JPH0312442B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing magnetic powder for high-density recording. Conventionally, magnetic recording media used for video recording, digital recording, etc. are widely used in which acicular particles of γ-Fe ₂ O ₃ , CrO ₂ , etc. are coated and oriented on a support. In recent years, there has been a desire for further improvement in recording density, and the minimum unit of recording is entering the submicron range. In such high-density recording, in order to obtain a sufficient S/N ratio, it is necessary to make the particle size of the magnetic powder sufficiently smaller than the minimum recording unit. For example, in the case of video recording, magnetic powder having a length of about 0.3 μm is required for the shortest recording wavelength of about 1 μm. By the way, the above γ-Fe ₂ O ₃ , CrO ₂
It is difficult to obtain acicular particles with sufficient magnetic properties and a length of 0.3 μm or less, and it has become clear that they cannot be used for higher-density recording. Ta. On the other hand, in current recording media, uniaxial anisotropy is imparted to the magnetic recording layer, and signals are recorded in the direction of the easy axis of magnetization. Generally, a medium is used that is coated and oriented so that the recording direction and its easy axis direction are parallel. In addition to γ-Fe ₂ O ₃ , CrO _{2 ,} etc., particles with this uniaxial easy axis of magnetization include
Hexagonal ferrites such as Ba ferrite are promising. However, this kind of ferrite is
The coercive force is too large, and recording by the head cannot be performed satisfactorily as it is, so it is necessary to perform atomic substitution to control the coercive force. However, even with this type of ferrite-based magnetic powder, it has been difficult to obtain magnetic powder with excellent magnetic properties even though the particle size of the magnetic powder is within a range suitable for high-density recording. The present inventors have developed a magnetic powder for high-density magnetic recording that has a particle size of 0.3 μm or less and is mechanically well separated without sintering agglomeration, which is required for uniform dispersion in paint. In addition, in order to provide substituted hexagonal ferrite (substituted magnetoplumbite ferrite), we conducted various experimental studies and found that a glass-forming substance and a ferrite raw material were mixed, melted, amorphized, and heat treated. In the so-called glass crystallization method, in which fine magnetoplumbite-type ferrite particles are precipitated in a glass matrix, by adopting specific heat treatment conditions, it is possible to achieve a particle size distribution with a maximum particle size of 0.3 μm or less, which is necessary for high-density magnetic recording. The inventors discovered that it was possible to obtain a magnetic powder that was uniform and had excellent magnetic properties, leading to the completion of the present invention. That is, the present invention provides a glass-forming material and a general formula
AO・nFe ₂ O ₃ (However, A is at least one selected from Ba, Sr, and Pb, and can be partially replaced with Ca)
A raw material mixture containing the basic components of magnetoplumbite-type ferrite shown in In the method for producing magnetic powder for magnetic recording, the method comprises the steps of precipitating substituted magnetoplumbite type ferrite fine particles with controlled coercive force, and then extracting the fine particles from a glass matrix. High-density magnetic material is heated at a heating rate of 200°C/hour or less, and then heated at a temperature such that the resulting fine particles have an n value of 6.2 or less to produce fine particles with a maximum particle size of 0.3 μm or less. The present invention relates to a method for producing magnetic powder for recording. The present invention will be explained in detail. The magnetoplumbite type ferrite produced by the method of the present invention is represented by the general formula AO.n {(F _e1-x Mx) ₂ O ₃ }. In the formula, A is at least one element selected from Ba, Sr, and Pb;
It is also possible to replace the moiety with Ca. A preferred element is Ba. In addition, M is a substitution component for controlling the coercive force of magnetoplumbite ferrite, and includes Co, Ti, In, Zn, Ge, Nb, Zr, V,
Use La etc. alone or in combination of two or more.
