JP2022142565A - Magnetic nanoparticle, composite magnetic material and electronic component - Google Patents

Abstract

To provide magnetic nanoparticles capable of combining high magnetization, high resonance frequency and oxidation resistance, a composite magnetic material comprising the magnetic nanoparticles, and an electronic component comprising the composite magnetic material.SOLUTION: Magnetic nanoparticles each have a core part made of an iron-cobalt alloy particle and a shell part covering at least a part of the surface of the core part and made of cobalt. The average thickness of the shell parts is preferably 1 or more to 18 nm or less. The average particle diameter of the core parts is preferably 5 or more to 30 nm or less. Provided that the average particle diameter of the core parts is defined as d and the average particle diameter of the magnetic nanoparticles is defined as D, the d and D preferably satisfy the relation of 0.10<(d/D)3<0.50.SELECTED DRAWING: Figure 1

Description

The present invention relates to magnetic nanoparticles, composite magnetic materials and electronic components.

2. Description of the Related Art In recent years, the frequency bands used by wireless communication devices such as mobile phones and personal digital assistants have been increasing, and high-frequency regions such as the GHz band are being used. As electronic components used in such a high frequency range, high frequency devices such as filters and diplexers equipped with inductors using a magnetic material having a relatively high magnetic permeability even in a high frequency range are used.

Composite magnetic materials in which nanometer-scale magnetic particles are dispersed in insulating resin are used as core materials for inductors in such high-frequency devices because of their low eddy current loss, high degree of freedom in shape, and high dimensional accuracy. is being developed.

For example, Patent Document 1 describes magnetic metal particles made of Fe or Fe—Co alloy, an alloy layer made of Fe—Ni alloy or Fe—Co—Ni alloy covering the surface of the magnetic metal particles, and covering the surface of the alloy layer. It discloses core-shell magnetic nanoparticles consisting of an Fe--Ni alloy or an oxide layer consisting of an oxide of an Fe--Co--Ni alloy.

JP 2013-125901 A

However, when an oxide, nickel, or an alloy containing nickel is applied to the shell, as in the magnetic nanoparticles described in Patent Document 1, although the oxidation resistance is good, the magnetization of the magnetic nanoparticles is low. , there is a problem that the resonance frequency is low and the magnetic loss increases in the high frequency region.

The present invention has been made in view of such a situation, and its object is to provide magnetic nanoparticles capable of achieving both high magnetization, high resonance frequency, and oxidation resistance, and a composite magnetic material to which the magnetic nanoparticles are applied. , to provide an electronic component containing the composite magnetic material.

Aspects of the present invention are as follows.
[1] A magnetic nanoparticle having a core made of iron-cobalt alloy particles and a shell made of cobalt covering at least a part of the surface of the core.

[2] The magnetic nanoparticles according to [1], wherein the shell portion has an average thickness of 1 nm or more and 18 nm or less.

[3] The magnetic nanoparticles according to [1] or [2], wherein the core portion has an average particle size of 5 nm or more and 30 nm or less.

[4] where d is the average particle size of the core portion and D is the average particle size of the magnetic nanoparticles, satisfying the relationship that d and D are 0.10<(d/D) ³ <0.50; The magnetic nanoparticles according to any one of [1] to [3].

[5] A composite magnetic material comprising the magnetic nanoparticles described in [1] to [4] and an insulating material.

[6] An electronic component comprising the composite magnetic material according to [5].

According to the present invention, there are provided magnetic particles capable of achieving both high magnetization, high resonance frequency, and oxidation resistance, a composite magnetic material to which the magnetic particles are applied, and an electronic component including the composite magnetic material. can be done.

FIG. 1 is a schematic cross-sectional view of magnetic nanoparticles according to this embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail in the following order based on specific embodiments shown in the drawings.
1. Magnetic nanoparticles2. Method for producing magnetic nanoparticles3. Composite magnetic material4. electronic components

(1. Magnetic nanoparticles)
The magnetic nanoparticles according to this embodiment are usually handled in the form of powder containing a plurality of magnetic nanoparticles. The average particle size of the powder containing multiple magnetic nanoparticles may be selected according to the application. In this embodiment, the average particle size of the powder is preferably in the range of 5 to 30 nm.

