JPH0776363B2 - Method for producing ferromagnetic metal particles - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing ferromagnetic metal particles, and more specifically to the production of ferromagnetic metal particles having excellent dispersibility and suitable as a raw material for high-density magnetic recording media. It is about the method.

[Conventional technology]

Conventionally, needle-shaped iron oxide particles have been mainly used as magnetic powder for magnetic recording media, but with the commercialization of digital audio tape (DAT), 8 mm video tape, high-density floppy disk, etc., high coercive force has been achieved. Ferromagnetic metal iron particles with (H _c ), high saturation magnetization (σ _s ) and high squareness ratio (σ _r / σ _s ) have come to be used. The ferromagnetic metal iron particles are generally obtained by heating and reducing needle-shaped fine particles mainly composed of α-iron oxyhydroxide or iron oxide in a reducing gas stream such as hydrogen, which is suitable for high density magnetic recording. Therefore, the ferromagnetic metal iron particles are required to be made finer year by year. However, the finer the particles, the more likely the particles to sinter during heating and reduction, and the magnetic properties of the magnetic powder deteriorate. In addition, the coercive force (H _c ) changes because the compatibility with the binder and lubricant used during coating varies depending on the type and amount of the dissimilar metals present on the surface of the magnetic powder.
There is a demand for a technique for controlling the type and amount of different metals on the surface of the magnetic powder while maintaining basic magnetic characteristics such as the saturation magnetization amount (σ _s ).

In order to solve these problems, a method of solid-solving a metal such as aluminum in α-iron oxyhydroxide, a method of depositing a different metal such as nickel, aluminum, or silicon on α-iron oxyhydroxide, etc. Various proposals have been made.

[Problems to be Solved by the Invention]

Japanese Patent Publication No. 59-17161 discloses that an iron compound containing at least one of FeOOH and Fe ₂ O ₃ as a main component is aluminum (A
l) A method of forming a solid solution of a compound and then reducing the compound is shown. This method is said to be effective for obtaining ferromagnetic metal particles without sintering between particles without adhering a sintering inhibitor to the acicular particles. However, the solid solution of Al
If the amount of the compound is small, the effect of preventing sintering decreases,
Sintering occurs during reduction, deteriorating magnetic properties. Also, if the amount of Al compound to be dissolved is large, FeOOH in which Al is dissolved
Alternatively, it has a serious drawback that the needle-like properties of Fe ₂ O ₃ particles are lost. That is, the amount of Al solid solution is 0.5 to 3 as Al atom with respect to the iron atom of α-iron oxyhydroxide particles in which Al is solid solution.
In the range of wt%, acicularity is not significantly broken, but in this range, the amount of solid solution of Al is small, so the effect of preventing sintering during the subsequent reduction cannot be sufficiently obtained, and the reduction The magnetic properties of the obtained ferromagnetic metal particles, especially the coercive force and the squareness ratio are deteriorated. If the solid solution amount of Al exceeds 3% by weight, FeOOH
Or, because the acicularity of Fe ₂ O ₃ collapses, the acicular ratio of the ferromagnetic metal particles obtained by reduction cannot be maintained, the coercive force and squareness ratio decrease, and the solid solution amount of Al is large. Therefore, the reducibility is suppressed, and it becomes difficult to obtain a high saturation magnetization amount. Further, in this method, since Al is dissolved as a solid solution, the amount of Al atoms on the surface of the ferromagnetic metal particles obtained by reduction is very small.

Further, in JP-A-63-109105, a nickel (Ni) compound is deposited on the surface of α-iron oxyhydroxide particles, and then a silicon compound and an aluminum compound are deposited in alkaline water to reduce the compound. It shows how to do it.
In this method, in the composition ratio of nickel, silicon and aluminum, ferromagnetic metal particles having relatively excellent magnetic properties can be obtained, especially when the amount of silicon compound attached is large and the amount of Al compound attached is small. If the amount of the Al compound deposited is large, the ferromagnetic metal particles obtained by the reduction are broken, and the magnetic properties, especially the coercive force and the squareness ratio are deteriorated. For this reason, there is a drawback that the amount of foreign main metal on the surface cannot be controlled, especially the amount of silicon cannot be reduced or the amount of Al cannot be increased while maintaining the basic magnetic properties of the magnetic powder.

An object of the present invention is to eliminate the above-mentioned drawbacks of the prior art, to provide a method for producing ferromagnetic metal particles which has excellent dispersibility and magnetic properties and which can control the amount of dissimilar metals existing on the surface thereof. Especially.

