JPS59152226A - Treatment of magnetic powder - Google Patents

Abstract

PURPOSE:To obtain Co-containing iron oxide magnetic powder having excellent mangentic characteristics, age stability and thermal stability, by covering the surface of an acicular iron oxide magnetic particle with a layer of a bivalent iron oxide and with a Co-containing iron oxide layer, and heating the laminated powder under specific condition. CONSTITUTION:An iron oxide layer is applied to the surface of an acicular iron oxide magnetic particles such as gamma-Fe2O3 by using ferrous sulfate etc. containing bivalent iron ion. The coated surface is further coated with a Co-containing iron oxide layer using cobalt sulfate and ferrous sulfate. The laminated iron oxide particle is heat-treated at 200-400 deg.C in an inert gas atmosphere or in vacuum. A high-quality iron oxide layer uniformly containing bivalent iron ion and cobalt is formed by the treatment. The cobalt ion is oriented along the surface magnetic field of the acicular iron oxide magnetic particle, and magnetic particles having large uniaxial anisotropy along the direction of long axis and having improved coercivity can be obtained by this process.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for processing cobalt-containing iron oxide magnetic powder suitable for use in magnetic recording media. Another object of the present invention is to provide the above cobalt-containing iron oxide magnetic powder.

Iron oxide magnetic powder containing cobalt has a higher coercive force than iron oxide magnetic powder commonly used as a recording element in conventional magnetic recording media, and is therefore useful as a recording element in high-performance magnetic recording media.

Various cobalt-containing iron oxide magnetic powders have been proposed. For example, iron oxide magnetic powder is dispersed in an aqueous solution containing a cobalt salt or a cobalt salt and an iron salt, and then an alkali is added to the powder. Those obtained by adding an aqueous solution to form an iron oxide layer containing cobalt on the particle surface of the iron oxide magnetic powder, and those obtained by dispersing the iron oxide magnetic powder in an alkaline aqueous solution containing cobalt and hydrothermally treating it to form cobalt. There are products in which iron oxide magnetic powder is dissolved as a solid solution.

However, although the conventional cobalt-containing iron oxide magnetic powder obtained by these methods has a high coercive force, it is still not fully satisfactory, and in particular, those obtained by solid dissolving cobalt in iron oxide magnetic powder have a high coercivity over time. Poor physical and thermal stability.

As a result of extensive research in view of the current situation, the inventors first formed an iron oxide layer containing divalent iron ions on the particle surface of acicular iron oxide magnetic powder, and then formed a layer on the iron oxide layer. When an iron oxide layer containing cobalt is formed on the iron oxide magnetic powder and this is heat-treated at a temperature of 200 to 400°C in an inert gas atmosphere or vacuum, two layers of oxidation formed on the particle surface of the iron oxide magnetic powder are formed. As the composition of the iron layer becomes the same, it becomes uniform, and the large amount of divalent iron ions makes it easier for cobalt ions to move, and the surface magnetic field of the acicular iron oxide magnetic powder causes cobalt ions to move in the direction of the magnetic field. The inventors have discovered that it is possible to obtain a cobalt-containing iron oxide magnetic powder that is oriented, has a higher coercive force, and has uniaxial anisotropy with excellent stability over time and thermal stability, leading to the present invention.

In this invention, the iron oxide layer formed on the surface of the acicular iron oxide magnetic powder includes an iron oxide layer containing divalent iron ions as the lower layer, and an iron oxide layer containing cobalt on top of the iron oxide layer containing divalent iron ions. It is preferable to form the layer in layers, and in this way, two layers are formed in the lower layer.
When an iron oxide layer containing valent iron ions is interposed, the cobalt ions contained in the upper iron oxide layer become more mobile due to the large amount of 21i11i iron ions.

Therefore, by heat treatment at a relatively low temperature of 400°C or less,
Cobalt ions are not diffused into the needle-shaped iron oxide magnetic powder and are easily and uniformly dissolved in the lower iron oxide layer, making the composition of both the upper and lower iron oxide layers the same, and divalent iron ions. A good iron oxide layer uniformly containing cobalt and cobalt is formed, and a cobalt-containing iron oxide magnetic powder with high coercive force is obtained. Furthermore, the cobalt ions, which have become more mobile due to the large amount of divalent iron ions present in the iron oxide layer on the surface, are oriented in the direction of the magnetic field by the surface magnetic field of the acicular iron oxide magnetic powder, and the direction of easy magnetization is changed. Since it is in the same direction as the acicular iron oxide magnetic powder, large uniaxial anisotropy occurs in the acicular direction, further improving coercive force and improving stability over time and thermal stability.

