JPS61227921A - Ferromagnetic fine powder and its production - Google Patents

Abstract

PURPOSE:To obtain the ferromagnetic fine powder wherein the viscosity or a magnetic coating soln. becomes low level and the extremely excellent dispersion properties are exhibited by making the specific surface area a specified value and above and making the levigation retention a specified value and below. CONSTITUTION:The ferromagnetic fine powder having >=36m<2>/g (preferably <=150m<2>/g) specific surface area and <=2% levigation retention on 350 meshes is obtained. Herein the levigation retention on 50 meshes is shown as the percentage of retention of the coarse particles remaining on a 350 mesh-sieve after the ferromagnetic material of a specified amount is put on a sieve of 350 meshes and immersed in water or sufficiently washed inn the running water. As an example of a substance constituting the ferromagnetic fine powder, gamma-iron oxide, Co-modified iron oxide and an alloy contg. iron atom are shown and especially the Co-modified iron oxide is preferable. For example, the above-mentioned ferromagnetic fine powder can be produced by subjecting a dispersing soln. of the ferromagnetic fine powder which contains the secondary particles and has >=35m<2>/g specific surface area to the classifying operation and recovering the ferromagnetic fine powder of <=2% retention on 350 meshes.

Description

Detailed Description of the Invention [Field of the Invention] The present invention relates to a ferromagnetic fine powder and a method for producing the same, and more specifically, a cobalt-modified iron oxide fine powder useful as a magnetic recording material used in magnetic recording media. The present invention relates to ferromagnetic fine powders such as ferromagnetic powders and methods for producing the same.

[Background of the Invention] In recent years, demands for higher density and higher signal-to-noise ratio for magnetic recording media have become increasingly strong, and in order to meet these demands, various improvements have been made. Their properties are particularly dependent on the properties of the particulate magnetic material that exhibits the magnetic recording function, so there is a particular need for improvements in the magnetic material. With the aim of improving the properties of magnetic materials, many studies have already been carried out on making the particles even finer than before, and magnetic recording media that have achieved significantly higher signal-to-noise ratios and higher densities have been achieved by making these particles finer. Already obtained.

As mentioned above, by making the magnetic materials used in magnetic recording media finer, noise components in particular are reduced.
As a result, a high signal-to-noise ratio has been achieved. However, as the particle size of magnetic materials progresses, many problems have arisen.

Generally, magnetic recording media are produced by uniformly dispersing fine particulate magnetic material (ferromagnetic fine powder) in a binder solution (a solution consisting of a binder and a solvent) to prepare a magnetic coating solution, and applying this to a support such as a plastic sheet. The solvent is then removed by evaporation or other means. When the particle size of magnetic materials progresses, that is, when the magnetic materials used become finer particles than before, there is a noticeable tendency for problems to occur in the preparation, storage, and coating operations of this magnetic coating liquid. becomes.

For example, there is a large increase in the viscosity of magnetic coating liquids containing magnetic materials that have become fine particles, difficulty in dispersing them into primary particles, and a tendency to reagglomerate once dispersed into primary particles. Examples include becoming stronger. Fine particulate magnetic materials consist of primary particles and secondary particles formed by agglomeration of multiple primary particles, and when preparing a coating liquid from conventional fine particulate magnetic materials, it is necessary to A considerable portion of the particles was deagglomerated and dispersed as secondary particles during the preparation of the coating solution, and coating was performed in this state. However, as the particle size of magnetic materials progresses, it becomes difficult to disperse secondary particles into primary particles, and even when dispersed, there is a tendency for them to aggregate again in the coating liquid to form secondary particles. On the other hand, an increase in the viscosity of the coating solution is undesirable because it hinders the preparation of a uniform coating solution and also inhibits the formation of a homogeneous coating layer and a coating layer with a uniform thickness when coating a support.

In order to solve these problems, improvement measures such as improving the performance of the dispersion device, using a dispersion aid, and surface treatment of the magnetic material have been investigated.

By utilizing these methods, it is possible to use magnetic materials with advanced fine grains, and high SN of magnetic recording media can be achieved.
The ratio and density have been achieved.