[Means for Solving the Problems]

The present invention relates to (1) α-iron oxyhydroxide in which aluminum is solid-dissolved by performing an oxidation reaction in an aqueous suspension containing a ferrous salt, an alkali compound and an aluminum compound through an oxygen-containing gas. The particles were synthesized, filtered, washed with water, suspended again in water to obtain a water suspension, and a nickel compound was added to the surface of the α-iron oxyhydroxide particles in which the aluminum was solid-dissolved in the suspension. After the deposition, the silicon compound and the alumina hydrate are further deposited in an alkaline suspension, the obtained particles are filtered and dried, and then in a non-reducing gas atmosphere, 400 ° C or more. The present invention relates to a method for producing ferromagnetic metal particles, which comprises heating and calcining at 2, then reducing.

The present invention also provides (2) α-containing a solid solution of the aluminum.
In depositing a nickel compound on the surface of iron oxyhydroxide particles, an aqueous suspension of the particles is made into a suspension of an aqueous organic acid solution having a pH of 4.0 or less, and then a salt of nickel is added, followed by a basic substance. Is added to adjust the pH to 7 to 12, and then aged at 70 ° C. or higher, to the method for producing ferromagnetic metal particles according to (1).

Furthermore, the present invention provides (3) the method of depositing a silicon compound and an alumina hydrate on the surface of α-iron oxyhydroxide particles having a solid solution of aluminum and having a nickel compound deposited thereon. After adding a basic substance to the suspension as needed to maintain the pH of the suspension at 7 or more, an aqueous solution of silicic acid or silicate and an aluminum salt or alumina sol is added simultaneously or sequentially, The temperature of the suspension
The method for producing ferromagnetic metal particles according to (1) or (2) is characterized in that the temperature is 70 ° C. or higher.

In the present invention, the aluminum compound according to claim (1) is preferably at least one selected from the group consisting of an inorganic salt of aluminum, an organic acid salt of aluminum and an alumina sol. It is preferable that the amount of aluminum atom is 0.5 to 3% by weight with respect to the iron atom of the α-iron oxyhydroxide particles that are solid-solved.

The nickel salt according to claim (2) is preferably at least one of nickel sulfate, nitrate, chloride, acetate and oxalate.

Furthermore, the aqueous solution of silicic acid or silicic acid salt according to claim (3) is an aqueous solution of orthosilicic acid, an aqueous solution of metasilicic acid, an aqueous silica sol, an ammonia-stabilized aqueous silica sol, or an aluminum-modified aqueous solution. It is preferable that at least one kind of the silica sol and the sodium silicate aqueous solution be used, and that the aqueous solution of the aluminum salt be at least one kind of the inorganic salt of aluminum and the organic acid salt of aluminum.

As the aluminum compound used in the present invention, aluminum sulfate, aluminum chloride, aluminum nitrate,
At least one of an inorganic salt of aluminum such as aluminum phosphate and aluminate, an organic acid salt of aluminum such as aluminum lactate, and alumina sol can be used, and among these, aluminum sulfate and sodium aluminate are preferable.

The solid solution amount of the aluminum compound is 0.5 to 3 wt% as Al atom with respect to the iron atom of α-iron oxyhydroxide particles in which Al is solid-solved (hereinafter referred to as Al solid solution α-iron oxyhydroxide particles). % Is preferable. If the solid solution amount of Al is less than 0.5% by weight, the ferromagnetic powder particles obtained by reduction may be cracked or sintered, and if it exceeds 3% by weight, acicular particles of the solid solution α-iron oxyhydroxide particles of Al are dissolved. Since the property is lost, the acicular ratio of the ferromagnetic metal particles obtained by reduction cannot be maintained, and the coercive force may decrease.

Examples of the ferrous salt used in the present invention include sulfuric acid first
Examples include iron and ferrous chloride.

Examples of the alkaline compound used in the present invention include sodium hydroxide and potassium hydroxide. The amount of the alkaline compound used is preferably 1.5 equivalents or more with respect to the ferrous salt.

The reaction temperature when synthesizing the Al solid solution α-iron oxyhydroxide particles is preferably 5 to 60 ° C. Air is preferable as the oxygen-containing gas.

In the present invention, the nickel compound can be adhered to the surface of the Al solid solution α-iron oxyhydroxide particles obtained above in the following manner, for example.

