Patent Publication Number: US-8123874-B2

Title: Soft magnetic material, dust core, method for manufacturing soft magnetic material, and method for manufacturing dust core

Description:
TECHNICAL FIELD 
     The present invention relates to a soft magnetic material, a dust core, a method for manufacturing the soft magnetic material, and a method for manufacturing the dust core. For example, the present invention relates to a soft magnetic material that does not easily cause magnetic saturation and provides excellent direct current (DC) bias characteristics when used for a magnetic core of an inverter or the like, a dust core, a method for manufacturing the soft magnetic material, and a method for manufacturing the dust core. 
     BACKGROUND ART 
     A magnetic steel sheet has been used as a soft magnetic material utilized for an iron core of a static apparatus such as a transformer, a choke coil, and an inverter. However, a dust core is investigated as an alternative material of the magnetic steel sheet. 
     In general, the waveform of a current applied to a coil of a static apparatus includes a direct-current component together with an alternating-current component. When a DC current increases, the inductance of the coil decreases. As a result, the impedance decreases, thereby causing a problem in that, for example, an output decreases or a power conversion efficiency drops. Therefore, a soft magnetic material used for a static apparatus is required to have characteristics such as a low inductance drop with an increase in a DC current, that is, excellent DC bias characteristics and a low loss (low iron loss). 
     However, dust cores are inferior to magnetic steel sheets in terms of DC bias characteristics. This is because an inductance drop with an increase in a DC current is caused by magnetic saturation of soft magnetic materials. Specifically, the magnetic field applied to soft magnetic materials becomes large with increasing DC current. Consequently, magnetic saturation decreases magnet permeability. Since inductance is proportional to magnetic permeability, inductance drops. 
     To improve the DC bias characteristics of dust cores, a method for manufacturing a core and the core are disclosed in Japanese Unexamined Patent Application Publication No. 2004-319652 (Patent Document 1). Patent Document 1 discloses that an irregular soft magnetic powder having a particle size of 5 to 70 μm is used.
     [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2004-319652   

     DISCLOSURE OF INVENTION 
     Problems to be Solved by the Invention 
     However, in the core disclosed in Patent Document 1, only a range of a particle size is specified, and therefore there exists the variation in the particle size of the powder within the range described above. Accordingly, when the powder is molded, the uniformity of the inside of the core is decreased and there is still room for improvement in terms of DC bias characteristics. 
     To solve the problem described above, it is an object of the present invention to provide a soft magnetic material, a dust core, a method for manufacturing the soft magnetic material, and a method for manufacturing the dust core that can improve DC bias characteristics. 
     Means for Solving the Problems 
     A soft magnetic material of the present invention includes a plurality of metal magnetic particles. In the soft magnetic material, a coefficient of variation Cv (σ/μ), which is a ratio of a standard deviation (σ) of a particle size of the metal magnetic particles to an average particle size (μ) thereof, is 0.40 or less and a circularity Sf of the metal magnetic particles is 0.80 or more and 1 or less. 
     A method for manufacturing a soft magnetic material of the present invention includes a preparation step of preparing a plurality of metal magnetic particles. In the preparation step, the metal magnetic particles whose coefficient of variation Cv (σ/μ), which is a ratio of a standard deviation (σ) of a particle size to an average particle size (μ), is 0.40 or less and whose circularity Sf is 0.80 or more and 1 or less are prepared. 
     In the soft magnetic material and the method for manufacturing the soft magnetic material of the present invention, the particle size distribution of the Metal magnetic particles can be uniformized by controlling the coefficient of variation Cv of the metal magnetic particles to 0.40 or less. Thus, the uniformity of the inside of a compact made of the soft magnetic material by compacting can be improved. This can facilitate the domain wall motion in a magnetization process and improve DC bias characteristics. Furthermore, because a distortion arising on a surface of each of the metal magnetic particles when the soft magnetic material is pressure-molded can be reduced by controlling the circularity Sf of the metal magnetic particles to 0.80 or more, the DC bias characteristics can be improved. In a case where the external shape of the metal magnetic particles is completely spherical, the circularity Sf of the metal magnetic particles is 1. 
     “The standard deviation (σ) of a particle size” mentioned herein means a value calculated from the particle size of the metal magnetic particles measured by a laser diffraction/scattering particle size distribution analysis method. “The average particle size (μ) of the metal magnetic particles” mentioned herein means a particle size of a particle at which the cumulative sum of the masses of particles starting from the smallest particle size reaches 50% in a histogram of particle sizes of the metal magnetic particles measured by a laser diffraction/scattering particle size distribution analysis method, that is, a 50% particle size. “The circularity of the metal magnetic particles” is specified by the following Eq. 1. In Eq. 1, the area and circumference of the metal magnetic particles can be determined by an optical method. For example, in the optical method, the area and circumference are statistically calculated from a projection image of each of the metal magnetic particles obtained by projecting the metal magnetic particles to be measured, using a commercially available image-processing device.
 
Circularity=4π×(Area of Metal Magnetic Particle)/(Square of Circumference of Metal Magnetic Particle)  (Eq. 1)
 
     In the soft magnetic material described above, the metal magnetic particles preferably have an average particle size of 1 μm or more and 70 μm or less. 
