JPS6131057B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to _{granular magnetite} ( _FeO
≦1, hereinafter simply referred to as granular magnetite. ) Particulate powder and its manufacturing method. The high coercive force granular magnetite particles of the present invention are mainly used for magnetically isotropic magnetic recording media, particularly floppy disks. In recent years, with the spread of office computers, word processors, etc., floppy disks have been widely used as magnetic recording media for inputting and outputting information. This floppy disk is a magnetically isotropic magnetic recording medium made by housing a disk made of a polyester base in the form of a sheet and coated with magnetic particles for magnetic recording on one or both sides in a vinyl chloride jacket. be. BACKGROUND ART Today, as magnetic recording and reproducing devices such as office computers and word processors become smaller and lighter, there is an increasing need for higher performance in floppy disks, which are magnetic recording media. That is, high recording density characteristics, high output characteristics, and stability of reproduction output are required. The properties of magnetic particles for magnetic recording suitable for satisfying the above requirements for floppy disks include having a high coercive force Hc, a large saturation magnetization σs, and magnetic stability, and being magnetically isotropic. It has a particle form. As is well known, the magnitude of the coercive force of magnetic particles for magnetic recording depends on shape anisotropy, crystal anisotropy, strain anisotropy, or their interaction. Currently, the main types of magnetic particles used for magnetic recording are acicular magnetite particles or acicular maghemite particles, which have magnetic anisotropy due to the shape anisotropy of the acicular particles. This was an attempt to obtain high coercive force by utilizing . These are generally made by reacting an aqueous ferrous salt solution with an aqueous alkaline solution and oxidizing it in the air (usually called a "wet reaction"). It is obtained by heating and reducing at a temperature of 0.degree. C. to obtain acicular magnetite particles, or by heating and oxidizing this in air at 200.degree. C. to 300.degree. C. to obtain acicular maghemite particles. These particles obtain high coercive force by making maximum use of the shape anisotropy derived from their shape. The above-mentioned acicular magnetite particles or acicular maghemite particles generally have a coercive force of 300 Oe.
With regard to coercive force, it is suitable as a magnetic particle powder to obtain the high recording density characteristics and high output characteristics currently required for floppy disks. It has been pointed out in the past that there is a drawback that the reproduced output fluctuates due to the strongly anisotropic particle morphology. That is, the reproduction output fluctuations are detailed as follows. When a floppy disk is manufactured using needle-shaped magnetic particles, the disk rotates and undergoes an angular displacement of 90° due to the relative positional relationship between the magnetic head and magnetic recording disk. The playback output changes each time. This is described, for example, in Japanese Patent Publication No. 55-39049 as follows. ``Magnetic recording media created by orienting needle-shaped magnetic materials are extremely difficult to record when used in a disk-like or sheet-like manner with a spiral, concentric, or arcuate recording track. There are disadvantageous drawbacks, i.e., spiral, concentric and arcuate recording of magnetic sheets produced by the conventional method of forcibly orienting acicular particles in the direction of the magnetic field and then drying and solidifying the magnetic paint. "The reproduction characteristics vary greatly depending on the scanning direction of the magnetic head." A method for reducing the above-mentioned reproduction output fluctuations that occur when using magnetic particle powder exhibiting an acicular shape is to apply a disturbing magnetic field and apply the acicular magnetic particles in random directions. (Japanese Patent Publication No. 55-39049) A method of suppressing the orientation of acicular magnetic particles in the coating direction by employing a gravure roll coating method. (Japanese Unexamined Patent Publication No. 54-33007), etc. are disclosed. The present inventors believe that if granular iron oxide, which is magnetically isotropic, is used, the resulting magnetic coating film will also be magnetically isotropic, without the need for the complicated coating method described above. It was thought that if used in a floppy disk, there would be no fluctuation in the reproduction output in the recording and reproduction characteristics. Generally, granular magnetite particles are well known as granular iron oxide that is magnetically isotropic.
Granular magnetite particles are obtained by a wet synthesis method from an aqueous solution containing Fe ²⁺ . The representative ones are as follows. That is, granular magnetite particles are prepared using a ferrous salt such as ferrous sulfate, and are prepared by using a ferrous salt such as ferrous sulfate.
An aqueous solution with a pH of 10 or higher containing Fe(OH) ₂ colloid obtained by adding an equivalent amount or more of a basic substance (alkali).