However, the particulate magnetic material that has become possible to use by using the above-mentioned improvement means has a particle size of 3 () m2/g.
It has a specific surface area of about 35 m''/g.

On the other hand, today there is a growing demand for higher signal-to-noise ratios and higher density, and it is necessary to put into practical use fine particulate magnetic materials having a specific surface area of 40d1g to 50rrf/g. Magnetic materials used for magnetic recording include:
γ-iron oxide (γ-Fe2O3), cobalt modified iron oxide,
A typical example is a ferromagnetic material such as an alloy containing iron atoms. Although it is possible to produce fine particulate magnetic materials with such a high specific surface area from such ferromagnetic materials by using known atomization technology, It has not been put into practical use because the problems associated with increasing the specific surface area become even greater. Among these, the serious problem is that the viscosity of the dispersion liquid (magnetic coating liquid) becomes high, and the fact that it is difficult to achieve sufficient dispersion using ordinary dispersion equipment is fatal.

It is not impossible to uniformly disperse such a ferromagnetic fine powder with a high specific surface area in a binder solution and to lower the viscosity using conventional techniques.

For example, there is a method of using more dispersion aid. However, in this method, the mechanical strength of the magnetic layer formed by coating and drying a magnetic coating liquid on a support decreases, and blooming occurs.

As a result, there is a problem in that it is difficult to obtain a magnetic recording medium with sufficient durability and aging characteristics.

There is also a method of using a large amount of binder, and although this method avoids a decrease in the mechanical strength of the magnetic layer, it also reduces the saturation magnetic flux (B m ), making it impossible to obtain high output. The improvement in ratio becomes insufficient.

Furthermore, there is a method of increasing the amount of solvent used in the binder solution in order to reduce the viscosity. However, this has the disadvantage that the time required for dispersion increases.

[Objective and summary of the invention] The present invention solves the above problems and has a specific surface area of 36
rn''/g, especially cobalt-modified iron oxide ferromagnetic powder, can be put to practical use as a magnetic material for magnetic recording.

That is, an object of the present invention is to provide a magnetic material for magnetic recording consisting of a ferromagnetic fine powder having a high specific surface area, in which the viscosity of a magnetic coating liquid obtained by dispersing the same is uniform in the ferromagnetic fine powder. An object of the present invention is to provide a ferromagnetic fine powder that can be dispersed at a low level that does not impede the uniformity of the coated layer and the uniformity of the thickness of the coated layer in the coating operation on a support. .

Another object of the present invention is to provide a magnetic material for magnetic recording comprising ferromagnetic fine powder having a high specific surface area and exhibiting a high degree of dispersibility.

Another object of the present invention is to provide a fine ferromagnetic powder with a sharp coercive force distribution.

A further object of the present invention is to provide a method for efficiently producing ferromagnetic fine powder made of the above-mentioned excellent magnetic substance fine powder.

The present invention is made of ferromagnetic fine powder and has a specific surface area of 38nf.
/g or more (preferably 150 m2/g or less, and more preferably 100 m''/g or less), and a 350 mesh water sieve residual rate of 2% or less. It is.

Here, the 350 mesh water sieve retention rate refers to the coarse particles that remain on the sieve after a certain amount of ferromagnetic material is placed on a 350 mesh sieve, soaked in water, or thoroughly washed under running water. Say the rate.

Examples of substances constituting the ferromagnetic fine powder of the present invention include γ
- Iron oxide (γ-Fe20z), cobalt-modified iron oxide, and iron atom-containing alloys may be mentioned. In the present invention, cobalt-modified iron oxide ferromagnetic fine powder is particularly preferred.

The above-mentioned ferromagnetic fine powder can be obtained by subjecting a dispersion of ferromagnetic fine powder containing secondary particles and having a specific surface area of 36 m''/g or more to a classification operation to achieve a 350 mesh residual rate of 2%.
It can be produced by the method for producing magnetic fine powder characterized by collecting the following ferromagnetic fine powder.