     In the method for manufacturing the soft magnetic material described above, in the preparation step, the metal magnetic particles having an average particle size of 1 μm or more and 70 μm or less are preferably prepared. 
     By controlling the average particle size of the metal magnetic particles to 1 μm or more, an increase in a coercive force and a hysteresis loss of a dust core made of the soft magnetic material can be suppressed without decreasing the flowability of the soft magnetic material. By controlling the average particle size of the metal magnetic particles to 70 μm or less, an eddy current loss arising in a high-frequency range of 1 kHz or more can be effectively reduced. 
     The soft magnetic material described above preferably further includes an additive composed of at least one of a metallic soap and an inorganic lubricant with a hexagonal crystal structure. In the soft magnetic material, a ratio of the additive to the plurality of metal magnetic particles is preferably 0.001% by mass or more and 0.2% by mass or less. 
     The method for manufacturing the soft magnetic material described above preferably further includes an addition step of adding an additive composed of at least one of a metallic soap and an inorganic lubricant with a hexagonal crystal structure, a ratio of the additive to the plurality of metal magnetic particles being 0.001% by mass or more and 0.2% by mass or less. 
     By controlling the ratio of the additive to 0.001% by mass or more, the flowability of the metal magnetic particles can be improved due to high lubricity of the metallic soap and the inorganic lubricant with a hexagonal crystal structure. This can improve the filling properties of the soft magnetic material when the soft magnetic material is filled in a die. As a result, since the density of a compact into which the soft magnetic material is molded can be increased, the DC bias characteristics can be improved. By controlling the ratio of the additive to 0.2% by mass or less, a decrease in the density of a compact into which the soft magnetic material is molded can be suppressed. This can prevent the degradation of the DC bias characteristics. 
     The soft magnetic material described above preferably further includes an insulating coated film that surrounds a surface of each of the metal magnetic particles. 
     The method for manufacturing the soft magnetic material described above preferably further includes an insulating coated film formation step of forming an insulating coated film on a surface of each of the metal magnetic particles. 
     Since the insulating coated film surrounds a surface of each of the metal magnetic particles having a circularity Sf of 0.80 or more, the insulating coated film is formed between the metal magnetic particles in a compact. As a result, the metal magnetic particles can be effectively insulated, thereby decreasing an eddy current loss. Thus, an iron loss can be effectively reduced in a high-frequency range. 
     Particularly in a case where the soft magnetic material further includes at least one of a metallic soap and an inorganic lubricant with a hexagonal crystal structure, the damage to the insulating coated film can be further reduced when the soft magnetic material is molded. Consequently, the insulation properties between the metal magnetic particles can be further improved even in a high temperature atmosphere, thereby further decreasing an eddy current loss. Thus, an iron loss can be more effectively reduced in a high-frequency range. 
     In the soft magnetic material described above, the insulating coated film is preferably composed of at least one material selected from the group consisting of a phosphoric acid compound, a silicon compound, a zirconium compound, and a boron compound. 
     In the method for manufacturing the soft magnetic material described above, in the insulating coated film formation step, the insulating coated film composed of at least one material selected from the group consisting of a phosphoric acid compound, a silicon compound, a zirconium compound, and a boron compound is preferably formed. 
     Because these materials have excellent insulation properties, an eddy current flowing between the metal magnetic particles can be more effectively suppressed. 
     In the soft magnetic material described above, the insulating coated film is preferably one insulating coated film; the metal magnetic particles each preferably includes another insulating coated film that surrounds a surface of the one insulating coated film; and the other insulating coated film preferably contains a thermosetting silicone resin. 
     In the method for manufacturing the soft magnetic material described above, a insulating coated film formation step preferably includes one insulating coated film formation step of forming the insulating coated film as one insulating coated film; and another insulating coated film formation step of forming another insulating coated film that surrounds a surface of the one insulating coated film. In the other insulating coated film formation step, the other insulating coated film containing a thermosetting silicone resin is preferably formed. 
     The one insulating coated film is protected by the other insulating coated film, whereby the temperature increase of the insulating coated film can be suppressed by the other insulating coated film during the heat-treatment of the soft magnetic material. Thus, the soft magnetic material in which the heat-resistance of the insulating coated film is improved is achieved. The material described above has high heat-resistance while increasing the bonding strength between composite magnetic particles each including each of the metal magnetic particles and the insulating coated film. 
     A dust core of the present invention is manufactured using the soft magnetic material. A method for manufacturing a dust core of the present invention includes the steps of manufacturing a soft magnetic material using the method for manufacturing the soft magnetic material; and manufacturing the dust core by compacting the soft magnetic material. 
     Advantages 
     As is seen, in the soft magnetic material and the method for manufacturing the soft magnetic material of the present invention, the plurality of metal magnetic particles whose coefficient of variation Cv is 0.40 or less and circularity Sf is 0.80 or more and 1 or less are included, which can improve DC bias characteristics. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view showing a soft magnetic material according to an embodiment of the present invention. 
         FIG. 2  is an enlarged sectional view of a dust core according to the embodiment of the present invention. 
