JP3031383B2 - Method for producing ferromagnetic iron oxide particles containing cobalt - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has a high coercive force suitable for magnetic iron oxide particles for high-density recording, has a good coercive force distribution and squareness ratio, and has excellent blackness. And a method for producing cobalt-coated magnetic iron oxide particles.

[0002]

2. Description of the Related Art In recent years, as the size and weight of magnetic recording and reproducing devices have been reduced, it has been required to improve the recording density characteristics of magnetic recording media such as magnetic tapes and magnetic disks.

In order to improve the recording density characteristics of a magnetic recording medium, the magnetic material particles used must be as fine as possible and have a high coercive force. This fact can be seen, for example, in the "Magnetic Recording Media Comprehensive Data Book" (Showa 60), pages 185 to 187, published by Sogo Electronic Research. Whether audio equipment, video equipment, or floppy disk drives, emphasis is placed on miniaturization, weight reduction, and operability based on high-density, short-wavelength recording technology. In addition, as a magnetic coating technology that matches this, a super-smooth thin film with a thickness of 1 to 2 microns using fine magnetic particles with a high coercive force was used. High-grade (HG) type using powder. In the future development of higher image quality, ultra-fine magnetic iron oxide powder is used on the tape side. Dramatic improvement in image quality is expected. ” It is as described that. Further, even in recent years, there has been no end to the demand for finer magnetic iron oxide particles.

On the other hand, video equipment usually employs a mechanism for detecting the end of a running tape with light, so that a malfunction occurs when the light transmittance of a video tape used increases. However, there is a problem that the light transmittance increases as the Co-coated γ-Fe ₂ O ₃ particles, which are currently most frequently used for video tapes, become finer.

Attempts have been made to improve the light transmittance by adding a large amount of non-magnetic filler such as carbon black. However, when a non-magnetic material such as carbon black is used, high-density recording is hindered.

Therefore, as a method of improving the light transmittance without adding a non-magnetic filler, a magnetic iron oxide coated with Co on the surface of acicular magnetite particles or acicular beltride compound particles having excellent blackness is used. Particle powders have been used.

[0007]

The above-mentioned acicular magnetite particles and acicular beltride compound particles have a surface of Co.
The technical means for applying is well known, and it is also well known that magnetic iron oxide particles having high coercive force and excellent blackness are used.

[0008] However, acicular magnetite particles have a problem that they oxidize and generate heat when handled in air. In addition, the magnetic iron oxide particle powder after Co deposition has the advantage that a high saturation magnetization value and excellent electric resistance can be obtained. However, the ferrous iron contained therein is gradually oxidized and the magnetic properties deteriorate over time. There is a problem that this occurs, and the tendency becomes more prominent as the particle size becomes smaller.

In addition, the acicular belt-ride compound particles contain Co
The same problem occurs when the deposition is performed, though not as much as the magnetite particles. In addition, after each heat treatment step of dehydration and reduction of the acicular goethite particles, it is difficult to control the degree of oxidation and control the proportion of ferrous iron contained in the oxidation treatment step of obtaining the beltlide compound particles. There is a problem that the quality of the belt-ride compound particles and the Co-coated magnetic iron oxide particles obtained therefrom vary greatly.

[0010] On the other hand, it is well known that if ferrous salts are applied to acicular maghemite particles, magnetite is formed and blackening can be achieved. It is conceivable to add.

However, when a large amount of ferrous salt and cobalt salt are simultaneously added to the suspension of acicular maghemite particles, the blackness can be improved, but the magnetic properties, especially the coercive force distribution and the squareness ratio, can be improved. Gets worse.

When ferrous salt is added after Co deposition, the blackness is insufficient and the magnetic properties are unstable, so that deterioration over time occurs.

Therefore, it has been proposed to add a ferrous salt to a suspension of acicular maghemite particles and then perform Co deposition, for example, Japanese Patent Application Laid-Open No. 51-23697,
JP-A-58-79822, JP-A-58-7982
No. 3, JP-A-58-79824, JP-A-58-79824
JP-A-107924 and JP-A-61-17426.

However, in the technical means for performing an oxidation reaction disclosed in JP-A-51-23697, it is difficult to magnetize acicular maghemite particles when treated in an oxidizing atmosphere. Particles and goethite particles may be generated.

