阅读说明：本技术 软磁性合金及磁性部件 (Soft magnetic alloy and magnetic component ) 是由 天野一 原田明洋 吉留和宏 堀野贤治 松元裕之 荒健辅 长谷川晓斗 野老诚吾 于 2019-02-15 设计创作，主要内容包括：本发明提供一种具有高饱和磁通密度、低矫顽力及高电阻率的软磁性合金。所述软磁性合金的组成式(Fe<Sub>(1-(α+β))</Sub>X1<Sub>α</Sub>X2<Sub>β</Sub>)<Sub>(1-(a+b+c+d+e))</Sub>M<Sub>a</Sub>Si<Sub>b</Sub>Cu<Sub>c</Sub>X3<Sub>d</Sub>B<Sub>e</Sub>,X1为选自Co及Ni中的1种以上,X2为选自Ti、V、Mn、Ag、Zn、Al、Sn、As、Sb、Bi及稀土元素中的1种以上,X3为选自C及Ge中的1种以上,M为选自Zr、Nb、Hf、Ta、Mo及W中的1种以上,0.030≤a≤0.120,0.020≤b≤0.175,0≤c≤0.020,0≤d≤0.100,0≤e≤0.030,α≥0,β≥0,0≤α+β≤0.55。(The invention provides a soft magnetic alloy having a high saturation magnetic flux density, a low coercive force and a high resistivity. The composition formula (Fe) of the soft magnetic alloy (1‑(α+β)) X1 α X2 β ) (1‑(a+b+c+d+e)) M a Si b Cu c X3 d B e X1 is more than 1 selected from Co and Ni, X2 is selected from Ti. More than 1 of V, Mn, Ag, Zn, Al, Sn, As, Sb, Bi and rare earth elements, X3 is more than 1 selected from C and Ge, M is more than 1 selected from Zr, Nb, Hf, Ta, Mo and W, a is more than or equal to 0.030 and less than or equal to 0.120, b is more than or equal to 0.020 and less than or equal to 0.175, C is more than or equal to 0 and less than or equal to 0.020, d is more than or equal to 0 and less than or equal to 0.100, e is more than or equal to 0 and less than or equal to 0.030, α is more than or equal to 0, β is more than or equal to 0, and more than or equal.)

1. A soft magnetic alloy characterized in that,

is represented by the composition formula (Fe)_(1-(α+β))X1_αX2_β)_{(1-(a+b+c+d+e))}M_aSi_bCu_cX3_dB_eThe soft magnetic alloy of the composition is provided with a soft magnetic alloy,

x1 is at least one member selected from the group consisting of Co and Ni,

x2 is more than 1 selected from Ti, V, Mn, Ag, Zn, Al, Sn, As, Sb, Bi and rare earth elements,

x3 is at least one member selected from the group consisting of C and Ge,

m is more than 1 selected from Zr, Nb, Hf, Ta, Mo and W,

0.030≤a≤0.120，

0.020≤b≤0.175，

0≤c≤0.020，

0≤d≤0.100，

0≤e≤0.030，

α≥0，

β≥0，

0≤α+β≤0.55。

2. the soft magnetic alloy according to claim 1,

0≤e≤0.010。

3. the soft magnetic alloy according to claim 1 or 2,

0≤e<0.001。

4. a soft magnetic alloy as described in any one of claims 1 to 3,

0.730≤1-(a+b+c+d+e)≤0.930。

5. a soft magnetic alloy as described in any one of claims 1 to 4,

0≤α{1-(a+b+c+d+e)}≤0.40。

6. a soft magnetic alloy as described in any one of claims 1 to 5,

α＝0。

7. a soft magnetic alloy as described in any one of claims 1 to 6,

0≤β{1-(a+b+c+d+e)}≤0.030。

8. a soft magnetic alloy as described in any one of claims 1 to 7,

β＝0。

9. a soft magnetic alloy as described in any one of claims 1 to 8,

α＝β＝0。

10. a soft magnetic alloy as described in any one of claims 1 to 9,

has a nano-heterostructure with initial crystallites present in the amorphous state.

11. The soft magnetic alloy of claim 10, wherein,

the average grain size of the initial microcrystals is 0.3-10 nm.

12. A soft magnetic alloy as described in any one of claims 1 to 9,

the soft magnetic alloy has a structure composed of Fe-based nanocrystals.

