JPH03123003A - Magnetic metal powder, manufacture thereof, and film for magnetic recording medium using the magnetic powder - Google Patents

Abstract

PURPOSE:To improve corrosion-resisting property by a method wherein a silicon- aluminum compound layer is formed on the surface of a needlelike Geothite, a magnetite is formed by reducing and hydrating the abovementioned layer, a non-ferrous transition metal compound layer is formed on its surface, a silicon compound layer is formed, and they are reduced. CONSTITUTION:A silicon-aluminum compound layer is formed on the surface of a needle-like Geothite, magnetite is formed by reducing and dehydrating the above- mentioned material, a non-ferrous transition metal compound layer is formed on its surface, a silicon compound layer is formed thereon, and they are reduced. The formation of the silicon-aluminum compound layer on the surface of the needle-like Geothite and the formation of the non-ferrous transition metal compound layer on the surface of the magnetite are conducted by the deposition of an insoluble substance from a solution state of each material and the precipitation and the like of a colloidal compound. Cobalt or nickel is used as a nonferrous transition element. As a result, the magnetic metal powder and the film for a magnetic recording medium, having excellent magnetic characteristics and oxidation-resistant property, can be obtained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing metal magnetic powder used for magnetic recording, and in particular to a method for manufacturing metal magnetic powder that has a large specific surface area, high coercive force, and excellent corrosion resistance. It is about the method.

The present invention also relates to a coating film for magnetic recording media that uses ferromagnetic metal powder and has excellent corrosion resistance and magnetic properties.

[Problems to be solved by conventional techniques and inventions] In recent years,
There have been remarkable developments in various recording methods, and among them, the progress in making magnetic recording and reproducing devices smaller and lighter has been remarkable. Along with this, there is an increasing demand for higher performance for magnetic recording media such as magnetic tapes and magnetic disks.

In order to satisfy these requirements for magnetic recording, magnetic powders with high coercive force and high saturation magnetization are required. Conventionally, acicular magnetite or magnetite or these magnetic iron oxide powders modified with cobalt, so-called cobalt-containing iron oxides, have been used as magnetic powders for magnetic recording, but in order to obtain higher output media, Metal magnetic powders with higher coercive force and saturation magnetization, so-called metal magnetic powders, are beginning to be used.

Various methods have been proposed for producing metal magnetic powder, but generally speaking, due to economic advantages, acicular goethite or iron oxide particles obtained by heating and dehydrating it are processed using hydrogen, etc. A method of reducing metal iron by heating in a reducing gas atmosphere is used, but since reduction is performed at high temperatures in this method, particles tend to fuse together and the shape collapses, so the performance is not fully satisfactory. Since this is not possible, various proposals have been made. An example of this is when goethite is treated with water glass.
A method in which reduction is carried out by firing (Japanese Patent Publication No. 63-49722), and a method in which a silicon compound is attached to the surface of goethite after it is heated and dehydrated (Japanese Patent Application Laid-open No. 59-1988)
No. 80901), goethite is coated with aluminum phosphate and subjected to thermal reduction (Japanese Unexamined Patent Publication No. 63-6
7705) etc.

However, these proposals can only solve the problem when the particle size is relatively large, and are not satisfactory in the case of fine metal magnetic powder particles close to 60 m/g, which correspond to recent high-density recording.

There is another problem with fine metal magnetic powder. In other words, metal magnetic powder has the drawback that it is chemically unstable and its saturation magnetization decreases over time. Various proposals have been made to solve this drawback, and the present inventors have also developed a surface transition metal layer. It was discovered that oxidation resistance was improved by the formation of
3-62205, Japanese Patent Application Laid-Open No. 1-164006), these solutions show remarkable effects when the particle system is relatively thick, but when it comes to fine particles, they are superior to those without these treatments. Although it shows oxidation resistance, it has not yet achieved a fully satisfactory effect.

The purpose of the present invention is to prevent sintering of particles together during the production stage of such fine-grained metal magnetic powder, and to provide a method for producing metal magnetic powder that exhibits excellent magnetic properties and excellent oxidation resistance, and a method using the same. The present invention aims to provide a coating film for magnetic recording media.

