Fe–Co–N SOFT MAGNETIC MATERIAL

Abstract

Provided is an Fe–Co–N soft magnetic material that exhibits higher magnetization properties than an electromagnetic pure iron material or an Fe–Si electrical steel material and costs less than permendur. An Fe–Co–N soft magnetic material according to the present invention is characterized in that the Co concentration, expressed as [Co] (unit: at%), is 0.4 ≤ [Co] ≤ 25, the N concentration, expressed as [N] (unit: at%), is 0 < 1.5 – 0.15 × [Co] ≤ [N] ≤ 7 – 0.2 × [Co], and the remainder is composed of Fe and unavoidable impurities, wherein when a partial radial distribution function that indicates the relationship between the Co atom existence rate and the interatomic distance centered on an N atom is determined for the soft magnetic material, the integral value of the Co atom existence rate for interatomic distances of up to 0.26 nm is 40% or less with respect to the integral value of the Co atom existence rate for interatomic distances of up to 1 nm.

Description

Fe-Co-N soft magnetic material

[0001] The present invention relates to soft magnetic material technology, and more particularly to Fe-Co-N (iron-cobalt-nitrogen) based soft magnetic materials having a body-centered tetragonal (bct) crystal structure.

[0002] Electromagnetic pure iron sheets and electromagnetic steel sheets (e.g., 0.01 to 3 mm thick) are widely used as iron cores for electrical machinery (e.g., rotating electrical machines and transformers) by laminating multiple sheets together. High conversion efficiency between electrical energy and magnetic energy is important for iron cores, so high magnetic flux density and low iron loss are essential. To increase magnetic flux density, it is desirable for soft magnetic materials to have high saturation magnetic flux density Bs or saturation magnetization Ms. Iron-based materials with high Bs / Ms include Fe-Co alloy materials and Fe-N martensitic materials.

[0003] Among currently commercially available soft magnetic bulk materials, permendur (49Fe-49Co-2V mass% = 50Fe-48Co-2V atomic%, Bs ≒ 2.4 T, Ms ≒ 240 emu / g) is well known as the material with the highest Bs / Ms. However, the material cost of Co is 100 to 200 times higher than that of Fe, although this varies depending on market conditions, making permendur a very expensive material. In other words, in Fe-Co alloy materials, reducing the Co content can reduce material costs accordingly.

[0004] However, Fe-Co alloy materials have the unfortunate problem of decreasing Bs / Ms when the Co content is reduced. Therefore, it is thought that the decrease in Bs / Ms due to the decrease in Co content can be compensated for by the generation / formation of an Fe-N martensite phase. Various research and development efforts have been reported regarding the generation / formation of an Fe-N martensite phase.

[0005] For example, Patent Document 1 (JP 2021-102799 A) describes a soft magnetic steel sheet containing 1.2 atomic % or less of carbon and 9 atomic % or less of nitrogen, with the total concentration of the carbon and the nitrogen being 0.01 atomic % or more and 10 atomic % or less, the concentration of the nitrogen being higher than the concentration of the carbon, and the remainder being iron and unavoidable impurities, and which is divided into an α phase (ferrite phase), an α' phase (FeN phase), an α" phase (Fe 16 The soft magnetic steel sheet is characterized in that it is composed of an α phase (N2 phase) and a γ phase (austenite phase), the α phase is the main phase, the volume fraction of the α″ phase is 10% or more, and the volume fraction of the γ phase is 5% or less.

[0006] According to Patent Document 1, it is possible to provide a soft magnetic steel sheet made of iron-nitrogen martensite that has a higher saturation magnetic flux density than pure iron, and it is also possible to provide an iron core and a rotating electric machine that have a higher conversion efficiency between electrical energy and magnetic energy than iron cores made of pure iron by using this soft magnetic steel sheet.

[0007] Furthermore, Patent Document 2 (JP 2022-167614 A) discloses a magnetic material containing iron and nitrogen, which contains body-centered tetragonal (bct) crystals containing iron and nitrogen, and in which the molar ratio of iron to nitrogen contained in the crystals is greater than 8 and less than or equal to 32. According to Patent Document 2, iron nitride α″-Fe 16 It is said that it is possible to provide a magnetic material that can obtain a saturation magnetic flux density higher than N2, as well as an iron core and a rotating electric machine that use this material.