13. The soft magnetic alloy of claim 12, wherein,

the average grain diameter of the Fe-based nanocrystalline is 5-30 nm.

14. The soft magnetic alloy according to any one of claims 1 to 13,

the soft magnetic alloy is in the shape of a thin strip.

15. The soft magnetic alloy according to any one of claims 1 to 13,

the soft magnetic alloy is in the form of a powder.

16. A magnetic component, wherein,

the soft magnetic alloy according to any one of claims 1 to 15.

Technical Field

The present invention relates to a soft magnetic alloy and a magnetic component.

Background

In recent years, nanocrystalline materials have become the mainstream of soft magnetic materials for magnetic parts, particularly for power inductors. For example, patent document 1 describes an Fe-based soft magnetic alloy having a fine crystal grain size. The nanocrystalline material can obtain a higher saturation magnetic flux density and the like than conventional crystalline materials such as FeSi and amorphous materials such as FeSiB.

However, with the progress of further higher frequencies and smaller sizes of magnetic components, particularly power inductors, there is a demand for soft magnetic alloys that can provide magnetic cores having both higher dc superposition characteristics and low magnetic core loss (magnetic loss).

Disclosure of Invention

Technical problem to be solved by the invention

As a method for reducing the core loss of the magnetic core, it is conceivable to reduce the coercive force and the specific resistance of the magnetic material constituting the magnetic core. In addition, as a method for obtaining high dc superposition characteristics, it is conceivable to increase the saturation magnetic flux density of the magnetic material constituting the magnetic core.

The invention aims to provide a soft magnetic alloy and the like with high saturation magnetic flux density, low coercive force and high resistivity.

Means for solving the problems

In order to achieve the above object, a soft magnetic alloy according to the present invention is characterized in that: is represented by the composition formula (Fe)_(1-(α+β))X1_αX2_β)_{(1-(a+b+c+d+e))}M_aSi_bCu_cX3_dB_eThe soft magnetic alloy of the composition is provided with a soft magnetic alloy,

x1 is at least one member selected from the group consisting of Co and Ni,

x2 is more than 1 selected from Ti, V, Mn, Ag, Zn, Al, Sn, As, Sb, Bi and rare earth elements,

x3 is at least one member selected from the group consisting of C and Ge,

m is more than 1 selected from Zr, Nb, Hf, Ta, Mo and W,

0.030≤a≤0.120，

0.020≤b≤0.175，

0≤c≤0.020，

0≤d≤0.100，

0≤e≤0.030，

α≥0，

β≥0，

0≤α+β≤0.55。

the soft magnetic alloy of the present invention has the above-described characteristics, and therefore can easily have a structure that can be easily converted into an Fe-based nanocrystalline alloy by heat treatment. The Fe-based nanocrystalline alloy having the above characteristics has preferable soft magnetic properties of high saturation magnetic flux density and low coercive force, and further has high specific resistance.

The soft magnetic alloy of the present invention may have 0. ltoreq. e.ltoreq.0.010.

The soft magnetic alloy of the present invention may have 0. ltoreq. e < 0.001.

The soft magnetic alloy of the present invention may have a composition of 0.730. ltoreq.1- (a + b + c + d + e). ltoreq.0.930.

In the soft magnetic alloy of the present invention, α {1- (a + b + c + d + e) } may be 0. ltoreq.0.40.

In the soft magnetic alloy of the present invention, α may be 0.

In the soft magnetic alloy of the present invention, 0. ltoreq. beta {1- (a + b + c + d + e) } 0.030 may be used.

In the soft magnetic alloy of the present invention, β may be 0.

In the soft magnetic alloy of the present invention, α ═ β ═ 0 may be used.

The soft magnetic alloy of the present invention may also have a nano-heterostructure in which primary crystallites are present in an amorphous state.

In the soft magnetic alloy of the present invention, the average grain size of the primary crystallites may be 0.3 to 10 nm.

The soft magnetic alloy of the present invention may have a structure composed of Fe-based nanocrystals.

In the soft magnetic alloy of the present invention, the Fe-based nanocrystals may have an average particle size of 5 to 30 nm.

The soft magnetic alloy of the present invention may have a thin strip shape.

The soft magnetic alloy of the present invention may be in the form of powder.