First, the Al-dissolved α-iron oxyhydroxide particles obtained in the above synthesis are washed with water until the pH of the aqueous suspension becomes 10.0 or less. Insufficient washing with water is not preferable because ions such as sodium adhering to the surface of the particles promote sintering in the heating reduction step. Next, an organic acid, preferably a water-soluble carboxylic acid such as acetic acid is added to the water suspension of the washed particles, or the washed particles are added to water containing an organic acid to have a pH of 4.0 or less, preferably Is an aqueous suspension of pH 3.5-2.0, and A
l Disperse the solid solution α-iron oxyhydroxide particles uniformly to a single particle. Then, an inorganic salt or organic acid salt of nickel is added to the aqueous suspension, and then a basic substance such as ammonia or monoethanolamine is added to adjust the pH to 7.0 to 12.0, more preferably pH 8.0 to 11.0. A suspension is prepared and aged at 70 ° C. or higher, preferably 90 ° C. or higher to deposit nickel oxide or hydroxide on the surface of the Al solid solution α-iron oxyhydroxide particles. The aging time is preferably 1 to 2 hours.

As the nickel salt, sulfate, nitrate, chloride,
Acetate, oxalate and the like can be used, but acetate is particularly preferable. The amount of nickel deposited is 0.3 as a nickel atom relative to the iron atom of the Al solid solution α-iron oxyhydroxide particles.
~ 10 wt% is preferred. If it is less than 0.3% by weight, the dispersibility and reducibility of the particles are poor, and it tends to cause a decrease in the high saturation magnetization and sintering, and if it exceeds 10% by weight, the ferromagnetic metal particles obtained by the reduction have α-Fe phase In addition, a γ- (Fe, Ni) phase may appear, which may deteriorate the magnetic properties.

In the present invention, the deposition of the silicon compound and the alumina hydrate on the surface of the Al solid solution α-iron oxyhydroxide particles coated with nickel can be performed, for example, as follows.

First, cool the suspension of Al solid solution α-iron oxyhydroxide particles coated with nickel, or 70 ℃ or more, preferably
Maintaining the pH of the suspension at 7.0 or higher while maintaining the temperature at 90 ° C or higher and adding a basic substance, preferably ammonia, as necessary in order to prevent elution of the deposited nickel, silicic acid or silicic acid. An aqueous salt solution and an aluminum salt solution or alumina sol are gradually added. As for the method of addition, the aqueous solution of silicic acid or silicate may be added first, the aqueous solution of aluminum salt or the alumina sol may be added first, or both may be added simultaneously. After that, it is aged, and the aging time is preferably 1 to 2 hours. When cooled and added, it is preferable to heat the suspension to 70 ° C. or higher, preferably 90 ° C. or higher for aging.

As the aqueous solution of silicic acid or silicate, orthosilicic acid, various silicic acid aqueous solutions such as metasilicic acid, silica sol,
Ammonium-stabilized silica sol, aluminum-modified silica sol, silicate aqueous solution and the like are used. The amount of silicon deposited from these aqueous solutions or sols is preferably 0.5 to 7% by weight as silicon atoms with respect to the iron atoms of Al solid solution α-iron oxyhydroxide. If it is less than 0.5% by weight, there is no effect of preventing sintering, and if it exceeds 7% by weight, the reduction is suppressed and the desired high saturation magnetization amount may not be obtained.

As the aqueous solution of the aluminum salt, an aqueous solution of an inorganic salt such as aluminum sulfate, aluminum chloride, aluminum nitrate, aluminum phosphate and sodium aluminate, or an organic acid salt such as aluminum formate, aluminum acetate and aluminum lactate is used. The amount of aluminum deposited from these aqueous solutions or alumina sol depends on the Al solid solution α
-0.5 to 7% by weight of aluminum atom is preferable, and more preferably 1 to 5 with respect to iron atom of iron oxyhydroxide.
% By weight. If the amount is less than 0.5% by weight, the amount of Al existing on the surface of the magnetic powder particles obtained by reduction is too small, so sintering prevention and dispersibility are not improved, and if it exceeds 7% by weight, reduction is suppressed, The desired high saturation magnetization amount may not be obtained.

The suspension thus obtained containing the nickel compound, the silicon compound, and the Al solid solution α-iron oxyhydroxide coated with the alumina hydrate is necessary after separation by a method such as filtration. Washed with water and then dried to obtain dry α-iron oxyhydroxide. The drying temperature at this time is 100-180 ℃
Is preferred.