         FIG. 3  is a schematic view showing the particle size distribution of a metal magnetic particle according to the embodiment of the present invention and the particle size distribution of a metal magnetic particle of a known example. 
         FIG. 4A  is a schematic view showing a shape of the metal magnetic particle according to the embodiment of the present invention. 
         FIG. 4B  is a schematic view showing a shape of a metal magnetic particle of the known example. 
         FIG. 5  is a schematic view showing another soft magnetic material according to the embodiment of the present invention. 
         FIG. 6  is a flowchart showing a method for manufacturing the soft magnetic material according to the embodiment of the present invention. 
         FIG. 7  is a schematic view showing another dust core according to the embodiment of the present invention. 
         FIG. 8  is a graph showing a relationship between magnetic field and flux density according to the embodiment of the present invention. 
         FIG. 9  is a graph showing a relationship between DC current and inductance according to the embodiment of the present invention. 
         FIG. 10  is a schematic view showing a device for measuring DC bias characteristics in Examples. 
         FIG. 11  is a graph showing DC bias characteristics in Examples. 
     
    
    
     REFERENCE NUMERALS 
     
         
         
           
               10  metal magnetic particle 
               20  insulating coated film 
               20   a  one insulating coated film 
               20   b  another insulating coated film 
               30  composite magnetic particle 
               40  additive 
               50  insulation 
           
         
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     An embodiment of the present invention will now be described with reference to drawings. The same or corresponding parts in the drawings are designated by the same reference numerals, and the descriptions are not repeated. 
       FIG. 1  is a schematic view showing a soft magnetic material according to an embodiment of the present invention. As shown in  FIG. 1 , the soft magnetic material according to this embodiment includes a plurality of composite magnetic particles  30  each having a metal magnetic particle  10  and an insulating coated film  20  that surrounds a surface of the metal magnetic particle  10 ; and an additive  40  composed of at least one of a metallic soap and an inorganic lubricant with a hexagonal crystal structure. 
       FIG. 2  is an enlarged sectional view of a dust core according to the embodiment of the present invention. The dust core of  FIG. 2  is manufactured by compacting and heat-treating the soft magnetic material of  FIG. 1 . In the dust core of this embodiment, as shown in  FIGS. 1 and 2 , the plurality of composite magnetic particles  30  are bonded to each other by an insulation  50  or by the engagement of the projections and indentations of the composite magnetic particles  30 . The insulation  50  is the one into which the additive  40 , resins (not shown), and the like included in the soft magnetic material are changed during the heat treatment. 
     In the soft magnetic material and the dust core of the present invention, a coefficient of variation Cv (σ/μ), which is a ratio of the standard deviation (σ) of the particle size of the metal magnetic particle  10  to its average particle size (μ), is 0.40 or less and the circularity Sf of the metal magnetic particle  10  is 0.80 or more and 1 or less. 
     The coefficient of variation Cv of the metal magnetic particle  10  is 0.40 or less, preferably 0.38 or less, more preferably 0.36 or less. Because the particle size distribution can be uniformized by controlling the coefficient of variation Cv to 0.40 or less, the uniformity of the inside of a compact made of the soft magnetic material can be improved. This can facilitate the domain wall motion in a magnetization process and improve DC bias characteristics. The DC bias characteristics can be further improved by controlling the coefficient of variation Cv to 0.38 or less. The DC bias characteristics can be more effectively improved by controlling the coefficient of variation Cv to 0.36 or less. Although the coefficient of variation Cv preferably has a smaller value, it is 0.001 or more in terms of, for example, ease of manufacturing. 
       FIG. 3  is a schematic view showing the particle size distribution of the metal magnetic particle  10  according to the embodiment of the present invention and the particle size distribution of a metal magnetic particle of a known example. As shown in  FIG. 3 , since the coefficient of variation of the metal magnetic particle  10  according to this embodiment (invention example in  FIG. 3 ) is 0.40 or less, the standard deviation (σ) of its particle size, that is, the variation of its particle size is smaller than that in the known example. 
     The circularity Sf of the metal magnetic particle  10  is 0.80 or more and 1 or less, preferably 0.91 or more and 1 or less, more preferably 0.92 or more and 1 or less. Because a distortion arising on a surface of the metal magnetic particle when the soft magnetic material is molded can be reduced by controlling the circularity Sf to 0.80 or more, the DC bias characteristics can be improved. The DC bias characteristics can be further improved by controlling the circularity Sf to 0.91 or more. The DC bias characteristics can be more effectively improved by controlling the circularity Sf to 0.92 or more. In a case where the external shape of the metal magnetic particle is completely spherical, the circularity Sf of the metal magnetic particle is 1. 
       FIG. 4A  is a schematic view showing a shape of the metal magnetic particle  10  according to the embodiment of the present invention.  FIG. 4B  is a schematic view showing a shape of a metal magnetic particle  11  of the known example. As shown in  FIGS. 4A and 4B , since the circularity Sf of the metal magnetic particle  10  according to this embodiment is 0.80 or more and 1 or less, the metal magnetic particle  10  is more spherical than the metal magnetic particle  11  of the known example. 