[0008] JP 2021-102799 A JP 2022-167614 A

[0009] In recent years, from the perspective of protecting the global environment, the application fields of electromechanical devices using iron cores have been expanding, and as a result, there has been a strong demand for higher output, higher efficiency, and smaller size for these electromechanical devices. To meet these demands for higher output and smaller size, improving the magnetic properties of the iron core (e.g., Bs / Ms) is an important factor. Naturally, reducing the cost of iron cores is one of the most important challenges for industrial products.

[0010] Research and development has begun on the creation / formation of an Fe-N martensite phase by infiltrating and diffusing N atoms into Fe-Co alloy materials. However, it is difficult for N atoms to infiltrate and diffuse into Fe-Co alloy materials, making it difficult to create / form an Fe-N martensite phase, and many aspects remain unclear. In other words, the properties of Fe-Co-N alloy materials are still not well understood.

[0011] Therefore, the primary object of the present invention is to provide an Fe—Co—N based soft magnetic material that can exhibit higher magnetic properties (e.g., Ms) than electromagnetic pure iron materials (Bs≈2.2 T, Ms≈220 emu / g) and Fe—Si based electromagnetic steel materials (Bs≈1.8 T, Ms≈180 emu / g), and that can be manufactured at a lower cost than permendur.

[0012] (I) One aspect of the present invention provides an Fe-Co-N based soft magnetic material comprising Fe (iron), Co (cobalt), and N (nitrogen), wherein the concentration of Co expressed as [Co] (unit: atomic %) is 0.4≦[Co]≦25, and the concentration of N expressed as [N] (unit: atomic %) is 0<1.5−0.15×[Co]≦[N]≦7−0.2×[Co], with the remainder consisting of Fe and inevitable impurities, and wherein, when a partial radial distribution function showing the relationship between the interatomic distance around an N atom in the soft magnetic material and the abundance ratio of Co atoms is calculated, the integral of the abundance ratio of Co atoms in the interatomic distance range of 0.26 nm or less is 40% or less relative to the integral of the abundance ratio of Co atoms in the interatomic distance range up to 1 nm.

[0013] The present invention can freely combine the following improvements and modifications to the Fe-Co-N based soft magnetic material (I) according to the present invention. (i) In the partial radial distribution function, the integral value of the abundance rate of the Co atoms in the interatomic distance range of 0.26 nm or less is smaller than the integral value of the abundance rate of the Co atoms in the interatomic distance range of more than 0.26 nm and 0.50 nm or less. (ii) When the constituent phases of the soft magnetic material are identified by X-ray diffraction (XRD) measurement, the α″ phase (Fe 16N2 phase and / or (Fe, Co) 16 N2 phase) is less than 10% by volume, the total of γ phase (austenite phase), γ' phase (Fe4N phase and / or (Fe,Co)4N phase) and ε phase (Fe3N phase and / or (Fe,Co)3N phase) is 5% by volume or less, and the balance is nitrogen-deficient α' phase (Fe8N 1-x phase and / or (Fe,Co)N 1-x phase, 0<x<1) and / or α phase (ferrite phase). (iii) The average lattice constant of the c-axis of the nitrogen-deficient α' phase is greater than 0.289 nm and not more than 0.309 nm. (iv) In the soft magnetic material, a portion of the Fe and / or the Co is substituted with 1 atomic % or less of Ni (nickel), 1 atomic % or less of Mn (manganese), 1 atomic % or less of Cu (copper), and 1 atomic % or less of Cr (chromium). (v) In the soft magnetic material, less than half of the N is substituted with C (carbon).

[0014] According to the present invention, it is possible to provide an Fe-Co-N based soft magnetic material that can exhibit a higher Ms than electromagnetic pure iron materials and Fe-Si based electromagnetic steel materials, and that can be made less expensive than permendur.