The dried α-iron oxyhydroxide obtained is heated and calcined to once form acicular hematite, and then reduced. The heating and calcination is usually performed in a non-reducing gas atmosphere such as argon, nitrogen and air at a temperature of 400 ° C. or higher, preferably 400 to 800 ° C. The reduction is usually performed in a hydrogen stream at a temperature of 300 to 600 ° C. These temperatures are appropriately determined depending on the size of the Al solid solution α-iron oxyhydroxide particles, the specific surface area, the deposition amount of various metals, and the like.

〔Example〕

Hereinafter, the present invention will be described in detail with reference to Examples. In the following examples,% means% by weight unless otherwise specified.

Example 1 An aqueous solution prepared by mixing 5.6 kg of 27% aqueous sodium hydroxide solution with 21.0 g of aqueous sodium aluminate solution (Al concentration: 10%) was used.
After adding 11.4 kg of 5% ferrous sulfate aqueous solution, add 20% air.
Stirring while blowing at a speed of / min, keeping the temperature at 30 ° C
Al solid solution α-iron oxyhydroxide particles were synthesized. The target value of the solid solution amount of Al was 1.0% as Al atom with respect to the iron atom of the Al solid solution α-iron oxyhydroxide particles, but the actually measured solid solution amount was 1.0%.

The obtained particles of Al solid solution α-iron oxyhydroxide were filtered and washed with water until the pH of the aqueous suspension of the particles became 9.0 or less. The A
l 30% acetic acid aqueous solution was added to 8000 g of a suspension of solid-solution α-iron oxyhydroxide particles dispersed again in water (the particle concentration: 1.5%) to adjust the pH of the water suspension to 3.0. After stirring for a minute,
Add 297.4 g of nickel acetate aqueous solution (Ni concentration: 2.41%) prepared beforehand and stir for another 30 minutes to 28%
Aqueous ammonia was gradually added to adjust the pH of the aqueous suspension to 10.3. After stirring this for 30 minutes, raise the temperature of the water suspension to 90 ° C.
And aged for 60 minutes.

While keeping the temperature of the water suspension at 90 ° C., 75.5 g of an orthosilicic acid aqueous solution (Si concentration: 1.0%) was gradually added and stirred for 30 minutes, and then an aluminum sulfate aqueous solution (Al concentration: 1.5%) 201 .
4 g was gradually added and stirred for 60 minutes. The aqueous suspension was cooled to 30 ° C., then filtered and washed with water to obtain a cake of α-iron oxyhydroxide particles on which a nickel compound, a silicon compound and a salt of aluminum were adhered. The cake was dried overnight at 130 ° C. to obtain dry α-iron oxyhydroxide particles having the solid solution amount and the adhered amount shown in Table 1.

100 g of the obtained dried α-iron oxyhydroxide particles was heated and calcined at 650 ° C. for 30 minutes in N ₂ atmosphere, then reduced at a flow rate of H ₂ flow rate of 50 / min and a temperature of 500 ° C. for 4 hours, and extracted into toluene. ℃,
It was air-dried for 24 hours in a thermostatic chamber with relative humidity of 60% to obtain a dried ferromagnetic metal powder. TEM of this powder (transmission electron microscope, 3
FIG. 1 shows a photograph of 0000 times (a length of 3 cm on the drawing corresponds to a length of 1 μm). The specific surface area of this powder and the magnetic characteristics in a magnetic field of 10 kOe were measured, and the results are shown in Table 1.

Further, 55 g of the powder, 12.4 g of binder made of vinyl chloride acetate and polyurethane, 0.7 g of curing agent, 3.8 g of polishing agent, 2.8 g of dispersant and 171 g of solvent made of toluene, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone.
Were collectively charged in a sand mill and stirred at 1850 rpm for 2 hours to obtain a coating material. This was oriented on a polyester film in a magnetic field of 3000 gauss to prepare a tape.
The magnetic properties of this tape in a magnetic field of 5 kOe were measured, and the results are shown in Table 1.

It can be seen from FIG. 1 and Table 1 that the obtained ferromagnetic metal particles have excellent shape retention, dispersibility and magnetic properties.