In the technical means disclosed in JP-A-59-107924 and JP-A-61-17426, in which a ferrous salt is added and then a cobalt salt is added, acicular maghemite particles are added. If a large amount of ferrous salt is added at one time, unfavorable results such as an increase in coercive force distribution may occur.
As can be seen from the examples of JP-A No. 24, the amount of ferrous iron added is less than 10% by weight, and the blackness is still insufficient at this level. Further, when the outermost layer is made of only a cobalt compound, it is not preferable because the reaction of the cobalt compound hardly occurs and the electric resistance increases.

Also, JP-A-58-79822, JP-A-58-79823 and JP-A-58-79824.
In the technical means disclosed in the above publication, γ-Fe ₂ O ₃ powder is dispersed in an aqueous solution containing an alkali and treated at a temperature lower than the boiling point, which is also as high as possible. It is not preferable to perform the addition treatment at such a high temperature because the coercive force distribution becomes large, and as described above, a large amount of ferrous salt is added at once to form an intermediate oxide, Subsequent washing with water, drying and, if necessary, further heat treatment or aging treatment are not preferred because the coercive force distribution becomes large.

In view of the above-mentioned problems, the present invention has a high coercive force, good coercive force distribution, squareness ratio, stable electric resistance and magnetic properties over time, and light transmittance, that is, An object of the present invention is to obtain cobalt-coated magnetic iron oxide particles having excellent blackness.

[0018]

The present inventor has made various studies on the above-mentioned technical problems, and as a result, has arrived at the present invention.

That is, according to the present invention, an aqueous alkali hydroxide solution and an aqueous ferrous salt solution are added to an aqueous dispersion of acicular maghemite particles in a non-oxidizing atmosphere at a temperature of 40 ° C. or less and mixed to obtain a pH of 7 or more. The suspension is stirred at a temperature in the range of more than 40 ° C. and less than the boiling point to produce beltride compound particles containing 7 to 15% by weight of ferrous iron. Is cooled to 40 ° C. or less, then an aqueous solution of a cobalt salt and an aqueous solution of a ferrous salt are added , mixed , and stirred at a temperature of 40 ° C. or less under a non-oxidizing atmosphere to thereby obtain the belt-ride compound particles in the liquid. Forming a cobalt compound and an iron compound on the surface of the particles, and then subjecting the suspension to a stirring treatment in a non-oxidizing atmosphere at a temperature in a range of higher than 60 ° C. and lower than the boiling point, followed by filtration, washing and drying. Depending on 10-20 weight A process for producing cobalt-containing ferromagnetic iron oxide particles, characterized in that to obtain good cobalt-containing ferromagnetic iron oxide particles in blackness containing ferrous.

Next, various conditions for implementing the present invention will be described.

The acicular maghemite particles in the present invention are acicular or spindle-shaped maghemite particles, and may contain 0 to 10% by weight, preferably less than 5% by weight of ferrous iron. Ni, Si, Al, Z
Particles containing one or more of n, P, Ba, Sr, Ca, Pb and the like can also be used.

The aqueous ferrous salt solution in the present invention includes:
An aqueous solution of ferrous sulfate, ferrous chloride or the like can be used.

As the aqueous alkali hydroxide solution in the present invention, an aqueous solution of sodium hydroxide, potassium hydroxide, aqueous ammonia or the like can be used.

As the aqueous solution of the cobalt salt in the present invention, an aqueous solution of cobalt sulfate, cobalt chloride, cobalt nitrate or the like can be used.

In the present invention, a ferrous hydroxide colloid is formed by adding an aqueous ferrous salt solution and an aqueous alkali hydroxide solution to an aqueous dispersion of acicular maghemite particles. The addition amount of the aqueous ferrous salt solution is 5 to 18% by weight in terms of Fe with respect to the acicular maghemite particles. By adding the ferrous salt aqueous solution, it is possible to generate a beltride compound particle containing 7 to 15% by weight of ferrous iron.

When the content of ferrous iron in the beltride compound particles is less than 7% by weight, the degree of blackness is insufficient. Since it is necessary to increase the addition amount of the ferrous salt, it is not preferable because magnetic temporal deterioration and coercive force distribution increase. If it exceeds 15% by weight, sufficient blackness can be obtained, but the belt-ride reaction proceeds rapidly, so that needle-like belt-ride particles grow rapidly and the coercive force distribution becomes undesirably large. .

The temperature for adding the aqueous ferrous salt solution and the aqueous alkali hydroxide solution in the present invention is not higher than 40 ° C. When the temperature exceeds 40 ° C., the belt-ride reaction starts rapidly at the same time as the addition, which is not preferable because the coercive force distribution may increase.

