JP2018148103A - Powder for magnetic core and production method thereof, dust core and magnetic film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide powder for magnetic core composed of soft magnetic particles of nano size where ferrite is produced uniformly on the surface.SOLUTION: Powder for magnetic core is composed of particles for magnetic core including soft magnetic particles composed of Fe and Ni and having granularity of 5-400 nm, magnetite on the surface thereof, and spinel-type ferrite on the surface of the soft magnetic particles having magnetite. Such a soft magnetic is obtained by performing ferrite plating of the oxide particles, where magnetite is generated on the surface of the soft magnetic particles, in an alkaline aqueous solution. Since the isoelectric point of magnetite exists on the weak acidic side, the oxide particles in the alkaline aqueous solution are in a state charged with negative charges, adsorbs cations becoming the raw material of ferrite strongly, and are not aggregated but dispersed. Powder for magnetic core composed of soft magnetic particles of nano size, where uniform ferrite coating is formed, is thus obtained.SELECTED DRAWING: Figure 2A

Description

  The present invention relates to a powder for a magnetic core made of soft magnetic particles having spinel ferrite (also simply referred to as “ferrite”) on its surface.

  There are many products that use electromagnetism around us, such as transformers, motors, generators, speakers, induction heaters, and various actuators. Many of these products use an alternating magnetic field. In order to efficiently obtain a large alternating magnetic field locally, a magnetic core (soft magnet) is usually provided in the alternating magnetic field.

  The magnetic core is required to have not only high magnetic characteristics in an alternating magnetic field but also a small amount of high-frequency loss (hereinafter simply referred to as “iron loss” regardless of the material of the magnetic core) when used in an alternating magnetic field. The iron loss includes eddy current loss, hysteresis loss, and residual loss. In particular, it is important to reduce eddy current loss that increases in proportion to the square of the frequency of the alternating magnetic field.

  As such a magnetic core, there is a powder magnetic core obtained by press-molding soft magnetic particles (each particle of magnetic core powder) covered with an insulating film. The dust core is used in various electromagnetic devices because of its low eddy current loss and high shape freedom. However, if the insulating film is formed of nonmagnetic silicon-based resin, phosphate, or the like, the (saturated) magnetic flux density of the dust core can be reduced. Therefore, it has been proposed to use ferrite as the insulating film, and there is a description related to Patent Document 1 below.

  There is also a magnetic film as a magnetic core. The magnetic film is a magnetic body formed by printing a paste obtained by adding a solvent to soft magnetic fine particles and kneading them on a substrate or the like. The description related to the magnetic film is in Non-Patent Document 1 below. However, in Non-Patent Document 1, a nonmagnetic resin is used for the insulating material, and improvement in the (saturated) magnetic flux density of the magnetic film cannot be expected.

WO2003 / 015109

Y. Shirakata, et al, IEEE. Trans. Magn., 2008, 44, 2100.

  Patent Document 1 describes, for example, magnetic fine particles in which a NiZn ferrite film having an average thickness of 15 nm is provided on the surface of very fine nanoparticles (carbonyl iron powder particles) having an average particle diameter of 70 nm. Further, it is also described that the high-frequency relative permeability of the molded body made of the particles has a real part exceeding 10 under 2 GHz (Example 3 of Patent Document 1).

  However, there is no description about the electrical characteristics (specific resistance, etc.) of the molded body. According to the research of the present inventor, the method as described in Patent Document 1 cannot form a uniform ferrite film on the surface of nano-sized fine pure iron particles.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide magnetic core particles (powder) or the like in which ferrite is uniformly present on the surface of nano-sized fine soft magnetic particles. .

As a result of diligent research to solve this problem, the present inventor has made magnetite (Fe ₃ O ₄ ) intervene between the soft magnetic particles and the ferrite so that the ferrite can be evenly distributed on the surface of the fine soft magnetic particles. I succeeded in making it exist. By developing this result, the present invention described below has been completed.

<Magnetic core powder>
(1) The magnetic core powder of the present invention is composed of Fe and Ni, a soft magnetic particle having a particle size of 5 to 400 nm, a magnetite on the surface of the soft magnetic particle, and a surface of the soft magnetic particle having the magnetite. And magnetic core particles including spinel ferrite.