In addition, when the ferromagnetic fine powder is a cobalt-modified iron oxide ferromagnetic fine powder, the specific surface area including secondary particles is 36 m''/
The desired ferromagnetic iron oxide fine powder can also be obtained by a method consisting of recovering a ferromagnetic iron oxide fine powder with a 350 mesh residual rate of 2% or less from a dispersion of ferromagnetic iron oxide fine powder weighing more than 30 g, and then modifying the ferromagnetic fine powder with cobalt. Ferromagnetic fine powder can be obtained efficiently.

[Detailed Description of the Invention] In the following, cobalt-modified iron oxide ferromagnetic fine powder, which is a preferred embodiment of the present invention, will be explained as an example. γ
- It can also be applied to iron oxide ferromagnetic fine powder and iron atom-containing alloy ferromagnetic fine powder.

Specific surface area is 36rr? Cobalt-modified iron oxide magnetic fine powder with a content of 350 mesh water sieve retention of 2% or more and a 350 mesh water sieve retention rate of 2% or less is a fine powder of ferromagnetic iron oxide in the form of fine particles with a specific surface area of about 36 d/g or more (e.g., Fe2O3, Fe50a , F e O
x (4/3 < x < 3/2), etc.). And this specific surface area is about 36
The fine particulate ferromagnetic iron oxide having a specific surface area of about 60 m''/g or more can be produced by firing the fine particulate ferromagnetic iron oxide having a specific surface area of about 60 m''/g or more. The preparation and firing of fine particulate hydrated iron oxide having a specific surface area of about 60 rrf/g or more can be easily carried out using known methods, so a detailed description of these points will be omitted. do.

A typical example of a method for preparing a cobalt-modified iron oxide magnetic fine powder having a specific particle size distribution from a ferromagnetic iron oxide fine powder having a specific surface area of about 36rrf/g or more is to carry out cobalt modification in advance and then change the particle size distribution. There are two methods: one in which the particle size distribution is adjusted, and the other in which cobalt modification is carried out after adjusting the particle size distribution.

The particle size distribution of the ferromagnetic iron oxide fine powder or its cobalt-modified product is adjusted in the state of a dispersion. That is,
After dispersing fine particulate iron oxide in a solvent inert to iron oxide, a fine ferromagnetic powder with an adjusted particle size distribution is recovered from the dispersion, and then the fine ferromagnetic powder is modified with cobalt. Alternatively, a method is used in which fine ferromagnetic powder whose particle size has not been adjusted is modified with cobalt in a dispersion liquid, and then the particle size of the cobalt-modified product is adjusted.

Examples of solvents that are inert to the iron oxides mentioned above include water,
Alcohols (e.g. methanol, ethanol, propatool, butanol), ketones (e.g. acetone, methyl ethyl ketone, methyl imbutyl ketone), esters (e.g. ethyl acetate, butyl acetate), paraffins (e.g.
pentane, hexane, heptane, 2-ethylhexane)
, alicyclic compounds (eg, cyclohexane), aromatic compounds (eg, benzene, toluene, xylene), and the like. These solvents can be used alone or as a mixture.

Methods for modifying iron oxide with cobalt are already known, and the cobalt modification in the present invention can be carried out by any method selected from these methods. A typical example of this method is a method in which γ-iron oxide particles are treated with a cobalt salt (e.g., cobalt sulfate, cobalt chloride, cobalt nitrate) in an alkaline aqueous solution.
Ru.

Further, if desired, cobalt modification may be carried out using one or more salts of metals such as ferrous iron, ferric iron, manganese, zinc, nickel, and chromium.

The cobalt-modified fine particulate iron oxide is used as it is if the particle size distribution has been adjusted, or after adjusting the particle size distribution if the particle size distribution is not adjusted, it is separated from the solvent and dried to produce the ferromagnetic fine particles of the present invention. It becomes a powder.

In the present invention, the particle size distribution of the fine powder is adjusted by removing coarse particles (agglomerates of secondary particles in the form of relatively large lumps) from the dispersion liquid at certain intervals. The specific method is as follows: (1) Vigorously stir the dispersion. A method of reducing coarse particles by promoting the separation of secondary particles in raw materials.

(2) A method of removing coarse particles by allowing the dispersion to stand still to precipitate the coarse particles. At this time, the concentration of the dispersion is adjusted as necessary.