[Failure to solve the problem]

The present inventors investigated the process of obtaining metal magnetic powder from goethite, and found that reduction was interrupted at the stage of magnetite produced during reduction, and a non-ferrous transition metal compound was formed on the surface of this magnetite. By forming a silicon compound layer on top of the silicon compound layer for reduction to metal magnetic powder, it is easy to prevent sintering and maintain the shape during reduction, and the oxidation resistance of the obtained metal magnetic powder is further improved. The present inventors have discovered that magnetite is excellent, and furthermore, that the effect can be further exhibited by using magnetite having a specific history, leading to the completion of the present invention.

That is, the present invention forms a silicon and/or aluminum compound layer on the surface of acicular goethite, reduces and dehydrates this to form magnetite, forms a nonferrous transition metal compound layer on the surface, and further forms a silicon compound layer. The present invention relates to a method for producing metal magnetic powder, which is characterized by forming and reducing metal magnetic powder.

According to the present invention, it is possible to obtain a fine metal magnetic powder with an excellent shape, and especially when obtaining magnetite from goethite, it is possible to obtain
As shown in Publication No. 901, instead of heating and dehydrating or firing in a non-reducing atmosphere and then thermally reducing, a silicon and/or aluminum compound layer is formed on the surface of goethite and then directly reduced. By using magnetite, the performance of the finally obtained metal magnetic powder can be improved.

As in the method of the present invention, the reduction is stopped at the magnetite stage, and a non-ferrous transition metal element compound layer is formed on this magnetite, and then a silicon compound layer is further formed on the magnetite to perform reduction to metal magnetic powder. Although it is not clear why the oxidation resistance of the metal magnetic powder is even better, and the reason why the performance of the metal magnetic powder improves when it is reduced to magnetite without heat dehydration or calcination in a non-reducing atmosphere is unclear. , magnetite with a non-ferrous transition metal compound layer formed thereon is more likely to form a dense layer than goethite or magnetite (an oxide of magnetite) with a non-ferrous transition metal layer formed thereon; It is assumed that one of the reasons for this is that the particles become fused and their shape collapses due to the high temperature.

The axial ratio and size of the acicular goethite used in the present invention may be as long as it is generally used as a raw material for metal magnetic powder, but the effect of the present invention is more noticeable when the particle size is small. .

The goethite referred to in the present invention is a powder mainly composed of hydrated iron oxide expressed by the general formula Fe0O1l, and has a lattice spacing of 4.18
±0.05.2.69±0.05.2.45±0.05
It has a main peak in X-ray diffraction at a position corresponding to angstroms, and when heated in the atmosphere, it loses weight by about 12% and produces hematite, which contains a small amount of metal elements other than iron. There is no problem even if it is included. Also,
The magnetite referred to in the present invention is an oxide mainly composed of iron oxide, and the interplanar spacing is 2.97±0.05.2.53±0.0.
5.2.It has a main peak in X-ray diffraction at a position corresponding to 10 ± 0.05 angstroms, and increases in weight by 2.5% or more when heated in the atmosphere. This refers to a state in which no diffraction peaks corresponding to goethite, hematite, or metallic iron are observed.

These typical powder X-ray diffraction patterns are shown in FIG. In FIG. 1, (8) is a diffraction pattern diagram of goethite, (B) is a diffraction pattern diagram of magnetite, and (C) is a diffraction pattern diagram of metallic iron.

In the present invention, silicon and/or
Alternatively, the aluminum compound layer is formed by precipitation of insoluble matter from a solution state, deposition of a colloidal compound, or the like. Specific examples include a method in which an aqueous solution of a water-soluble compound such as water glass or sodium aluminate is added to a slurry of goethite, and then insoluble hydroxides are precipitated by adjusting the pH of the system, calcium salts, phosphates, etc. Examples include a method of precipitating an insoluble salt, and a method of adding a metal alkoxide such as tetraethoxysilane or triisopropoxyaluminum to a goethite slurry to precipitate a hydrolyzate. When a method of precipitating insoluble hydroxide is used, the silicon and/or aluminum compound layer can be easily formed by keeping the system heated for 30 minutes to 24 hours after the p It RFj section 4.

The silicon compound layer and the aluminum compound layer may be formed singly or in combination, but in particular, when the aluminum compound layer is formed after the silicon compound layer is formed, the performance of the finally obtained metal magnetic powder is excellent.