In this way, heat treatment of acicular iron oxide magnetic powder, in which an iron oxide layer containing divalent iron ions is first formed on the particle surface, and then an iron oxide layer containing cobalt is further formed on the particle surface, is performed using nitrogen. It is preferable to conduct the reaction at a temperature of 200 to 400°C in an inert gas atmosphere such as gas, helium gas, or argon gas, or in vacuum. In an oxidizing gas atmosphere, divalent iron ions contained in the lower layer of the powder surface are Since it is oxidized into trivalent iron ions, it becomes difficult for cobalt ions to diffuse into this layer, making it impossible to obtain a high coercive force. Further, in a reducing gas atmosphere, divalent iron ions present in the underlying iron oxide layer may be diffused into the interior of the iron oxide magnetic powder at the same time as the reduction, which is not preferable. In addition, if the heat treatment temperature is below 200°C, the cobalt ions contained in the upper iron oxide layer become difficult to move, so the cobalt ions may diffuse evenly into the lower iron oxide layer or become acicular iron oxide magnetic powder. Due to the surface magnetic field, the cobalt ions are no longer oriented in the acicular direction, making it impossible to sufficiently improve the coercive force, and when carried out at temperatures above 400°C, cobalt ions diffuse into the acicular iron oxide magnetic powder. Since the coercive force decreases and the stability over time and thermal stability also deteriorate, the heat treatment is preferably carried out at a temperature of 200 to 400°C, more preferably 250 to 350°C.

Figure 1 shows that an iron oxide layer containing divalent iron ions and an iron oxide layer containing cobalt are sequentially laminated on the surface of acicular γ-Fe, 03 powder, and the cobalt content is 5. weight%
Magnetic powder (coercive force 611 oersted, square shape 0.47
) is heat-treated in nitrogen gas for 3 hours, and the relationship between heat treatment temperature, coercive force, and square shape is shown in graph form. Laminated 2N iron oxide layer with cobalt content of 7
Magnetic powder (coercive force: 814 Oersted, square shape: 0.05 Oe, coercive force: Co/F e = 1/2 in weight%)
50) is heat treated in nitrogen gas for 3 hours, and the relationship between heat treatment temperature, coercive force, and square shape is shown in graph form. In both figures, graph A shows the relationship between coercive force and heat treatment temperature. Graph B shows the relationship between the square shape and the heat treatment temperature.

As is clear from these graphs, the coercive force of any magnetic powder decreases significantly when the heat treatment temperature is below 200°C or above 400°C, but at temperatures between 200 and 400°C
It can be seen that good coercive force and square shape were obtained at a temperature of 200 to 400°C, and that the heat treatment temperature is preferably 200 to 400°C.

In addition, the coercive force of both is at a heat treatment temperature of 300 to 350°C.
The square shape ' increases rapidly from around 300 to 350°C, where cobalt ions begin to diffuse inside the iron oxide magnetic powder. It can be seen that the optimum temperature is just before diffusion begins. Furthermore, the magnetic powder obtained by this heat treatment has a square shape close to 0.5 or 0.5.
5, indicating that this has axial anisotropy.

Figure 3 shows that γ-Fe203 powder is treated in an alkaline aqueous solution containing cobalt to dissolve cobalt into a solid solution without forming an iron oxide layer containing divalent iron ions or cobalt on the powder particle surface. The cobalt content obtained by
% by weight of magnetic powder (coercive force 446 oerstesod rectangular 0.
45) is heat treated in nitrogen gas for 3 hours, and the relationship between heat treatment temperature and coercive force (graph A) and the relationship between heat treatment temperature and square shape (graph B) are graphed. As is clear from this graph, although the coercive force of this type of cobalt solid solution iron oxide magnetic powder is improved by heat treatment, it is extremely small, which indicates that the particle surface of the iron oxide magnetic powder contains divalent iron ions and cobalt. It can be seen that if the iron oxide layer is not formed, no significant effect can be obtained even if heat treatment is performed.

The iron oxide layer containing divalent iron ions and the iron oxide layer containing cobalt that are formed on the surface of the acicular iron oxide magnetic powder are formed by first applying the acicular iron oxide magnetic powder such as γ-Fe203 powder. Heat-treated in a solution containing divalent iron ions such as a ferrous salt aqueous solution, then dispersed in an aqueous solution containing cobalt salt or cobalt salt and iron salt, and reacted by adding an alkaline aqueous solution to this. Alternatively, it can be formed by dispersing acicular iron oxide magnetic powder in water, adding iron salt and cobalt salt thereto, mixing and melting the powder, and then adding an aqueous alkali solution to cause a reaction.

The acicular iron oxide magnetic powder used here is γ-
Ffl1203 powder, Fe50 cow powder and γ-Fe2
Iron oxide magnetic powder having an oxidation state intermediate between 7-Fe, 03, and Fe3O4, which is obtained by partially reducing O3 in a hydrogen stream, is preferably used.

Further, as the iron salt, ferrous chloride, ferrous sulfate, ferrous nitrate, etc. are preferably used, and as the cobalt salt, cobalt chloride, cobalt sulfate, cobalt nitrate, etc. are preferably used.

As the alkali, caustic soda is usually used, and its suitable amount is preferably at least equivalent to the total amount of iron salt and cobalt salt.