Specifically, In, Co-Ti, Co-Zr, Co-Ge, Co
-La, Co-V, Zn-Ti, Zn-V, Zn-Ge, Zn
Examples include -Zr and Zn-Nb, with Co-Ti and Co-Zr being particularly preferred. By controlling the amount of substitution element (represented by x in the formula), magnetic powder having a desired coercive force can be obtained. In addition, n is the composition ratio of AO and (F _e1-x Mx) ₂ O _3, and theoretically it is desirable to take a value of 6 in order to obtain a perfect magnetoplumbite type ferrite, but in practice it is 5.0 to 6.2. If so, it is sufficient. In the first step of the production method of the present invention, the above-mentioned glass-forming substance and a raw material mixture consisting of a basic component and a substituted component of magnetoplumbite-type ferrite are mixed and melted. This raw material mixture is a mixture of oxides, carbonates, etc. of each metal element constituting the magnetoplumbite ferrite represented by the above general formula, or a mixture thereof, which has been previously converted into ferrite through a solid phase reaction. , the blending amount as ferrite is determined by the values of x and n in the general formula. The blending ratio of the magnetoplumbite-type ferrite component and the glass-forming substance is such that the amount of the glass-forming substance is lower than the equimolar amount of AO of the magnetoplumbite-type ferrite component and the glass-forming substance. . In the mixing and melting process, the raw material mixture is generally mixed in a well-known mixer, and then heated and melted in an inert container made of platinum or the like using a well-known means such as high-frequency heating. The atmosphere during melting may be air. The glass-forming substance used in the present invention is a material that forms glass together with each component of magnetoplumbite-type ferrite, and specifically,
Examples include B ₂ O ₃ , P ₂ O ₅ , B ₂ O ₃ −SiO ₂ system, etc.
Particularly preferred is B ₂ O ₃ . This melt is then rapidly cooled to become amorphous. For the amorphization, for example, known means such as the so-called single roll method in which the melt is dropped into a metal roll rotating at high speed or a metal drum, the rolling quenching method, or the centrifugal quenching method can be employed.
The thickness of the obtained amorphous body must be 80 μm or less, particularly preferably 60 μm or less. If the thickness exceeds 80 μm, the quenching effect will not be sufficient and some magnetoplumbite-type ferrite will have already precipitated in the amorphous material before the next heat treatment, and the heat treatment will cause This is not preferable because it grows into coarse particles on the micron order. Although the obtained amorphous body contains the elements that make up magnetoplumbite ferrite, it has not yet crystallized, and heat treatment promotes crystallization. be done. Although the mechanism of crystallization of magnetoplumbite-type ferrite in a glass matrix is not necessarily clear, it is presumed to be roughly as follows. In other words, in the low-temperature region where ferrite fine particles begin to precipitate from an amorphous state, a nucleus is first formed centered on the _three Fe ₂ O components, but this nucleus or microcrystal has an abnormally higher Fe ₂ O content than the stoichiometric composition. There are many ₃ components. In other words, n in the general formula AO·nFe ₂ O ₃ of the magnetoplumbite type is much larger than 6, and at this point it seems that a normal ferrite lattice is not formed. Both the saturation magnetization and coercive force of these microcrystals are much smaller than their original values. However, as the heat treatment temperature is increased, n approaches its original value and the magnetic properties become better. As a result of studies conducted by the present inventors, it has become clear that in order to have sufficient magnetic properties, it is necessary to heat-treat at least the above-mentioned ferrite at a temperature such that n is 6.2 or less.