     The average particle size (g) of the metal magnetic particle  10  is preferably 1 μm or more and 70 μm or less, more preferably 1 μm or more and 65 μm or less, more preferably 20 μm or more and 60 μm or less. By controlling the average particle size of the metal magnetic particle  10  to 1 μm or more, an increase in a coercive force and a hysteresis loss of the dust core made of the soft magnetic material can be suppressed without decreasing the flowability of the soft magnetic material. By controlling the average particle size to 20 μm or more, an increase in a coercive force and a hysteresis loss of the dust core made of the soft magnetic material can be further suppressed. By controlling the average particle size of the metal magnetic particle  10  to 70 μm or less, an eddy current loss arising in a high-frequency range of 1 kHz or more can be effectively reduced. By controlling the average particle size to 65 μm or less, an eddy current loss can be more effectively reduced. By controlling the average particle size to 60 μm or less, an eddy current loss can be far more effectively reduced. 
     Examples of the material forming the metal magnetic particle  10  include iron (Fe), iron (Fe)-aluminum (Al) alloys, iron (Fe)-silicon (Si) alloys, iron (Fe)-nitrogen (N) alloys, iron (Fe)-nickel (Ni) alloys, iron (Fe)-carbon (C) alloys, iron (Fe)-boron (B) alloys, iron (Fe)-cobalt (Co) alloys, iron (Fe)-phosphorus (P) alloys, iron (Fe)-nickel (Ni)-cobalt (Co) alloys, iron (Fe)-aluminum (Si) alloys, iron (Fe)-aluminum (Al)-chromium(Cr) alloys, iron (Fe)-aluminum (Al)-manganese (Mn) alloys, iron (Fe)-aluminum (Al)-nickel (Ni) alloys, iron (Fe)-silicon (Si)-chromium(Cr) alloys, iron (Fe)-silicon (Si)-manganese (Mn) alloys, and iron (Fe)-silicon (Si)-nickel (Ni) alloys. The metal magnetic particle  10  may be made of a single metal or an alloy. 
     The soft magnetic material shown in  FIG. 1  and the dust core shown in  FIG. 2  preferably further include the insulating coated film  20  that surrounds a surface of the metal magnetic particle  10 . The insulating coated film  20  functions as an insulating layer between the metal magnetic particles  10 . The electric resistivity ρ of the dust core obtained after compacting the soft magnetic material can be increased by coating the metal magnetic particle  10  with the insulating coated film  20 . This can suppress an eddy current flowing between the metal magnetic particles  10  and reduce an eddy current loss of the dust core. 
     The average film thickness of the insulating coated film  20  is preferably 10 nm or more and 1 μm or less. An eddy current loss can be effectively suppressed by controlling the average film thickness of the insulating coated film  20  to 10 nm or more. Share fracture of the insulating coated film  20  during compacting can be prevented by controlling the average film thickness of the insulating coated film  20  to 1 μm or less. Furthermore, since the ratio of the insulating coated film  20  to the soft magnetic material does not become too high, the flux density of the dust core obtained after compacting the soft magnetic material can be prevented from significantly decreasing. 
     The “average thickness” mentioned herein is determined by deriving an equivalent thickness, taking into account the film composition measured by composition analysis (transmission electron microscope energy dispersive X-ray spectroscopy (TEM-EDX)) and the element contents measured by inductively coupled plasma-mass spectroscopy (ICP-MS), and then by directly observing the film using a TEM image and confirming that the order of magnitude of the equivalent thickness derived above is a proper value. 
     The insulating coated film  20  is preferably composed of at least one material selected from the group consisting of a phosphoric acid compound, a silicon compound, a zirconium compound, and a boron compound. Because these materials have excellent insulation properties, an eddy current flowing between the metal magnetic particles  10  can be effectively suppressed. Specifically, the insulating coated film  20  is preferably composed of silicon oxide, zirconium oxide, or the like. In particular, a coating layer that coats a surface of the metal magnetic particle can be further thinned by using a phosphate-containing metal oxide for the insulating coated film  20 . This is because the flux density of the composite magnetic particles  30  can be increased by using such a metal oxide and the magnetic characteristics thereof are improved. 
     The insulating coated film  20  may be composed of a metal such as Fe (iron), Al (aluminum), Ca (calcium), Mn (manganese), Zn (zinc), Mg (magnesium), V (vanadium), Cr (chromium), Y (yttrium), Ba (barium), or Sr (strontium). It may be composed of a metal oxide of a rare-earth element, a metal nitride, a metal oxide, a metal phosphate compound, a metal borate compound, a metal silicate compound, or the like. 
     The insulating coated film  20  may also be composed of an amorphous phosphate compound of at least one material selected from the group consisting of Al (aluminum), Si (silicon), Mg (magnesium), Y (yttrium), Ca (calcium), Zr (zirconium), and Fe (iron), and an amorphous borate compound of the at least one material. 
     The insulating coated film  20  may also be composed of an amorphous oxide compound of at least one material selected from the group consisting of Si, Mg, Y, Ca, and Zr. 
