WO2018193900A1 - Composite magnetic material, motor and method for producing composite magnetic material - Google Patents

Abstract

A composite magnetic material which contains a soft magnetic material S and a hard magnetic material H, and which is characterized in that: the soft magnetic material S contains iron or an iron alloy; and at least a part of the surface of the soft magnetic material S is covered by a crystalline iron oxide.

Description

Composite magnetic material, motor, and method of manufacturing composite magnetic material

The present invention relates to a composite magnetic material, a motor, and a method for manufacturing the composite magnetic material.

As a high-performance magnet, a neodymium magnet (composition: Nd ₂ Fe ₁₄ B or the like) is known. Neodymium magnets are widely used because of their large residual magnetic flux density and coercive force.

Neodymium magnets contain neodymium, a rare earth element, as an essential component. Since rare earth elements are expensive and may be unstable in supply, there is a demand for suppressing the amount of rare earth elements used. Therefore, attempts have been made to produce high-performance magnets while suppressing the amount of rare earth elements used.

JP 2011-35006 A (Patent Document 1) discloses a hard magnetic phase core containing epsilon iron oxide (ε-Fe ₂ O ₃ ), alpha iron (α-Fe), and at least one of the cores. A core-shell type magnetic material having a soft magnetic phase shell covering the portion is described. In Patent Document 1, ε-Fe ₂ O _{3 is used} as a hard magnetic phase having a high coercive force, and α-Fe is used as a soft magnetic phase having a high saturation magnetic flux density, and both are magnetically coupled by an exchange coupling action. A composite magnet is manufactured.

In a magnetic material using iron or iron alloy, iron or iron alloy may be exposed on the surface of the magnetic material. This is particularly noticeable when iron or an iron alloy is used as a shell of a core-shell type magnetic material as described in Patent Document 1.

Iron and iron alloys are easily oxidized by air and moisture. Therefore, if the iron or iron alloy constituting the magnetic material is exposed on the surface, it is oxidized by air or moisture, and the magnetic properties of the magnetic material are deteriorated. That is, the composite magnetic material containing iron or an iron alloy has a problem of low stability over time.

Therefore, in view of the above-described problems, an object of the present invention is to provide a magnetic material having high temporal stability, which is a composite magnetic material containing iron or an iron alloy.

A composite magnetic material as one aspect of the present invention is a composite magnetic material containing a soft magnetic material and a hard magnetic material, wherein the soft magnetic material contains iron or an iron alloy, and at least a surface of the soft magnetic material A part is covered with crystalline iron oxide.

It is a figure which shows typically the structure of the composite magnetic material which concerns on 1st Embodiment. It is a figure which shows typically the structure of the composite magnetic material which concerns on 1st Embodiment. It is a flowchart which shows the manufacturing method of the composite magnetic material which concerns on 1st Embodiment. It is a figure which shows typically the structure of the composite magnetic material which concerns on 2nd Embodiment. It is a figure which shows typically the structure of the composite magnetic material which concerns on 2nd Embodiment.

Hereinafter, embodiments of the present invention will be described. It should be noted that the present invention is not limited to the following embodiments, and is appropriately modified with respect to the following embodiments based on ordinary knowledge of those skilled in the art without departing from the spirit of the present invention. Those with improvements and the like are also included in the scope of the present invention.

(First embodiment)
The composite magnetic material according to the present embodiment includes a soft magnetic material and a hard magnetic material, the soft magnetic material includes iron or an iron alloy, and at least a part of the surface of the soft magnetic material is coated with crystalline iron oxide. Has been.

Here, in this specification, the “soft magnetic material” refers to a material having a small coercive force and a large saturation magnetic flux density. In the present specification, “hard magnetic material” refers to a material having a large coercive force.

The composite magnetic material according to the present embodiment has a fine structure in which two phases of a soft magnetic material phase (soft magnetic phase) and a hard magnetic material phase (hard magnetic phase) are adjacent to each other in the order of nm (nanometer). Has a mixed structure. By having such a fine mixed structure, an exchange coupling action can be exerted between the soft magnetic phase and the hard magnetic phase. When an exchange coupling action is acting between the soft magnetic phase and the hard magnetic phase, when the switching magnetic field is applied, the magnetization reversal of the soft magnetic phase is suppressed by the magnetization of the exchanged hard magnetic phase. At this time, the magnetization curve behaves as if the soft magnetic phase and the hard magnetic phase are single-phase magnets due to the exchange coupling action. For this reason, a magnetization curve having both a large saturation magnetic flux density of the soft magnetic phase and a large coercivity of the hard magnetic phase is realized. As a result, a high energy product (BH) _max can be realized. A magnet in which an exchange coupling action is exerted between the soft magnetic phase and the hard magnetic phase as described above is known as a nanocomposite magnet or an exchange spring magnet.

FIG. 1 is a diagram schematically showing a structural example of a composite magnetic material according to the first embodiment. As shown in FIG. 1A and FIG. 1B, the composite magnetic material 101 according to the present embodiment has a sea-island structure having an island portion containing a hard magnetic material H in a sea portion containing a soft magnetic material S. Further, the composite magnetic material 101 includes crystalline iron oxide O that covers at least a part of the surface of the soft magnetic material S.

(Soft magnetic material S)
The soft magnetic material S includes iron or an iron alloy. The soft magnetic material S preferably contains α-Fe (alpha iron) or an FeM alloy. Here, M represents at least one element selected from the group consisting of Co, Ni, Al, Ga, and Si, and the composition ratio of each element in the FeM alloy can be arbitrarily selected. Among these, the soft magnetic material S preferably contains α-Fe, and is particularly preferably made of α-Fe. Note that the iron or iron alloy included in the soft magnetic material S does not necessarily have crystallinity.

The soft magnetic material S is a material having a saturation magnetic flux density larger than that of the hard magnetic material H. The saturation magnetic flux density of the soft magnetic material S is not particularly limited, but is preferably 50 emu / g or more, and more preferably 100 emu / g or more.

(Hard magnetic material H)
The hard magnetic material H is a material having a larger coercive force than the soft magnetic material S. The coercive force of the hard magnetic material H is not particularly limited, but is preferably 500 Oe or more, and more preferably 1000 Oe or more.

The hard magnetic material H preferably contains ε-Fe ₂ O ₃ (epsilon iron oxide). Since ε-Fe ₂ O ₃ is a material having a particularly large coercive force among iron-based oxide materials, the hard magnetic material H contains ε-Fe ₂ O ₃ , so that the energy product of the composite magnetic material 101 ( BH) _max can be further increased.

If hard magnetic material H comprises _ε-Fe 2 _{O 3,} a portion of the Fe atoms in the _ε-Fe 2 _{O 3} may be substituted by other metal elements. A part of Fe atoms in ε-Fe ₂ O ₃ may be substituted with at least one element selected from the group consisting of Co, Ni, Al, and Ga. Also, if the hard magnetic material H comprises _ε-Fe 2 _{O 3,} the content of _ε-Fe 2 _{O 3} in the hard magnetic material H is preferably 100 vol% or less than 50 vol%, 70 vol% More preferably, it is 100 volume% or less.