The stirring time after the addition is preferably selected from the range of 10 to 60 minutes. If less than 10 minutes, the ferrous hydroxide colloid is not sufficiently formed,
Further, even if the stirring treatment is performed for more than 60 minutes, there is no industrial significance.

The order of adding the aqueous ferrous salt solution and the aqueous alkali hydroxide solution may be either earlier or simultaneously.

The heating and stirring treatment for producing the beltlide compound particles in the present invention is carried out in a temperature range of more than 40 ° C. and less than the boiling point. When the temperature is lower than 40 ° C., the reactivity is not sufficient, and it is difficult to form a sufficient belt ride. If the boiling point is exceeded, it is not industrial because an apparatus such as an autoclave is required.

In the present invention, the stirring time for forming the beltride compound particles is preferably selected from the range of 30 to 120 minutes. The required time varies depending on the surface area of the acicular maghemite particles serving as nuclei, the reaction temperature, and the amount of ferrous salt added, so it is difficult to specify exactly. However, if the time is less than 30 minutes, the reaction is sufficiently performed. If the reaction time exceeds 120 minutes, the reaction has already been sufficiently performed, so that it has no industrial significance.

In the present invention, after forming the beltride compound particles, the mixture is cooled to a temperature of 40 ° C. or less. 40
The temperature of not more than ℃ is suppressed before the cobalt salt aqueous solution and the ferrous salt to be added are sufficiently mixed to react on the surface of the beltlide compound particles and suppress the increase in coercive force distribution. That's why.

In the present invention, the amount of the aqueous solution of the cobalt salt added to the acicular maghemite particles is 0.1% in terms of Co with respect to the powder.
The amount is more than 1% by weight, preferably more than 0.5% by weight. In order to obtain a high coercive force, a large amount of a cobalt salt may be added, and there is no particular limitation. Usually, it is added up to about 10.0% by weight.

In the present invention, the amount of the aqueous ferrous salt solution for producing the cobalt compound and the ferrous compound on the surface of the beltride compound particles is 5 to 10 in terms of Fe with respect to the acicular maghemite particles. % By weight. If the amount is less than 5% by weight, a high coercive force cannot be effectively derived in the formation with the cobalt compound. If it exceeds 10% by weight, it is not preferable because magnetic deterioration with time and coercive force distribution become large.

The order of adding the aqueous solution of the cobalt salt and the aqueous solution of the ferrous salt is preferably such that the aqueous solution of the cobalt salt is added first or at the same time in order to make high coercive force effective.

In the present invention, the stirring treatment until the cobalt compound and the ferrous compound are formed on the surface of the beltlide compound particles is performed at a temperature of 40 ° C. or less. 40
The temperature is set to not more than ℃ in order to obtain a high coercive force and an excellent coercive force distribution by suppressing a rapid reaction and sufficiently mixing the cobalt compound and the ferrous compound.

The stirring time in this case is 30 to 120.
It is preferable to select from the range of minutes. If the time is less than 30 minutes, there is a possibility that the mixture cannot be sufficiently mixed. Even if it exceeds 120 minutes, there is no industrial significance.

In the present invention, the heating and stirring treatment after the formation of the cobalt compound and the iron compound on the surface of the beltride compound particles is carried out in a temperature range of more than 60 ° C. and less than the boiling point. If the temperature is lower than 60 ° C., the deposition reaction on the beltride compound particles becomes extremely slow, and sufficient magnetic properties cannot be obtained. When the boiling point is exceeded, the coating treatment can be performed, but an apparatus such as an autoclave is required.

The stirring time in this case is preferably selected from the range of 30 to 900 minutes. If the time is less than 30 minutes, the deposition treatment is not sufficient, and if it exceeds 900 minutes, there is no industrial significance. A practically desirable range is 30 to 600 minutes.

The pH of the suspension of each addition and each stirring treatment in the present invention is performed in an alkaline range of 7 or more. When the pH is lower than 7, each reaction may not be sufficiently performed.

Each addition and each stirring treatment in the present invention are performed in a non-oxidizing atmosphere. This is because, in a non-oxidizing atmosphere, ferrous iron is suppressed from being oxidized so that the belt-ride is effectively performed, and a high coercive force and an excellent coercive force distribution are obtained in the deposition process. It is also to prevent independent generation of goethite particles and granular magnetite particles.
In addition, it is also to minimize the oxidation of ferrous iron to ferric iron in each reaction to the minimum necessary, and to leave magnetic iron oxide particles having excellent blackness while leaving as much ferrous iron as possible.
The non-oxidizing atmosphere is desirably performed under a flow of an inert gas such as N ₂ or Ar gas.