     Although a case where a composite magnetic particle constituting a soft magnetic material has an insulating coated film with one layer is shown above, the composite magnetic particle constituting the soft magnetic material may have an insulating coated film with a plurality of layers as described below. 
       FIG. 5  is a schematic view showing another soft magnetic material according to the embodiment of the present invention. In the other soft magnetic material according to this embodiment, as shown in  FIG. 5 , the insulating coated film  20  includes one insulating coated film  20   a  and another insulating coated film  20   b . The one insulating coated film  20   a  surrounds a surface of the metal magnetic particle  10  and the other insulating coated film  20   b  surrounds a surface of the one insulating coated film  20   a.    
     The one insulating coated film  20   a  has substantially the same structure as the insulating coated film  20  shown in  FIGS. 1 and 2 . 
     A silicone resin, a thermoplastic resin, a non-thermoplastic resin, or a metal salt of higher fatty acid is preferably used as the other insulating coated film  20   b . Specifically, a thermoplastic resin such as thermoplastic polyimide, thermoplastic polyamide, thermoplastic polyamide-imide, polyphenylene sulfide, polyethersulfone, polyetherimide or polyether ether ketone, high-molecular-weight polyethylene, or wholly aromatic polyester; a non-thermoplastic resin such as wholly aromatic polyimide or non-thermoplastic polyamide-imide; or a metal salt of higher fatty acid such as zinc stearate, lithium stearate, calcium stearate, lithium palmitate, calcium palmitate, lithium oleate, or calcium oleate is preferably used. In particular, the insulating coated film  20   b  is preferably composed of a thermosetting silicone resin. These organic materials can also be used as a mixture. The high-molecular-weight polyethylene is polyethylene with a molecular weight of 100 thousands or more. 
     Each of the one insulating coated film  20   a  and the other insulating coated film  20   b  is not necessarily constituted by a single layer. Each of the one insulating coated film  20   a  and the other insulating coated film  20   b  may be constituted by a plurality of layers. 
     The soft magnetic material shown in  FIG. 1  and the dust core shown in  FIG. 2  preferably further include the additive  40  composed of at least one of a metallic soap and an inorganic lubricant with a hexagonal crystal structure. 
     Examples of the metallic soap include zinc stearate, lithium stearate, calcium stearate, lithium palmitate, calcium palmitate, lithium oleate, and calcium oleate. Examples of the inorganic lubricant with a hexagonal crystal structure include boron nitride, molybdenum disulfide, tungsten disulfide, and graphite. 
     The additive  40  is preferably included such that the ratio of the additive  40  to the plurality of metal magnetic particles  10  is 0.001% by mass or more and 0.2% by mass or less, more preferably 0.001% by mass or more and 0.1% by mass or less. By controlling the ratio of the additive  40  to 0.001% by mass or more, the flowability of the metal magnetic particles  10  can be improved due to high lubricity of the metallic soap and the inorganic lubricant with a hexagonal crystal structure. This can improve the filling properties of the soft magnetic material when the soft magnetic material is filled in a die. As a result, since the density of a compact into which the soft magnetic material is molded can be increased, the DC bias characteristics can be improved. By controlling the ratio of the additive  40  to 0.2% by mass or less, a decrease in the density of a compact into which the soft magnetic material is molded can be suppressed. This can prevent the degradation of the DC bias characteristics. 
     In particular, because the metallic soap and the inorganic lubricant with a hexagonal crystal structure constituting the additive  40  can impart good lubricity that suppresses damage to the insulating coated film  20 , the damage to the insulating coated film  20  can be further reduced when the soft magnetic material is molded. As a result, the bonding strength between the metal magnetic particles  10  adjoining each other is maintained even in a high-temperature environment, which can further reduce an eddy current loss. Thus, an iron loss can be more effectively reduced in a high-frequency range. 
     The average particle size of the additive  40  is preferably 2.0 μm or less. By controlling the average particle size to 2.0 μm or less, the damage to the insulating coated film  20  can be further reduced when the soft magnetic material is pressure-molded, which can further reduce an iron loss. 
     “The average particle size of the additive  40 ” mentioned herein means a particle size of a particle at which the cumulative sum of the masses of particles starting from the smallest particle size reaches 50% in a histogram of particle sizes measured by a laser scattering/diffraction method, that is, a 50% particle size D. 
     The soft magnetic material shown in  FIG. 1  may further include a lubricant or the like other than the additive  40  described above and a resin (not shown). 
     A method for manufacturing the soft magnetic material of the present invention will now be described with reference to  FIG. 6 .  FIG. 6  is a flowchart showing a method for manufacturing the soft magnetic material according to the embodiment of the present invention. 
     As shown in  FIG. 6 , a preparation step (S 11 ) of preparing a plurality of metal magnetic particles  10  is conducted first. In the preparation step (S 11 ), the metal magnetic particles  10  whose coefficient of variation Cv (σ/μ), which is the ratio of the standard deviation (σ) of the particle size of the metal magnetic particles  10  to the average particle size (μ) of the metal magnetic particles  10 , is 0.4 or less and whose circularity Sf is 0.8 or more and 1 or less, are prepared. 