[0015] 3A is a schematic perspective view showing a unit lattice of an Fe-Co alloy; FIG. 3B is a schematic perspective view showing a portion of a unit lattice of the α″ phase; FIG. 3C is a schematic partial schematic view of an example of a supercell having an alloy composition of 123Fe-5Co, viewed from the a-axis direction; FIG. 3D is a schematic partial schematic view of an example of a supercell having the same 123Fe-5Co alloy composition as FIG. 3A but with Co atom positions different from those in FIG. 3A, viewed from the a-axis direction; FIG. 3E is a schematic partial schematic view of a supercell having an alloy composition of 123Fe-5Co-4N, obtained by introducing N atoms into crystal structure A, viewed from the a-axis direction; FIG. 3F is a schematic partial schematic view of a supercell having an alloy composition of 123Fe-5Co-4N, obtained by introducing N atoms into crystal structure B, viewed from the a-axis direction. 1 is a schematic partial schematic diagram of a quartz-crystal cell viewed from the a-axis direction; 2 is a graph showing the relationship between the interatomic distance between Co atoms and N atoms, the energy of the system, and the magnetic moment; 3 is a graph showing the partial radial distribution function of Co atoms when focusing on N atoms in crystal structures A' and B'; 4 is a graph showing the partial radial distribution function of Fe atoms when focusing on N atoms in crystal structures A' and B'; and 5 is a graph showing the relationship between the Co concentration and N concentration in an Fe-Co-N alloy, where the magnetic properties are higher than those of pure Fe and are improved by nitriding treatment / N atom introduction.

[0016] [Background and basic concept of the present invention] The inventors have reduced the Co content compared to Permendur to reduce material costs, and have made up for the decrease in Ms due to the decrease in Co content by using iron nitride martensite phases with a bct structure (α″ phase, Fe 16 N2 phase and / or (Fe, Co) 16 However, as mentioned above, it is difficult for N atoms to penetrate and diffuse into Fe-Co alloy materials, making it difficult to generate and form the α″ phase.

[0017] Therefore, the present inventors have conducted extensive research into methods for efficiently generating / forming the α″ phase by nitriding Fe-Co based alloy materials. For Fe-Co based alloy materials (Fe-Co-N alloy materials) that had been nitrided under various conditions, precise mass measurements were performed using an electronic balance, nitrogen concentration analysis was performed using an electron probe microanalyzer (EPMA), detected phases were identified using wide-angle X-ray diffraction (WAXD) measurements, and magnetization measurements were performed using a vibrating sample magnetometer (VSM).

[0018] However, the measurement results showed that the Ms varied more than expected depending on the sample, and some samples did not reach the expected Ms. After a more detailed investigation, it was thought that the Ms might be relatively small in samples in which the α” phase was detected in the WAXD measurement.

[0019] The inventors of the present invention believed that unexplained properties of Fe-Co-N based soft magnetic materials might be contributing to the measurement results, and conducted first-principles calculations to thoroughly study the properties of Fe-Co-N based soft magnetic materials. As a result, they found that in Fe-Co-N based soft magnetic materials, Fe-N bonds (where the nearest neighbor of an N atom is an Fe atom) contribute to improving Ms, while Co-N bonds (where the nearest neighbor of an N atom is a Co atom) inhibit Ms improvement. In other words, they found that the Ms of the soft magnetic material can be effectively improved by generating Fe-N bonds while minimizing Co-N bonds. The present invention is based on this finding.

[0020] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the embodiments described herein, and can be appropriately combined with or improved on known techniques without departing from the technical concept of the invention.

[0021] [Embodiments] (First-principles calculation) First, a brief description of first-principles calculation will be given. Materials consist of atoms, and atoms consist of atomic nuclei and electrons. First-principles density functional theory, which is a type of first-principles calculation, is a technique for determining the wave function of electrons that obeys the Schrödinger equation based on a functional of electron density. The ground-state wave function of electrons can be determined by minimizing the energy of the system based on the variational principle. First-principles density functional theory solves the Schrödinger equation, the most fundamental principle that electrons obey, and therefore obtains a solution that is faithful to the principles of quantum mechanics.