The magnetic member according to the present invention is made of the soft magnetic alloy.

Detailed Description

Embodiments of the present invention will be described below.

The soft magnetic alloy of the present embodiment has a composition formula (Fe)_(1-(α+β))X1_αX2_β)_{(1-(a+b+c+d+e))}M_aSi_bCu_cX3_dB_eA soft magnetic alloy having the following composition,

x1 is at least one member selected from the group consisting of Co and Ni,

x2 is more than 1 selected from Ti, V, Mn, Ag, Zn, Al, Sn, As, Sb, Bi and rare earth elements,

x3 is at least one member selected from the group consisting of C and Ge,

m is more than 1 selected from Zr, Nb, Hf, Ta, Mo and W,

0.030≤a≤0.120，

0.020≤b≤0.175，

0≤c≤0.020，

0≤d≤0.100，

0≤e≤0.030，

α≥0，

β≥0，

0≤α+β≤0.55。

the soft magnetic alloy having the above composition is amorphous, and is easily a soft magnetic alloy containing no crystal phase having a grain size of more than 15 nm. In addition, when the soft magnetic alloy is heat-treated, Fe-based nanocrystals are likely to precipitate. Furthermore, soft magnetic alloys containing Fe-based nanocrystals tend to have high saturation magnetic flux density, low coercivity, and high electrical resistivity. Further, the oxidation resistance is also likely to be high.

In other words, the soft magnetic alloy having the above composition is easily used as a starting material for the soft magnetic alloy in which Fe-based nanocrystals are precipitated.

The Fe-based nanocrystal is a crystal having a nanoscale particle diameter and a bcc (body-centered cubic lattice structure) crystal structure of Fe. In the present embodiment, it is preferable to precipitate Fe-based nanocrystals having an average particle size of 5 to 30 nm. The saturation magnetic flux density of the soft magnetic alloy in which such Fe-based nanocrystals are precipitated tends to be high, and the coercivity tends to be low. Further, the resistivity is also likely to be high.

The soft magnetic alloy before heat treatment may be entirely composed of only amorphous grains, but is preferably composed of amorphous grains and primary crystallites having a grain size of 15nm or less, and has a nano-heterostructure in which the primary crystallites are present in the amorphous grains. By having a nano-heterostructure in which initial crystallites are present in an amorphous state, Fe-based nanocrystals are easily precipitated at the time of heat treatment. In the present embodiment, the average particle size of the primary crystallites is preferably 0.3 to 10 nm.

Hereinafter, each component of the soft magnetic alloy of the present embodiment will be described in detail.

M is at least 1 selected from Zr, Nb, Hf, Ta, Mo and W. Further, the type of M is preferably composed of only 1 or more species selected from Nb, Hf and Zr. When the kind of M is 1 or more selected from Nb, Hf and Zr, the saturation magnetic flux density tends to be high and the coercivity tends to be low.

The content (a) of M satisfies 0.030. ltoreq. a.ltoreq.0.120. The content (a) of M is preferably 0.050. ltoreq. a.ltoreq.0.100. When a is small, a crystal phase composed of crystals having a particle diameter of more than 15nm is easily generated in the soft magnetic alloy before heat treatment, Fe-based nanocrystals cannot be precipitated by heat treatment, and the coercivity is easily increased. When a is large, the saturation magnetic flux density tends to be low.

The content (b) of Si satisfies 0.020 or more and 0.175 or less. The content (b) of Si preferably satisfies 0.030. ltoreq. b.ltoreq.0.100. When b is small, the coercive force tends to be high. Further, when b is large, the saturation magnetic flux density tends to be low.

Further, as the content (a) of M is smaller, the content (b) of Si is larger, and thus good characteristics tend to be obtained. Conversely, the larger the content (a) of M, the smaller the Si content, and the better the characteristics tend to be obtained.

The content (c) of Cu satisfies that c is more than or equal to 0 and less than or equal to 0.020. That is, Cu may not be contained. The smaller the Cu content is, the higher the saturation magnetic flux density is, and the larger the Cu content is, the lower the coercive force tends to be. When c is too large, the saturation magnetic flux density becomes too low.