     In the preparation step (S 11 ), the plurality of metal magnetic particles  10  described above are prepared. These metal magnetic particles  10  are prepared by, for example, atomizing iron having a certain composition by an atomizing method, a water-atomizing method, or the like. In particular, in the preparation step (S 11 ), the metal magnetic particles  10  having an average particle size of 1 μm or more and 70 μm or less are preferably prepared. 
     As shown in  FIG. 6 , a first heat-treatment step (S 12 ) of heat-treating the plurality of metal magnetic particles  10  is then conducted. In the first heat-treatment step (S 12 ), the plurality of metal magnetic particles  10  are heat-treated at a temperature of, for example, 700° C. or more and less than 1400° C. Before the heat-treatment, there are many defects such as distortions and grain boundaries caused by thermal stress or the like in an atomizing process inside the metal magnetic particles  10 . These defects can be reduced by conducting the heat-treatment on the metal magnetic particles  10  in the first heat-treatment step (S 12 ). The first heat-treatment step (S 12 ) may be omitted. 
     As shown in  FIG. 6 , an insulating coated film formation step (S 13 ) of forming an insulating coated film  20  on a surface of each of the metal magnetic particles  10  is then conducted. In the insulating coated film formation step (S 13 ), the insulating coated film  20  described above (or one insulating coated film  20   a  and another insulating coated film  20   b ) is formed on a surface of each of the metal magnetic particles  10 . Thus, a plurality of composite magnetic particles  30  are produced. 
     In the insulating coated film formation step (S 13 ), the insulating coated film  20  composed of a phosphate can be formed by, for example, subjecting the metal magnetic particles  10  to phosphating treatment. Solvent spraying or sol-gel treatment using a precursor can be used as the method for forming the insulating coated film  20  composed of a phosphate instead of the phosphating treatment. Alternatively, the insulating coated film  20  may instead be formed of a silicon organic compound. This insulating coated film can be formed by wet coating treatment using an organic solvent, direct coating treatment with a mixer, or the like. 
     In the insulating coated film formation step (S 13 ), the insulating coated film  20  composed of at least one material selected from the group consisting of a phosphoric compound, a silicon compound, a zirconium compound, and a boron compound is preferably formed. Specifically, the insulating coated film  20  composed of iron phosphate, manganese phosphate, zinc phosphate, calcium phosphate, silicon phosphate, zirconium phosphate, or the like is preferably formed. 
     In a case where the soft magnetic material having the one insulating coated film  20  with a plurality of layers is manufactured, as shown in  FIG. 5 , the insulating coated film formation step (S 13 ) includes an insulating coated film step of forming the insulating coated film  20  as one insulating coated film  20   a  and another insulating coated film formation step of forming another insulating coated film  20   b  that surrounds a surface of the one insulating coated film  20   a . The other insulating coated film  20   b  is preferably contains a thermosetting silicone resin. 
     In a case where an insulating coated film with two layers shown in  FIG. 5  is formed, each of the metal magnetic particles  10  having the one insulating coated film  20   a  is mixed with an additive  40  added in an addition step (S 14 ) described below to form the other insulating coated film  20   b.    
     Instead of the method described above, the other insulating coated film  20   b  may be formed by mixing or spraying a silicone resin dissolved in an organic solvent and then by drying the silicone resin to remove the organic solvent. 
     As shown in  FIG. 6 , the addition step (S 14 ) of adding the additive  40  composed of at least one of a metallic soap and an inorganic lubricant with a hexagonal crystal structure, such that the ratio of the additive  40  to the plurality of metal magnetic particles  10  is 0.001% by mass or more and 0.2% by mass or less, is then conducted. In the addition step (S 14 ), the metal magnetic particles  10  are mixed with the additive  40 . The mixing method is not limited. Examples of the method include mechanical alloying, vibrating ball mill, planetary ball mill, mechanofusion, coprecipitation, chemical vapor deposition (CVD), physical vapor deposition (PVD), plating, sputtering, vapor deposition, and sol-gel methods. A resin or another additive may be optionally added. 
     Through the steps (S 11  to  514 ) described above, the soft magnetic material of this embodiment is obtained. To manufacture the dust core of this embodiment, the following steps will be further conducted. 
     A compacting step (S 21 ) of filling a die with the resultant soft magnetic material and compacting it is then conducted. In the compacting step (S 21 ), the soft magnetic material is pressure-molded at a pressure of 390 MPa or more and 1500 MPa or less. As a result, a compact into which the soft magnetic material is pressure-molded is obtained. The compacting is preferably conducted in an inert gas atmosphere or a reduced-pressure atmosphere. In this case, a mixed powder can be prevented from being oxidized by oxygen in the air. 
     If the addition step (S 14 ) is conducted, the additive  40  composed of at least one of a metallic soap and an inorganic lubricant with a hexagonal crystal structure is present between the composite magnetic particles  30  adjoining each other. This prevents the composite magnetic particles  30  from being rubbed hard each other in the compacting step (S 21 ). Since the additive  40  exhibits good lubricity, the insulating coated film  20  formed on the outer surface of each of the composite magnetic particles  30  is not broken. This can maintain the form in which the insulating coated film  20  coats a surface of each of the metal magnetic particles  10 . Consequently, the insulating coated film  20  can function as an insulating layer between the metal magnetic particles  10  with certainty. 