Example 1 Acicular r with a major axis diameter of 0.3μ and an axial ratio (major axis diameter/minor axis diameter) of 8
After dispersing 1000g of Fe2O3 powder in 5β of water, 350g of ferrous sulfate and 55% of caustic soda were added to it.
00g was added and mixed and dissolved, and the mixture was heated at a temperature of 40°C for 2 hours to react. Next, 330 g of cobalt sulfate and 320 g of ferrous sulfate were added thereto and mixed and dissolved. Further, an aqueous solution 51 of caustic soda in which 1100 g of caustic soda was dissolved was added and reacted at 45° C. for 8 hours. After the reaction was completed, it was washed with water, filtered, and dried. The coercive force of this magnetic powder after drying is 670
At Oersted, the saturation magnetization is 77.4 emu/g
, the square shape was 0.49.

Next, this magnetic powder was heat-treated at 300° C. for 3 hours in nitrogen gas to obtain a cobalt-containing iron oxide magnetic powder having −axis anisotropy.

Example 2 The same acicular r-Fe203 powder 1 used in Example 1
After dispersing 000g in water 51, 330g of cobal sulfate) and 650g of ferrous sulfate were added and mixed and melted, and then a caustic soda aqueous solution 51 in which 1100g of caustic soda was dissolved was added and reacted at 45°C for 8 hours. . After the reaction was completed, it was washed with water, filtered, and dried. The coercive force of this magnetic powder after drying is 650
The saturation magnetization amount was 77.2 emu 7g in Oersted, and 0.49 in the square shape.

Next, this magnetic powder was heat-treated at 300° C. for 3 hours in nitrogen gas to obtain a cobalt-containing iron oxide magnetic powder having −axis anisotropy.

Comparative Example 1 The same acicular γ-Fe203 powder 1 as used in Example 1
After dispersing 000g in water 51, add 330g of cobal sulfate, mix and dissolve, and then add 11g of caustic soda.
Add 100g of caustic soda aqueous solution at 5℃ to 45℃.
The reaction was carried out at ゛C for 8 hours.

After the reaction was completed, it was washed with water, filtered, and dried. The coercive force of this magnetic powder after drying is 390 oersted, and the saturation magnetization is 69
．． 4emu 7g, square shape 0.48.

Next, this magnetic powder was heat-treated at 300° C. for 3 hours in nitrogen gas to obtain a cobalt-containing iron oxide magnetic powder.

Comparative Example 2 In Comparative Example 1, Fe3O was used instead of γ-Fe2O3.
A cobalt-containing iron oxide magnetic powder was obtained in the same manner as in Comparative Example 1, except that Comparative Example 1 was used and the heat treatment temperature was changed from 300°C to 450°C.

For the cobalt-containing iron oxide magnetic powder obtained in each example and each comparative example, the coercive force, saturation magnetization amount, and square shape were measured, and the change in coercive force over time was measured to examine the stability over time. To investigate thermal stability, heating demagnetization was measured. The change in coercive force over time shows that the obtained magnetic powder is 150
The coercive force was measured after being heated to 60° C., then rapidly cooled, and held at 60° C. for one week, and the amount of change in coercive force was determined as a percentage.

In addition, thermal demagnetization is performed by saturated magnetizing the obtained magnetic powder and measuring the saturated residual magnetization, then heating this magnetic powder at 60°C for 2 hours, measuring the residual magnetization, and calculating the residual magnetization at this time. The amount of decrease was determined as a percentage.

The table below shows the results.

3 As is clear from the above table, the cobalt-containing iron oxide magnetic powders obtained in Examples 1 and 2 have a much higher coercive force than the cobalt-containing iron oxide magnetic powder obtained in Comparative Example 1. The rate of change in magnetic force and heating demagnetization are small, which means that the processing method of the present invention provides cobalt-containing iron oxide magnetic powder that has excellent magnetic properties, stability over time, and thermal stability. I understand. Furthermore, the square shape of the product obtained in Example 1 was 0.51, and the square shape of the product obtained in Example 2 was 0.50. It can be seen that the iron oxide magnetic powder contained has uniaxial anisotropy.

[Brief explanation of drawings]

1 to 3 are diagrams showing the relationship between heat treatment time, coercive force, and square shape of magnetic powder obtained by the treatment method of the present invention and other treatment methods. To old KuOOO 2 animals defense 4Rz.熥蔜蔜 KUO00

Claims (1)

[Claims] 1. An iron oxide layer containing divalent iron ions is formed on the particle surface of acicular iron oxide magnetic powder, and an iron oxide layer containing cobalt is further formed on the iron oxide layer. It is heated at a temperature of 200 to 400°C in a gas atmosphere or vacuum to make the composition of the iron oxide layer containing divalent iron ions and the iron oxide scrap containing cobalt the same and uniform, and to form acicular oxides. A method for processing magnetic powder, which comprises orienting cobalt ions in the direction of the magnetic field using a surface magnetic field of the iron magnetic powder to produce a cobalt-containing iron oxide magnetic powder having uniaxial anisotropy.