The ferrous iron contained in the cobalt-containing ferromagnetic iron oxide particles obtained in the present invention is from 10 to 20% by weight, more preferably from 15 to 18% by weight. If the amount is less than 10% by weight, sufficient blackness cannot be obtained, and if it exceeds 20% by weight, it is necessary to obtain belt-ride compound particles having a high ferrous content. In this case, the reaction can be obtained by repeating the belt-ride reaction a plurality of times (divided into two or more times) in order to suppress the particle growth of the belt-ride compound particles, but the process becomes complicated and the industrial efficiency is impaired. .

[0043]

As described above, the particle powder obtained by coating Co on acicular magnetite particles and acicular beltride compound particles has excellent blackness, but may have problems in workability and magnetic temporal stability. Are known.

The present inventor has found that the cause is that the acicular magnetite particles obtained by each heat treatment of heating dehydration and reduction of the acicular goethite particles and the magnetite particles are heated and oxidized to perform difficult control during the rapid oxidation reaction. It was considered that the acicular beltride compound particles obtained as described above had high activity and caused magnetic degradation with time in air.

It has been considered that it is difficult to improve the magnetic deterioration over time by performing Co deposition using such acicular magnetite particles or acicular beltride compound particles having a high activity.

Therefore, for example, needle-like maghemite particles are dispersed in a suspension containing ferrous hydroxide colloid disclosed in Japanese Patent Publication Nos. If the technical means of generating acicular magnetite particles by heating and stirring in an oxidizing atmosphere is used, magnetite particles excellent in blackness can be obtained without performing dry heat treatment, and subsequently, in the suspension, It was noted that Co deposition could be performed.

However, if ferrous iron is used as a belt-ride compound containing more than 15% by weight, rapid grain growth occurs, and even when Co is deposited, the reaction of Co is unlikely to occur, and a high coercive force is obtained. I couldn't.

From the above results, when the amount of ferrous iron contained in the acicular belt-ride compound particles is controlled to 7 to 15% by weight, it is more excellent to apply a cobalt compound and an iron compound to the surface of the particles. It was found that Co-coated magnetic iron oxide particles having excellent magnetic properties and blackness were obtained. It should be noted that the amount of ferrous iron contained to form the beltride compound particles can be arbitrarily controlled by a wet method.

As shown in FIG. 1, when the cobalt salt was kept constant in the acicular maghemite particles and the amount of the ferrous salt added was varied, the deposition treatment was carried out in a non-oxidizing atmosphere. From the relationship between the linear absorption coefficient when taped and the content of ferrous iron in the Co-coated magnetic iron oxide particles, which is a substantial measure for evaluating the fact that it should be contained in an amount exceeding 10% by weight. I understood. However, in FIG. 1, a mark indicates a comparative example, and a mark indicates an example. Hereinafter, the same applies to FIGS. 2 and 3.

However, as shown in FIG. 2, the relationship between the coercive force distribution and the ferrous content in the same manner as in FIG. 1 indicates that the case where the cobalt salt and the ferrous salt were added simultaneously (curve a), the coercive force distribution deteriorates as the ferrous iron content increases.

As shown in FIG. 3, the relationship between the squareness ratio and the content of ferrous iron also deteriorates as the ferrous iron content increases, as in FIG.

However, the ferrous salt of the present invention is divided and charged, and first, a beltride compound particle containing 7 to 15% by weight of ferrous iron is added, and then a cobalt salt and a ferrous salt are added. When Co-coated magnetic iron oxide particles containing 10 to 20% by weight of the generated ferrous iron (indicated by curve b),
As shown in FIGS. 1, 2 and 3, it can be seen that the coercive force distribution and the squareness ratio are significantly improved with a sufficient linear absorption coefficient.

Further, ΔHc showing the stability over time of the magnetic characteristics
In the case of using the acicular magnetite particle powder (Comparative Example 1) and acicular beltride compound particle powder (Comparative Example 3) obtained by a gas phase reduction method (dry method),
It can be seen that the Co-coated magnetic iron oxide particles obtained according to the present invention have a small ΔHc and are superior to those obtained by the present invention. Here, ΔHc is a value obtained by measuring the coercive force Hc after leaving it in a constant temperature bath at a temperature of 60 ° C. and a relative humidity of 90% for two weeks, and subtracting the initial Hc value.