Moreover, the magnetic nanoparticles according to the present embodiment have a structure in which at least part of the surface of the iron-cobalt (FeCo) alloy particles is covered with metallic cobalt (Co). That is, as shown in FIG. 1, the magnetic nanoparticles 1 according to the present embodiment are core-shell type particles in which the core portion 2 is made of an iron-cobalt alloy particle and the shell portion 10 is made of cobalt. In this embodiment, as shown in FIG. 1, the shell portion 10 preferably covers the entire surface of the core portion 2 .

An iron-cobalt alloy has a large magnetization and tends to increase the magnetic permeability. On the other hand, since the magnetic anisotropy of the iron-cobalt alloy is weak, the resonance frequency is lowered. In addition, although it is not as easy as iron, it is easily oxidized, so oxidation progresses even if it is in contact with oxygen for a short time.

Therefore, in the present embodiment, the shell portion covering the surface of the iron-cobalt alloy particles is made of cobalt. The magnetic anisotropy of cobalt is stronger than the magnetic anisotropy of the iron-cobalt alloy, and the exchange coupling between the iron-cobalt alloy and cobalt at the interface between the core and the shell results in the magnetic nanoparticle as a whole. Resonance frequency can be increased.

Furthermore, since cobalt is superior in oxidation resistance to iron-cobalt alloys, the progress of oxidation of the magnetic nanoparticles can be suppressed by covering the surfaces of the iron-cobalt alloy particles with cobalt.

Therefore, by making the structure of the magnetic nanoparticles the core-shell structure, it is possible to suppress the decrease in magnetization due to the oxidation of the magnetic nanoparticles and increase the resonance frequency. Furthermore, since it has sufficient magnetic anisotropy, the magnetic loss tangent can be reduced.

In the present embodiment, the average diameter of the core portion, that is, the average particle diameter of the iron-cobalt alloy particles is preferably 5 nm or more, more preferably 8 nm or more, and even more preferably 10 nm or more. When the average particle size is within the above range, the magnetic nanoparticles can be prevented from becoming superparamagnetic and losing magnetic anisotropy, and the magnetization of the core portion can be maintained at a high level.

On the other hand, the average particle size of the iron-cobalt alloy particles is preferably 30 nm or less, preferably 20 nm or less, and more preferably 15 nm or less. When the average particle size is within the above range, the magnetization of the core portion is exchange-coupled with the magnetization of the shell portion to the center, and the resonance frequency can be kept high.

Further, in the present embodiment, the average thickness of the shell portion is preferably 1 nm or more, more preferably 1.5 nm or more, and even more preferably 2 nm or more. By covering the surface of the iron-cobalt alloy particles with cobalt within the above range, the oxidation resistance of the magnetic nanoparticles is sufficiently ensured.

On the other hand, the average thickness of the shell portion is preferably 18 nm or less, preferably 10 nm or less, and more preferably 5 nm or less. Cobalt is less magnetized than iron-cobalt alloys, so by limiting the average thickness of the shell portion, it is possible to suppress the decrease in magnetization as magnetic nanoparticles. As a result, a decrease in magnetic permeability of the magnetic nanoparticles can be suppressed.

Furthermore, in the present embodiment, when the average particle size of the iron-cobalt alloy particles is d and the average particle size of the magnetic nanoparticles is D, 0.10<(d/D) ³ <0. 50, and more preferably 0.30≦(d/D) ³ ≦0.50. The above relationship indicates the volume ratio of the core portion to the magnetic nanoparticles. If the volume of the core is too small, the magnetization of the magnetic nanoparticles tends to be low and the high magnetization of the iron-cobalt alloy cannot be effectively utilized. On the other hand, if the volume of the core is too large, the magnetic anisotropy of the core becomes dominant over the strong magnetic anisotropy of the shell, so the resonance frequency tends to decrease and the magnetic loss tangent tends to increase. In this embodiment, the average particle diameter D of the magnetic nanoparticles is a value obtained by adding twice the average thickness of the shell portion to the average particle diameter d of the iron-cobalt alloy particles.