(2) By using the magnetic core powder of the present invention, a powder magnetic core (soft magnetic core) excellent in magnetic characteristics (permeability, saturation magnetization, etc.) and electrical characteristics (specific resistance, etc.) can be obtained. In particular, the magnetic core powder of the present invention is suitable for a dust core used at a high frequency. The reason is considered as follows.

First, the magnetic core powder of the present invention is composed of soft magnetic particles composed of Fe and Ni (also simply referred to as “Fe—Ni particles”). Fe-Ni particles have better magnetic properties (such as magnetic permeability) than pure Fe particles or the like (see FIG. 5). Next, since the Fe—Ni particles are very fine and have insulating (spinel) ferrite (MFe ₂ O ₄ ) on the surface, the surface of the Fe—Ni particles has eddy currents even under a high frequency. Is difficult to flow. Therefore, the dust core made of fine Fe—Ni particles having ferrite on the surface is excellent in magnetic characteristics and electrical characteristics.

<Method for producing magnetic core powder>
(1) The present invention can be grasped not only as a magnetic core powder but also as a production method thereof. That is, the present invention comprises an oxidation step in which soft magnetic particles comprising Fe and Ni and having a particle size of 5 to 400 nm are oxidized to obtain oxidized particles that generate magnetite on the surface of the soft magnetic particles; And a ferrite generation step of obtaining magnetic core particles in which spinel ferrite is generated on the surface of the oxidized particles.

(2) According to the production method of the present invention, a magnetic core powder in which ferrite is uniformly distributed on the surface of fine Fe—Ni particles can be obtained. The reason is considered as follows.

  In the present invention, before ferrite is generated on the surface of the soft magnetic particles, magnetite is previously generated on the surface of the soft magnetic particles. Here, the magnetite has a pH (isoelectric point) at which the surface potential (zeta potential) becomes zero and is on the slightly acidic side (see Fig. 4). Fig. 4 (bottom): "Measurement of zeta potential", Bunkeki 5,251, (2004)).

For this reason, the soft magnetic particles (oxidized particles) having magnetite on the surface are in a state in which the surface is charged to a negative charge in an alkaline aqueous solution. As a result, positively charged particles existing in the alkaline aqueous solution, that is, cations (M ²⁺ , Fe ^2+, etc.) are strongly adsorbed by the Coulomb force on the surface of the oxidized particles.

  Moreover, normally, each fine particle having a nanoscale particle size has a large surface energy and easily aggregates. For this reason, conventionally, it has been difficult to generate ferrite uniformly for each surface of nano-sized fine soft magnetic particles. However, since the fine oxidized particles of the present invention are charged with surface charges having the same sign, they are repelled in an alkaline aqueous solution and are likely to be dispersed without agglomeration.

  As described above, the oxidized particles according to the present invention are excellent in dispersibility and adsorptivity of a cation serving as a raw material of ferrite in an alkaline aqueous solution. It is considered that ferrite was uniformly formed on the surface of each particle after “plating”).

Incidentally, as is apparent from FIG. 4, fine pure Ni particles have an (electrically strong) alkaline side of the isoelectric point of oxide (NiO) formed on the surface thereof. For this reason, the particle | grains which have NiO cannot exhibit the adsorptivity and dispersibility which were mentioned above in alkaline aqueous solution. Similarly, fine pure Fe particles also have an isoelectric point of an oxide (Fe ₂ O ₃ ) mainly formed on the surface thereof on the alkaline side. For this reason, particles mainly containing Fe ₂ O ₃ cannot exhibit the adsorptivity and dispersibility as described above in an alkaline aqueous solution.

  Even soft magnetic particles that easily aggregate in an alkaline aqueous solution may have excellent dispersibility in an acidic aqueous solution. However, as apparent from FIG. 4, such soft magnetic particles are in a state of being charged to a positive charge, so that the adsorptivity of the cation serving as a raw material for ferrite is inferior. For this reason, it is difficult to uniformly generate ferrite on the surface of such particles.

<Dust core>
The present invention can be grasped not only as the above-described magnetic core powder, but also as a powder magnetic core obtained by pressure molding it. The dust core is used in an alternating magnetic field in a high frequency range (for example, 0.1 to 1000 MHz or 1 to 100 MHz) because it has excellent magnetic characteristics and electrical characteristics, regardless of use or usage environment. Suitable for soft magnetic cores.