The amount of the silicon and/or aluminum compound layer is 2 to 10%, 4 to 15%, preferably 2.5 to 7%, and 6 to 12%, respectively, as an atomic ratio of silicon and aluminum to iron atoms in goethite. . When a silicon compound and an aluminum compound are used together, the ratio of silicon to aluminum does not need to be particularly regulated; however, when forming an aluminum compound layer after forming a silicon compound layer, the atomic ratio of silicon/aluminum = 1/ 2 to about 115 exhibits the effect of lamination.

By forming a calcium compound layer prior to forming such a silicon and/or aluminum compound layer, it becomes easier to control the magnetic properties, particularly the coercive force, of the finally obtained metal magnetic powder.

The calcium compound layer is formed, for example, by adding a calcium acetate solution to a goethite slurry and then neutralizing it with an alkali.

In the present invention, the content of each element is analyzed by dissolving the sample powder in hydrochloric acid for Fe and non-ferrous transition metal elements;
, AI, and Ca are melted in an alkali to form a homogeneous solution and then subjected to TCP emission spectrometry.

In the method of the present invention, the reduction of goethite to magnetite is carried out by reducing goethite 1-1 after forming a silicon and/or aluminum compound layer to 250 to 350
This is done by keeping the temperature at °C. At this time, heat dehydration may be performed in air prior to reduction, but direct heat dehydration in a reducing atmosphere is preferred in order to obtain metal magnetic powder with better characteristics.

In the present invention, the formation of a non-ferrous transition metal compound layer on the magnetite surface is carried out by precipitation of insoluble matter from a solution state, deposition of a colloidal compound, etc., similar to the formation of a silicon and/or aluminum compound layer. As a specific example, a method is used in which an aqueous solution of a water-soluble transition metal compound such as cobalt sulfate or nickel sulfate is added to an alkaline slurry of magnetite to precipitate cobalt hydroxide or nickel hydroxide. At this time, it is also effective to use a ferrous salt such as ferrous sulfate to simultaneously precipitate ferrous hydroxide. In this case as well, the transition metal compound layer is firmly formed by keeping the system in a heated state after hydroxide precipitation.

For the subsequent formation of a silicon compound layer, the same method as for goethite is used.

The amount of the non-ferrous transition metal compound layer formed on the magnetite surface is determined by the amount of iron in the metal magnetic powder (combined iron used to form the non-ferrous transition metal compound layer and iron in magnetite) as a non-ferrous transition metal element. amount) of 15 atom % or more, preferably 20 atom % or more. If the non-ferrous transition metal element is less than 15 at %, oxidation resistance will be insufficient. Furthermore, there is no need to specify an upper limit for the amount of nonferrous transition metal elements, but care must be taken because if the amount exceeds 50 at. be.

As the non-ferrous transition metal element used in the present invention, cobalt or nickel is particularly preferable in terms of imparting oxidation resistance.

1 2 The outermost silicon compound layer of the metal magnetic powder of the present invention preferably has an atomic ratio of 1 to 4% with respect to the nonferrous transition metal compound layer. If the amount of the silicon compound layer is too little or too much than this range, it becomes difficult to obtain preferable magnetic properties.

The magnetite having a non-ferrous transition metal compound layer and a silicon compound layer thus obtained can be reduced as is, but it can also be further treated with a thermosetting resin such as a phenol resin or a furan resin. Examples of treatments with thermosetting resins include water-soluble organic solvents (acetone,
This is done by adding a solution (such as ethanol) to the magnetite slurry to insolubilize it.

The coating film for a magnetic recording medium of the present invention containing the metal magnetic powder excellent in oxidation resistance and magnetic properties obtained as described above is produced according to a conventional method. For example, a magnetic paint is prepared by dispersing and mixing this metal magnetic powder with a binder resin, an organic solvent, and other necessary components, and this magnetic paint is coated onto a substrate such as a polyester film using the doctor blade method, graphia, or reverse method. , by any means such as roll coating, and if necessary, after orientation in a magnetic field, drying.

Here, as the binder resin, polyvinyl chloride resin,
Conventional binder resins such as vinyl chloride-vinyl acetate copolymers, cellulose resins, butyral resins, polyurethane resins, polyester resins, epoxy resins, polyether resins, isocyanate compounds, etc. is also used.

Examples of organic solvents include ketone solvents such as cyclohexanone, methyl ethyl ketone, and methyl isobutyl ketone, ester solvents such as ethyl acetate and butyl acetate, aromatic hydrocarbon solvents such as Hensen, toluene, and xylene, and sulfoxides such as dimethyl sulfoxide. solvent,
Solvents suitable for dissolving the binder resin used, such as ether solvents such as tetrahydrofuran and dioxane, may be used alone or in combination of two or more without particular limitation.