(Crystalline iron oxide O)
The crystalline iron oxide O according to this embodiment covers at least a part of the surface of the soft magnetic material S. The crystalline iron oxide O preferably covers 50% to 100% of the surface of the soft magnetic material S, more preferably 70% to 100%, more preferably 90% to 100%. % Or less is particularly preferred. In addition, the surface of the soft magnetic material S here refers to the surface portion of the soft magnetic material S exposed to the outside in a state where the crystalline iron oxide O is removed. In the present embodiment, since the soft magnetic material S forms the sea part in the sea-island structure, the above-mentioned “surface of the soft magnetic material S” can be rephrased as “the surface of the sea part”.

Since the soft magnetic material S contains iron or an iron alloy as described above, it is easily oxidized or corroded by oxygen, moisture, etc. in the atmosphere when placed in contact with the atmosphere. The characteristics will deteriorate. In particular, when the soft magnetic material S and the hard magnetic material H are magnetically coupled in the composite magnetic material 101, when the saturation magnetic flux density of the soft magnetic material S decreases, the saturation magnetic flux of the composite magnetic material 101 as a whole. As the density decreases, the coercive force also decreases because the exchange coupling force decreases. However, in this embodiment, at least a part of the surface of the soft magnetic material S is covered with crystalline iron oxide O. Crystalline iron oxide O acts as a protective layer and can suppress the oxidation or corrosion of the soft magnetic material S. Thereby, the fall of the magnetic characteristic of the soft magnetic material S can be suppressed, and the temporal stability of the composite magnetic material 101 can be improved.

The crystalline iron oxide O preferably forms a dense film that covers the surface of the soft magnetic material S. Thereby, the penetration | invasion of oxygen and a water | moisture content in air | atmosphere can be blocked, and the fall of the magnetic characteristic of the soft magnetic material S can be suppressed more effectively.

The thickness of the crystalline iron oxide O is preferably 5 nm or more and 500 nm or less, more preferably 5 nm or more and 200 nm or less, and further preferably 5 nm or more and 100 nm or less. By setting the thickness of the crystalline iron oxide O to 5 nm or more, rapid oxidation and corrosion of the soft magnetic material S can be suppressed, and deterioration of the magnetic properties of the soft magnetic material S can be suppressed. Moreover, since the saturation magnetic flux density of the composite magnetic material 101 will fall if the thickness of crystalline iron oxide O is too thick, it is preferable not to be too thick. Specifically, by setting the thickness of the crystalline iron oxide O to 500 nm or less, it is possible to suppress a temporal decrease in the magnetic characteristics of the soft magnetic material S without greatly reducing the saturation magnetic flux density. it can.

The crystalline iron oxide O is not particularly limited as long as it has crystallinity, but is preferably Fe ₃ O ₄ (magnetite). Fe ₃ O ₄ has a particularly high effect of blocking the intrusion of oxygen and moisture in the atmosphere among crystalline iron oxides, and can suppress the deterioration of the magnetic properties of the soft magnetic material S more effectively.

As described above, α-Fe (alpha iron) or an FeM alloy is preferably used as the soft magnetic material S, but Fe ₃ O ₄ which is crystalline iron oxide O obtained by oxidizing this from the surface. (Magnetite) also functions as a soft magnetic material. Therefore, the crystalline iron oxide O has a function of protecting the soft magnetic material S and suppressing oxidation or corrosion, and magnetically couples with the hard magnetic material H, thereby exhibiting magnetic properties as a whole of the composite magnetic material. It also has a function to When a protective layer for suppressing oxidation is formed on the surface with silica or resin as in the past, silica and resin do not have a function as a magnetic material, so the magnetic characteristics of the composite magnetic material as a whole are large. It will decline. However, according to the present embodiment, the protective layer can also have a function as a magnetic material, and a composite magnetic material having a high stability over time can be realized without greatly degrading the magnetic properties of the entire composite magnetic material. can do.

In the present embodiment, the composite magnetic material 101 includes not only the soft magnetic material S that contains iron or an iron alloy and is easily oxidized or corroded, but also is typically an oxide that is hard to be oxidized or corroded. H is also included. Therefore, the progress of oxidation and corrosion is slower than in the case of a magnetic material composed of only the soft magnetic material S that is easily oxidized or corroded. As a result, a composite magnetic material with high stability over time can be realized even with crystalline iron oxide O having a relatively thin thickness of 5 nm to 500 nm as described above. In addition, since the progress of oxidation and corrosion is relatively slow, even if the heating temperature or the heating time for forming the crystalline iron oxide O by oxidizing the soft magnetic material S is increased, the softness can be increased. Only the vicinity of the outermost surface of the magnetic material S can be efficiently oxidized. Therefore, the crystallinity of the crystalline iron oxide O to be formed and the denseness of the film can be further increased, and a composite magnetic material having high transit stability can be realized.

(Constituent elements of composite magnetic materials)
In the composite magnetic material 101 according to this embodiment, when the total amount of the composite magnetic material 101 is 100% by mass, the content of the Nd element is preferably 0% by mass or more and 3% by mass or less, and 0% by mass or more. More preferably, it is 1 mass% or less. It is particularly preferable that the composite magnetic material 101 does not substantially contain an Nd element. Thus, the cost of the composite magnetic material 101 can be reduced by reducing the content of the Nd element in the composite magnetic material 101.

(Construction)
The composite magnetic material 101 according to the present embodiment has a sea-island structure having a sea part including the soft magnetic material S and an island part including the hard magnetic material H.

In this embodiment, the sea portion includes the soft magnetic material S and the island portion includes the hard magnetic material H. However, the sea portion includes the hard magnetic material H and the island portion includes the soft magnetic material S. Good. In this case, when a part of the island part is exposed from the sea part as shown in FIG. 1B, it is sufficient that at least part of the surface of the exposed island part is covered with the crystalline oxide O. .

It is preferable that the soft magnetic material S and the hard magnetic material H are magnetically coupled by an exchange coupling action. Therefore, when the distance at which the exchange coupling action works from the interface between the island and the sea (hereinafter referred to as “exchange coupling distance”) is a, in the composite magnetic material 101, the average between two adjacent islands The distance d preferably satisfies d ≦ 2a. That is, it is preferable that the average distance between two adjacent islands is not more than twice the exchange coupling distance.

When the soft magnetic material S contains α-Fe, the average distance d between two adjacent islands is preferably 2 nm or more and 20 nm or less.

It is preferable that the average particle size of the particulate island portion including the hard magnetic material H is so large that the coercive force of the hard magnetic material H does not decrease. Also, if the hard magnetic material H comprises ε-Fe ₂ O _3, an average particle size of the particulate island portion comprising hard magnetic material H is the extent to which ε-Fe ₂ O ₃ it is possible to maintain the epsilon structure Small is preferable. Specifically, the average particle diameter of the particulate island portion containing the hard magnetic material H is preferably 5 nm or more and 60 nm or less, and more preferably 10 nm or more and 40 nm or less.