<Others>
Unless otherwise specified, “x to y” in this specification includes a lower limit value x and an upper limit value y. A range such as “a to b” can be newly established with any numerical value included in various numerical values or numerical ranges described in the present specification as a new lower limit value or upper limit value.

It is a X-ray-diffraction (XRD) pattern figure of the Fe-Ni particle | grains (sample 1) oxidized. It is a XRD pattern figure of the Fe-Ni particle (sample 1) which carried out the ferrite plating process. A photograph (STEM image) of the surface of Fe-Ni particles (sample 1) plated with ferrite after oxidation treatment observed with a scanning transmission electron microscope (STEM) and an element mapping image using an energy dispersive X-ray spectrometer (STEM-EDX) It is. It is the STEM image and element mapping image which were obtained by observing the surface of the Fe-Ni particle (sample C1) plated with ferrite without oxidation treatment. It is a STEM image and an element mapping image obtained by observing the surface of Fe particles (sample C2) plated with ferrite after oxidation treatment. It is the STEM image and element mapping image which were obtained by observing the surface of Ni particle (sample C3) plated with ferrite. It is the table | surface which shows the relationship between pH, a zeta potential, or a particle size, and the relationship between each substance and an isoelectric point. It is a figure which shows the relationship between the component composition and initial permeability which concern on a Fe-Ni alloy.

  One or two or more components arbitrarily selected from the present specification may be added to the above-described components of the present invention. The contents described in the present specification can be appropriately applied not only to the magnetic core powder of the present invention but also to the production method thereof and the dust core using the same. The content related to the method can also be a component related to the object. Which embodiment is the best depends on the target, required performance, and the like.

《Core particles / Core powder》
(1) Soft magnetic particles (soft magnetic powder)
The soft magnetic particles according to the present invention are made of an alloy of Fe and Ni. It is preferable that 30 to 90 atomic%, further 40 to 85 atomic% of Ni is included with respect to the entire soft magnetic particles (100 atomic%), and the balance is Fe. Even if Ni is too little or too much, magnetic properties (especially magnetic permeability) will be lowered, and high magnetic properties of the dust core (core) will not be obtained (see Fig. 5). 5 Non-ferrous materials ").

  In addition to the impurities, the soft magnetic particles may contain a total of 6 atomic% or less, further 1 atomic% or less of modifying elements that can improve magnetic properties. Examples of such modifying elements include Mo, Cu, and Cr.

  The soft magnetic particles preferably have a particle size of 5 to 400 nm, 10 to 300 nm, and more preferably 20 to 200 nm. An excessively large particle size is not preferable because eddy current loss of the dust core increases in a high frequency range. Soft magnetic particles having an excessively small particle size are undesirable because they lead to a decrease in magnetic flux density, an increase in hysteresis loss, and a decrease in availability and handleability.

  The “particle size” as used herein is a value indicating the diameter of soft magnetic particles, and is measured from the average value of the maximum diameters measured for about 10 to 100 particles in a STEM or TEM image.

Examples of the method for producing the soft magnetic powder include a mechanical pulverization method, an atomization method, and a reduction method. A powder having a very small particle size can be obtained, for example, by a mechanical grinding method.
Atomized powder composed of substantially spherical particles has low aggression between particles, and a decrease in specific resistance due to destruction of a ferrite coating or the like during molding of a dust core is suppressed. In addition, it is preferable that the powder before the oxidation treatment described later is prepared in a non-oxidizing atmosphere.

(2) Magnetite Although it is preferable that the entire surface of the soft magnetic particles is uniformly coated with magnetite (Fe ₃ O ₄ ), it may be present only on at least a part of the surface of the soft magnetic particles. The magnetite according to the present invention is sufficient if the surface of the soft magnetic particles is charged to a negative charge in an alkaline aqueous solution.

Further, it is not necessary that all oxides present on the surface of the soft magnetic particles are magnetite, and other iron oxides (hematite (Fe ₂ O ₃ ), wustite (FeO), etc.) and nickel oxide (NiO, etc.) are mixed. You may do it. However, since magnetite also has a rust-preventing effect on soft magnetic particles, it is more preferable that it is present thinly and uniformly on the surface of soft magnetic particles.