[0022] In the present invention, calculations were performed using a supercell in order to handle alloys and take lattice distortion into account. The supercell was formed by stacking four unit cells in each of the vertical, horizontal, and height directions (4 × 4 × 4 = 64 unit cells). The Fe-Co alloys targeted in the present invention have a body-centered cubic (bcc) structure, meaning that two metal atoms exist in each unit cell. In other words, in the present invention, calculations were performed using a supercell of "64 unit cells × 2 atoms = 128 metal atoms."

[0023] Here is a brief example of a calculation. The experimental lattice length of the α phase (ferrite phase, bcc) of pure Fe is said to be 0.286 nm (0.1 nm = 1 Å), and the experimental saturation magnetization Ms is said to be 220 emu / g. In contrast, the calculated values ​​obtained using first-principles density functional theory are a lattice length of 0.283 nm and an Ms of 219.4 emu / g. The difference between the experimental and calculated values ​​is within about 1%, respectively, which can be said to show a sufficiently good agreement.

[0024] Possible factors that cause differences between experimental and calculated values ​​include lattice defects and unavoidable impurities that may exist in the actual material, and errors due to measurement, because first-principles calculations usually do not take into account lattice defects, unavoidable impurities, or measurement errors.

[0025] (Fe-Co Alloy) The Fe-Co alloy will now be described. FIG. 1 is a perspective schematic diagram showing a unit lattice of an Fe-Co alloy. As shown in FIG. 1, the unit lattice 101 of the Fe-Co alloy has a bcc structure, with Fe atoms and Co atoms randomly occupying metal atom sites 102 in accordance with the composition ratio of the alloy. It is known that when the Co ratio exceeds 50 atomic %, an ordered phase appears in which Fe atoms and Co atoms are regularly aligned. However, in the present invention, since the Co content is less than 48 atomic % of permendur, the alloy is considered to be in a disordered phase (Fe atoms and Co atoms are randomly arranged).

[0026] It is known that the magnetic moment of an Fe-Co alloy changes depending on the composition ratio of Fe to Co. When the Co concentration is increased relative to pure Fe, the magnetic moment increases and reaches a maximum value, and then decreases as the Co concentration is further increased. This upward convex curve, with the Co concentration on the horizontal axis and the magnetic moment on the vertical axis, is known as the Slater-Pauling curve.

[0027] The magnetic moment is a vector quantity that represents the magnitude and direction of magnetic force, and is expressed as the vector sum of the intrinsic magnetic moment of a proton, the intrinsic magnetic moment of an electron, and the magnetic moment of the electron's orbital motion. The larger the magnetic moment, the stronger the magnetic force. The magnetic moment of a bulk material is mainly determined by the spin angular momentum of the electron.

[0028] Magnetization is defined as the magnetic moment per unit mass. Once the crystal structure (i.e., atomic arrangement) of a material is determined, the Ms of that material can be calculated using first-principles density functional theory.

[0029] (α″ and α' phases) Figure 2 shows the α″ phase (Fe 16 N2 phase and / or (Fe, Co) 16 The unit cell of the α" phase is expressed as eight unit cells (2 × 2 × 2) of the α phase. 16 The a-axis and b-axis lattice constants of the N2 phase are reported to be 0.572 nm, and the c-axis lattice constant is reported to be 0.629 nm.

[0030] The α" phase contains 16 metal atoms (in this invention, Fe atoms or Co atoms) and 2 N atoms in the unit cell. The α" phase has a bct structure because the c-axis length is extended compared to the α phase (bcc structure) due to the intrusion of N atoms. The space group of the α" phase is assigned to I4 / mmm, and the metal atoms occupy three independent crystallographic sites (4e site, 8h site, and 4d site), with the N atom occupying the 2a site. In the α" phase, the N atoms are regularly arranged and are bonded to metal atoms in the 4e site and 8h site. The metal atoms in the 4d site do not bond to N atoms.

[0031] On the other hand, the α' phase (Fe8N phase and / or (Fe,Co)8N phase) has a composition formula half that of the α" phase, but the unit cell of the α' phase is expressed as containing two metal atoms, just like the bcc structure α phase. For this reason, experimentally measured literature values ​​have been reported to give the a-axis and b-axis lattice constants of the Fe8N phase as 0.289 nm, and the c-axis lattice constant as 0.316 nm.