(Production method of composite magnetic material)
FIG. 2 is a flowchart showing a method for manufacturing a composite magnetic material according to this embodiment. In the method for manufacturing a composite magnetic material according to the present embodiment, a first step (S201) for forming a precursor material having a soft magnetic material S and a hard magnetic material H, and a second step for oxidizing the precursor material (S201). S202). Hereinafter, these steps will be described.

[1] First Step of Forming Precursor Material Having Soft Magnetic Material S and Hard Magnetic Material H This step is a precursor material having a soft magnetic material S containing iron or an iron alloy and a hard magnetic material H. Is a step of forming.

This step may be a step of preparing particles of the soft magnetic material S and particles of the hard magnetic material H, and mixing them at an appropriate mixing ratio. Alternatively, the precursor material may be formed by heat-treating (or firing) after mixing and compression molding these. The heat treatment is preferably performed in any of an inert gas atmosphere, a reducing atmosphere, and a vacuum.

When α-Fe is used as the soft magnetic material S, iron oxide or iron hydroxide nanoparticles are generated using a chemical process in solution, and the generated nanoparticles are heat-treated in a reducing atmosphere to form α -Fe nanoparticles can be synthesized relatively easily. Further, α-Fe nanoparticles can be directly synthesized without passing through iron oxide or iron hydroxide by adding a reducing agent such as NaBH ₄ to a solution containing iron ions to reduce iron ions.

When ε-Fe ₂ O ₃ is used as the hard magnetic material H, iron oxide or iron hydroxide nanoparticles are generated using a chemical process in solution, and the generated nanoparticles are heated in an oxidizing atmosphere. Thus, ε-Fe ₂ O ₃ particles can be synthesized relatively easily. As the chemical process in the solution, for example, a reverse micelle method or a sol-gel method using iron nitrate hydrate as a starting material can be used. In the step of synthesizing the _ε-Fe 2 _{O 3} particles, the surface of the _ε-Fe 2 _{O 3} particles may be added step of coating with silica (SiO _2).

In addition, a dispersion in which the particles of the other material are dispersed in a solution in which the raw material of one of the soft magnetic material S and the hard magnetic material H is dissolved is prepared. Alternatively, a method of precipitating magnetic material particles or precursor particles thereof may be used. Thereafter, the obtained composite particle powder may be heat-treated.

For example, particles of hard magnetic material H (hard magnetic particles) are dispersed in a solution in which at least one transition metal element contained in the soft magnetic material S is ionized and dissolved to obtain a dispersion. Thereafter, while stirring the dispersion, an additive such as a pH adjusting agent (typically a basic solution) or a reducing agent is added to the dispersion to precipitate the particles containing the transition metal. At this time, the particles to be precipitated may be particles of the intended soft magnetic material S, or may be precursor particles that can be converted into the soft magnetic material S by a subsequent heat treatment or the like. Since the hard magnetic particles are dispersed in the dispersion, the ions are present around the hard magnetic particles in the dispersion so as to surround the hard magnetic particles. In this state, the ions react to precipitate particles or precipitates containing the transition metal element in the ions, so that the particles or precipitates are deposited around the hard magnetic particles. Even if the soft magnetic material S and the hard magnetic material H are interchanged, the composite magnetic material can be formed by the same method.

For example, ammonia which is a pH adjuster in an aqueous solution containing Fe ³⁺ ions obtained by dissolving a raw material containing trivalent iron such as iron (III) chloride, iron (III) sulfate, or iron (III) nitrate in water. When water is added to change the pH, iron hydroxide (Fe (OH) ₃ ) can be precipitated. According to this method, the average particle size of the precipitated iron hydroxide particles depends on the deposition conditions, but is generally about 5 nm to 15 nm. By heat-treating the iron hydroxide particles in a reducing atmosphere, α-Fe particles that are the soft magnetic material S can be obtained.

Moreover, when ammonia water as a pH adjusting agent is added to an aqueous solution containing Fe ²⁺ ions obtained by dissolving a raw material containing divalent iron such as iron (II) chloride in water, Fe ₃ is changed. O ₄ particles can be precipitated. According to this method, although the average particle diameter of the precipitated Fe ₃ O ₄ particles depends on the deposition conditions, it is generally about 13 nm to 100 nm. By heat-treating the Fe ₃ O ₄ particles in a reducing atmosphere, α-Fe particles that are the soft magnetic material S can be obtained.

Further, when NaBH ₄ as a reducing agent is added to an aqueous solution containing Fe ²⁺ ions obtained by dissolving a raw material containing divalent iron such as iron (II) chloride in water to reduce Fe ²⁺ ions, α -Fe nanoparticles can be deposited directly.

[2] Second Step of Oxidizing Precursor Material This step is a step of oxidizing the precursor material obtained in the first step. Thereby, the soft magnetic material S exposed on the surface of the precursor material is oxidized to generate crystalline iron oxide.

As a method of oxidizing the precursor material, a method of heat-treating in an oxidizing atmosphere can be mentioned. As the oxidizing atmosphere, any one of air, water vapor, oxygen, and a mixed gas of oxygen and an inert gas (argon, nitrogen, helium) can be used.

Soft magnetic material S is easily oxidized because it contains iron or an iron alloy. Therefore, when it is taken out into the atmosphere, there is a possibility that oxidation starts to proceed at that time. Therefore, this step is preferably performed continuously from the first step.

When ε-Fe ₂ O ₃ is used as the hard magnetic material H, when ε-Fe ₂ O ₃ is heat-treated at a temperature higher than 800 ° C. in an oxidizing atmosphere, its crystal structure is transformed from the epsilon phase to the alpha phase. . Therefore, in this case, the temperature range in the heat treatment in the second step is preferably 200 ° C. or higher and 800 ° C. or lower, and more preferably 250 ° C. or higher and 700 ° C. or lower.

(magnet)
The composite magnetic material according to the present embodiment can be formed into a desired shape into a nanocomposite magnet. The nanocomposite magnet according to the present embodiment includes a soft magnetic material and a hard magnetic material, the soft magnetic material includes iron or an iron alloy, and the surface of the soft magnetic material is coated with crystalline iron oxide. The nanocomposite magnet according to the present embodiment may be a sintered magnet or a bonded magnet.

[1] Sintered magnet A composite magnet material according to the present embodiment is formed into a desired shape, and the obtained molded body is heat-treated in an inert atmosphere or under vacuum to obtain a sintered magnet. Moreover, a sintered magnet can be obtained also by sintering a molded object by plasma activated sintering (PAS: Plasma Activated Sintering) or discharge plasma sintering (SPS: Spark Plasma Sintering). Moreover, an anisotropic sintered magnet is obtained by shaping in a magnetic field.