When the shell portion is made of a magnetic oxide, although the oxidation resistance is improved, the anisotropic magnetic field of the magnetic oxide is lower than that of cobalt, and the resonance frequency cannot be increased. Rather, the resonance frequency may be lowered.

Further, when the shell portion is made of metallic nickel (Ni) or an alloy containing nickel, the anisotropic magnetic field of nickel and the alloy containing nickel is lower than that of cobalt, and the magnetic loss tangent tends to deteriorate.

In the iron-cobalt alloy, the content of iron is preferably 50 mol% or more and 90 mol% or less, more preferably 60 mol% or more and 80 mol% or less, in 100 mol% of the total of iron and cobalt. When the content of iron is within the above range, an iron-cobalt alloy with high magnetization is likely to be obtained.

Moreover, the iron-cobalt alloy may contain elements other than iron and cobalt within the range in which the effects of the present invention can be obtained. Specifically, in 100 mol % of the iron-cobalt alloy, the total content of elements other than iron and cobalt is preferably 5 mol % or less, more preferably 2 mol % or less. Impurity elements other than iron and cobalt include chromium, manganese, nickel, carbon, oxygen, nitrogen, sulfur, and phosphorus, and these elements are usually contained as unavoidable impurities.

In addition, cobalt may contain elements other than cobalt within the range in which the effects of the present invention can be obtained. Specifically, in 100 mol % of cobalt, the total content of elements other than cobalt is preferably 3 mol % or less, more preferably 2 mol % or less. Examples of elements other than cobalt include carbon, oxygen, nitrogen, chlorine, and phosphorus, and these elements are usually contained as unavoidable impurities.

The total content of iron and nickel in 100 mol % of cobalt is preferably 3 mol % or less, more preferably 1 mol % or less.

From the above, it is preferable that the purity of the iron-cobalt alloy and cobalt constituting the magnetic nanoparticles is high in order to enhance the desired magnetic properties.

The magnetic nanoparticles only need to have the above-described core portion and shell portion, and for example, an oxide film resulting from natural oxidation may be formed on the surface of the shell portion.

(2. Method for producing magnetic nanoparticles)
Next, a method for producing magnetic nanoparticles will be described. In the present embodiment, magnetic nanoparticles are produced using a thermal decomposition reaction of a metal carbonyl compound, which is a liquid phase synthesis method.

First, iron-cobalt alloy particles to be the core are produced. An iron carbonyl compound and a cobalt carbonyl compound are prepared as raw materials. Examples of iron carbonyl compounds include pentacarbonyl iron (Fe(CO) ₅ ) and nonacarbonyl diiron (Fe ₂ (CO) ₉ ). Pentacarbonyl iron is preferred in this embodiment. Examples of cobalt carbonyl compounds include octacarbonyl dicobalt (Co ₂ (CO) ₈ ) and tetracobalt dodecacarbonyl (Co ₄ (CO) ₁₂ ). Octacarbonyl dicobalt is preferred in this embodiment.

Next, a solvent and a dispersant that constitute the liquid phase are prepared. Examples of the solvent include those that are well mixed with the liquid metal carbonyl compound and that can dissolve the solid metal carbonyl compound well. Further, the solvent must have a melting point of room temperature or lower and a boiling point of the metal carbonyl compound's decomposition temperature or lower. Furthermore, the solvent should not react with the metal carbonyl compound and the metal nanoparticles produced. Examples of such solvents include organic solvents, and in the present embodiment, for example, kerosene, dichlorobenzene, 1-octadecene, and 1,2,3,4-tetrahydronaphthalene.

The dispersant is not particularly limited as long as it promotes the thermal decomposition of the metal carbonyl compound and can satisfactorily disperse the generated iron-cobalt alloy particles without agglomeration. In this embodiment, examples of such dispersants include oleylamine, oleic acid, trioctylphosphine oxide, trioctylamine, and tribenzylamine.

The prepared solvent and dispersant are mixed in a reaction vessel and the mixed solution is heated. The heating temperature may be any temperature at which thermal decomposition of the metal carbonyl compound proceeds. In this embodiment, it is preferably within the range of 150 to 300.degree.