[0032] The α' phase also differs from the α" phase in that N atoms are not arranged in an orderly fashion (disorderly arranged). For this reason, when precise measurements of the crystal structure are carried out, a superlattice peak resulting from the orderly arrangement of N atoms is observed in the α" phase, whereas no such superlattice peak is observed in the α' phase. This difference allows the two phases to be distinguished from one another.

[0033] (Fe-Co-N alloy) To investigate the properties of Fe-Co-N alloy, we constructed two types of Fe-Co alloy supercells (128 metal atoms) with the same alloy composition but different Co atom positions, and performed various calculations of the states before and after nitriding treatment using first-principles density functional theory.

[0034] Figure 3A is a partial schematic diagram of an example of a supercell having an alloy composition of 123Fe-5Co, viewed from the a-axis direction. Figure 3B is a partial schematic diagram of an example of a supercell having the same alloy composition of 123Fe-5Co as Figure 3A but with a different Co atom position than Figure 3A, viewed from the a-axis direction. The alloy composition of 123Fe-5Co is approximately 3.9 atomic % Co. The crystal structure of Figure 3A is referred to as crystal structure A, and the crystal structure of Figure 3B is referred to as crystal structure B.

[0035] When the Ms of the crystal structures A and B was calculated using the first-principles density functional method, equivalent values ​​were obtained. From this, it can be said that the magnetic properties of Fe-Co alloys in the concentration region where Co atoms are randomly arranged are largely determined by the Co concentration, and are hardly affected by the Co atom positions. This is consistent with what has been confirmed experimentally with actual materials.

[0036] Next, nitriding was performed on crystal structures A and B, and N atoms were introduced into crystal structures A and B so as to be regularly arranged, thereby constructing a supercell with a composition of 123Fe-5Co-4N.

[0037] Figure 4A is a partial schematic diagram of a supercell, viewed from the a-axis direction, in which N atoms have been introduced into crystal structure A to form a 123Fe-5Co-4N alloy. Figure 4B is a partial schematic diagram of a supercell, viewed from the a-axis direction, in which N atoms have been introduced into crystal structure B to form a 123Fe-5Co-4N alloy. The 123Fe-5Co-4N alloy composition is approximately 3.8 atomic % Co and approximately 3.0 atomic % N. The crystal structure in Figure 4A is referred to as crystal structure A', and the crystal structure in Figure 4B is referred to as crystal structure B'.

[0038] The lattice constant change rate and Ms of the crystal structures A' and B' were calculated using first-principles density functional theory. The lattice constant change rate represents the rate of change from the equilibrium lattice constant calculated for the crystal structures A and B before nitriding treatment. The results are shown in Table 1.

[0039]

[0040] It was confirmed that both crystal structures A' and B' had improved Ms compared to crystal structures A and B before nitriding. Furthermore, as shown in Table 1, the rate of change in c-axis length was much larger than the rates of change in a-axis length and b-axis length. This is consistent with the experimental results of actual materials, in which nitriding (intrusion of N atoms) changes the bcc structure to a bct structure. A detailed comparison of the results in Table 1 reveals that crystal structure B' has a larger rate of change in c-axis length and a larger Ms than crystal structure A'.

[0041] In addition, in first-principles calculations covering 100 atoms or more, a difference of 0.1 emu / g in Ms can be considered to be a significant difference. This indicates that there is a large difference in the magnetic properties (here, Ms) between crystal structures A' and B'.

[0042] The results shown in Table 1, where a slight difference in the atomic arrangement within the crystal structure causes a clear difference in the Ms characteristics even though the alloy composition and crystal structure are identical, as in crystal structures A' and B', have not been reported previously for Fe-Co-N alloys and are a phenomenon discovered for the first time in the present invention. Therefore, the inventors conducted further detailed research into the effect of differences in the atomic arrangement within the crystal structure on the magnetization characteristics.

[0043] The inventors suspected that the Co-N bond in the Fe-Co-N alloy may have some effect on the magnetic properties, and so constructed a separate crystal model to investigate the interaction between Co atoms and N atoms in the Fe-Co-N alloy and performed first-principles calculations.

