JPS58161706A - Production of ferromagnetic powder - Google Patents

Abstract

PURPOSE:To produce ferromagnetic powder which has excellent magnetic characteristics suitable for recording and reproducing by subjecting to solid solution treatment of cobalt on the surface of magnetic acicular iron oxide powder then grinding the same to specific shapes. CONSTITUTION:Acicular iron oxide powder such as gamma-Fe2O3 or Fe3O4 having >=5 axial ratio is dispersed in an aq. soln. contg. cobalt salt such as cobalt sulfate, ferrous salt such as ferrous sulfate and further alkali such as caustic soda. The dispersion is heated under stirring to react so that iron oxide film contg. Co is formed on the surface of the acicular iron oxide powder. The iron powder is heated to >=250 deg.C in air to subjected to solid solution treatment of Co uniformly and to reduce the content of the bivalent iron contained therein. Such acicular iron oxide powder is grounded to <=5 axial ratio (long axial ratio/short axial ratio). The ferromagnetic powder which is suited for recording and reproduction in the longitudinal direction as with magnetic tapes and for recording and reproduction in the circumferential (transverse) and vertical (thickness) directions as with magnetic discs is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION In addition to recording and reproducing in the longitudinal direction of conventional magnetic tapes, this invention is particularly applicable to recording and reproducing in the circumferential direction (width direction) of magnetic disks, etc.
The present invention relates to a method for producing ferromagnetic powder suitable for recording and reproducing in the vertical direction (thickness direction).

Cobalt-containing iron oxide magnetic powder has a higher magnetic force than general iron oxide magnetic powder that does not contain cobalt, and is prized for use in high-density recording.

Conventionally, when applying this type of powder to magnetic tape, etc.
It has been thought that it is desirable in terms of magnetic properties to have as much acicularity as possible and to utilize its shape anisotropy.

In fact, by orienting the acicular powder in the longitudinal direction of the tape and recording and reproducing in the oriented direction, good recording and reproducing characteristics were obtained.

For this reason, the above-mentioned conventional cobalt-containing iron oxide magnetic powder has an axial ratio (major axis ratio/minor axis ratio) of greater than 5, which can be easily and stably obtained from goethite etc. by a very common manufacturing method. γ-Fez with excellent needle-like properties
Magnetic iron oxide powder such as 03 powder is used as a starting material, and cobalt is introduced into the powder in a manner that does not impair the acicularity as much as possible. As a typical example, if the acicular magnetic iron oxide powder is treated in an aqueous solution containing a cobalt salt, a ferrous salt, and an alkali, an iron oxide layer containing cobalt is formed on the particle surface of the powder. There is a known method to do this.

By the way, cobalt-containing iron oxide magnetic powder has greater crystal anisotropy than iron oxide magnetic powder by uniformly dissolving the cobalt inside the powder9 and reducing the divalent iron content in the powder. (triaxial anisotropy) is insulted.

Given such crystal anisotropy, the squareness ratio (σr10S
), and it can be effectively applied not only to recording and reproducing in the longitudinal direction as with conventional magnetic tapes, but also to recording and reproducing in the circumferential direction and in the vertical direction as in magnetic disks.

From the above point of view, an iron oxide layer containing cobalt is formed on the surface of the acicular iron oxide powder particles according to the conventional manufacturing method, and then heat-treated in air at high temperature to form an iron oxide layer on the surface side. An attempt was made to obtain a 1-magnetic powder with a good squareness ratio (σr//1s) by uniformly dissolving the cobalt contained in the particles and oxidizing the divalent iron in the powder to trivalent iron. Ta.

However, the cobalt-containing iron oxide magnetic powder obtained by the above method does not necessarily have a sufficient squareness ratio (S of σ), and when it is made into a tape, it does not have sufficient squareness in the longitudinal and width directions. And the magnetic properties in all directions in the thickness direction could not be satisfied with high variation.