Also, an iron carbonyl compound, a cobalt carbonyl compound, and a solvent are mixed to obtain a raw material mixed solution. Here, the particle size of the iron-cobalt alloy particles can be controlled by the concentration of the metal carbonyl compound in the raw material mixed solution. For example, when the concentration of the metal carbonyl compound in the raw material mixed solution is increased, iron-cobalt alloy particles with a small particle size can be easily obtained. Further, by adjusting the molar ratio of iron in the iron carbonyl compound and cobalt in the cobalt carbonyl compound, the composition ratio of iron and cobalt in the obtained iron-cobalt alloy particles can be adjusted.

The raw material mixed solution thus obtained is injected into a heated mixed solution of a solvent and a dispersant to cause a thermal decomposition reaction of the metal carbonyl compound. Although the reaction time is not particularly limited as long as it is longer than the time required for the thermal decomposition reaction to proceed sufficiently, it is about 15 minutes to 4 hours in this embodiment. The particle size of the iron-cobalt alloy particles can be controlled by the injection speed of the raw material mixed solution. For example, when the injection speed is slowed down, iron-cobalt alloy particles with a large particle size are likely to be obtained.

When the raw material mixed solution is injected into the heated mixed solution, the metal carbonyl compound is thermally decomposed in the presence of the dispersant, iron and cobalt are mixed and bonded at the atomic level, and nanometer-scale iron-cobalt alloy particles are formed. Generate.

The produced iron-cobalt alloy particles and the solution are subjected to solid-liquid separation to obtain iron-cobalt alloy particles. In this embodiment, solid-liquid separation is performed by centrifugation.

Next, a shell portion made of cobalt is formed on the generated iron-cobalt alloy particles to produce magnetic nanoparticles.

A cobalt carbonyl compound is prepared as a raw material for the shell portion. Examples of cobalt carbonyl compounds include octacarbonyl dicobalt (Co ₂ (CO) ₈ ) and tetracobalt dodecacarbonyl (Co ₄ (CO) ₁₂ ). Octacarbonyl dicobalt is preferred in this embodiment.

Next, a solvent and a dispersant that constitute the liquid phase are prepared. The solvent is not particularly limited as long as it is well mixed with the liquid metal carbonyl compound and can well dissolve the solid metal carbonyl compound. Examples of such solvents include organic solvents, and in the present embodiment, for example, dichlorobenzene and 1,2,3,4-tetrahydronaphthalene.

The dispersant is not particularly limited as long as it promotes the thermal decomposition of the metal carbonyl compound and can satisfactorily disperse the magnetic nanoparticles having shell portions without agglomeration. In this embodiment, such dispersants are exemplified by oleylamine, oleic acid, trioctylphosphine oxide, triphenylphosphine, and dioctylamine.

First, the produced iron-cobalt alloy particles, a cobalt carbonyl compound, a solvent, and a dispersant are mixed in a reaction vessel, and the mixed solution is heated. Here, the thickness of the shell portion can be adjusted by adjusting the molar ratio between the iron and cobalt constituting the iron-cobalt alloy particles and the cobalt in the cobalt carbonyl compound.

In addition, the temperature to heat should just be the temperature which thermal decomposition of a cobalt carbonyl compound progresses. In this embodiment, it is preferably within the range of 150 to 250.degree.

Heating the mixed solution causes a thermal decomposition reaction of the cobalt carbonyl compound, and cobalt precipitates on the surface of the iron-cobalt alloy particles, thereby producing magnetic nanoparticles having a shell portion. The reaction time is not particularly limited as long as it is longer than the time required to obtain the desired thickness of the shell portion, but in this embodiment, it is about 3 to 60 minutes.

The generated magnetic nanoparticles and the solution are subjected to solid-liquid separation to obtain magnetic nanoparticles. In this embodiment, solid-liquid separation is performed by magnetic separation.

Note that the iron-cobalt alloy particles and the magnetic nanoparticles have a very small particle size and a large specific surface area. Therefore, the process of producing magnetic nanoparticles is preferably carried out under an inert atmosphere in order to suppress oxidation of the core and shell.