Fe ²⁺ in the ferrous salt aqueous solution can be removed by maintaining the temperature in the range of 60 to 100°C and passing an oxidizing gas through it (Japanese Patent Publication No. 44-668), or by adding an alkali to the ferrous salt aqueous solution. 5 to 30 mol% is suspended as an iron precipitate consisting of Fe ²⁺ , and the total Fe ²⁺ concentration in the suspension is 1
Prepare a suspension in the range of ~2 mol/kg, then maintain the temperature of this suspension at 60-90°C, aerate the oxidizing gas with sufficient stirring, and Continuously add alkali so that the pH is always maintained in the range of 5 to 6 (Special Publication No. 49-35520
(No. Publication). The granular magnetite particle powder obtained by the wet synthesis method as described above is magnetically isotropic and has a large saturation magnetization, but it is not suitable as a material for magnetic recording media.
There was only one with a maximum coercive force of about 120 Oe. On the other hand, Co-added magnetite particles exist as granular magnetite particles having a high coercive force. To obtain a coercive force of about 300 Oe, approximately
It is necessary to add 5wt% (Co/total Fe) of Co, and although increasing the amount of Co added is effective in improving coercive force, it is extremely sensitive to changes in magnetic properties over time, pressure, and temperature. It has been pointed out that it is unstable, making it unsuitable as a magnetic powder for floppy disks. In view of the above, the inventors of the present invention have discovered that Co, etc.
Various studies have been conducted to obtain granular magnetite particles with high coercive force without adding any metals other than Fe. Then, _{the present inventors synthesized granular magnetite (FeO X} ^Fe ₂ O ₃
However, high coercive force granular magnetite ( _FeO Fe ₂ O ₃ , however, 0<X
The present invention was completed by discovering that ≦1) particle powder can be obtained. That is, the present invention provides strain anisotropy obtained by applying internal stress to granular magnetite particles with a specific surface area of 1.0 to 6.0 m ² /g synthesized from an aqueous solution containing Fe ²⁺ by impact and grinding. ^High coercive force granular _{magnetite particle} powder consisting ^of granular magnetite ( _FeO Synthesized granular magnetite with _a coercive force of 120 Oe _or less ( _FeO
≦1) Granular magnetite with a coercive force of 130 Oe or more that has been given strain anisotropy by using particles as a starting material and applying internal stress to the starting material by impact and grinding using a vibrating mill or a rotary mill ( FeO _X・Fe ₂ O ₃
However, this is a method for producing high coercive force granular magnetite particles by obtaining particles (0<X≦1). Next, the technical background that led to the completion of the present invention and the structure of the present invention will be described. As mentioned above, granular magnetite particles synthesized from an aqueous solution containing Fe ²⁺ by the conventional method have the following properties:
It has a low coercive force. As a result of various studies on methods for improving the coercive force of granular magnetite particles without adding different metals other than Fe such as Co, the present inventors discovered a coercive force synthesized from an aqueous solution containing Fe ²⁺ . magnetic force
When _starting particles _are granular magnetite ( _FeO We have obtained the knowledge that the coercive force of can be improved. This phenomenon will be explained as follows by extracting some of the many experimental examples conducted by the present inventor. Figure 1 shows the relationship between the coercive force of granular magnetite particles synthesized from an aqueous solution containing Fe ²⁺ and the operating time of the vibration mill, obtained by applying internal stress using a vibration mill. It is. In Figure 1, curves A, B, and C are, respectively,
BET specific surface area is 1.5m ² /g, ^{3.2m 2} /g and 5.6
This is the case when a starting granular magnetite particle powder of m ² /g is used. As is clear from FIG. 1, as the operating time of the vibration mill increases, the coercive force of the starting granular magnetite particles tends to improve. Although the theoretical elucidation of the phenomenon of improving the coercive force of the granular magnetite particles in the present invention is not yet clear, the inventor of the present invention thinks as follows. As described above, the magnitude of the coercive force of magnetic particles for magnetic recording depends on any one of shape anisotropy, crystal anisotropy, and strain anisotropy, or the interaction thereof. The present inventor studied the above-mentioned factors that cause an increase in coercive force, and found that the present invention relates to an increase in coercive force of granular magnetite particles that are magnetically isotropic, and that the present invention relates to an increase in coercive force of granular magnetite particles that are magnetically isotropic. Since no dissimilar metal is added to the granular magnetite particles, no crystal structure change occurs in the granular magnetite particles. Therefore, the coercive force improvement in the present invention is not due to shape anisotropy or crystal anisotropy, but due to strain anisotropy. We believe that this is due to the following. The relationship between coercive force and strain anisotropy is generally expressed by the following equation (1). Coercive force Hc∽λ×σ/Is...