[2] Bonded magnet A bonded magnet is obtained by blending and molding the composite magnetic material according to the present embodiment and a binder (binder). As the binder, a resin material such as a thermoplastic resin or a thermosetting resin, a low melting point metal such as Al, Pb, Sn, Zn, or Mg, or an alloy made of these low melting point metals can be used. The composite magnetic material can be formed into a desired shape by compression molding or injection molding the mixture of the composite magnetic material and the binder. An anisotropic bonded magnet can be obtained by molding the composite magnetic material in a magnetic field.

(motor)
The composite magnetic material according to the present embodiment can be suitably used as a material for forming a rotor (rotor) in a motor. That is, the motor according to the present embodiment includes a magnet, and the magnet includes the composite magnetic material according to the present embodiment.

(Second Embodiment)
FIG. 3 is a diagram schematically showing an example of the structure of the composite magnetic material according to the second embodiment. As shown in FIGS. 3A and 3B, the composite magnetic material 301 according to the present embodiment includes a core portion including the hard magnetic material H, a shell portion including the soft magnetic material S covering at least a part of the core portion, A core-shell structure. Further, the composite magnetic material 301 includes crystalline iron oxide O that covers at least a part of the surface of the soft magnetic material S. Descriptions similar to those in the first embodiment, such as the hard magnetic material H, the soft magnetic material S, and the crystalline iron oxide O included in the composite magnetic material 301, are omitted as appropriate.

(Construction)
The composite magnetic material 301 according to the present embodiment has a core-shell structure having a core portion including the hard magnetic material H and a shell portion including the soft magnetic material S that covers at least a part of the core portion. As shown in FIG. 3, the composite magnetic material 301 may be an aggregate of a plurality of core-shell particles. At this time, a closed gap that does not communicate with the outside may be formed inside the composite magnetic material 301. In this case, as shown in FIG. 3B, the composite magnetic material 301 may have a crystalline oxide O also on the surface of the void.

It is preferable that the soft magnetic material S and the hard magnetic material H are magnetically coupled by an exchange coupling action. Therefore, when the distance at which the exchange coupling action works from the interface between the core portion and the shell portion (hereinafter referred to as “exchange coupling distance”) is a, the thickness t of the shell portion satisfies t ≦ a. preferable. That is, the thickness of the shell part is preferably equal to or less than the exchange coupling distance.

When the soft magnetic material S contains α-Fe, the thickness t of the shell portion is preferably 1 nm or more and 20 nm or less, and more preferably 1 nm or more and 10 nm or less.

It is preferable that the average particle diameter of the core portion including the hard magnetic material H is large so that the coercive force of the hard magnetic material H does not decrease. When the hard magnetic material H contains ε-Fe ₂ O ₃ , the average particle size of the core portion containing the hard magnetic material H is so small that ε-Fe ₂ O ₃ can maintain the epsilon structure. preferable. Specifically, the average particle size of the core portion including the hard magnetic material H is preferably 5 nm or more and 60 nm or less, and more preferably 10 nm or more and 40 nm or less.

(Production method of composite magnetic material)
The composite magnetic material 301 according to the present embodiment can also be manufactured by the same method as in the first embodiment. At this time, the first step (the first step of forming a precursor material having the soft magnetic material S and the hard magnetic material H) is to prepare particles of the hard magnetic material H and process the particles. It may be a step of forming a shell of the soft magnetic material S on the surface of the hard magnetic material H. For example, when ε-Fe ₂ O ₃ is used as the hard magnetic material H, the synthesized ε-Fe ₂ O ₃ particles may be heat-treated in a reducing atmosphere after the ε-Fe ₂ O ₃ particles are synthesized. Thereby, a part of ε-Fe ₂ O ₃ is reduced from the surface, and α-Fe which is the soft magnetic material S is formed.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the technical scope of the present invention is not limited to the following examples. Note that “%” used below is based on mass unless otherwise specified.

[Comparative Example 1]
In Comparative Example 1, α-Fe nanoparticles and ε-Fe ₂ O ₃ particles were respectively prepared, mixed, and heat-treated, so that a composite magnetic material containing α-Fe and ε-Fe ₂ O ₃ was used. 1 was produced.

(Production of α-Fe nanoparticles)
ΑFe nanoparticles, which are soft magnetic materials, were prepared by the following procedure.

First, iron nitrate hydrate _{(Fe (NO 3) 3 ·} 9H 2 O) and 6g weighed and dissolved in pure water 75 mL, to obtain a nitric acid aqueous solution of iron. While stirring 75 mL of 28% aqueous ammonia, an aqueous iron nitrate solution was added to the aqueous ammonia to precipitate iron hydroxide (Fe (OH) ₃ ). The precipitated iron hydroxide was collected by filtration, washed thoroughly with pure water, and then vacuum dried to obtain iron hydroxide nanoparticles. As a result of measuring the particle size of the obtained iron hydroxide nanoparticles by a dynamic light scattering method (DLS), the volume-based average particle size was 8 nm.

Next, the obtained iron hydroxide nanoparticles were put in an alumina crucible, and the iron hydroxide nanoparticles were heat-treated in a reducing atmosphere to obtain α-Fe nanoparticles. A mixed gas of 2% hydrogen-98% nitrogen was used as the atmospheric gas during the heat treatment, and the flow rate of the mixed gas was 300 sccm. The temperature during the heat treatment was 500 ° C., held at 500 ° C. for 5 hours, and then cooled to room temperature. As a result of measuring the particle diameter of the obtained α-Fe nanoparticles by a dynamic light scattering method (DLS), the volume-based average particle diameter was 25 nm. Further, as a result of analyzing the crystal structure of the obtained α-Fe nanoparticles by XRD, a diffraction peak of α-Fe (alpha iron) was confirmed, and diffraction peaks derived from other crystal structures were not confirmed.

(Preparation of ε-Fe ₂ O ₃ particles)
Ε-Fe ₂ O ₃ particles, which are hard magnetic materials, were prepared by the following procedure.

(1) First, two kinds of micelle solutions (micelle solution (A) and micelle solution (B)) were prepared as follows.

(1-1) 30 mL of pure water, 92 mL of n-octane, and 19 mL of 1-butanol were placed in a reaction vessel and mixed. There, iron nitrate hydrate _{(Fe (NO 3) 3 ·} 9H 2 O) was added 6 g, was sufficiently dissolved with stirring. Next, cetyltrimethylammonium bromide as a surfactant is added in such an amount that the molar ratio represented by (number of moles of pure water) / (number of moles of surfactant) is 30, and stirring is performed. Dissolved. Thereby, a micelle solution (A) was obtained.

(1-2) In a separate reaction vessel, 10 mL of 28% aqueous ammonia was mixed with 20 mL of pure water and stirred, and then 92 mL of n-octane and 19 mL of 1-butanol were further added and stirred well. In the solution, cetyltrimethylammonium bromide as a surfactant is such that the molar ratio represented by (number of moles of (pure water + water in ammonia water)) / (number of moles of surfactant) is 30. Add in volume and dissolve by stirring. Thereby, a micelle solution (B) was obtained.

(2) The micelle solution (B) was added dropwise to the micelle solution (A) while thoroughly stirring the micelle solution (A). After completion of the dropwise addition, the mixture was continuously stirred for 30 minutes.