  Since magnetite itself has a low magnetic permeability, its film thickness is preferably not more than the film thickness of ferrite, preferably 5 nm or even 3 nm or less. The “film thickness” in the present specification is specified as follows, for example. Measure with STEM and element mapping. The film thickness is obtained by subtracting the distribution diameter of Ni, which is a constituent element of soft magnetic particles (core), from the distribution diameter of constituent elements (M, Fe) of ferrite. This measurement is performed at two arbitrarily selected measurement positions (positions rotated by 90 °) for each particle. The same operation is performed on a total of three particles arbitrarily extracted from the powder. The arithmetic average value of the total six film thicknesses thus obtained may be obtained and used as the “film thickness” of ferrite. The same applies to the determination of the thickness of the ferrite.

(3) Spinel-type ferrite Ferrite is a magnetic material having insulating properties, and is, for example, a cubic compound represented by MFe ₂ O ₄ (M: a metal element that becomes a divalent cation). Specifically, M is composed of one or more of Fe, Mn, Ni, Zn, Cu, Mg, Sr and the like. M preferably contains at least Mn. Since ferrite containing Mn (and Zn) has a larger specific resistance and magnetic moment (saturation magnetization) than ferrite containing other M elements, it has electrical characteristics (such as resistivity) and magnetic characteristics (such as magnetic flux density). Can be compatible at a high level.

  Ferrite is more preferable as it is uniformly distributed on the surface of soft magnetic particles (oxidized particles). However, as long as the specific resistance of the dust core can be ensured, the entire surface may not necessarily be uniformly coated. Ferrite can also contain a modifying element or an inevitable impurity in addition to M, Fe and O described above.

  The ferrite (coating) is preferably 5 to 100 nm, more preferably 10 to 50 nm, although it depends on the particle size of the soft magnetic particles. If the film thickness is too small, the electrical characteristics (specific resistance) of the dust core will be reduced. Since ferrite has a saturation magnetization smaller than that of soft magnetic particles (Fe—Ni particles), if the film thickness is excessive, the magnetic properties of the powder magnetic core deteriorate, which is not preferable.

"Production method"
(1) Oxidation process The magnetite produced | generated on the surface of a soft-magnetic particle (Fe-Ni particle) is obtained by oxidation treatment. The magnetite produced on the surface of the Fe—Ni particles is sufficient even if it is thin, and the oxidizing conditions are sufficient from the viewpoint of suppressing the production of Fe ₂ O _{3 and the} like. Therefore, for example, an oxidation process may be performed in which the soft magnetic particles are heated in an oxidizing atmosphere (for example, in an air atmosphere) at 50 to 200 ° C. or 80 to 150 ° C. for 1 to 20 hours or further 5 to 10 hours.

  Incidentally, it is possible to form magnetite on the surface of Fe-Ni particles even in a normal atmosphere. However, such a method is not efficient because it takes a long time, and it is not easy to reliably generate magnetite for each particle surface.

(2) Ferrite generation process (ferrite plating process)
There are various ferrite plating methods. An aqueous solution method in which a powder to be treated is immersed in a reaction solution (reference document: Japanese Patent Application Laid-Open No. 2013-191839), and a spray method in which the reaction solution is sprayed on the powder to be processed (reference document: Japanese Patent Application Laid-Open No. 2000-260688) No. 2014-183199), a one-part method using a reaction solution containing urea (reference document: JP-A-2006-127042), and the like. It is possible to produce the ferrite according to the present invention by any method.

  However, in the production method of the present invention, the oxidized particles whose isoelectric point is slightly acidic are negatively charged in the alkaline aqueous solution, and ferrite is uniformly generated on the particle surface. Therefore, in the case of the present invention, an aqueous solution method, particularly a two-component method using a reaction solution and a pH adjusting solution is preferable.

  Specifically, the ferrite generation step according to the present invention includes a first treatment step of adding a reaction liquid containing M and Fe into water or an aqueous solution in which oxidized particles are dispersed, and an aqueous solution after the first treatment step. It is preferable to have a second treatment step of adding a pH adjusting solution to the solution. In addition, it is preferable when the pH of the aqueous solution in a 1st process process shall be 3-6, and the pH of the aqueous solution in a 2nd process process shall be 7-12 and also 8-10.

  The first treatment process and the second treatment process may be repeated depending on the desired film thickness of the ferrite and the like. Moreover, it is preferable to perform a washing process for removing unnecessary substances after the ferrite production process. The washing step is performed, for example, by washing with ethanol after washing with water. Unnecessary items to be washed are ferrite particles that have not contributed to the coating of Fe—Ni particles, chlorine, sodium, sulfate ions, etc. contained in the treatment liquid (reaction liquid, pH adjustment liquid).