In addition, various additives commonly used in magnetic paints,
For example, a lubricant, an abrasive, an antistatic agent, etc. may be added as appropriate.

The magnetic recording medium coating film thus formed can be cut into a tape or disk shape depending on the intended use, and assembled into a tape or disk shape to be used as a highly reliable, high-performance magnetic recording medium.

〔Example〕

EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to these Examples.

Example 1 1 kg of goethite (long axis diameter: 0.18 pitch, axial ratio: 8) was mixed with 1 kg of a 3% aqueous solution of Boyz 530 (manufactured by Kao ■). After dispersing and stirring for about 1 hour with a TK homomixer SL model (manufactured by Tokushu Kika Kogyo ■),
9%) was added thereto, and stirring was continued for an additional hour. After that, dilute nitric acid was added to lower the pH of the system to 6.5, and after refluxing for 1 hour with stirring, the system was filtered, washed, and dried to form goethite (Si/Fe atomic ratio -3,1/ 10
0) was obtained.

This X-ray diffraction diagram is shown in (8) of FIG. For X-ray diffraction, RAD-RC manufactured by Rigaku Denki ■ was used, and the tube voltage was 55 kv.
Tube current 200mA CuK, slit width DS IR
3O,3,SS 1, scan speed 1°/m i
I went with n.

This silicon compound-treated goethite was heated in a retort furnace (inner volume: 30ρ) with hydrogen/nitrogen-1/1 mixed gas at 50 p, 7
The temperature was raised to 300°C at a rate of 2.5'C/min while flowing at 2.5'C/min, and then kept at 300°C, and reduction was performed until the peaks of goethite and hematite were no longer seen in X-ray diffraction to obtain magnetite. This X-ray diffraction diagram is shown in FIG. 1(B).

Next, add 500g of this magnetite to 370g of caustic soda.
Aqueous solution 23 containing 725 g of ferrous sulfate heptahydrate and 490 g of cobalt sulfate heptahydrate while blowing nitrogen gas into the slurry dispersed in the aqueous solution 2.71 containing g! After adding 4
The reaction was carried out at 0°C for 6 hours, and then the temperature was raised and refluxed for 6 hours to form a 56 cohalt compound layer (Co/Fe atomic ratio -20/100) on the surface.
After cleaning, the voids are 530 and 15
After adding 35 g of water glass and thoroughly dispersing it, the pH of the system was adjusted to 5.5 with dilute nitric acid, and a silicon compound (Si
/ (Co+Fe) atomic ratio -1.6/100) was deposited.

The metal magnetic powder precursor obtained as above was sized into 48 to 64 meshes, and heated at 450°C for 7 hours in a hydrogen stream with a gas linear velocity of 7 cm/sec in a fluidized bed furnace with an inner diameter of 6.2+++n+.
Reduction was performed at 500°C for 3 hours. After completion of the reduction, the mixture was cooled to 30° C. in a nitrogen stream, and then the oxygen concentration was gradually increased from 500 ppm at a gas linear velocity of 7 cm/sec, and finally the atmosphere was released to obtain metal magnetic powder 1.

Next, the following paint composition was mixed for 6 hours in a batch-type sand mill, and 2.5 parts by weight of Coronate (manufactured by Nippon Polyurethane Industries, Ltd.) was added to the mixture, and after further mixing for 15 minutes, it was filtered to prepare glass. The peas were separated and the magnetic paint was prepared. This coating material was applied onto a PET film having a thickness of 10 mounds so that the dry film thickness was 3 mounds, and after magnetic field orientation treatment, it was dried to form a magnetic layer on the PET film. Next, a mirror finish was obtained by calendering to obtain a coating film 1.

Tables 1 and 2 show the magnetostatic properties of the obtained metal magnetic powder and coating film.
Shown below.