[0054]

Next, the present invention will be described with reference to examples and comparative examples.

The major axis diameter and the axial ratio (major axis diameter / minor axis diameter) of the particles in the following Examples and Comparative Examples are average values of numerical values measured from electron micrographs, and the specific surface area is BE.
It is shown by the value measured by the T method. The magnetic properties of the needle-like magnetic iron oxide particles are described in "Vibration sample magnetometer VSM-3S-1".
5 "(manufactured by Toei Industry Co., Ltd.) and an external magnetic field of 10 KO
e.

The content of ferrous iron was measured by charging magnetic iron oxide particles into a flask, replacing with an inert gas, adding and mixing a mixed acid of sulfuric acid and phosphoric acid while heating and dissolving, followed by heating and dissolving. Was determined by a redox titration method.

The coercive force distribution (SFD) was measured using a sheet sample piece, a differential curve of the coercive force was obtained by using a differentiating circuit of the magnetometer, and the half width of this curve was measured. Then, this value was obtained by dividing the peak value by the coercive force.

The light transmittance of the magnetic sheet was represented by a linear absorption coefficient measured using a “photoelectric spectrophotometer UV-2100” (manufactured by Shimadzu Corporation). The linear absorption coefficient is defined by the following equation, and the larger the value, the more difficult it is to transmit light. Linear absorption coefficient (μm ⁻¹ ) = ln (1 / t) / FT t: Light transmittance at λ = 900 nm (−) FT: Thickness of magnetic layer of film used for measurement (μm) Note that linear absorption coefficient is 1 If it is 0.2 or more (film thickness 4.0 μm), it is possible to satisfy the light transmittance of 0.8% or less defined by the VHS standard. Further, the linear absorption coefficient is 1.4.
If it is above, it can be said that the blackness of the Co-coated magnetic iron oxide particles is sufficiently excellent.

The evaluation of blackness was represented by the linear absorption coefficient, which is a substantial evaluation measure as described above.

A sheet-like sample piece was prepared by adding magnetic iron oxide particles, a resin and a solvent at the following ratios to 100 cc of a polybin, and then mixing and dispersing with a paint conditioner for 6 hours. Is applied to a thickness of 50 μm using an applicator on a polyethylene terephthalate film of
s in a magnetic field.

3 mmφ still ball 800 parts by weight Magnetic iron oxide particle powder 100 parts by weight Polyurethane resin having sodium sulfonate group 20 parts by weight Cyclohexanone 83.3 parts by weight Methyl ethyl ketone 83.3 parts by weight Toluene 83.3 parts by weight

Example 1 Acicular maghemite particles (average major axis diameter 0.22 μm,
Axial ratio (major axis diameter / minor axis diameter) 8.0, BET specific surface area 35 m
² / g, coercive force 380 Oe, ferrous iron 3.0% by weight) 20
To an aqueous dispersion obtained by dispersing 0 g in 1330 ml of water, 272 ml of a 10 mol / l NaOH aqueous solution was added, N ₂ gas was flown to make a non-oxidizing atmosphere, and the temperature of the suspension was brought to 35 ° C. . Thereafter, the treatment was performed in a non-oxidizing atmosphere.

0.9 mol / l of FeSO ₄ was added to the suspension.
398 ml of an aqueous solution (the amount of Fe corresponds to 10.0% by weight with respect to the acicular maghemite particle powder) was added over 30 minutes. After the addition, stirring was continued for 30 minutes. Maintained for minutes to produce dark brown precipitate particles.

A part of the formed black-brown precipitate particles was extracted, separated by filtration, washed with water and dried, and the ferrous content of the obtained particle powder was measured to be 12.0% by weight.

After cooling the suspension to 35 ° C.,
113 ml of an aqueous solution of CoSO ₄ mol / l (Co amount is
This corresponds to 3.0% by weight based on the acicular maghemite particle powder. ) Was added in 10 minutes. Then, 0.9mol
298 ml of an aqueous solution of FeSO ₄ / l (the amount of Fe corresponds to 7.5% by weight based on the powder of the acicular maghemite particles) was added over 30 minutes, and the stirring treatment was continued for 30 minutes after the addition. Further, the temperature is raised to 100 ° C. in a non-oxidizing atmosphere,
The mixture was held for 5 hours and heated and stirred to form black precipitate particles.