Example 2 In Example 1, an aluminum sulfate aqueous solution (Al concentration:
1.5%) 201.4 g, sodium aluminate aqueous solution (Al concentration: 1.
0%) 302.1 g except that the treatment was performed in the same manner as in Example 1 to prepare a ferromagnetic metal powder and a tape using the powder, and the characteristics thereof were examined. The results are shown in Table 1.

From Table 1, it can be seen that the obtained ferromagnetic metal particles have excellent magnetic properties.

Example 3 In Example 1, an ortho-silicic acid aqueous solution (Si concentration: 1.0
%) 75.5 g, sodium silicate aqueous solution (Si concentration: 1.0%) 75.
A ferromagnetic metal powder and a tape were prepared using the same treatment as in Example 1 except that the amount was changed to 5 g, and their characteristics were examined. The results are shown in Table 1.

It can be seen from Table 1 that the ferromagnetic metal particles have excellent magnetic properties.

Example 4 In Example 1, an ortho-silicic acid aqueous solution (Si concentration: 1.0
%) 75.5 g was changed to 188.7 g, and aluminum sulfate aqueous solution (Al concentration: 1.5%) 201.4 g was changed to 55.4 g
Ferromagnetic metal powders and tapes were prepared using the powders by the same treatment as above, and their properties were investigated. The results are shown in Table 1.

From Table 1, it can be seen that the obtained ferromagnetic metal particles have excellent magnetic properties.

Comparative Example 1 After the aluminum-solid-dissolved α-iron oxyhydroxide particles synthesized in Example 1 were filtered and washed without coating treatment, the obtained cake was dried at 130 overnight and dried α-oxy water. Iron oxide particles were obtained. 100 g of the obtained dried α-iron oxyhydroxide particles was reduced at a H ₂ flow rate of 50 / min at a temperature of 400 ° C. for 5 hours and then air-dried in the same manner as in Example 1 to obtain a ferromagnetic metal powder. A TEM photograph of this powder is shown in FIG. As is clear from FIG. 2, the powder was sintered even though the reduction temperature was low. Using this powder, tapes were prepared in the same manner as in Example 1 and their characteristics were examined. The results are shown in Table 1, and the magnetic properties such as coercive force and squareness ratio were far inferior to those of the examples. Comparative Example 1 shows that if the solid solution amount of aluminum is small, there is no effect in preventing sintering.

Comparative Example 2 An aqueous solution prepared by mixing 94.5 g of a sodium aluminate aqueous solution (Al concentration: 10%) with 5.6 kg of a 27% sodium hydroxide aqueous solution,
After adding 11.4 kg of 5% ferrous sulfate aqueous solution, add 20% air.
The mixture was stirred while being blown at a rate of / min, and the temperature was kept at 30 ° C to synthesize α-iron oxyhydroxide particles in which aluminum was dissolved. The target value of solid solution aluminum is 4.5% with respect to the iron atom of α-iron oxyhydroxide in which aluminum is solid solution.
However, the actual amount of solid solution was 3.7%, which is smaller than this. This aqueous suspension of α-iron oxyhydroxide particles in which aluminum was dissolved was filtered and washed with water, and
It was dried at 0 ° C. overnight to obtain dry α-iron oxyhydroxide particles.

Of the α-iron oxyhydroxide particles in which this aluminum is dissolved
A TEM photograph is shown in FIG. As is clear from FIG. 3, the particles lacked acicularity and were unsuitable as raw materials for ferromagnetic metal particles having high coercive force and high squareness ratio. The particles were reduced in the same manner as in Comparative Example 1 to obtain a ferromagnetic metal powder, and a tape was produced using the powder, and their characteristics were examined. The results are shown in Table 1, and as expected, the magnetic properties were inferior in coercive force and squareness. A TEM photograph of the ferromagnetic metal powder is shown in FIG. 4. The acicularity was broken, and sintering was also seen in part. It has been found that when the solid solution amount of aluminum increases, the coercive force and the squareness ratio are particularly deteriorated because the acicularity of the α-iron oxyhydroxide particles is impaired.

Comparative Example 3 After adding 11.4 kg of a 5% ferrous sulfate aqueous solution to 5.6 kg of a 27% aqueous sodium hydroxide solution, stirring was performed while blowing air at a rate of 20 / min, and the temperature was kept at 30 ° C. to solidify aluminum. Non-dissolved α-iron oxyhydroxide particles were synthesized. The particles were washed with water until the aqueous suspension of α-iron oxyhydroxide had a pH of 9.0 or less. Example 1 except that this α-iron oxyhydroxide in which aluminum was not dissolved was used.
In the same manner as above, deposition, firing and reduction treatment were carried out to obtain a ferromagnetic metal powder. A TEM photograph of this powder is shown in FIG. As is clear from FIG. 5, the ferromagnetic metal powder produced by this method had particle breakage. A tape was prepared using this powder in the same manner as in Comparative Example 1, and the characteristics of the powder and the tape were examined. The results are shown in Table 1, and it was found that the holding force and the squareness ratio were particularly lowered due to the breakage of the particles.