X3 is at least 1 selected from C and Ge. The content (d) of X3 satisfies that d is more than or equal to 0 and less than or equal to 0.100. That is, X3 may not be contained. The content (d) of X3 is preferably 0. ltoreq. d.ltoreq.0.050. When the content of X3 is too large, the saturation magnetic flux density tends to be low, and the coercivity tends to be high.

The content (e) of B satisfies that e is more than or equal to 0 and less than or equal to 0.030. That is, B may not be contained. Furthermore, it is preferably 0. ltoreq. e.ltoreq.0.010, and it is more preferably substantially free of B. The term "substantially free of B" means that 0. ltoreq. e < 0.001. When the content of B is large, the saturation magnetic flux density tends to be low, and the coercivity tends to be high.

The Fe content (1- (a + b + c + d + e)) is not particularly limited, but preferably satisfies 0.730. ltoreq.1- (a + b + c + d + e). ltoreq.0.930. Can also meet the requirement that the content of 1- (a + b + c + d + e) is more than or equal to 0.780 and less than or equal to 0.930. When the above range is satisfied, the saturation magnetic flux density is easily increased and the coercive force is easily lowered.

In the soft magnetic alloy of the present embodiment, a part of Fe may be replaced with X1 and/or X2.

X1 is at least 1 selected from Co and Ni. The content (α) of X1 may be α ═ 0. That is, X1 may not be contained. The number of atoms in the entire composition is set to 100 at%, and the number of atoms in X1 is preferably 40 at% or less. That is, it is preferable to satisfy 0. ltoreq. α {1- (a + b + c + d + e) }. ltoreq.0.40.

X2 is more than 1 selected from Ti, V, Mn, Ag, Zn, Al, Sn, As, Sb, Bi and rare earth elements. The content (β) of X2 may be β ═ 0. That is, X2 may not be contained. The number of atoms in the entire composition is set to 100 at%, and the number of atoms in X2 is preferably 3.0 at% or less. That is, it is preferable to satisfy 0. ltoreq. beta {1- (a + b + c + d + e) } 0.030.

The range of the substitution amount of Fe for X1 and/or X2 is 0. ltoreq. alpha. + beta. ltoreq.0.55. When α + β >0.55, it is difficult to produce an Fe-based nanocrystalline alloy by heat treatment, and even if an Fe-based nanocrystalline alloy can be produced, the coercive force tends to be high.

The soft magnetic alloy according to the present embodiment may contain other elements than the above as inevitable impurities. For example, the total content of elements other than the above elements may be less than 1% by weight based on 100% by weight of the soft magnetic alloy.

The method for producing the soft magnetic alloy of the present embodiment will be described below.

The method for producing the soft magnetic alloy of the present embodiment is not particularly limited. For example, there is a method of manufacturing a thin strip of the soft magnetic alloy of the present embodiment by a single-roll method. Further, the thin strip may be a continuous thin strip.

In the single-roll method, first, pure metals of the respective metal elements included in the finally obtained soft magnetic alloy are prepared and weighed so as to have the same composition as that of the finally obtained soft magnetic alloy. Then, pure metals of the respective metal elements are melted and mixed to produce a master alloy. The melting method of the pure metal is not particularly limited, and for example, a method of melting the pure metal by high-frequency heating after evacuating the chamber may be used. In addition, the master alloy and the finally obtained soft magnetic alloy composed of Fe-based nanocrystals are generally the same composition.

Next, the prepared master alloy is heated and melted to obtain molten metal (melt). The temperature of the molten metal is not particularly limited, and may be set to 1200 to 1500 ℃.

In the single roll method, the thickness of the obtained ribbon can be adjusted mainly by adjusting the rotation speed of the roll, and for example, the thickness of the obtained ribbon can be adjusted by adjusting the distance between the nozzle and the roll, the temperature of the molten metal, and the like. The thickness of the ribbon is not particularly limited, and may be, for example, 5 to 30 μm.

Before the heat treatment described later, the ribbon is amorphous without crystals having a particle size of more than 15 nm. The Fe-based nanocrystalline alloy can be obtained by subjecting the amorphous thin strip to a heat treatment described later.

Further, the method for confirming whether or not crystals having a particle size of more than 15nm are contained in the ribbon of the soft magnetic alloy before the heat treatment is not particularly limited. For example, the presence or absence of crystals having a particle size of more than 15nm can be confirmed by ordinary X-ray diffraction measurement.