     In the addition step (S 14 ), instead of or in addition to the additive  40 , another lubricant or resin may be added. 
     A second heat-treatment step (S 22 ) of heat-treating the compact obtained by compacting is then conducted. In the second heat-treatment step (S 22 ), the compact is heat-treated, for example, at a temperature between 575° C. and the pyrolysis temperature of the insulating coated film  20 . There are many defects inside the compact after compacting. These defects can be removed by conducting the second heat-treatment step (S 22 ). Furthermore, since the second heat-treatment step (S 22 ) is conducted at a temperature less than the pyrolysis temperature of the insulating coated film  20 , the insulating coated film  20  does not deteriorate due to the second heat-treatment step (S 22 ). The second heat-treatment step (S 22 ) changes the additive  40  into an insulation  50 . 
     After the second heat-treatment step (S 22 ), appropriate processing such as extrusion or cutting processing is optionally conducted on the compact to complete the dust core shown in  FIG. 2 . 
     Through the steps (S 11  to S 14  and S 21  to S 22 ) described above, the dust core of this embodiment shown in  FIG. 2  can be manufactured. In a case where the soft magnetic material having the insulating coated film  20  with two layers is used, a dust core shown in  FIG. 7  can be manufactured.  FIG. 7  is a schematic view showing another dust core according to the embodiment of the present invention. 
     As described above, the soft magnetic material according to the embodiment of the present invention includes the metal magnetic particles  10  whose coefficient of variation Cv (σ/μ), which is the ratio of the standard deviation (σ) of the particle size to the average particle size (μ), is 0.40 or less and whose circularity Sf is 0.80 or more and 1 or less. As shown in  FIGS. 3 ,  4 A, and  4 B, since the coefficient of variation Cv (σ/μ) is 0.40 or less, the variation in the particle size of the metal magnetic particles  10  can be reduced (uniform particle size distribution can be achieved). This can improve the uniformity of the inside of the dust core made of the soft magnetic material, thereby facilitating the domain wall motion in a magnetization process. Since the circularity Sf of the metal magnetic particles  10  is 0.8 or more, a distortion arising on a surface of each of the metal magnetic particles  10  when the soft magnetic material is pressure-molded can be reduced. As shown in  FIG. 8 , a combined effect of the coefficient of variation Cv and the circularity Sf of the metal magnetic particles  10  can improve flux density in a B-H curve. As a result, a reduction in inductance caused by an increase in a DC current can be suppressed as shown in  FIG. 9 . In other words, the DC bias characteristics can be improved.  FIG. 8  is a graph showing a relationship between magnetic field and flux density according to the embodiment of the present invention.  FIG. 9  is a graph showing a relationship between DC current and inductance according to the embodiment of the present invention. In  FIGS. 8 and 9 , the one described as an invention example shows the dust core made of the soft magnetic material including the metal magnetic particles  10  of this embodiment. 
     EXAMPLES 
     In these Examples, an effect provided by including metal magnetic particles whose coefficient of variation Cv (σ/μ) is 0.40 or less and circularity Sf is 0.80 or more was examined. 
     Examples 1 to 4 
     In Example 1, the soft magnetic material manufactured by the method described in the embodiment above was used. Specifically, in the preparation step (S 11 ), a metal magnetic particle containing 99.6% by weight or more of iron and the balance that is composed of incidental impurities such as 0.3% by weight or less of O and 0.1% by weight or less of C, N, P, Mn, or the like was prepared by water-atomizing an iron powder. The average particle sizes of the metal magnetic particles in Examples 1 to 4 were selected as described in Table. The coefficient of variation Cv and the circularity Sf of the metal magnetic particles in Examples 1 to 4 were as described in Table. The coefficient of variation Cv of the metal magnetic particles was calculated by measuring the particle size distribution of the targeted soft magnetic material (a plurality of metal magnetic particles) using a laser diffraction/scattering particle size distribution analysis method. The circularity Sf was statistically calculated from projection images of the metal magnetic particles whose area and circumference were measured, on the basis of Eq. (1) described above. 
     In the insulating coated film formation step (S 13 ), the insulating coated film composed of iron phosphate was then formed by conducting phosphating treatment. 
     In the addition step (S 14 ), 0.1% by mass of zinc stearate as a metallic soap was added in Examples 1 to 3. In Example 4, 0.1% by mass of ethylenebisstearamide that is a lubricant with a non-hexagonal crystal structure was added. Furthermore, 0.3% by mass of a methylsilicone resin was added. Thus, the soft magnetic materials of Examples 1 to 4 were obtained. 
     In the compacting step (S 21 ), a pressure of 1000 MPa was applied to the soft magnetic material to make a compact. In the second heat-treatment step (S 22 ), the compact was heat-treated at 500° C. in a nitrogen stream atmosphere for one hour. Thus, the dust core of Example 1 to 4 was manufactured. 