(3. Composite magnetic material)
A composite magnetic material according to the present embodiment includes the magnetic nanoparticles described above and an insulating material, and has a predetermined shape (for example, a core). As such a composite magnetic material, a material in which the powder containing the magnetic nanoparticles is dispersed in an insulating resin is exemplified. Examples of the insulating resin include polystyrene, acrylic resin, and epoxy resin.

As a method of obtaining the composite magnetic material according to the present embodiment, a method of mixing a powder containing magnetic nanoparticles and a resin, filling the mixture in a mold, and press-molding is exemplified. In this embodiment, the nanoparticles in the composite material are preferably well dispersed in the resin so as not to generate eddy current loss. Specifically, first, a powder containing a weighed amount of magnetic nanoparticles is added to a solvent in which a resin is dissolved, and the resulting dispersion is dried (solvent is removed) to obtain a solid. By molding the obtained solid, a composite magnetic material in which magnetic nanoparticles are dispersed in a resin can be obtained. As a method of dispersing, an ultrasonic homogenizer, a shaker mixer, a dispersing machine such as a bead mill, a kneading machine such as a kneader, a triple roll, or the like may be used, or a combination thereof may be used. In order to more reliably prevent contact between magnetic nanoparticles, magnetic nanoparticles coated with a resin on the surface may be used for molding.

(4. Electronic parts)
The electronic component according to the present embodiment is not particularly limited as long as it has the above composite magnetic material. For example, an electronic component in which the composite magnetic material described above is applied to the core of an inductor is exemplified.

Although the embodiments of the present invention have been described above, the present invention is by no means limited to the above embodiments, and may be modified in various ways within the scope of the present invention.

EXAMPLES The present invention will be described in more detail below using examples, but the present invention is not limited to these examples.

(Production of iron-cobalt alloy particles)
In an inert atmosphere, iron-cobalt alloy particles, which serve as the core portion of the magnetic nanoparticles, were produced by the method shown below. Pentacarbonyl iron (Fe(CO) ₅ ) and octacarbonyl dicobalt (Co ₂ (CO) ₈ ) were prepared as raw materials for iron-cobalt alloy particles. Also, kerosene was prepared as a solvent, and oleylamine was prepared as a dispersant. Magnetic nanoparticles of Example 3 were produced as follows.

First, a mixed solution of 210 mL of kerosene and 3.75 g of oleylamine was put into a reactor and heated to 160°C. Next, 2.19 g of pentacarbonyl iron, 0.48 g of octacarbonyl dicobalt, and 14 mL of kerosene were mixed to obtain a raw material mixed solution.

The raw material mixed solution was injected into a reaction vessel heated to 160° C. at an injection rate of 28 mL/min, and held for 1 hour in order to allow the thermal decomposition reaction of iron pentacarbonyl and dicobalt octacarbonyl to proceed sufficiently.

After holding for 1 hour, the reaction vessel was cooled to room temperature, the reaction solution was diluted twice with ethanol to promote aggregation of the nanoparticles, and then solid-liquid separation was performed by centrifugation to produce an iron-cobalt alloy. Particles were obtained. The obtained iron-cobalt alloy particles were washed with ethanol. The average particle size of the obtained iron-cobalt alloy particles was 11 nm.

The average particle size of the iron-cobalt alloy particles was measured as follows. The iron-cobalt alloy particles were observed with a transmission electron microscope (TEM, JEM-2100FCS manufactured by JEOL Ltd.), and the observed image was subjected to image processing for calculation.

(Formation of shell portion)
Next, in an inert atmosphere, a metal or alloy to be the shell portion was formed on the surfaces of the obtained iron-cobalt alloy particles by the following method. Octacarbonyl dicobalt was prepared as a raw material for the shell portion. Also, dichlorobenzene was prepared as a solvent, and oleic acid, trioctylphosphine oxide, and oleylamine were prepared as dispersants.

0.193 g of the obtained iron-cobalt alloy particles, 1.57 g of octacarbonyl dicobalt, 150 mL of dichlorobenzene, 0.318 g of oleic acid, 0.146 g of trioctylphosphine oxide, and 4.65 g of oleylamine were reacted. It was put into a container and heated to 180° C. and held for 5 minutes so that the octacarbonyl dicobalt was thermally decomposed to form a shell.