In order to overcome the above-mentioned problem, the inventors conducted a trial-and-error study, and after forming an iron oxide layer containing cobalt on the particle surface of iron oxide magnetic powder such as γ-Fez03 powder,
Alternatively, after further heat treatment as described above, the -treated powder was pulverized to an appropriate size to reduce its axial ratio. It was discovered that good results were obtained in the ratio (σr/l01s) and the magnetic properties were significantly improved compared to the above-mentioned non-pulverized material, leading to the completion of this invention.

That is, the present invention includes a step of treating acicular magnetic iron oxide powder in an aqueous solution containing a cobalt salt, a ferrous salt, and an alkali to form an iron oxide layer containing cobalt on the particle surface of the powder; In addition to the step of heat-treating the treated powder obtained in this step at a temperature of 250°C or higher in air, the treated powder after passing through either one of the above two steps is treated with an axial ratio (good axial ratio/ This relates to a method for producing ferromagnetic powder characterized by a step of pulverizing it until it has a minor axis ratio of 5 or less, which enables it to be used not only for recording and reproducing in the longitudinal direction like conventional magnetic tapes, but also for magnetic It becomes possible to provide a method for producing ferromagnetic powder suitable for recording and reproducing in the circumferential direction and vertically, such as on a disk.

The acicular magnetic iron oxide powder used in this invention includes 1-Fe2O3 powder, Fe3O4 powder, or magnetic powder in an intermediate oxidation state of these powders, which have an axial ratio of more than 5 and can be easily and stably obtained by a known method. Either can be used.

In the first step of the present invention, the acicular iron oxide powder as described above is dispersed in an alkaline aqueous solution containing a cobalt salt and a ferrous salt and treated at a temperature of 50° C. or lower.
An iron oxide layer containing cobalt is formed thereon by using the above powder as a nucleus crystal.

The obtained treated powder can be used as a recording element suitable for recording and reproducing in the lateral direction like a conventional magnetic tape, but in order to give it crystal anisotropy, it is heated in air at 250 °C as a second step. Even if heat treatment is performed at a temperature of 600°C or higher, it is not possible to impart sufficient crystal anisotropy. That is, even though the heat treatment uniformly dissolves the cobalt contained in the surface side iron oxide layer inside the particles and reduces the divalent iron content inside the powder, spoiling the crystal anisotropy, Its magnetic properties are not as expected.

The third step of the present invention is to prepare the treated powder after the first step, that is, the iron oxide magnetic powder in which an iron oxide layer containing cobalt is formed on the particle surface, or the treated powder after the second step, that is, the particle surface. Iron oxide magnetic powder is formed by forming an iron oxide layer containing cobalt and then subjecting it to the above-mentioned heat treatment.
The purpose is to crush it to an axial ratio of 5 or less.

When pulverized in this way, the magnetic properties of the final fS treated powder are significantly improved, especially when t and the squareness 1 ratio (σ
r/aS).

In other words, when an iron oxide layer containing cobalt is formed on the particle surface and then pulverized before heat treatment, the magnetic properties are significantly improved during the heat treatment process. The effect is that the magnetic properties can be further improved compared to the above.

In order to obtain such an effect, the amount of cobalt required in the first step is approximately 2 parts by weight or more per 100 parts by weight of iron oxide magnetic powder, and the amount of cobalt required in the second step, that is, the retention after homogeneous solid solution, is approximately 2 parts by weight or more per 100 parts by weight of iron oxide magnetic powder. It is desirable that the magnetic force is at least 500 degrees. In addition, the final divalent iron content is Fe
It is desirable that the 2+/Fe 3+ ratio be 1/100 or less.

As the crusher in the third step, a kneader such as a KRC kneader (continuous kneader) or a pressure kneader, a ball mill, a three-roll mill, or the like is used. The pulverization may be wet or dry. The degree of pulverization is desirably kept at the above-mentioned axial ratio of 5 or less, usually 1.5 to 2 or more, and if it is smaller than this, the thermal stability will deteriorate, so it is not preferable.

When a magnetic tape or other magnetic recording medium is manufactured by using the thus obtained Kobaru according to the present invention as a recording element, the magnetic recording medium can be used in both the longitudinal direction and the width direction.