[0044] Specifically, we first constructed a supercell (128 atoms) by replacing one Fe atom with one Co atom in the crystal structure of pure Fe with a bcc structure. Furthermore, we placed one N atom at the 2a site (see Figure 2) so that it was closest to the Co atom. Next, we gradually moved the N atom to sites farther away from the Co atom, and calculated the energy and magnetic moment changes of the system using first-principles density functional theory. The results are shown in Figure 5.

[0045] Figure 5 is a graph showing the relationship between the interatomic distance between Co atoms and N atoms, the energy of the system, and the magnetic moment. As shown in Figure 5, the energy is large when the Co-N interatomic distance is small (when a Co-N bond is present), and decreases as the Co-N interatomic distance increases. On the other hand, the magnetic moment is small when a Co-N bond is present, and increases as the Co-N interatomic distance increases. These results indicate that the Co-N bond is undesirable in Fe-Co-N alloys from the perspectives of the system energy and magnetic moment.

[0046] Here, we closely examined the supercells of crystal structures A' and B' with the alloy composition 123Fe-5Co-4N. In crystal structure A', two of the 24 metal atoms (4e site, 8h site) coordinated with four N atoms (2a site) were Co atoms. On the other hand, in crystal structure B', all 24 metal atoms coordinated with four N atoms were Fe atoms. From these results, it can be said that in Fe-Co-N alloys, the Fe-N bond has the effect of actively improving the magnetic properties, while the Co-N bond has the effect of suppressing the improvement of magnetic properties.

[0047] The degree / quantity of Fe-N and Co-N bonds in an Fe-Co-N alloy can be determined by calculating the partial radial distribution function, which defines the distribution (abundance rate) of atoms surrounding a given atom as a function of distance, and allows us to understand the local structure.

[0048] In the present invention, the partial radial distribution function is calculated by first-principles calculation using a supercell. However, the partial radial distribution function can also be calculated by experimental techniques for real materials. Experimental techniques utilize X-ray scattering, electron scattering, etc. For example, the radial distribution function can be calculated by correcting the diffraction intensity obtained by irradiating X-rays onto a measurement sample and then performing a Fourier transform.

[0049] Fig. 6 is a graph showing the partial radial distribution functions of Co atoms when focusing on N atoms in the crystal structures A' and B'. As shown in Fig. 6, it can be seen that the partial radial distribution functions of Co atoms in the crystal structures A' and B' are significantly different from each other.

[0050] In crystal structure A', a large peak is observed in a region at a distance of 0.26 nm or less from the N atom, and a relatively small peak is observed in a region at a distance of more than 0.26 nm and less than 0.5 nm from the N atom. On the other hand, in crystal structure B', no clear peak is observed in a region at a distance of 0.26 nm or less from the N atom, but rather, relatively small peaks are observed in a region at a distance of 0.22 nm to 0.4 nm and a region at a distance of more than 0.4 nm and less than 0.6 nm from the N atom.

[0051] The peak in the region of 0.26 nm or less in crystal structure A' indicates a high abundance of Co atoms nearest to N atoms, strongly suggesting the formation of Co-N bonds. The peak in the region of more than 0.26 nm and less than 0.5 nm in crystal structure A' and the peaks in the region of 0.22 nm to 0.4 nm and the region of more than 0.4 nm to 0.6 nm in crystal structure B' indicate Co atoms that are not bonded to N atoms because the interatomic distance is too long for chemical bonding.

[0052] Fig. 7 is a graph showing the partial radial distribution functions of Fe atoms when focusing on N atoms in crystal structures A' and B'. As shown in Fig. 7, the partial radial distribution functions of Fe atoms in crystal structures A' and B' are almost the same, and it can be seen that there is a large peak in the region within a distance of 0.26 nm from the N atom.

[0053] The peak in the region of distances of 0.26 nm or less indicates a high abundance of Fe atoms nearest to the N atoms, strongly suggesting the formation of Fe-N bonds. This is a reasonable result, considering that both crystal structures A' and B' have the alloy composition 123Fe-5Co-4N (mostly Fe atoms) and that the N atom is coordinated in the 2a site (a site surrounded by six metal atoms).