After holding for 5 minutes, the reaction vessel was cooled to room temperature, solid-liquid separation was performed by magnetic separation, and magnetic nanoparticles in which a shell portion was formed on the surface of iron-cobalt alloy particles (core portion) were obtained. The obtained magnetic nanoparticles were washed with ethanol. The average thickness of the shell portion of the obtained magnetic nanoparticles was 3 nm.

The average thickness of the shell portion was measured as follows. The average thickness of the shell part is an elemental mapping image by electron energy loss spectroscopy (EELS) in which the magnetic nanoparticles are observed in the scanning transmission electron microscope (STEM) mode of a transmission electron microscope (JEM-2100FCS manufactured by JEOL Ltd.). was calculated by image processing.

The average particle size D of the magnetic nanoparticles was obtained by adding twice the average thickness of the shell portion to the average particle size d of the iron-cobalt alloy particles.

Iron-cobalt alloy particles were produced in the same manner as in Example 3 so that the average particle diameter of the iron-cobalt alloy particles was the value shown in Table 1, and the average thickness of the shell part was the value shown in Table 1. Then, a shell portion was formed to obtain magnetic nanoparticles of Examples 1, 2, 4-10 and Comparative Examples 1-5. In Comparative Example 1, no shell portion was formed. In Comparative Examples 2 to 5, octacarbonyl dicobalt, pentacarbonyl iron, and nickel acetylacetonate (Ni(acac) ₂ ) were prepared as raw materials for the shell portion, and used according to the composition of the shell portion. .

The average particle size of the iron-cobalt alloy particles was adjusted by changing the concentration of pentacarbonyl iron and octacarbonyl dicobalt in the raw material mixed solution and the injection speed of the raw material mixed solution. Also, the average thickness of the shell portion was adjusted by changing the molar ratio of the metal constituting the iron-cobalt alloy particles and the metal contained in the raw material of the shell portion.

In addition, the magnetic nanoparticles obtained by scanning transmission electron microscopy (STEM) were observed and elemental analysis was performed by electron energy loss spectroscopy (EELS). It was confirmed that the core portion was made of an iron-cobalt alloy, and the shell portion was made of a metal contained in the raw material of the shell portion or an alloy thereof.

Furthermore, the magnetization σ _p of the obtained magnetic nanoparticles was measured as follows. The magnetic nanoparticles dried in a glove box were placed in a special capsule and sealed with paraffin, and the saturation magnetization was measured using a vibrating sample magnetometer (VSM, manufactured by Tamagawa Seisakusho Co., Ltd.). Table 1 shows the results.

(Preparation of Composite Magnetic Material)
Using the obtained magnetic nanoparticles, a composite magnetic material was produced by the method described below. The magnetic nanoparticles and mesitylene as a solvent were weighed at a weight ratio of 20:80. The weighed magnetic nanoparticles were dispersed in mesitylene to obtain a magnetic slurry. Next, polystyrene as an insulating material and mesitylene as a solvent were weighed at a weight ratio of 20:80. A weighed amount of polystyrene was dissolved in mesitylene to obtain a resin solution. The magnetic slurry and the resin solution were weighed and mixed at a weight ratio of 1:1. The mixed solution was dried in a nitrogen atmosphere at 100° C. for 30 minutes to volatilize mesitylene to obtain a composite magnetic material. Note that the above steps were performed under an inert atmosphere.

In the obtained composite magnetic material, the filling rate of the magnetic nanoparticles was 10% by volume.

The composite magnetic material thus obtained was put into a mold in the atmosphere and molded under conditions of a molding pressure of 0.25 ton/cm ² and a molding temperature of 100° C., and the size was 7 mm in outer diameter, 3 mm in inner diameter, and 1 mm in thickness. A certain toroidal shaped compact was obtained. The following evaluations were carried out on the obtained compacts.