(1) However, λ: Magnetostriction constant Is: Saturation magnetization σ: Internal stress In equation (1), if the internal stress σ increases or the saturation magnetization Is decreases, If so, the coercive force will improve. In the present invention, since the saturation magnetization Is of the product granular magnetite particles is constant and hardly changes compared to the saturation magnetization Is of the starting material granular magnetite particles, the increase in internal stress causes the granular magnetite to This is thought to be due to the improvement in the coercive force of the particles. That is, it is thought that the improvement in the coercive force of the granular magnetite particles in the present invention is due to strain anisotropy occurring in the granular magnetite particles by applying internal stress through impact and grinding. Blowing and grinding can be easily imparted by using a vibratory or rotary mill. Next, various conditions for implementing the present invention will be described. The starting material used in the present invention is granular magnetite particle powder obtained by the wet synthesis method described above. In addition to the above-mentioned method, the wet synthesis method includes a well-known method using a coprecipitation reaction between ferrous ion Fe ²⁺ and ferric ion Fe ³⁺ . The coprecipitation reaction method uses a ferrous salt aqueous solution such as ferrous sulfate and a ferric salt aqueous solution such as ferric sulfate to form a mixed iron aqueous solution with a ratio of Fe ²⁺ :Fe ³⁺ of 1:2. In this method, granular magnetite particles are synthesized by adding one equivalent or more of an alkaline aqueous solution such as NaOH to the mixed iron aqueous solution, and heating and mixing at a temperature of 50 to 100°C. In the present invention, a well-known vibrating mill and rotary mill can be used as means for applying internal stress by impact and grinding. Vibratory mills and rotary mills use a horizontal cylinder that vibrates or rotates at low speed to place a medium (steel ball or rod) in the
It is about 40% sealed and has a working mechanism of "blow" caused by the falling of the medium lifted by the vibration or rotation of the cylinder, and "friction" as the medium rolls around. It is thought that strain anisotropy is produced in the granular magnetite particles by applying internal stress through the effects of this "blow" and "friction". The present invention configured as described above has the following effects. That is, according to the invention, a high coercive force, especially 130 Oe
Since it is possible to obtain magnetically isotropic granular magnetite particles having the above properties, it is possible to obtain magnetically isotropic granular magnetite particles with excellent recording density characteristics and output characteristics without fluctuations in the reproduction output, which are currently most required. It can be used as a magnetic powder for recording media, especially floppy disks. Furthermore, in the production of magnetic paint, the granular magnetite particles obtained by the present invention can be applied to the surface of the material disk without applying a disturbing magnetic field or using conventional techniques such as gravure rolls, which are complicated application means. By this alone, it is possible to obtain a magnetically isotropic magnetic recording medium with stable reproduction output without being affected by the structural relative positional relationship between the magnetic head and the magnetic recording disk. Next, the present invention will be explained by examples. In addition, the specific surface area of the particles in the above experimental examples and the following examples was measured by the BET method, and the magnetic measurement was performed using a P-1 type VSM manufactured by Toei Kogyo Co., Ltd. with an external magnetic field of 10 KOe. <Production of Starting Materials> Starting materials A, B, and C having various properties were produced by a wet synthesis method in which an aqueous solution containing Fe ²⁺ was used for an oxidation reaction at a pH of 10 or higher. The properties of starting materials A, B and C are shown in Table 1. Example 1 Using a vibrating mill with a pot internal volume of 3.734 mm and a 12.7 mmφ steel ball of 7 kg as the medium,
500 g of starting material A was sealed in the pot, and the pot was operated for 10 minutes at a motor speed of 1600 rpm. The specific surface area of the obtained granular magnetite particles is 2.0 m ² /g, and the magnetic properties are coercive force 261 Oe,
The saturation magnetization was 88.3 emu/g. Examples 2 to 9 Granular magnetite particle powder was obtained in the same manner as in Example 1, except that the type of starting material, the type of processing equipment, and the operating time were varied. Table 1 shows various properties of the obtained granular magnetite particle powder. Example 10 Using a ball mill having a pot internal volume of 500 ml and containing 1.5 kg of 12.4 mmφ steel balls as the medium, 70 g of starting material B was sealed in the pot, and the pot was operated at a rotational speed of 75 rpm for 120 minutes. The specific surface area of the obtained granular magnetite particles was 4.6 m ² /g, and the magnetic properties were a coercive force of 250 Oe,
The saturation magnetization was 87.2 emu/g. 【table】

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the coercive force of granular magnetite particles synthesized from an aqueous solution containing Fe ²⁺ and the operating time of the vibration mill, obtained by applying internal stress using a vibration mill. be.

Claims (1)

[Claims] 1. Strain anisotropy obtained by applying internal stress to granular magnetite particles with a specific surface area of 1.0 to 6.0 m ² /g synthesized from an aqueous solution containing Fe ²⁺ by impact and grinding. Specific surface area 2.0 to 10.0
_A high coercive force granular magnetite particle powder consisting of ^granular _magnetite ( _FeO 2 Coercive force synthesized from an aqueous solution containing Fe ²⁺
_Starting material _is granular magnetite ( _FeO A method for producing _a high coercive force granular magnetite particle powder _, which is characterized by obtaining granular magnetite ( _FeO .