The ribbon before heat treatment may not contain any primary crystallites having a particle size of less than 15nm at all, but preferably contains primary crystallites. That is, the ribbon before heat treatment is preferably a nano-heterostructure composed of an amorphous state and the primary crystallites present in the amorphous state. The particle size of the primary crystallites is not particularly limited, but is preferably in the range of 0.3 to 10nm in average particle size.

The presence or absence of the above-described primary crystallites and the observation method of the average particle size are not particularly limited, and can be confirmed by obtaining a selected area diffraction image, a nanobeam diffraction image, a bright field image, or a high-resolution image using a transmission electron microscope with respect to a sample flaked by ion milling, for example. When the diffraction pattern is amorphous in the case of using a selective area diffraction pattern or a nanobeam diffraction patternForm annular diffraction, and on the other hand, when the material is not amorphous, form diffraction spots due to a crystal structure, and when a bright field image or a high resolution image is used, the magnification is 1.00 × 10⁵～3.00×10⁵The presence or absence of the primary crystallites and the average particle size can be observed visually.

The temperature, rotation speed, and atmosphere inside the chamber of the roller are not particularly limited. For amorphization, the roll temperature is preferably set to 4 to 30 ℃. The average particle size of the primary crystallites tends to decrease as the rotation speed of the roll increases, and it is preferable to set the average particle size to 30 to 40m/sec so as to obtain primary crystallites having an average particle size of 0.3 to 10 nm. The atmosphere inside the chamber is preferably set to the atmosphere if cost is taken into consideration.

The heat treatment conditions for producing the Fe-based nanocrystalline alloy are not particularly limited. The preferable heat treatment conditions vary depending on the composition of the soft magnetic alloy. In general, the heat treatment temperature is preferably 400 to 600 ℃ and the heat treatment time is preferably 10 minutes to 10 hours. However, there may be a case where a preferable heat treatment temperature and heat treatment time are out of the above ranges depending on the composition. The atmosphere during the heat treatment is not particularly limited. The reaction may be carried out in an active atmosphere such as air or in an inert atmosphere such as Ar gas.

The method for calculating the average particle diameter of the Fe-based nanocrystalline alloy obtained is not particularly limited. For example, it can be calculated by observation using a transmission electron microscope. Further, a method for confirming that the crystal structure is bcc (body-centered cubic lattice structure) is not particularly limited. For example, the confirmation can be performed by using X-ray diffraction measurement.

As a method for obtaining the soft magnetic alloy of the present embodiment, there is a method for obtaining a powder of the soft magnetic alloy of the present embodiment by, for example, a water atomization method or a gas atomization method, in addition to the above-described single roll method. Hereinafter, the gas atomization method will be described.

In the gas atomization method, a molten alloy at 1200 to 1500 ℃ is obtained in the same manner as in the single-roll method. Then, the molten alloy is sprayed into the chamber to produce powder.

In this case, the preferable nano-heterostructure can be easily obtained by setting the gas ejection temperature to 4 to 30 ℃ and setting the vapor pressure in the chamber to 1hPa or less.

After the powder is produced by the gas atomization method, the powder is heat-treated at 400 to 600 ℃ for 0.5 to 10 minutes, whereby the powder is prevented from being sintered to coarsened, the diffusion of elements is promoted, the thermodynamic equilibrium state can be reached in a short time, the strain and stress can be removed, and the Fe-based soft magnetic alloy having an average particle diameter of 10 to 50nm can be easily obtained.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment.

The shape of the soft magnetic alloy of the present embodiment is not particularly limited. As described above, the shape of a thin strip or a powder may be exemplified, and a block shape may be considered.

The use of the soft magnetic alloy (Fe-based nanocrystalline alloy) of the present embodiment is not particularly limited. For example, magnetic components are cited, and among them, a magnetic core is particularly cited. Can be suitably used as a magnetic core for inductors, particularly power inductors. The soft magnetic alloy of the present embodiment can be applied to a thin film inductor and a magnetic head, in addition to the magnetic core.

Hereinafter, a method of obtaining a magnetic component, particularly a core and an inductor, from the soft magnetic alloy of the present embodiment will be described, but the method of obtaining a core and an inductor from the soft magnetic alloy of the present embodiment is not limited to the following method. Further, as applications of the magnetic core, in addition to the inductor, a transformer, a motor, and the like can be cited.