<Paint formulation> Metal magnetic powder 100 parts by weight lecithin
2 Carbon black 3 〃 T-Alumina 5 VAGH” 15 Nimboran 23
04" 10 〃Methyl ethyl ketone
150〃 Toluene 50〃 Cyclohexanone 75〃 Note) *1: Manufactured by Union Carbide, vinyl chloride/vinyl acetate/polyvinyl alcohol copolymer *2: Manufactured by Nippon Polyurethane Industry ■, polyurethane resin Example 2 3 in Example 1 Instead of No. Keiso, aqueous sulfuric acid solution (AhOi content 2.1%) 3! Goethite (Al/Fe atomic ratio -11,4/100) having an aluminum compound layer was obtained in the same manner as in Example 1 except that the pl+ of the system was set to 7.0 with an ammonia aqueous solution. After reduction to magnetite in a retort furnace in the same manner as in Example 1, a cobalt compound layer (Co/Fe atomic ratio -20/100), followed by a silicon compound layer (St/
(Co+Fe) atomic ratio=1.6/100) was formed, and reduction and air oxidation were performed to obtain metal magnetic powder 2 and coating film 2.

Tables 1 and 2 show the magnetostatic properties of the obtained metal magnetic powder and coating film.
Shown below.

Example 3 As in Example 1, 1 kg of Gesai) was added to Poise 530 (
After dispersing and stirring the mixture in a 3% aqueous solution (manufactured by Kao ■) for about 1 hour using a TK homomixer SL model (manufactured by Tokushu Kika Kogyo ■), 70 g of No. 3 Keiso (29% SiO□) was added. , stirring was continued for an additional hour.

Thereafter, dilute nitric acid was added to lower the pH of the system to 6.5, and the mixture was refluxed with stirring for 1 hour. Then, cool to below 50°C, sulfuric acid hand aqueous solution (to 1□08 minutes 2.1%) 3f
f and bring the system's pl+ to 7.0 with an ammonia aqueous solution.
After that, it is refluxed again, filtered, washed, and dried to produce goethite (Al/Si/Fe atomic ratio -11, 4/3.1.
/100) was obtained.

Thereafter, in the same manner as in Example 1, magnetite was reduced in a furnace, and then a cobalt compound layer (C) was formed.

/Pe atomic ratio -20/100), followed by a silicon compound layer (Si/(Co+Fe) atomic ratio -1.6/100), and reduction and air oxidation were performed to obtain metal magnetic powder 3 and further coating film 3. Ta.

Tables 1 and 2 show the magnetostatic properties of the obtained metal magnetic powder and coating film.
Shown below.

Examples 4 to 13 Goethite with a silicon and/or aluminum compound layer formed thereon was reduced to magnetite in the same manner as in Examples 1 to 3 by changing the amount of silicon and aluminum, and then by changing the amount of cobalt. After forming the cobalt compound layer, changing the amount of silicon to form a silicon compound layer and reducing it to obtain metal magnetic powders 4 to 13,
Furthermore, coating films 4 to 8 were obtained using metal magnetic powders 4 to 8.

Tables 1 and 2 show the magnetostatic properties of the obtained metal magnetic powder and coating film.
Shown below.

Comparative Example 1 The silicon compound layer-containing goethite obtained by the same operation as in Example 1 was reduced to magnetite, and then boiled in hydrogen at 350 °C.
The metal magnetic powder 21 was obtained by reduction at °C. This X-ray diffraction diagram is shown in FIG. 1(C).

Using this metal magnetic powder 21, a coating film 21 was obtained in the same manner as in Example 1.

Tables 1 and 2 show the magnetostatic properties of the obtained metal magnetic powder and coating film.
Shown below.

The metal magnetic powder 21 is very easy to rust and is heated at 60°C.
/90%R) σs is 85emu/g after 11 weeks storage
, Hc was also less than 8000e, and it was so unstable that it was essentially no longer a metal magnetic powder.

Comparative Example 2 Magnetite was obtained by reducing silicon compound layer-containing Gesai 1- obtained in the same manner as in Example 1.

This magnetite was added again to a 3% aqueous solution of Boies 530.
Spread out time. To this slurry, add 50 g of Resin H (phenolic resin manufactured by Marukubi Petrochemical ■) and 0.3 I of ethanol!
After dropping a solution dissolved in , and stirring for 1 hour, the mixture was filtered, washed, and dried to obtain magnetite coated with phenol resin. This magnetite is heated in a hydrogen stream in a retort I furnace.
Reduction was performed at 50°C to obtain metal magnetic powder 22, and further coating film 2 was obtained.
I got 2. Similar to Comparative Example 1, this metal magnetic powder 22 was also unstable.

Tables 1 and 2 show the magnetostatic properties of the obtained metal magnetic powder and coating film.
Shown below.