  Furthermore, it is preferable to dry the filtered powder after the washing step. The drying process may be natural drying, but by performing heat drying or vacuum drying, the magnetic core powder can be produced efficiently.

<Application>
The magnetic core powder of the present invention is a soft magnetic core (powder magnetic core) used in power conversion circuits (inverters and converters) that require high-frequency operation, and other components of various actuators used in the high-frequency range. It is preferable to be used for such as.

  The powder magnetic core of the present invention is preferably subjected to heat treatment (annealing or the like) for removing processing distortion or the like that causes hysteresis as appropriate after the pressure forming of the magnetic core powder.

  Treated powders obtained by subjecting various nanoparticle powders to ferrite plating were manufactured, and the surface of each particle was observed. Based on such an example, the present invention will be described more specifically below.

<Production of sample>
(1) Soft magnetic powder and oxidized powder As a raw material powder (soft magnetic powder), a fine powder composed of Fe-50 at% Ni (manufactured by Sigma-Aldrich Co. LLC. 676426 / simply referred to as “Fe-Ni powder”) is prepared. did. The particle size of this Fe—Ni powder was 50 nm.

  The Fe—Ni powder was placed in a heating furnace and heated in an air atmosphere at 100 ° C. for 6 hours. Thus, an oxidized powder obtained by oxidizing the Fe—Ni powder was obtained (oxidation step).

(2) Reaction solution and pH adjusting solution As a reaction solution used for ferrite plating, an aqueous solution (reaction solution) in which FeSO ₄ · 7H ₂ O and MnSO ₄ · 5H ₂ O (molar ratio 3: 2) are dissolved in ion-exchanged water. Was prepared. The concentration of this reaction solution was 33 mmol / L with respect to 250 ml of the reaction solution.

  A NaOH aqueous solution (2.5 mol / L) was also prepared as a pH adjusting solution.

(3) Pretreatment Ion exchange water (500 ml) was placed in a flask and nitrogen bubbling was performed. Thus, dissolved oxygen that oxidizes Fe ²⁺ was removed in advance. The above-mentioned oxidized powder (0.5 g) was put into the ion-exchanged water after this treatment, and was dispersed by being vibrated with an ultrasonic horn. Stirring with an ultrasonic horn was continued until the ferrite plating process was completed. Thereby, the dispersibility of oxidation powder can be improved.

(4) Ferrite plating treatment (ferrite production process)
The reaction liquid was introduced into the water in which the oxidized powder was dispersed using a micro pump at a flow rate of 3.1 ml / min (first treatment step). A pH adjusting solution was further introduced into this aqueous solution. The pH of the aqueous solution was set to 8 (second treatment step) by controlling the amount of pH adjusting solution introduced. The water temperature during the treatment was 70 to 90 ° C.

  After completion of the ferrite plating treatment, the filtered powder was washed with water and further washed with ethanol to remove Cl and the residue (washing step). The washed powder was dried by drying in vacuum (room temperature) in the air (drying process). In this way, a magnetic core powder made of soft magnetic particles subjected to ferrite plating was obtained (Sample 1).

<Comparative sample>
(1) Oxidation-untreated powder A powder obtained by applying the same ferrite plating treatment to an as-obtained Fe-Ni powder (simply referred to as "oxidation-untreated powder") without the above-described oxidation treatment was also produced ( Sample C1).

(2) Pure Fe powder A powder obtained by subjecting pure Fe powder (QSI-Nano Iron / particle size: 50 nm, manufactured by Quantum sphere Inc.) to the oxidation treatment and ferrite plating treatment described above was also manufactured (Sample C2).

(3) Pure Ni powder A powder obtained by subjecting pure Ni powder (Ni-60 / particle size: 60 nm, manufactured by Daiken Chemical Industry Co., Ltd.) to ferrite plating was also manufactured (Sample C3). The ferrite plating process was performed using M element as Fe. In other words, we aimed to generate magnetite (a kind of spinel ferrite) on the particle surface. NiO was present on the surface of the particles before the ferrite plating treatment.