(loss of magnetization)
The magnetization _σc of the composite magnetic material (molded body) was measured as follows. The saturation magnetization of the composite magnetic material was measured using a vibrating sample magnetometer (VSM, manufactured by Tamagawa Seisakusho Co., Ltd.). A magnetization ratio r (=σ _c /σ _p ) was calculated from the magnetization σ _p of the magnetic nanoparticle powder and the magnetization σ _c of the composite magnetic material measured above. Also, from the weight of the composite magnetic material and the weight of the magnetic nanoparticle powder contained in the composite magnetic material, the proportion of the magnetic nanoparticle powder in 100% by weight of the composite magnetic material (filling rate p) was calculated.

Since the magnetization of polystyrene contained in the composite magnetic material can be considered to be approximately 0, ideally the magnetization ratio r and the filling ratio p match. However, since molding of the composite magnetic material is performed in air and heated to 100° C., the magnetic nanoparticles dispersed in polystyrene are oxidized. In other words, part of the highly magnetized metal that constitutes the magnetic nanoparticles is converted to a less magnetized metal oxide. As a result, the magnetization σ _c of the composite magnetic material becomes lower than the magnetization exhibited when the magnetic nanoparticles contained in the composite magnetic material are not oxidized, and the magnetization ratio r becomes lower than the ideal value.

Therefore, the magnetization loss (1−(r/p)) is calculated from the magnetization ratio r and the filling factor p, and the magnetization loss reflects the degree of oxidation of the magnetic nanoparticles. In this example, the lower the loss of magnetization, the better, and samples with a loss of magnetization of 10% or less were judged to be good. Table 1 shows the results.

(complex relative permeability)
The resonance frequency, complex relative permeability at 1 GHz and magnetic loss tangent of the composite magnetic material were measured by the following methods. First, by the coaxial S-parameter method using a network analyzer (N5222A manufactured by Agilent Technologies), the real part (μ′) of the complex relative permeability of the toroidal-shaped composite magnetic material was obtained by changing the frequency from 100 MHz to 18 GHz. And the imaginary part (μ″) was measured. Then, at the loss peak indicated by μ″, the frequency at which μ″ was maximized was taken as the resonance frequency. The magnetic loss tangent was defined as μ″/μ′, which is the ratio of the imaginary part to the real part at 1 GHz.

A higher resonance frequency is preferable, and in this example, samples with a resonance frequency of 2.2 GHz or higher were judged to be good. Also, the smaller the magnetic loss tangent, the better. In this example, samples with a magnetic loss tangent of 15% or less were judged to be good. Further, a higher complex relative magnetic permeability is preferable, and in this example, it was determined that a sample having a complex relative magnetic permeability of 2.0 or more was preferable. Table 1 shows the results.

From Table 1, it was confirmed that when the shell portion formed on the surface of the iron-cobalt alloy particles (core portion) was metallic cobalt, the magnetization loss was low, and the resonance frequency and the magnetic loss tangent were good. .

On the other hand, it was confirmed that when the shell portion was not formed, oxidation progressed during molding of the composite magnetic material, resulting in a very high magnetization loss. It was confirmed that when the shell portion was made of iron, oxidation progressed rapidly during molding of the composite magnetic material, resulting in a very high magnetization loss. It was confirmed that when the shell portion was metallic nickel or an alloy containing nickel, although oxidation was suppressed, both the resonance frequency and the magnetic loss tangent were insufficient.

DESCRIPTION OF SYMBOLS 1... Magnetic nanoparticle 2... Core part 10... Shell part

Claims (6)

A magnetic nanoparticle having a core made of iron-cobalt alloy particles and a shell made of cobalt covering at least part of the surface of the core. 2. The magnetic nanoparticles according to claim 1, wherein the shell portion has an average thickness of 1 nm or more and 18 nm or less. 3. The magnetic nanoparticles according to claim 1, wherein the core portion has an average particle size of 5 nm or more and 30 nm or less. When the average particle size of the core portion is d and the average particle size of the magnetic nanoparticles is D, d and D satisfy a relationship of 0.10<(d/D) ³ <0.50. Item 4. Magnetic nanoparticles according to any one of Items 1 to 3. A composite magnetic material comprising the magnetic nanoparticles according to claim 1 and an insulating material. An electronic component comprising the composite magnetic material according to claim 5 .

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