[0054] Next, to confirm the uniqueness of the Fe-Co-N alloy, the α” phase ((Fe, Co) 16 A supercell (123Fe-5Co-16N) of the α-phase (123Fe-5Co-4N) was constructed and its magnetic properties were calculated. The alloy composition of 123Fe-5Co-16N is approximately 3.5 atomic % Co and approximately 11.1 atomic % N. For comparison, a supercell (128Fe) of the α-phase of pure Fe was constructed and its magnetic properties were calculated. The results are shown in Table 2. Table 2 also lists the magnetic properties of the previously calculated crystal structures B (123Fe-5Co), B' (123Fe-5Co-4N), and A' (123Fe-5Co-4N).

[0055]

[0056] As shown in Table 2, crystal structure B, which is an Fe-Co alloy, exhibits a higher Ms than the α phase of pure Fe, and crystal structure B', which is obtained by nitriding crystal structure B, also exhibits a higher Ms than crystal structure B. These are consistent with previous experimental results for actual materials. However, it was found that the α" phase (123Fe-5Co-16N, nitrided to the N content of the stoichiometric composition) based on crystal structure B exhibits a lower Ms than crystal structure B', and also a lower Ms than crystal structure A'.

[0057] As mentioned above, the difference in Ms between crystal structures A' and B' is thought to be due to the difference in the number and ratio of Co-N bonds in the atomic arrangement. The reason why the Ms of the α" phase (123Fe-5Co-16N) based on crystal structure B is lower than that of crystal structure A' is thought to be because the number and ratio of Co-N bonds in the atomic arrangement of 123Fe-5Co-16N is even greater than that of crystal structure A'.

[0058] From the viewpoint of magnetic properties, it is considered that the generation / formation of the α” phase, which increases the Co-N bond, is undesirable in Fe-Co-N alloys. This finding supports the conventional technical idea in Fe-N and Fe-Co alloys that "to improve magnetic properties, the α” phase (Fe 16 N2 phase and / or (Fe, Co) 16 This is a technical idea that is the exact opposite of the idea that "it is preferable to generate / form N2" and was discovered for the first time in the present invention.

[0059] (Fe-Co-N based soft magnetic material according to the present invention) As described above, the Fe-Co-N based soft magnetic material according to the present invention is characterized in that, when the concentration of Co is expressed as [Co] (unit: atomic %), 0.4 ≦ [Co] ≦ 25, and when the concentration of N is expressed as [N] (unit: atomic %), 0 < 1.5 − 0.15 × [Co] ≦ [N] ≦ 7 − 0.2 × [Co], with the remainder consisting of Fe and unavoidable impurities, and when the partial radial distribution function showing the relationship between the interatomic distance centered on an N atom and the abundance rate of Co atoms in the soft magnetic material is calculated, the integral value of the abundance rate of Co atoms in the interatomic distance range of 0.26 nm or less is 40% or less compared to the integral value of the abundance rate of Co atoms in the interatomic distance range up to 1 nm.

[0060] Each provision will be explained in detail below.

[0061] Co concentration and N concentration In order for the soft magnetic material to have magnetic properties that significantly exceed those of pure Fe, the Co concentration in the Fe-Co-N alloy is preferably 0.4 atomic % or more, more preferably 0.6 atomic % or more, and even more preferably 0.8 atomic % or more.

[0062] On the other hand, from the viewpoint of magnetic properties, it is preferable that the Co-N bonds in the Fe-Co-N alloy be as few as possible. As a result of investigations and studies by the inventors using percolation theory, it was found that Co-N bonds are unavoidable when the Co concentration is 32 atomic % or more. Therefore, it is desirable that the Co concentration be less than 32 atomic %. Furthermore, from the viewpoint of ensuring the amount of N introduced while avoiding Co-N bonds as much as possible and from the viewpoint of material costs, in the present invention, the Co concentration is preferably 25 atomic % or less, and more preferably 20 atomic % or less.