<< Observation >>
(1) XRD
The vicinity of the surface of the particles before the ferrite plating treatment (oxidized particles) according to Sample 1 and the particles after the ferrite plating treatment (particles for magnetic core) were observed by XRD. The results are shown in FIGS. 1A and 1B (both are simply referred to as “FIG. 1”).

(2) STEM and STEM-EDX
The particle surfaces after the ferrite plating treatment according to Samples 1 to C3 were observed with STEM and STEM-EDX. The STEM image and the mapping image of each element relating to each sample thus obtained are shown in FIGS. 2A, 2B, 3A, and 3B, respectively.

<Evaluation>
(1) XRD
As is apparent from FIG. 1A, it was confirmed that magnetite (Fe ₃ O ₄ ) was generated on the surface of the oxidized particles. Further, as apparent from FIG. 1B, it was also confirmed that spinel ferrite was generated on the surface of the oxidized particles by subjecting them to ferrite plating.

(2) STEM-EDX
As is clear from FIG. 2A, it was revealed that Mn (M element) was uniformly distributed on the particle surface according to Sample 1 in addition to Fe and Ni. From this, it can be said that the particle surface is uniformly coated with spinel ferrite (MnFe ₂ O ₄ ).

As is clear from FIG. 2B, Mn was not substantially detected from the particle surface of the sample C1. Therefore, it was also found that ferrite (MnFe ₂ O ₄ ) was not generated on the particle surface that was not oxidized.

As apparent from FIG. 3A, Mn was not substantially detected also from the particle surface according to Sample C2. It was also found that ferrite (MnFe ₂ O ₄ ) was not generated on the surface of particles made of pure Fe. The reason is presumed as follows.

Hematite (Fe ₂ O ₃ ) having an isoelectric point on the alkaline side is easily formed on the surface of the pure Fe particles. For this reason, pure Fe particles are inferior in cation adsorption and dispersibility in an alkaline aqueous solution even after oxidation treatment. As a result, it is considered that ferrite was not substantially generated on the particle surface.

As is clear from FIG. 3B, the same can be said for the sample C3 as for the sample C2. That is, NiO on the surface of pure Ni particles also has an isoelectric point on the alkaline side. For this reason, it is considered that the pure Ni particles also have poor adsorptivity and dispersibility in an alkaline aqueous solution, and ferrite (Fe ₃ O ₄ / M = Fe) was not substantially generated on the surface thereof.

  From the above, it can be seen that a uniform ferrite film can be formed on each surface of nanoscale fine Fe-Ni particles only when the oxidized powder obtained by oxidizing Fe-Ni powder is subjected to ferrite plating treatment. It was.

Claims (9)

Soft magnetic particles made of Fe and Ni and having a particle size of 5 to 400 nm,
Magnetite on the surface of the soft magnetic particles;
Spinel-type ferrite (MFe ₂ O ₄ , M: metal element that becomes a divalent cation) on the surface of the soft magnetic particles having the magnetite;
Magnetic core powder comprising particles for a magnetic core provided with   The magnetic core powder according to claim 1, wherein the soft magnetic particles contain 40 to 90 atomic% of Ni with respect to the entire soft magnetic particles, and the balance is Fe.   The magnetic core powder according to claim 1, wherein M contains at least Mn. An oxidation step of oxidizing oxidized soft magnetic particles composed of Fe and Ni and having a particle size of 5 to 400 nm to obtain oxidized particles that generate magnetite on the surface of the soft magnetic particles;
A ferrite generating step of obtaining particles for a magnetic core in which spinel ferrite is generated on the surface of the oxidized particles in an alkaline aqueous solution;
The manufacturing method of the powder for magnetic cores provided with this.   The said oxidation process is a process of heating the said soft-magnetic particle in 50-200 degreeC oxidizing atmosphere for 1 to 20 hours, The manufacturing method of the powder for magnetic cores of Claim 4. The ferrite generation step includes a first treatment step of adding a reaction liquid containing M and Fe into water or an aqueous solution in which the oxidized particles are dispersed;
A second treatment step of adding a pH adjusting solution to the aqueous solution after the first treatment step;
The manufacturing method of the powder for magnetic cores of Claim 4 or 5 which has these.   The powder magnetic core which consists of the powder for magnetic cores in any one of Claims 1-3.   The dust core according to claim 7, which is used in an alternating magnetic field of 0.1 MHz or higher.   A magnetic film comprising the magnetic core powder according to claim 1.