(3) While stirring the obtained mixed solution, 7.5 mL of tetraethoxysilane (TEOS) was added to the mixed solution, and stirring was continued as it was for 1 day. In this step, a silica layer was formed on the surface of the iron-containing particles in the mixed solution.

(4) The obtained solution was set in a centrifuge and centrifuged at 4500 rpm for 30 minutes to collect a precipitate. The collected precipitate was washed several times with ethanol.

(5) After the obtained precipitate was dried, it was placed in a firing furnace in the air atmosphere and heat-treated at 1150 ° C. for 4 hours.

(6) The heat-treated powder was dispersed in a 2 mol / L NaOH aqueous solution and stirred for 24 hours to remove the silica layer on the particle surface. Thereafter, filtration, washing with water and drying were performed to obtain ε-Fe ₂ O ₃ particles. Further, as a result of analyzing the crystal structure of the obtained ε-Fe ₂ O ₃ particles by XRD, a diffraction peak of ε-Fe ₂ O ₃ was confirmed, and a diffraction peak derived from other crystal structures was not confirmed. .

(Production of composite magnetic materials)
0.48 g and 0.2 g of αFe nanoparticles and ε-Fe ₂ O ₃ particles respectively produced by the above-mentioned methods were weighed and mixed in a nitrogen gas atmosphere using a planetary ball mill. Next, this mixed powder was processed with a pressure molding machine to obtain a molded body.

The obtained molded body was set in an electric furnace and heat-treated at 260 ° C. for 5 hours in an atmosphere of a mixed gas of hydrogen and nitrogen (2% H ₂ -98% N ₂ ). After cooling to room temperature, it was coarsely pulverized under a nitrogen gas atmosphere using a planetary ball mill. The powder obtained by coarse pulverization was set again in an electric furnace, and was heat-treated at 260 ° C. for 3 hours in an atmosphere of a mixed gas of hydrogen and nitrogen (2% H ₂ -98% N ₂ ). Got.

(Structural analysis of composite magnetic materials)
As a result of analyzing the crystal structure of the obtained composite magnetic material 1 by XRD, a diffraction peak of ε-Fe ₂ O _{3 and} a diffraction peak of α-Fe can be confirmed respectively, and diffraction peaks derived from other crystal structures are confirmed. Was not.

Further, as a result of observing the cross section of the particulate composite magnetic material 1 with a TEM, a sea-island structure in which a plurality of islands made of ε-Fe ₂ O ₃ exist in the sea made of α-Fe (continuous phase) was confirmed. . On the surface layer of α-Fe exposed on the particle surface, amorphous iron oxide was formed with a thickness of about 3 nm.

(Evaluation of magnetic properties of composite magnetic materials)
With respect to the obtained composite magnetic material 1, the stability over time of the magnetic properties was evaluated. Immediately after the production of the composite magnetic material, the residual magnetic flux density and the coercive force were measured using a vibrating sample magnetometer, stored in the atmosphere at room temperature for 30 days, and the residual magnetic flux density and the coercive force were measured again in the same manner. . The temporal stability of the magnetic properties was evaluated by the ratio (retention rate) of the residual magnetic flux density and coercive force after 30 days to the residual magnetic flux density and coercive force immediately after production. The results are shown in Table 1.

[Example 1]
In Example 1, the composite magnetic material 1 of Comparative Example 1 was used as a precursor material, and the precursor material was oxidized to include α-Fe and ε-Fe ₂ O ₃ and have Fe ₃ O ₄ on the surface. A composite magnetic material 2 was produced.

(Formation of crystalline iron oxide layer)
The composite magnetic material 1 obtained in the same manner as in Comparative Example 1 was set in an electric furnace, heat-treated at 350 ° C. for 2 hours while flowing air, and α-Fe exposed on the particle surface of the particulate composite magnetic material 1 A crystalline iron oxide layer was formed on the surface layer.

(Structural analysis of composite magnetic materials)
As a result of analyzing the crystal structure of the obtained composite magnetic material 2 by XRD, a diffraction peak of ε-Fe ₂ O _3, a diffraction peak of α-Fe, and a diffraction peak of Fe ₃ O ₄ can be confirmed, respectively. A diffraction peak derived from the crystal structure was not confirmed.

Moreover, as a result of observing the cross section of the particulate composite magnetic material 2 with a TEM, a sea-island structure in which a plurality of islands made of ε-Fe ₂ O ₃ exist in the sea made of α-Fe (continuous phase) was confirmed. . In addition, a protective layer of crystalline iron oxide having a thickness of about 200 nm was formed on the surface layer of αFe exposed on the particle surface.

(Evaluation of magnetic properties of composite magnetic materials)
Similar to Comparative Example 1, the temporal stability of the magnetic properties of the composite magnetic material 2 was evaluated. The results are shown in Table 1.

[Comparative Example 2]
In Comparative Example 2, ε-Fe ₂ O ₃ particles were produced in the same manner as in Comparative Example 1, and the produced ε-Fe ₂ O ₃ particles were subjected to a reduction treatment, whereby α-Fe and ε-Fe ₂ O ₃ The composite magnetic material 3 containing these was produced.

(Production of composite magnetic materials)
Ε-Fe ₂ O ₃ particles obtained in the same manner as in Comparative Example 1 were set in an electric furnace and heat-treated at 350 ° C. for 30 minutes in an atmosphere of a mixed gas of hydrogen and nitrogen (2% H 2 -98% N 2). . After cooling to room temperature, it was coarsely pulverized under a nitrogen gas atmosphere using a planetary ball mill. The powder obtained by the coarse pulverization was set again in an electric furnace, and was heat-treated at 350 ° C. for 30 minutes in a mixed gas atmosphere of hydrogen and nitrogen (2% H2-98% N2) to obtain a composite magnetic material 3 .

(Structural analysis of composite magnetic materials)
As a result of analyzing the crystal structure of the obtained composite magnetic material 3 by XRD, a diffraction peak of ε-Fe ₂ O _{3 and} a diffraction peak of αFe can be confirmed respectively, and diffraction peaks derived from other crystal structures are not confirmed. It was.

Further, as a result of observing the cross section of the particulate composite magnetic material 3 with a TEM, a core-shell structure composed of an ε-Fe ₂ O ₃ core and an α-Fe shell was confirmed. On the surface layer of the α-Fe shell, amorphous iron oxide was formed with a thickness of about 3 nm.

(Evaluation of magnetic properties of composite magnetic materials)
In the same manner as in Comparative Example 1, the temporal stability of the magnetic properties of the composite magnetic material 3 was evaluated. The results are shown in Table 1.

[Example 2]
In Example 2, the composite magnetic material 3 of Comparative Example 2 is used as a precursor material, and the precursor material is oxidized to include α-Fe and ε-Fe ₂ O ₃ and have Fe ₃ O ₄ on the surface. A composite magnetic material 4 was produced.