(3) A method of filtering the dispersion liquid using an appropriate filtration device to remove coarse particles. At this time, the concentration of the dispersion liquid is adjusted as necessary. As a filtration device, for example, 35
A zero mesh strainer is utilized.

Note that these methods can be used alone or in combination. However, as an industrial method for reducing coarse particles, the method using the filtration operation described in (3) above is advantageous.

The ferromagnetic fine powder whose particle size distribution has been adjusted as described above is separated from the liquid and then dried. Note that the separation operation from the liquid and the drying operation may be performed in one step.

The ferromagnetic fine powder that has undergone the drying process is generally obtained in the form of lumps, and therefore may be subsequently subjected to a light grinding process for the purpose of facilitating handling. Examples of crushing equipment used for this purpose include show crushers,
Examples include a roll crusher.

The ferromagnetic fine powder obtained through the above process is composed of cobalt-modified iron oxide particles, has a specific surface area of 36 m''/g or more, and has a 350 mesh water sieve retention rate of 2% or less, Ferromagnetic fine powder that conforms to this regulation has excellent dispersibility in binder solutions and also suppresses the formation of secondary particles.

The resulting magnetic coating liquid tends to be homogeneous and of low viscosity. Therefore, a magnetic recording medium manufactured using this magnetic coating liquid has a magnetic layer that is homogeneous and has a highly smooth surface. And as a result, the coercive force (Hc)
The distribution becomes sharper, and the transfer effect is also improved.

That is, according to the research of the present inventor, among the conventionally known ferromagnetic fine powders, 20rrf/g to 30tn
'/g, they cause an increase in the viscosity of the magnetic coating liquid containing them and a decrease in workability, and in magnetic recording media, coarse particles tend to deteriorate the magnetic recording properties. contains only about 0.1% at most.
Therefore, as long as a fine powder with a specific surface area of this level is used, there will be no particular problem in the actual preparation process and coating process of the magnetic coating liquid, and there will be no problem with deterioration of the characteristics of the magnetic recording medium. There wasn't. However, when the specific surface area of the ferromagnetic fine powder is about 35rrf/g, the content of coarse particles (weight ratio) is about 2%, and when the specific surface area is 40rrf/g, the content of coarse particles is about 2%. 2%
It turns out that it is not uncommon for the percentage to exceed 3% to 4%.

Even if such fine ferromagnetic powder containing coarse particles is dispersed, it is difficult to achieve homogeneous dispersion due to strong bonding between the fine particles within the coarse particles. When a magnetic layer is formed by using such a material, various characteristics of the magnetic recording medium naturally deteriorate. Furthermore, according to the research of the present inventor, as schematically shown in the attached FIG. 1,
It was found that the magnetic coating liquid (dispersion liquid) A containing a relatively large amount of coarse particles had a low viscosity at the initial stage of dispersion, but as the dispersion proceeded, the viscosity increased rapidly. On the other hand, when magnetic coating liquid B is prepared using the ferromagnetic fine powder with an adjusted particle size distribution provided by the present invention, the viscosity at the initial stage of dispersion tends to be relatively high, but the viscosity increases over time. After carrying out the 1-dispersion operation for a certain period of time with almost no change even after the elapse of It begins to show viscosity. In addition, the surface properties (surface smoothness) of the magnetic layer of the magnetic recording medium prepared using these magnetic coating liquids are as shown schematically in FIG. Magnetic coating liquid B using powder
provides a magnetic layer exhibiting good surface properties, whereas magnetic coating liquid A containing many coarse particles provides insufficient surface properties.

Next, the present invention will be explained in more detail with reference to Examples and Comparative Examples.

[Example 1] 8 fL of water was put into a 20IL stainless steel beaker, and while stirring with a dipper with a blade diameter of 150 mm, the specific surface a was 40 m.
2/g (7)y 2 kg of Fe 203 was added and dispersed for 30 minutes. The slurry (total amount: 10 kg) had a very high viscosity, and no supernatant could be formed even if it was allowed to stand still.