Examples of a method for obtaining a magnetic core from a soft magnetic alloy in a thin strip shape include a method of winding a soft magnetic alloy in a thin strip shape and a method of laminating the soft magnetic alloy. When the soft magnetic alloys in the form of thin strips are laminated via an insulator, a magnetic core having further improved characteristics can be obtained.

As a method for obtaining a magnetic core from a powder-shaped soft magnetic alloy, for example, a method of appropriately mixing a binder and then molding the mixture using a mold is given. Further, by subjecting the powder surface to oxidation treatment, an insulating coating, or the like before mixing with the binder, the resistivity is improved, and the magnetic core is more suitable for a high-frequency band.

The molding method is not particularly limited, and molding using a mold, press molding, and the like can be mentioned. The type of the binder is not particularly limited, and silicone resin may be mentioned. The mixing ratio of the soft magnetic alloy powder and the binder is also not particularly limited. For example, the binder is mixed in an amount of 1 to 10 mass% based on 100 mass% of the soft magnetic alloy powder.

For example, by mixing 1 to 5 mass% of a binder with 100 mass% of soft magnetic alloy powder and compression molding the mixture using a die, a space factor (powder packing ratio) of 70% or more and a value of 1.6 × 10⁴A magnetic core having a magnetic flux density of 0.45T or more and a resistivity of 1. omega. cm or more in an A/m magnetic field. The above characteristics are equal to or more than those of a general ferrite core.

For example, by mixing 1 to 3 mass% of a binder with 100 mass% of the soft magnetic alloy powder and compression molding the mixture with a mold under a temperature condition of the softening point of the binder or higher, a space factor of 80% or more can be obtained, and 1.6 × 10 is applied⁴A powder magnetic core having a magnetic flux density of 0.9T or more in an A/m magnetic field and a resistivity of 0.1. omega. cm or more. The above characteristics are superior to those of a normal powder magnetic core.

Further, the molded body constituting the magnetic core is subjected to heat treatment after molding as strain relief heat treatment, whereby the core loss is further reduced and the usefulness is improved. Further, the core loss of the magnetic core is reduced by reducing the coercive force of the magnetic material constituting the magnetic core.

Further, an inductance component is obtained by winding the core. The method of implementing the winding and the method of manufacturing the inductance component are not particularly limited. For example, a method of winding a winding at least 1 turn around the magnetic core manufactured by the above-described method is given.

In the case of using soft magnetic alloy particles, there is a method of manufacturing an inductance component by integrating the soft magnetic alloy particles by press molding in a state where the coil is incorporated in the magnetic body. In this case, an inductance component corresponding to a high frequency and a large current can be easily obtained.

In the case of using soft magnetic alloy particles, a soft magnetic alloy paste obtained by adding a binder and a solvent to the soft magnetic alloy particles and a conductor paste obtained by adding a binder and a solvent to a conductor metal for a coil are alternately printed and then heated and fired, whereby an inductance component can be obtained. Alternatively, a soft magnetic alloy sheet is produced using a soft magnetic alloy paste, a conductor paste is printed on the surface of the soft magnetic alloy sheet, and the soft magnetic alloy sheet and the conductor paste are laminated and fired, whereby an inductance component in which a coil is incorporated in a magnetic body can be obtained.

Here, in the case of manufacturing an inductance component using soft magnetic alloy particles, in order to obtain excellent Q characteristics, it is preferable to use soft magnetic alloy powder having a maximum particle diameter of 45 μm or less in terms of mesh diameter and a center particle diameter (D50) of 30 μm or less. In order to make the maximum particle diameter 45 μm or less in terms of mesh diameter, a sieve having a mesh size of 45 μm may be used, and only the soft magnetic alloy powder passing through the sieve may be used.

The Q value tends to decrease in the high frequency region as the maximum particle size of the soft magnetic alloy powder is larger, and particularly, in the case of using the soft magnetic alloy powder having the maximum particle size exceeding 45 μm in mesh diameter, the Q value may decrease significantly in the high frequency region. However, when the Q value in the high frequency region is not regarded as important, a soft magnetic alloy powder having a large dispersion can be used. Since the soft magnetic alloy powder having a large dispersion can be manufactured at a low price, the cost can be reduced when the soft magnetic alloy powder having a large dispersion is used.