetic iron oxide powder of Fe20/Fe30 was obtained. Table 1 These magnetic powders 3k9 were dispersed in an aqueous solution containing 1 mole of cobalt sulfate dissolved therein, and an aqueous solution containing 6 moles of NaOH dissolved therein was added thereto.

The temperature of this dispersion liquid was raised to 100 qo, and the water crossing was continued for 3 hours while maintaining this temperature. Next, the magnetic powder was taken out, thoroughly washed with water to remove the reaction solution, and then dried. Hc of the cobalt-containing iron oxide powder thus obtained. s, Fe20/Fe30 were as shown in Table 2. Table 2 These cobalt-containing iron oxide powders were heat-treated in air at heating temperatures and times as shown in Table 3.

s, Fe20/Fe30 magnetic powder was obtained. As is clear from the results in Table 3 and above, magnetic powder with high coercive force can be easily and reliably obtained by performing oxidation treatment according to the ferrous iron content.

[Brief explanation of drawings]

The drawing is a characteristic diagram showing the relationship between fe20/Fe30 of ferrous and ferric iron in a cobalt-containing iron oxide magnetic powder and the coercive force of the magnetic powder. Curve 1 is the case before the heat treatment of the present invention, and curves 2, 3, and 4 are the cases after the heat treatment of the present invention.

Claims (1)

[Claims] 1 Iron oxide magnetic powder is used as a core crystal, and a magnetic iron oxide layer containing cobalt is formed on the core crystal, and the ferrous iron is Fe^2^+
Cobalt-containing iron oxide magnetic powder containing 0.1 to 0.3 in /Fe^3^+ was heated at a temperature of 60° to 200°C until the ferrous iron content was 0.3 in Fe^2^+/Fe^3^+. A method for producing ferromagnetic powder, the method comprising increasing the coercive force by oxidation treatment until the coercive force becomes 0.05 to 0.13.