This slurry was diluted with water to give a total concentration of 501%, resulting in a highly fluid liquid. After 5 minutes of stirring, the liquid was drawn out into another beaker using a siphon. At this time, in order to avoid sucking out the coarse powder near the bottom, the suction port was placed near the liquid level, and the suction port was adjusted to be lowered sequentially as the liquid level decreased. After extracting the liquid, a slurry containing coarse undispersed particles remained in the beaker.The amount of undispersed solids in a classification operation of 0 or more was 57.
It was g. Therefore, the extracted liquid contains γ-Fe.
20. This means that it contains 1943g.

The extracted liquid is then allowed to stand still to form a supernatant,
By removing this supernatant, the total amount of slurry can be reduced to 10k.
g, and transferred to a 20-gram reaction vessel. To this was added 300 g of cobalt sulfate heptahydrate, and the resulting slurry was vigorously stirred while blowing nitrogen gas at a rate of 11/min. After 30 minutes, a sodium hydroxide aqueous solution (NaOH1,
Okg/2fL water) was added. Then, the slurry was heated to 95° C. while blowing nitrogen gas and kept at that temperature for 5 hours.Then, the slurry was washed with water to remove alkali and the like, dehydrated, and dried at 70° C. for 24 hours. The lumps produced by drying were crushed to a size of 2 mm or less in diameter to obtain cobalt-modified iron oxide (magnetic powder) in the form of fine particles. This was designated as sample 1.

[Comparative Example 1] 40 m2/g of γ-Fe2O3 was dispersed under the same conditions as in Example 1 to obtain a slurry with a total amount of 10 kg.

285 g of this slurry was taken and 285 g of water was added thereto. Using this diluted slurry, γ-Fe20. ．． A cobalt modification reaction was carried out.

After the reaction was completed, the reaction solution was divided into two parts, and one part was similarly washed with water, dehydrated, and then dried. The lumps produced by drying were crushed to a size of 2 mm or less in diameter to obtain cobalt-modified iron oxide fine powder. This was designated as comparative sample 1.

[Example 2] The other half of the reaction solution divided into two in Comparative Example 1 was passed through a 350 mesh sieve (diameter 300 mm). Since the sieve was prone to clogging, one reaction solution was not passed through it all at once, but the classification process was carried out while replenishing water little by little onto the sieve. The slurry that passed through the sieve was similarly filtered, washed with water, dehydrated, and then dried. The lumps produced by drying were crushed to a size of 2 mm or less in diameter to obtain cobalt-modified iron oxide fine powder. This was designated as sample 2.

[Example 3] γ-Fe2O3 with a specific surface area of 60 rtf/ as iron oxide
Cobalt-modified iron oxide fine powder was obtained in the same manner as in Example 1, except that sample 3 was used.

[Comparative Example 2] Cobalt-modified iron oxide fine powder was obtained in the same manner as in Example 3, except that the classification operation was not performed, and this was designated as Comparative Sample 2.

[Example 4] 40 m''/g of γ-Fe2O3 was dispersed under the same conditions as in Example 1 to obtain a slurry with a total amount of 10 kg.

5 kg of glass peas (diameter 1 mm) were added to this slurry, and the slurry was further stirred for 60 minutes. 1 of the obtained slurry
After separating the glass peas through a 6-mesh sieve,
The cancer fluid (slurry) was similarly denatured with cobalt. Since no coarse particles were observed in the slurry after cobalt modification, this slurry was used as sample 4.

[Example 5] The same treatment as in Example 1 was carried out except that instead of the modification with 300 g of cobalt sulfate heptahydrate, modification was performed with 200 g of cobalt sulfate heptahydrate and 500 g of ferrous sulfate heptahydrate. A cobalt-modified iron oxide fine powder was obtained, which was designated as Sample 5.

[Evaluation of cobalt-modified iron oxide fine powder] Each sample obtained in the above Examples and Comparative Examples was evaluated by the following method.

1) Specific surface area: Measured by BET method.

Unit: m2/g 2) 350 mesh water sieve residual rate: 350 g of sample
Place it in a mesh water bottle (diameter 300 mm), immerse it in a 0.1% hexametaphosphoric acid aqueous solution, and in that state, give the sieve sufficient vibration to filter as much of the sample as possible, and then expose the sieve to running water to remove particles from the filtrate. After carrying out this classification operation until no particles can be seen, the weight of the coarse particles remaining on the sieve is determined, and the weight ratio of the remaining coarse particles to the sample subjected to the test is determined. ~ In Table 3, it was described as water sieve residual rate).