Moreover, this temperature generally existed near the intermediate point between the glass crystallization temperature and the melting point. In short, the purpose of the present invention is to perform heat treatment at as high a temperature as possible in order to enhance the magnetic properties of the precipitated particles, while at the same time reducing the heating rate to prevent coarsening of the particle size due to high temperature heat treatment. This is intended to suppress grain growth. The reason why the glass becomes fine particles by reducing the heating rate is that many nuclei are formed due to the heat treatment at low temperature for a long time, and the viscosity of the glass is higher compared to rapid heating, which causes grain growth. This is thought to be because it is difficult. The heat treatment in the present invention is performed by heating at a heating rate of 200°C/hour or less, and when the resulting fine particles reach a temperature where the n value is 6.2 or less, the temperature is maintained for a certain period of time to prevent crystallization. This is done by stopping the heating when it has sufficiently progressed. At this time, if the heating rate exceeds 200℃/hour, the width of the grain size distribution of the obtained crystals will widen, and in order to keep the maximum grain size below 0.3 μm, which is necessary for high-density recording, the holding temperature should be around the crystallization temperature. It is necessary to set the temperature at a relatively low temperature, and satisfactory characteristics cannot be obtained in terms of saturation magnetization and coercive force. On the other hand, when heating at a temperature increase rate of 200°C/hour or less, the width of the grain size distribution of the crystals becomes narrower, so the holding temperature can be set close to the melting point, and the maximum grain size is 0.3 Magnetic powder having sufficient magnetic properties without exceeding μm can be obtained. Although there is no reason to specifically limit the lower limit of the temperature increase rate, it is preferably 35° C./hour or more from the viewpoint of workability in the manufacturing process. n
The temperature at which the value is 6.2 or less is, as described above, a temperature near the middle between the glass crystallization temperature and the melting point, and is generally a temperature of 650° C. or higher. Also, the time to be maintained at this temperature needs to be at least 30 minutes. Next, the heat-treated amorphous body is treated with dilute acid to dissolve and remove the glass matrix and separate the magnetoplumbite type ferrite fine particles. Examples of the dilute acids used in this case include organic acids and inorganic acids such as dilute acetic acid, dilute hydrochloric acid, and dilute nitric acid. The glass matrix was removed by this dilute acid treatment, and fine particles of magnetoplumbite-type ferrite were obtained in the form of a fine powder with a maximum particle size of 0.3 μm or less, which was mechanically separated by drying by a conventional method. Can produce magnetic powder. The magnetic powder thus obtained can be coated on a non-magnetic support together with a resin binder, a solvent, a dispersant, and other additives to produce a magnetic recording medium such as a magnetic tape. This magnetic recording medium uses conventionally commonly used γ-Fe ₂ O ₃ and CrO ₂
It has become clear that extremely high-density magnetic recording is possible compared to magnetic recording media using acicular particles such as the above. The present invention will be explained below with reference to Examples. Example 1 As a hexagonal ferrite with controlled coercive force,
A magnetoplumbite-type Ba ferrite in which some of the constituent iron atoms were replaced with Ti-Co atom pairs was selected, and B ₂ O ₃ was selected as the glass-forming substance. The amorphous composition created was B ₂ O ₃ ...31.0 mol%, BaO
...39.0 mol%, Fe ₂ O ₃ ... 22.56 mol%, TiO ₂ ...
...3.72 mol%, CoO...3.72 mol%. To create the amorphous material, first, the raw materials were sufficiently mixed using a mixer, and this mixture was placed in a platinum container having a nozzle at the tip. Next, the mixture was heated to 1350°C using a high-frequency heater to melt it, and gas pressure was applied from above the platinum container to transfer the mixed melt onto twin rolls with a diameter of 20 cm and a rotation speed of 100 rpm. The mixture was poured into water and rapidly cooled to produce an amorphous ribbon with a thickness of 60 μm. The crystallization temperature of this amorphous material was approximately 630°C.
After heat-treating this amorphous material in an electric furnace at a heating rate of 200°C/hour or less and a heat treatment temperature of 650°C or higher,
Ferrite particles were extracted with a 20% acetic acid solution. Average particle diameter (Dm) of several samples obtained in Table 1,
The maximum particle diameter (Dmax), the value obtained by dividing the half-width (△D) determined from the particle size distribution by Dm (△D/Dm), the saturation magnetization (6 g), and the coercive force (Hc) are shown. Comparative Example 1 The amorphous material prepared in Example 1 was heated at a heating rate of 200°C/
Heat treatment was performed at a heat treatment temperature of 650° C. or higher for more than an hour. The various properties of the several types of samples obtained are shown in Table 1 in comparison with those of Example 1. Example 2 In the same manner as in Example 1, _{B2O3 ...} 35.0 mol%, BaO...40.0 mol%, _{Fe2O3 ...} 18.80 mol%, _TiO2 ...3.10 mol%, CoO... An amorphous glass with a glass composition of 3.10 mol% was prepared. This amorphous material was heat-treated in an electric furnace at a heating rate of 200° C./hour or less and a heat treatment temperature of 650° C. or higher, and fine particles were extracted from the glass matrix in the same manner as in Example 1. Table 1 shows the properties of the several types of samples obtained. Comparative Example 2 The amorphous material prepared in Example 2 was heat-treated in an electric furnace at a heating rate of 200° C./hour or higher and a heat treatment temperature of 650° C. or higher. Various properties of the several types of samples obtained were investigated and compared with those of Example 2, as shown in Table 1.