(Formation of crystalline iron oxide layer)
The composite magnetic material 3 obtained in the same manner as in Comparative Example 2 was set in an electric furnace, heat-treated at 300 ° C. for 10 minutes while flowing air, and α-Fe exposed on the particle surface of the particulate composite magnetic material 1 A crystalline iron oxide layer was formed on the surface layer.

(Structural analysis of composite magnetic materials)
As a result of analyzing the crystal structure of the obtained composite magnetic material 4 by XRD, the diffraction peak of ε-Fe ₂ O ₃ , the diffraction peak of α-Fe, and the diffraction peak of Fe ₃ O ₄ can be confirmed, respectively. A diffraction peak derived from the crystal structure was not confirmed.

Further, as a result of observing a cross section of the particulate composite magnetic material 4 with a TEM, a core-shell structure composed of an ε-Fe ₂ O ₃ core and an α-Fe shell was confirmed. On the surface layer of the α-Fe shell, a protective layer of crystalline iron oxide having a thickness of about 10 nm was formed.

(Evaluation of magnetic properties of composite magnetic materials)
In the same manner as in Comparative Example 1, the temporal stability of the magnetic properties of the composite magnetic material 4 was evaluated. The results are shown in Table 1.

[Comparative Example 3]
In Comparative Example 3, the composite magnetic material 1 of Comparative Example 1 was used as a precursor material, and the precursor material was subjected to silica coating treatment, thereby containing α-Fe and ε-Fe ₂ O ₃ and having a silica on the surface. Material 5 was produced.

(Silica coating treatment)
5 g of the composite magnetic material 3 obtained in the same manner as in Comparative Example 1 was weighed and dispersed in 32 g of ethanol. Next, 9 mL of pure water and 2 mL of 28% ammonia water were added to this dispersion. Next, while stirring the composite magnetic material dispersion, 1 g of tetraethoxysilane (TEOS) diluted with 1.5 g of ethanol was added dropwise to the dispersion. After completion of dropping, the mixture was stirred for 24 hours. The solid content was collected by filtration, washed with ethanol and pure water, and dried.

(Structural analysis of composite magnetic materials)
As a result of observing the cross section of the particulate composite magnetic material 5 with a TEM, a core-shell structure composed of an ε-Fe ₂ O ₃ core and an α-Fe shell was confirmed. In addition, a silica protective layer having a thickness of about 20 nm was formed on the surface layer of the α-Fe shell.

(Evaluation of magnetic properties of composite magnetic materials)
Similar to Comparative Example 1, the temporal stability of the magnetic properties of the composite magnetic material 5 was evaluated. The results are shown in Table 1.

[Examples 3 to 5]
In Examples 3 to 5, the composite magnetic material 1 of Comparative Example 1 was used as a precursor material in the same manner as in Example 1 except that the oxidation treatment conditions were changed as shown in Table 1, and the precursor material was oxidized. . Thus, composite magnetic materials 6 to 8 containing α-Fe and ε-Fe ₂ O ₃ and having Fe ₃ O ₄ on the surface were produced.

(Evaluation of magnetic properties of composite magnetic materials)
In the same manner as in Comparative Example 1, the temporal stability of the magnetic properties of the composite magnetic materials 6 to 8 was evaluated. The results are shown in Table 1.

[Example 6]
In Example 6, Fe (OH) ₃ particles are precipitated in a dispersion liquid in which ε-Fe ₂ O ₃ particles are dispersed, and this is heat-treated in a reducing atmosphere, so that α-Fe and ε-Fe ₂ O ₃ are treated. The precursor material containing was produced. Thereafter, this was oxidized to produce a composite magnetic material containing α-Fe and ε-Fe ₂ O ₃ and having Fe ₃ O ₄ on the surface.

(Preparation of dispersion)
Iron nitrate hydrate _{(Fe (NO 3) 3 ·} 9H 2 O) and 6g weighed and dissolved in pure water 75 mL, to obtain a nitric acid aqueous solution of iron. Next, 0.36 g of ε-Fe ₂ O ₃ particles obtained in the same manner as in Comparative Example 1 was weighed and added to an aqueous iron nitrate solution, and sufficiently dispersed with an ultrasonic disperser to prepare a dispersion.

(Precipitation of precursor particles)
While stirring the prepared dispersion, 75 mL of 28% aqueous ammonia was added to precipitate Fe (OH) ₃ particles serving as α-Fe precursor particles, and Fe (OH) ₃ particles and ε-Fe ₂ O ₃ were precipitated. Composite particles containing the particles were formed. When the particle size of Fe (OH) ₃ particles in the obtained composite particles was observed by SEM, it was 10 nm to 20 nm.

(Preparation of precursor material (before coating))
Fe (OH) ₃ particles were reduced and converted to α-Fe to prepare a precursor material. 1 g of powder of composite particles of Fe (OH) ₃ particles and ε-Fe ₂ O ₃ particles was processed with a pressure molding machine to produce a molded body.

The obtained molded body was set in an electric furnace and heat-treated at 500 ° C. for 5 hours in a mixed gas of hydrogen and nitrogen (2% H ₂ -98% N ₂ ) (primary firing). The flow rate of the mixed gas was 300 sccm. After cooling to room temperature, it was coarsely pulverized under a nitrogen gas atmosphere using a planetary ball mill. The powder obtained by coarse pulverization was set again in an electric furnace, and was heat-treated at 260 ° C. for 3 hours in a mixed gas of hydrogen and nitrogen (2% H ₂ -98% N ₂ ) (secondary firing). A precursor material was obtained.

(Formation of crystalline iron oxide layer)
The obtained precursor material was set in an electric furnace, heat-treated at 350 ° C. for 2 hours while flowing air, and a crystalline iron oxide layer was formed on the surface layer of α-Fe exposed on the particle surface of the particulate precursor material. Thus, a composite magnetic material 9 was produced.

(Structural analysis of composite magnetic materials)
As a result of analyzing the crystal structure of the obtained composite magnetic material 9 by XRD, the diffraction peak of ε-Fe ₂ O ₃ , the diffraction peak of α-Fe, and the diffraction peak of Fe ₃ O ₄ can be confirmed, respectively. A diffraction peak derived from the crystal structure was not confirmed.

Moreover, as a result of observing the cross section of the particulate composite magnetic material 9 with a TEM, a sea-island structure in which a plurality of islands made of ε-Fe ₂ O ₃ exist in the sea made of α-Fe (continuous phase) was confirmed. . Further, a protective layer of crystalline iron oxide having a thickness of about 200 nm was formed on the surface layer of α-Fe exposed on the particle surface.

(Evaluation of magnetic properties of composite magnetic materials)
In the same manner as in Comparative Example 1, the temporal stability of the magnetic properties of the composite magnetic material 9 was evaluated. The results are shown in Table 1.

[Example 7]
In Example 7, Fe ₃ O ₄ particles were precipitated with a dispersion solution in which ε-Fe ₂ O ₃ particles were dispersed, and this was heat-treated in a reducing atmosphere, whereby α-Fe, ε-Fe ₂ O ₃ and A precursor material containing was prepared. Thereafter, this was oxidized to produce a composite magnetic material containing α-Fe and ε-Fe ₂ O ₃ and having Fe ₃ O ₄ on the surface.