3] Viscosity of magnetic coating liquid and characteristics as a magnetic recording material Samples (magnetic powder) were mixed into a composition according to the following recipe, and kneaded for 2 hours in a batch-type Ni-Goo.

The resulting mixture was diluted with methyl ethyl ketone and butyl acetate, and the diluted slurry was applied to a sand mill for dispersion treatment for 4 hours to obtain a magnetic coating liquid.

Below are blank samples (magnetic powder) 100 100 Carboxyl group-containing vinyl chloride/vinyl acetate copolymer 12. 12 Urethane resin 77 Butyl stearate 0.3 0.3 Methyl ethyl ketone 35 45 Butyl acetate
15 25 Fish JQ Color Base Methyl Ethyl Ketone 100 25 Butyl Acetate 50 60 Note) P: Sample 1.2.4
．． 5 and Comparative Sample 1.3 Q: Applied to Sample 3 and Comparative Sample 2 The viscosity of the obtained magnetic coating liquid was measured using a B-type viscometer. The viscosities are shown in Tables 1-3.

The above magnetic coating solution was applied to a polyethylene terephthalate support (thickness: 2 mm) using a doctor blade with a gap of 50 ILm.
A magnetic recording medium sample was prepared by coating the magnetic powder on a 3000 Gauss magnetic field and evaporating the solvent to form a magnetic layer.

Regarding this magnetic recording medium sample, gloss and various magnetic properties [coercive force (Hc), squareness ratio (SQ).

5FDI was measured. Gloss was evaluated in accordance with JIS-Z-8741 by measuring the intensity of reflected light when white light was incident on the surface of the magnetic layer at an angle of 45 degrees. The measured value is a relative value with the glass surface gloss as 100%.

Further, the magnetic properties were measured using a VSM (vibrating sample magnetometer).

Margin below Table 1 Sample Sample 1 Comparative sample 1 Sample 2 Specific surface ta 39.8 39.5 39.8 Water sieve retention rate 1.1 5.7 0.01 Gloss
95 71 102 Viscosity 51
105 48Hc 645 63
7 650SQ O,820,770,8
1SFD O, 430, 470, 41 or less Margin Table 2 Sample Sample 3 Comparative sample 2 Sample 4 Specific surface roughness 59.7 59.8 40.0 Water sieve retention rate
1.8 8.3 (ml gloss 85
49 96 Viscosity 71 2
09 48He 630 612
648SQ O, 690, 640, 81S
FD O, 590, 650, 43 or less margin Table 3 Sample Sample 5 Comparative sample 3 Specific surface area
38.0 3g, 0 water sieve residual rate 0.7
1 4.1 Gloss 90 65 Viscosity 50 115Hc
674 660SQ O,80
0,78 SFD O,440,49 [Reference Example 1] γ-Fe20. Comparative Example 1
Cobalt-modified iron oxide fine powder was produced in the same manner as above. The specific surface area of the obtained cobalt-modified iron oxide fine powder and 35
Table 4 shows the 0 mesh water sieve retention rate (measured by the method described above).

Table 4 Specific surface area 350 mesh water sieve residual rate 20.2
m2/g O, 01%25.0 0.0
2 30.1 0.05 32.3 0.38 34.9 0.97 36.7 2.2 38.1 4.3 39.5 5.7 59.5 8.7 From the above results, the specific surface area is It can be seen that with small cobalt-modified iron oxide fine powders, the amount of coarse particles produced is small even without a particular classification operation, but when the specific surface area exceeds about 36 rrf/g, the production of coarse particles becomes noticeable.