【table】

[Table] As is clear from Table 1, sufficient saturation magnetization is generally obtained when n of the extracted fine particles is smaller than about 6.2. However, as can be seen from the comparative example, when the temperature increase rate is about 300°C/hour, at high temperatures where n is 6.2 or less, the particle size becomes coarse, and some particles have a particle size of 0.3 μm or more. do. However, at a heating rate of 200°C/hour or less, n is 6.2 or less and has high saturation magnetization.
Moreover, it can be seen that fine particles with a maximum particle size controlled to 0.3 μm or less can be obtained. Furthermore, these fine particles were made into a paint and applied onto a film to make a prototype magnetic tape, with a particle size of approximately 0.3 μm.
The characteristics were compared with a magnetic tape coated with γ-Fe ₂ O ₃ . Table 1 compares the reproduction output (recording and reproduction using a ring head) at a recording wave number of 1×10 ⁴ cm ^-1 . From the same table, the samples with n<6.2, which were heat-treated at 200° C./hour or less, exceeded the conventional γ-Fe ₂ O ₃ in terms of output characteristics. Especially the heating rate
It can be seen that the tape properties of the samples heat-treated at 100°C/hour or less are excellent. Example 3 With the composition and heat treatment conditions shown in Table 2,
Magnetic powder was produced in the same manner as in Example 1. As a result, as shown in the same table, the particle size was 0.3μm in all cases.
below, Hc is below 900Oe, and σg is 57~
The value was 59emu/g. Comparative Example 3 Magnetic powder was produced in the same manner as in Example 1 except that sample a of Example 1 was made into an amorphous material and the thickness of the amorphous material was 100 μm. The magnetic powder obtained as a result had a maximum particle size of 2 μm and an average particle size of 0.3 μm, which was not optimal for high-density recording.

[Table] From the above results, substituted magnetoplumbite-type ferrite, whose particle size is controlled to 0.3 μm or less and has good magnetic properties, is more suitable for high-density recording than the current γ-Fe ₂ O ₃ particles. It is clear that the above-mentioned heat treatment conditions are necessary to produce fine particles by the glass crystallization method.

Claims (1)

[Claims] 1. A glass-forming substance and a compound with the general formula AO·nFe ₂ O ₃ (where A is at least one selected from Ba, Sr, and Pb, or a part of these is replaced with Ca) A raw material mixture containing the basic components of magnetoplumbite ferrite shown in In a method for producing magnetic powder, which comprises a step of precipitating substituted magnetoplumbite-type ferrite fine particles with controlled coercive force through heat treatment, and then extracting the fine particles from a glass matrix, The obtained amorphous material with a thickness of 80 μm or less is heated at a heating rate of 200° C./hour or less, and then heated and held at a temperature where the n value of the resulting fine particles is 6.2 or less, and the maximum particle size is 0.3 μm. A method for producing magnetic powder for high-density magnetic recording, characterized by producing the following fine particles. 2. The method for producing magnetic powder for high-density magnetic recording according to claim 1, wherein the magnetoplumbite-type ferrite is barium ferrite. 3. The method for producing magnetic powder for high-density magnetic recording according to claim 1 or 2, wherein the glass-forming substance is B ₂ O ₃ . 4 Claims 1 and 2, characterized in that the replacement component for coercive force control is Co-Ti.
A method for producing magnetic powder for high-density magnetic recording according to item 1 or 3. 5 The thickness of the amorphous material obtained by amorphization is
The method for producing magnetic powder for high-density magnetic recording according to claim 1, wherein the particle size is 60 μm or less. 6. The method for producing magnetic powder for high-density magnetic recording according to claim 1, wherein the heating temperature increase rate is 100° C./hour or less.