(Preparation of dispersion)
3 g of iron chloride hydrate (FeCl ₂ .4H ₂ O) was weighed and dissolved in 75 mL of pure water to obtain an aqueous iron chloride solution. Next, 0.36 g of ε-Fe ₂ O ₃ particles obtained in the same manner as in Comparative Example 1 was weighed and added to an aqueous iron chloride solution, and sufficiently dispersed with an ultrasonic disperser to prepare a dispersion.

(Precipitation of precursor particles)
While stirring the prepared dispersion, 75 mL of 28% ammonia water was added to precipitate Fe ₃ O ₄ particles serving as α-Fe precursor particles, and Fe ₃ O ₄ particles, ε-Fe ₂ O ₃ particles, Of composite particles were formed. When the particle size of Fe ₃ O ₄ particles in the obtained composite particles was observed by SEM, it was 50 nm to 80 nm.

(Preparation of precursor material (before coating))
The Fe ₃ O ₄ particles were reduced and converted to α-Fe to prepare a precursor material. 1 g of composite particles of Fe ₃ O ₄ particles and ε-Fe ₂ O ₃ particles were processed with a pressure molding machine to prepare a compact.

The obtained molded body was set in an electric furnace and heat-treated at 470 ° C. for 5 hours in a mixed gas of hydrogen and nitrogen (2% H ₂ -98% N ₂ ) (primary firing). The flow rate of the mixed gas was 300 sccm. After cooling to room temperature, it was coarsely pulverized under a nitrogen gas atmosphere using a planetary ball mill. The powder obtained by coarse pulverization was set again in an electric furnace, and was heat-treated at 260 ° C. for 3 hours in a mixed gas of hydrogen and nitrogen (2% H ₂ -98% N ₂ ) (secondary firing). A precursor material was obtained.

(Formation of crystalline iron oxide layer)
The obtained precursor material was set in an electric furnace, heat-treated at 350 ° C. for 2 hours while flowing air, and a crystalline iron oxide layer was formed on the surface layer of α-Fe exposed on the particle surface of the particulate precursor material. Thus, a composite magnetic material 10 was produced.

(Structural analysis of composite magnetic materials)
As a result of XRD analysis of the crystal structure of the obtained composite magnetic material 10, a diffraction peak of ε-Fe ₂ O _3, a diffraction peak of α-Fe, and a diffraction peak of Fe ₃ O ₄ can be confirmed, respectively. A diffraction peak derived from the crystal structure was not confirmed.

Moreover, as a result of observing a cross section of the particulate composite magnetic material 10 with a TEM, a sea-island structure in which a plurality of islands made of ε-Fe ₂ O ₃ exist in the sea made of α-Fe (continuous phase) was confirmed. . Further, a protective layer of crystalline iron oxide having a thickness of about 200 nm was formed on the surface layer of α-Fe exposed on the particle surface.

(Evaluation of magnetic properties of composite magnetic materials)
In the same manner as in Comparative Example 1, the temporal stability of the magnetic properties of the composite magnetic material 10 was evaluated. The results are shown in Table 1.

[Example 8]
In Example 8, as in Example 7, Fe ₃ O ₄ particles were precipitated in a dispersion liquid in which ε-Fe ₂ O ₃ particles were dispersed, and this was heat-treated in a reducing atmosphere, whereby α-Fe And a precursor material containing ε-Fe ₂ O ₃ was prepared. Thereafter, this was oxidized to produce a composite magnetic material containing α-Fe and ε-Fe ₂ O ₃ and having Fe ₃ O ₄ on the surface. In this example, the Fe ₃ O ₄ particles were precipitated such that the particle size of the precipitated Fe ₃ O ₄ particles was smaller than that in Example 7.

(Preparation of dispersion)
1.5 g of iron chloride hydrate (FeCl ₂ .4H ₂ O) was weighed and dissolved in 150 mL of pure water to obtain an aqueous iron chloride solution. That is, in this example, an iron chloride aqueous solution in which the concentration of iron chloride was ¼ that of Example 7 was prepared. Next, 0.18 g of ε-Fe ₂ O ₃ particles obtained in the same manner as in Comparative Example 1 was weighed and added to an aqueous iron chloride solution and sufficiently dispersed with an ultrasonic disperser to prepare a dispersion.

(Precipitation of precursor particles)
While stirring the prepared dispersion, 75 mL of 28% ammonia water was added to precipitate Fe ₃ O ₄ particles serving as α-Fe precursor particles, and Fe ₃ O ₄ particles, ε-Fe ₂ O ₃ particles, Of composite particles were formed. When the particle size of the Fe ₃ O ₄ particles in the obtained composite particles was observed by SEM, it was 10 nm to 30 nm.

(Preparation of precursor material (before coating))
The Fe ₃ O ₄ particles were reduced and converted to α-Fe to prepare a precursor material. 0.5 g of composite particles of Fe ₃ O ₄ particles and ε-Fe ₂ O ₃ particles were processed with a pressure molding machine to prepare a compact.

The obtained molded body was set in an electric furnace and heat-treated at 450 ° C. for 5 hours in a mixed gas of hydrogen and nitrogen (2% H ₂ -98% N ₂ ) (primary firing). The flow rate of the mixed gas was 300 sccm. After cooling to room temperature, it was coarsely pulverized under a nitrogen gas atmosphere using a planetary ball mill. The powder obtained by coarse pulverization was set again in an electric furnace, and was heat-treated at 260 ° C. for 3 hours in a mixed gas of hydrogen and nitrogen (2% H ₂ -98% N ₂ ) (secondary firing). A precursor material was obtained.

(Formation of crystalline iron oxide layer)
The obtained precursor material was set in an electric furnace, heat-treated at 350 ° C. for 2 hours while flowing air, and a crystalline iron oxide layer was formed on the surface layer of α-Fe exposed on the particle surface of the particulate precursor material. Thus, the composite magnetic material 11 was produced.

(Structural analysis of composite magnetic materials)
As a result of XRD analysis of the crystal structure of the obtained composite magnetic material 11, a diffraction peak of ε-Fe ₂ O _3, a diffraction peak of α-Fe, and a diffraction peak of Fe ₃ O ₄ can be confirmed, respectively. A diffraction peak derived from the crystal structure was not confirmed.

Moreover, as a result of observing the cross section of the particulate composite magnetic material 11 with a TEM, a sea-island structure in which a plurality of islands made of ε-Fe ₂ O ₃ exist in the sea made of α-Fe (continuous phase) was confirmed. . Further, a protective layer of crystalline iron oxide having a thickness of about 200 nm was formed on the surface layer of α-Fe exposed on the particle surface.

(Evaluation of magnetic properties of composite magnetic materials)
In the same manner as in Comparative Example 1, the temporal stability of the magnetic properties of the composite magnetic material 11 was evaluated. The results are shown in Table 1.