Comparative Example 4 α-containing no solid solution of aluminum prepared in Comparative Example 3
A ferromagnetic metal powder was obtained by performing the deposition treatment in the same manner as in Example 2 except that iron oxyhydroxide particles were used. In the TEM photograph of this powder, as in Comparative Example 3, the particles were torn. A tape was prepared using this powder in the same manner as in Comparative Example 1, and the characteristics of the powder and the tape were examined. The results are shown in Table 1, and like Comparative Example 3,
It was found that the coercive force and the squareness ratio were greatly reduced.

Comparative Example 5 α-which does not form a solid solution of aluminum produced in Comparative Example 3
A ferromagnetic metal powder was obtained by performing the deposition treatment in the same manner as in Example 4 except that iron oxyhydroxide particles were used. The TEM photograph of this powder also showed particle breakage. A tape was prepared using this powder in the same manner as in Comparative Example 1, and the characteristics of the powder and the tape were examined. The results are shown in Table 1, and although the characteristics were slightly improved as compared with Comparative Example 4 due to the change in the composition ratio of silicon and aluminum, they were inferior to those in Examples.

EFFECT OF THE INVENTION According to the present invention, α-iron oxyhydroxide particles are uniformly dissolved in aluminum, and then α
-Shape retention and dispersibility by depositing a nickel compound on the surface of iron oxyhydroxide particles, then depositing a silicon compound and alumina hydrate in an alkaline suspension, followed by firing and heat reduction. And ferromagnetic metal iron particles having excellent magnetic properties are obtained at the same time.

[Brief description of drawings]

FIG. 1 is a TEM of the ferromagnetic metal particle structure obtained in Example 1.
(Transmission Electron Microscope) Photograph, FIG. 2 is a TEM photograph of the ferromagnetic metal particle structure obtained in Comparative Example 1, and FIG. 3 is an Al solid solution α-oxyhydroxide synthesized in Comparative Example 2. Iron particle structure
TEM photograph, FIG. 4 and FIG. 5 are comparative examples 2 respectively.
3 is a TEM photograph of a ferromagnetic metal particle structure obtained in Comparative Example 3. FIG.

Claims (3)

[Claims] 1. An α-solution of aluminum which is solid-dissolved in an aqueous suspension containing a ferrous salt, an alkali compound and an aluminum compound, through which an oxygen-containing gas is passed to carry out an oxidation reaction.
After synthesizing iron oxyhydroxide particles and filtering and washing with water,
The suspension is again suspended in water to form a water suspension, and the surface of the α-iron oxyhydroxide particles in which the aluminum is solid-dissolved in the suspension is coated with a nickel compound, and then a silicon compound and alumina are further added. It is characterized in that a hydrate is deposited in an alkaline suspension, the obtained particles are filtered off, dried, then heated and baked at 400 ° C. or higher in a non-reducing gas atmosphere, and then reduced. Method for producing ferromagnetic metal particles. 2. When depositing a nickel compound on the surface of the α-iron oxyhydroxide particles in which aluminum is solid-dissolved, an aqueous suspension of the particles is suspended in a water-soluble carboxylic acid aqueous solution having a pH of 4.0 or less. The method for producing ferromagnetic metal particles according to claim 1, wherein after the solution is prepared, a nickel compound is added, a basic substance is then added to adjust the pH to 7 to 12, and the mixture is aged at 70 ° C or higher. Method. 3. When depositing a silicon compound and an alumina hydrate on the surface of α-iron oxyhydroxide particles having a solid solution of aluminum and having a nickel compound deposited thereon,
If necessary, a basic substance is added to the aqueous suspension of the particles to keep the pH of the suspension at 7 or more, and an aqueous solution of silicic acid or silicate and an aluminum salt or alumina sol is added simultaneously or sequentially. After that, the temperature of the suspension is set to 70 ° C. or higher, and the method for producing ferromagnetic metal particles according to claim (1) or (2).