[Reference Example 2] Sample 2 (specific surface area: 39.8 m″/g) and Comparative Sample I (specific surface area: 39.5 m″/g) were mixed at different quantitative ratios, and various 350 mesh water sieve residues were mixed. Cobalt-modified iron oxide fine powder of 10% was prepared.A magnetic coating liquid similar to that described above was prepared using this cobalt-modified iron oxide fine powder, and this magnetic coating liquid was similarly applied to the surface of polyethylene terephthalate.
An orientation treatment was performed to form a magnetic layer. The gloss of this magnetic layer was measured by the same method. Table 5 shows the cobalt-modified iron oxide fine powder used at 350 m, the residual ratio on a sieve water sieve, the viscosity of a magnetic coating liquid prepared using the magnetic powder, and the gloss of the magnetic layer. Note that the units of each measurement item in Table 5 are the same as described above.

Below margin No. 5 Water sieve residue ratio Viscosity Gloss 0.01 48 102 *0, 5
49 100 1.0 51 97 1 , 5 53 95 2.0 55 94 2 , 5 60 92 3.0 64 90 5.0 81 81 5.7 105 71 Note) Trees are the data of sample 2, and woody indicates data for comparative sample 1.

From the results in Table 5, the residual rate on the 350 mesh water sieve is 2.0%.
It can be seen that the viscosity increases at an accelerating rate from around the point where . It is also clear that the gloss decreases in inverse proportion to the increase in the percentage remaining on the 350 mesh water sieve.

[Brief explanation of the drawing]

Figure 1 shows the dispersion time and the dispersion time in magnetic coating liquid (dispersion liquid) A containing a relatively large amount of coarse particles and magnetic coating liquid B prepared using magnetic powder with adjusted particle size distribution provided by the present invention. 1 is a graph schematically showing the relationship between the viscosity and dispersion time of a coating liquid and the surface property [surface smoothness of a magnetic layer obtained using the magnetic coating liquid].

Claims (1)

[Scope of Claims] 1. A ferromagnetic fine powder having a specific surface area of 36 m^2/g or more and a 350 mesh water sieve retention rate of 2% or less. 2. The ferromagnetic fine powder according to claim 1, which has a specific surface area of 150 m^2/g or less. 3. The ferromagnetic fine powder according to claim 1, wherein the ferromagnetic fine powder has a residual rate on a 350-mesh water sieve of 1.5% or less. The ferromagnetic fine powder according to claim 1, wherein the ferromagnetic fine powder has a residual rate on a 4,350 mesh water sieve of 1% or less. 5. The ferromagnetic fine powder according to any one of claims 1 to 4, characterized in that the main component is cobalt-modified iron oxide. 6. By subjecting a dispersion of ferromagnetic fine powder containing secondary particles and having a specific surface area of 36 m^2/g or more to a classification operation,
A method for producing magnetic fine powder, which comprises recovering ferromagnetic fine powder having a 350 mesh residual ratio of 2% or less. 7. The specific surface area of the raw material ferromagnetic fine powder is 150 m^2/g
A method for producing ferromagnetic fine powder according to claim 6, characterized in that: 8. The method for producing ferromagnetic fine powder according to claim 6, characterized in that the ferromagnetic fine powder recovered from the dispersion has a 350 mesh water sieve retention rate of 1% or less. 9. The method for producing ferromagnetic fine powder according to any one of claims 6 to 8, wherein the main component of the ferromagnetic fine powder is cobalt-modified iron oxide. 10. After collecting ferromagnetic iron oxide fine powder with a 350 mesh residual ratio of 2% or less from a dispersion of ferromagnetic iron oxide fine powder containing secondary particles and having a specific surface area of 36 m^2/g or more, the fine powder A method for producing fine ferromagnetic powder characterized by modifying it with cobalt. 11. A dispersion of ferromagnetic iron oxide fine powder is subjected to a classification operation to collect ferromagnetic iron oxide fine powder with a 350 mesh residual ratio of 2% or less, and then the fine powder is modified with cobalt. A method for producing ferromagnetic fine powder according to claim 10. 12. The specific surface area of the raw material ferromagnetic iron oxide fine powder is 150 m
12. The method for producing a ferromagnetic fine powder according to claim 10 or 11, characterized in that the ferromagnetic fine powder is ^2/g or less. 13. 3 of the ferromagnetic iron oxide fine powder recovered from the above dispersion
12. The method for producing ferromagnetic fine powder according to claim 10 or 11, characterized in that the residual rate on a 50 mesh water sieve is 1% or less.