[Example 9]
In Example 9, a precursor material containing α-Fe and ε-Fe ₂ O ₃ was formed by precipitating α-Fe particles in a dispersion solution in which ε-Fe ₂ O ₃ particles were dispersed. Thereafter, this was oxidized to produce a composite magnetic material containing α-Fe and ε-Fe ₂ O ₃ and having Fe ₃ O ₄ on the surface.

(Preparation of dispersion)
3 g of iron chloride hydrate (FeCl ₂ .4H ₂ O) was weighed and dissolved in 75 mL of pure water to obtain an aqueous iron chloride solution. Next, 0.36 g of ε-Fe ₂ O ₃ particles obtained in the same manner as in Comparative Example 1 was weighed and added to an aqueous iron chloride solution, and sufficiently dispersed with an ultrasonic disperser to prepare a dispersion.

(Deposition of α-Fe particles)
2 g of NaBH ₄ as a reducing agent was weighed and dissolved in 20 mL of pure water to prepare a reducing agent solution. Next, a reducing agent solution was dropped while stirring the dispersion, thereby precipitating α-Fe particles to form composite particles of α-Fe particles and ε-Fe ₂ O ₃ particles. When the particle diameter of the α-Fe particles in the obtained composite particles was observed with an SEM, it was about 100 nm.

(Precursor material production)
1 g of composite particles of α-Fe particles and ε-Fe ₂ O ₃ particles were processed with a pressure molding machine to produce a compact.

The obtained molded body was set in an electric furnace and heat-treated at 400 ° C. for 5 hours in a nitrogen gas atmosphere (primary firing). The flow rate of nitrogen gas was 300 sccm. After cooling to room temperature, it was coarsely pulverized under a nitrogen gas atmosphere using a planetary ball mill. The powder obtained by coarse pulverization was set again in an electric furnace, and was heat-treated at 260 ° C. for 3 hours in a mixed gas of hydrogen and nitrogen (2% H ₂ -98% N ₂ ) (secondary firing). A precursor material was obtained.

(Structural analysis of composite magnetic materials)
As a result of analyzing the crystal structure of the obtained composite magnetic material 12 by XRD, a diffraction peak of ε-Fe ₂ O _3, a diffraction peak of α-Fe, and a diffraction peak of Fe ₃ O ₄ can be confirmed, respectively. A diffraction peak derived from the crystal structure was not confirmed.

Moreover, as a result of observing a cross section of the particulate composite magnetic material 12 with a TEM, a sea-island structure in which a plurality of islands made of ε-Fe ₂ O ₃ exist in the sea made of α-Fe (continuous phase) was confirmed. . Further, a protective layer of crystalline iron oxide having a thickness of about 200 nm was formed on the surface layer of α-Fe exposed on the particle surface.

(Evaluation of magnetic properties of composite magnetic materials)
Similar to Comparative Example 1, the temporal stability of the magnetic properties of the composite magnetic material 12 was evaluated. The results are shown in Table 1.

As shown in Table 1, in Comparative Examples 1 and 2, both the residual magnetic flux density retention rate and the coercive force retention rate were about 80%, while in Examples 1 to 9, the residual magnetic flux density retention rate and the coercive force retention rate were about 80%. Both magnetic retention rates were 99% or more, indicating very high stability over time. In Comparative Example 3, although silica was formed as the protective layer, the residual magnetic flux density retention rate and coercive force retention rate were both 85% or less, and the temporal stability was insufficient. From the above results, it was found that the temporal stability of magnetic properties can be improved by coating the surface with crystalline iron oxide.

The present invention is not limited to the above embodiment, and various changes and modifications can be made without departing from the spirit and scope of the present invention. Therefore, in order to make the scope of the present invention public, the following claims are attached.

This application claims priority on the basis of Japanese Patent Application No. 2017-081522 filed on Apr. 17, 2017 and Japanese Patent Application No. 2018-023556 filed on Feb. 13, 2018. All the descriptions are incorporated herein.

Claims (18)

1. A composite magnetic material containing a soft magnetic material and a hard magnetic material,
   The soft magnetic material includes iron or an iron alloy;
   A composite magnetic material, wherein at least a part of the surface of the soft magnetic material is coated with crystalline iron oxide.
2. 2. The composite magnetic material according to claim 1, wherein the composite magnetic material has a sea-island structure including a sea part including the soft magnetic material and an island part including the hard magnetic material.
3. The composite magnetic material according to claim 2, wherein 50% or more of the surface of the sea part is covered with the crystalline iron oxide.
4. 2. The composite magnetic material according to claim 1, further comprising: a core portion including the hard magnetic material; and a shell portion including the soft magnetic material covering at least a part of the core portion.
5. The composite magnetic material according to claim 4, wherein 50% or more of the surface of the shell portion is coated with the crystalline iron oxide.
6. 6. The composite magnetic material according to claim 1, wherein the crystalline iron oxide is Fe ₃ O ₄ .
7. The composite magnetic material according to any one of claims 1 to 6, wherein the soft magnetic material contains α-Fe.
8. The composite magnetic material according to any one of claims 1 to 7, wherein the hard magnetic material contains ε-Fe ₂ O ₃ .
9. The composite magnetic material according to any one of claims 1 to 8, wherein the soft magnetic material and the hard magnetic material are magnetically coupled.
10. The composite magnetic material according to any one of claims 1 to 9, wherein the content of the Nd element is 3% by mass or less.
11. The composite magnetic material according to any one of claims 1 to 10, wherein the crystalline iron oxide has a thickness of 5 nm to 500 nm.
12. A motor having a magnet,
    A motor, wherein the magnet contains the composite magnetic material according to any one of claims 1 to 11.
13. A first step of forming a precursor material comprising: a soft magnetic material comprising iron or an iron alloy; and a hard magnetic material;
    A second step of forming crystalline iron oxide on at least a part of the surface of the soft magnetic material exposed on the surface of the precursor material by oxidizing the precursor material. Method.
14. The first step includes a step of mixing the first particles including the hard magnetic material and the second particles including the soft magnetic material, and a step of heat-treating the mixture obtained by mixing. The method for producing a composite magnetic material according to claim 13, comprising:
15. In the first step, the soft magnetic material and the hard magnetic material are dispersed from the raw material in the dispersion solution in which particles of the other material are dispersed in a solution in which the raw material of the one material is dissolved. The method for producing a composite magnetic material according to claim 13, comprising a step of precipitating the magnetic material or the hard magnetic material, or a precursor thereof.
16. The hard magnetic material includes ε-Fe ₂ O ₃ ;
    The first step includes a step of reducing the surface of the first particle containing the hard magnetic material to form the soft magnetic material containing α-Fe on the surface of the first particle. The method for producing a composite magnetic material according to claim 13.
17. The method for producing a composite magnetic material according to claim 13, wherein the crystalline iron oxide is Fe ₃ O ₄ .
18. 18. The composite magnetism according to claim 13, wherein the oxidation treatment is a treatment of heating at a temperature of 250 ° C. or more and 700 ° C. or less in an atmosphere containing oxygen or water vapor. Material manufacturing method.

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