Comparative Example 3 575 g of silicon compound layer-containing goethite obtained in the same manner as in Example 1 was dissolved in water M2.7 containing 370 g of caustic soda.
While blowing nitrogen gas into the slurry dispersed in ff, a sulfuric acid cylinder - 725 g of iron heptahydrate, 490 g of cobalt sulfate superhydrate
Aqueous solution containing 23! was added, and reacted at 40"C for 6 hours to form a cobalt compound layer (Co/Fe atomic ratio -20") on the surface.
/100), and after cleaning, Boise 5
After adding 15 g of No. 30 and 35 g of water glass and sufficiently dispersing them, the PU of the system was adjusted to 5.5 with dilute nitric acid, and a silicon compound (S!/(Go+Fe) atomic ratio -1.6/100) was deposited.

Example 1 Using the metal magnetic powder precursor obtained as described above
Metal magnetic powder 23 was obtained by reduction in the same manner as above.

Table 1 shows the magnetostatic properties of the obtained metal magnetic powder.

The obtained metal magnetic powder 23 has Hc; 14200s,
σ5; 134 emu/g, Rs; 0.44
According to TEM observation, the particles were strongly fused together. Furthermore, the oxidation resistance was also insufficient.

Comparative Example 4 The aluminum compound layer-containing goezite obtained in the same manner as in Example 2 was reduced to obtain metal magnetic powder 24.

Table 1 shows the magnetostatic properties of the obtained metal magnetic powder.

The obtained metal magnetic powder has Hc; 14400e, σ8
; 140 emu/g, Rs; 0.46, and observation by TEM showed that particles were strongly fused together. Furthermore, the oxidation resistance was also insufficient.

Comparative Example 5 A co compound layer was formed on magnetite containing a silicon and aluminum compound layer in the same manner as in Example 3, and a metal magnetic powder precursor obtained by washing and drying without forming a silicon compound layer was used. A metal magnetic powder 25 was obtained by reduction in the same manner, and a coating film 25 was further obtained.

Tables 1 and 2 show the magnetostatic properties of the obtained metal magnetic powder and coating film.
Shown below.

The obtained metal magnetic powder has Ilc; 14100e, σ
8; 140 emu/g, Rs; 0.43, and observation by TIEM showed that the particles were severely fused together.

Comparative Example 6 3 4 A silicon compound layer-containing goezite obtained in the same manner as in Example 1 was kept at 300°C in air for 2 hours, dehydrated, and then heated to 500°C.
The mixture was kept at °C for 3 hours to obtain hematite containing a silicon compound layer, which was then reduced to magnetite using a hydrogen/nitrogen mixed gas. Using this magnetite, a Go compound layer and a silicon compound layer were formed and hydrogen reduction was performed in the same manner as in Example 1 to obtain a metal magnetic powder 26 and a coating film 26.

Tables 1 and 2 show the magnetostatic properties of the obtained metal magnetic powder and coating film.
Shown below.

*B.S. :60°C/90%RH Bs after storage for 30 days

[Brief explanation of drawings]

Figure 1 is an X-ray diffraction diagram, in which (8) is the goethite obtained in Example 1, (B) is the magnetite obtained in Example 1, and (C) is the one obtained in Comparative Example 1. It is an X-ray diffraction diagram of metal magnetic powder.

Claims (5)

[Claims] 1. Forming a silicon and/or aluminum compound layer on the surface of acicular goethite, reducing and dehydrating this to form magnetite, and forming a non-ferrous transition metal compound layer on the surface,
A method for producing metal magnetic powder, which further comprises forming a silicon compound layer and reducing it. 2. The method for producing metal magnetic powder according to claim 1, characterized in that in the step of forming a silicon and aluminum compound layer on the surface of the acicular goethite, the silicon compound layer is first formed, and then the aluminum compound layer is formed subsequently. . 3. In the formation of the non-ferrous transition metal compound layer, the amount of the non-ferrous transition metal element in the formed layer is 15 atomic % or more and 50 atomic % or less with respect to the amount of iron in the metal magnetic powder. A method for producing a metal magnetic powder according to claim 1 or 2. 4. A metal magnetic powder produced by the method according to claim 1, 2 or 3. 5. A coating film for a magnetic recording medium, comprising a metal magnetic powder produced by the method according to claim 1, 2 or 3.