Significantly improves magnetic properties such as squareness ratio and coercive force in all directions, including the vertical direction. In addition, since the particle size of the powder is reduced, the powder improves filling properties and surface properties when applied to magnetic tapes, etc., and also exhibits effects such as reducing noise.

Next, embodiments of this invention will be described.

Example 1 Acicular γ-Fe20. with a major axis diameter of 0.32μ and an axial ratio of 8. + powder (
Coercive force HC345 Oersted, saturation magnetization σs71e
Mu/f) 1800 t was dispersed in 121 water, then 60 l of cobalt sulfate and ferrous sulfate 1800 F were added and dissolved, and 212 was further dissolved in 6 t of water.
89 of caustic soda was added, and the mixture was allowed to react at a temperature of 45° C. for 8 hours while stirring. After the reaction is complete, wash with water.

The particles were dehydrated and dried to obtain magnetic powder in which an iron oxide layer containing cobalt was formed on the particle surface.

This magnetic powder has an axial diameter of approximately 0.32μ and an axial ratio of 8.
Hc 9570e, as 77.3 emu/l
, or/as051.

This magnetic powder was then heat treated in air at a temperature of 450° C. for 3 hours. Furthermore, this heat-treated powder 120 Or and water 3001 were put into a pressure kneader with an internal volume of 1 t, and
After kneading under pressure for 2 hours at a pressure of 9/crIi, the mixture was taken out from the kneader and dried.

The ferromagnetic powder according to the invention thus obtained had a major axis diameter of about 0.12 μm and an axial ratio of about 3.

Example 2 Magnetic powder with a cobalt-containing iron oxide layer formed on the particle surface obtained by the method of Example 1 was subjected to the same pulverization treatment as in Example 1, and then subjected to the same heat treatment as in Example 1. In this way, a ferromagnetic powder according to the present invention was obtained. The major axis diameter and axial ratio of this powder were both the same as in Example 1.

The results of examining the magnetic properties of the ferromagnetic powders of Examples 1 and 2 above and the treated powders obtained in the same manner as in Example 1 except that no pulverization treatment was performed for comparison are as follows. It was as shown in the table.

Table 1 A magnetic paint was prepared according to the following composition using each of the magnetic powders according to Examples 1 and 2 and Comparative Examples described above, in parts by weight based on 100 parts by weight of γ-Fe20a powder. A magnetic tape was obtained by coating and drying the mixture on a polyester base film having a thickness of 12 μm to a dry thickness of 3 μm.

Magnetic powder 100 parts by weight S-LEC A-5 (manufactured by Shimizu Chemical Industry Co., Ltd., a combination of vinyl chloride, vinyl acetate, and vinyl alcohol) 12.5 parts by weight Takelac E-551 (manufactured by Shikinichi Yakuhin Kogyo Co., Ltd., urethane prepolymer) 80 parts by weight Coronate L (manufactured by Nippon Polyurethane Industries Co., Ltd., trifunctional low molecular weight incyanate compound) 23 parts by weight Valve Handle
2 Carbon black
11 Mystilic acid
0.8〃Methyl in butyl ketone 6
4 Toluene 64 1 The results of investigating the magnetic properties of the above magnetic tape are as follows:
The results were as shown in Table 2 below.

As is clear from the above table, the ferromagnetic powder obtained by the method of the present invention can be used in any of the longitudinal direction (<j>), width direction (<ear>), and perpendicular direction (<R>) of the magnetic tape. It can be seen that the magnetic properties of the magnetic recording medium are also inferior to other excellent magnetic recording media.

Patent applicant: Hitachi Maxell, Ltd. Agent: Patent attorney Kunio Negi J.

Claims (1)

[Claims] 11) A step of treating acicular magnetic iron oxide powder in an aqueous solution containing a cobalt salt, a ferrous salt, and an alkali IJ'& to form an iron oxide layer containing cobalt on the particle surface of the powder, and this step In addition to the step of heat-treating the treated powder obtained in air at a temperature of 250°C or higher, the treated powder after passing through either one of the above two steps has an axial ratio (major axis ratio/minor axis ratio). A method for producing ferromagnetic powder, comprising the step of pulverizing until the ratio is 5 or less.