The black precipitated particles were separated by filtration, washed with water and dried by a conventional method to obtain black particle powder. As a result of X-ray diffraction, the obtained black particle powder was found to contain C and ferrous iron.
o It was an adhered magnetic iron oxide particle powder.

The obtained Co-coated magnetic iron oxide particles had an average major axis diameter of 0.22 μm and an axial ratio (major axis diameter / minor axis diameter).
7.0, coercive force 667 Oe, saturation magnetization 85.0 emu
/ G, ferrous iron 17.4% by weight. Further, ΔHc was −27 Oe, and the deterioration with time was small.

A sheet sample piece was prepared using the obtained Co-coated magnetic iron oxide particle powder, and the sheet characteristics obtained were as follows:
Coercive force 655 Oe, squareness ratio (Br / Bm) 0.795,
2. Coercive force distribution (SFD) 0.490, electrical resistance
8 × 10 ⁹ Ω / sq, linear absorption coefficient (λ = 900 nm)
1.65.

Examples 2 to 5, Comparative Examples 1 to 7 Types of magnetic iron oxide particles to be treated, alkali ion concentration, amount of ferrous salt aqueous solution for producing beltlide compound particles, temperature at addition and after addition Stirring time, the temperature of the production treatment, the temperature after cooling, the amount of the aqueous cobalt salt solution, the amount of the aqueous ferrous salt solution to be added after the addition of the aqueous cobalt salt solution, the temperature at the time of addition and the stirring time after the addition and after the temperature rise Co-coated magnetic iron oxide particles were obtained in the same manner as in Example 1 except that the deposition temperature and the stirring time were variously changed.

Tables 1 to 3 show main manufacturing conditions and various characteristics at this time.

[0071]

[Table 1]

The maghemite A of the particles to be treated is
Average long axis diameter 0.22 μm, axis ratio (long axis diameter / short axis diameter) 8.
0, BET specific surface area 35 m ² / g, coercive force 380 Oe,
Ferrous iron was 3.0% by weight, and maghemite B had an average major axis diameter of 0.22 μm, an axial ratio (major axis diameter / minor axis diameter) of 8.0, and B
The ET specific surface area was 35 m ² / g, the coercive force was 383 Oe, and ferrous iron was 0.0% by weight. Magnetite C had an average major axis diameter of 0.22 μm, an axial ratio (major axis diameter / minor axis diameter) of 8.0, BET
Specific surface area 35 m ² / g, coercive force 380 Oe, ferrous iron 1
And the belt ride D had an average major axis diameter of 0.22 μm, an axial ratio (major axis diameter / minor axis diameter) of 8.0, and BET.
Specific surface area 34 m ² / g, coercive force 350 Oe, ferrous iron 1
0.0% by weight.

[0073]

[Table 2]

[0074]

[Table 3]

[0075]

The cobalt-containing ferromagnetic iron oxide particles produced according to the present invention have a high coercive force as shown in the above examples, and have a coercive force distribution, squareness ratio, electric resistance and It is suitable for high-density recording because it has good magnetic properties over time and excellent blackness.

The cobalt-containing ferromagnetic iron oxide particles obtained in the present invention have an effect that the light transmittance can be improved without adding a non-magnetic filler.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the content of ferrous iron and the linear absorption coefficient of Co-coated magnetic iron oxide particles.

FIG. 2 is a diagram showing the relationship between the content of ferrous iron and the distribution of coercive force.

FIG. 3 is a diagram showing the relationship between the content of ferrous iron and the squareness ratio.

Claims (1)

(57) [Claims] 1. An aqueous alkali hydroxide solution and an aqueous ferrous salt solution are added to and mixed with an aqueous dispersion of acicular maghemite particles at a temperature of 40 ° C. or less in a non-oxidizing atmosphere to form a suspension having a pH of 7 or more. Stirring the suspension at a temperature in the range of more than 40 ° C. and less than the boiling point, thereby producing 7-15% by weight of ferrite-containing beltride compound particles. Then, an aqueous solution of a cobalt salt and an aqueous solution of a ferrous salt are added , mixed , and agitated at a temperature of 40 ° C. or less under a non-oxidizing atmosphere. A cobalt compound and an iron compound are formed, and the suspension is stirred under a non-oxidizing atmosphere at a temperature in the range of higher than 60 ° C. and lower than the boiling point. 20
A method for producing a cobalt-containing ferromagnetic iron oxide particle powder, comprising obtaining cobalt-containing ferromagnetic iron oxide particles having excellent blackness and containing ferrous iron by weight.