     Comparative Examples 1 to 4 
     The soft magnetic materials of Comparative Examples 1 to 4 were basically manufactured in the same manner as the soft magnetic material of Example 2. However, the coefficient of variation Cv, the circularity Sf, and the average particle size (0 were changed to the values described in Table below. The dust core of Comparative Examples 1 to 4 were manufactured in the same manner as in Example 1. 
     (Evaluation Method) 
     For each of the dust cores of Examples 1 to 4 and Comparative Examples 1 to 4, the DC bias characteristics and eddy current loss were measured. 
     Specifically, the DC bias characteristics were measured using a DC bias tester after test samples were set up as shown in  FIG. 10 .  FIG. 11  and Table show the results.  FIG. 10  is a schematic view showing a device for measuring DC bias characteristics in Examples.  FIG. 11  is a graph showing DC bias characteristics in Examples. In  FIG. 11 , the axis of ordinates represents the ratio (L xA /L 0A ) (unit: none) of inductance L xA  at x A to inductance L 0A  at 0 A and the axis of abscissas represents the current (unit: A) applied. L 8A /L 0A  in Table means the ratio of inductance L 8A  at 8 A to inductance L 0A  at 0 A. 
     After an iron loss was measured, an eddy current loss was evaluated by separating the iron loss into a hysteresis loss and an eddy current loss on the basis of the frequency dependency of the iron loss. Specifically, for each of the obtained dust cores of Examples 1 to 4 and Comparative Examples 1 to 4, a primary winding with 300 turns and a secondary winding with 20 turns were wound around a ring-shaped compact (after heat treatment) with an outer diameter of 34 mm, an inner diameter of 20 mm, and a thickness of 5 mm, to prepare magnetic characteristic measurement samples. The iron loss of these samples was measured at an excitation flux density of 1 kG (=0.1 T (tesla)) at various frequencies from 50 Hz to 10000 Hz using an alternating current (AC)-BH curve tracer. The eddy current loss was then calculated from the iron loss. Table shows the results. With the following three equations, the eddy current loss was calculated by fitting the frequency curve of the iron loss using a least-squares method.
 
(Iron Loss)=(Hysteresis Loss Coefficient)×(Frequency)+(Eddy Current Loss Coefficient)×(Frequency) 2  
 
(Eddy Current Loss)=(Eddy Current Loss Coefficient)×(Frequency) 2 
 
                                     TABLE 1                          Metal magnetic particle       Eddy                                                     Average           current           Coefficient of   Circularity   particle           loss           variation Cv   Sf   size (μm)   Lubricant   L SA /L OA     [kW/m 3 ]                                                     Example 1   0.40   0.80   120   Zn. St   0.79   61       Example 2   0.38   0.91   120   Zn. St   0.81   56       Example 3   0.36   0.92   70   Zn. St   0.80   31       Example 4   0.36   0.92   65   EBS   0.79   50       Comparative   0.48   0.76   121   Zn. St   0.70   76       example 1       Comparative   0.47   0.75   120   Zn. St   0.70   75       example 2       Comparative   0.47   0.83   123   Zn. St   0.72   61       example 3       Comparative   0.40   0.75   122   Zn. St   0.75   73       example 4                    
(Measurement Results)
 
     As evident from  FIG. 11  and Table, in Examples 1 to 4 in which the metal magnetic particles whose coefficient of variation Cv (σ/μ) is 0.4 or less and circularity Sf is 0.8 or more and 1.0 or less are included, inductance decreased less and DC bias characteristics were better than in Comparative Examples 1 to 3. 
     Comparing Example 1 with Comparative Example 4, in both of which the metal magnetic particles have substantially the same particle size and coefficient of variation, it was found that an eddy current loss could be suppressed as the circularity increased. Therefore, comparing Example 1 with Examples 2 to 4, in which the metal magnetic particles have a circularity of 0.91 or more, it was revealed that better bias characteristics and a lower eddy current loss could be achieved when the circularity was 0.91 or more. 
     Comparing Examples 3 and 4 with Example 1, in all of which the metal magnetic particles have substantially the same coefficient of variation Cv, better DC bias characteristics and a lower eddy current loss could be achieved when the average particle size was small. Moreover, comparing Example 3 with Example 4, a low hysteresis loss is achieved and the best characteristics are exhibited by improving the heat-resistance temperature of the insulating coated film using a metallic soap. 
     In Examples, as described above, it was confirmed that the DC bias characteristics of the soft magnetic materials including metal magnetic particles whose coefficient of variation Cv (σ/μ), which is the ratio of the standard deviation (σ) of the particle size to its average particle size (μ), is 0.40 or less and whose circularity Sf is 0.80 or more and 1 or less could be improved. 
     It should be considered that the embodiment and Examples disclosed herein are all exemplary and not restrictive. The scope of the present invention is determined by the appended claims but not the embodiment described above, and any modifications can be made within the spirit and scope of the appended claims or the equivalents. 
     INDUSTRIAL APPLICABILITY 
     The soft magnetic material, the dust core, the method for manufacturing the soft magnetic material, and the method for manufacturing the dust core according to the present invention can be applied to, for example, an iron core of a static apparatus such as a transformer, a choke coil, or an inverter.