[0063] When the Co concentration in the alloy is relatively low, the probability of generating / forming Co-N bonds is low, so the N concentration can be relatively high. On the other hand, when the Co concentration in the alloy is relatively high, the probability of generating / forming Co-N bonds is high, so the N concentration needs to be relatively low.

[0064] The inventors constructed numerous supercells with various Co and N concentrations and atomic arrangements in the Fe-Co-N alloy and calculated their magnetic properties using first-principles density functional theory to investigate the range in which the magnetic properties are higher than those of pure Fe and are improved by nitriding / introducing N atoms. The results are shown in Figure 8.

[0065] Fig. 8 is a graph showing the relationship between the Co concentration and the N concentration in an Fe-Co-N alloy at which the magnetic properties are higher than those of pure Fe and are improved by nitriding / introduction of N atoms. As shown in Fig. 8, as the Co concentration increases, the optimum N concentration decreases and the range of the optimum N concentration narrows.

[0066] The relationship between the Co concentration and the N concentration shown in Figure 8 can be expressed mathematically as follows: 0.4 ≦ [Co] ≦ 25 ... Equation (1) 0 < 1.5 - 0.15 × [Co] ≦ [N] ≦ 7 - 0.2 × [Co] ... Equation (2) where [Co] represents the Co concentration (unit: atomic %) and [N] represents the N concentration (unit: atomic %).

[0067] Partial radial distribution function: It has been found that it is desirable to have as few Co-N bonds as possible in Fe-Co-N alloys. The degree / amount of Co-N bonds in Fe-Co-N alloys can be determined by calculating the partial radial distribution function of the target material. The partial radial distribution function can be calculated by first-principles calculations using a supercell of the target material, or by correcting the diffraction intensity obtained by irradiating X-rays onto the actual material and then performing a Fourier transform.

[0068] As described above, the partial radial distribution function is defined by focusing on a certain atom and expressing the distribution (abundance rate) of atoms located around the atom as a function of distance, and can reveal the local structure. In the present invention, by focusing on N atoms and determining the partial radial distribution function of Co atoms, the degree / amount of Co-N bonds can be known.

[0069] As shown in Figure 6, the area within 0.26 nm from the N atom is considered to be the atomic region nearest to the N atom, so the observation of a strong peak in this area strongly suggests the formation of a Co-N bond. The area between 0.26 nm and 0.50 nm from the N atom is considered to be the atomic region of the second nearest neighbors to the N atom, and the area beyond 0.50 nm from the N atom is considered to be the atomic region of the third nearest neighbors or higher to the N atom, so it can be assumed that a Co-N bond is not formed.

[0070] Numerous calculations by the inventors have revealed that the magnetic properties can be improved by nitriding an Fe-Co alloy and introducing N atoms into the Fe-Co alloy if the integral of the abundance rate of Co atoms within 0.26 nm from an N atom is 40% or less relative to the integral of the abundance rate of Co atoms within 1 nm from an N atom. In addition, the magnetic properties can be further improved by nitriding an Fe-Co alloy and introducing N atoms into the Fe-Co alloy if the integral of the abundance rate of Co atoms within 0.26 nm from an N atom is smaller than the integral of the abundance rate of Co atoms within more than 0.26 nm and 0.50 nm from an N atom.

[0071] In the soft magnetic material of the present invention, the α″ phase (here, (Fe, Co)) in which N atoms are regularly arranged is a crystalline phase constituting the Fe-Co-N based soft magnetic material. 16 The generation / formation of a Ni-N2 phase is considered undesirable because it increases the probability of Co-N bonding. On the other hand, it has been confirmed experimentally and numerically that nitriding / introduction of N atoms in Fe-Co alloys improves magnetic properties. As mentioned above, the preferred range of N concentration is lower than the 11.1 atomic % N concentration in the stoichiometric composition of the α″ and α' phases. Furthermore, in the α' phase, N atoms are not regularly arranged (they are irregularly arranged).

[0072] Taking all these factors into consideration, in the present invention, the nitrogen-deficient α' phase ((Fe,Co)N) is obtained by avoiding Co-N bonds as much as possible during nitriding / introduction of N atoms. 1-xIt can be said that it is preferable to generate / form the nitrogen-deficient α' phase (0<x<1). In other words, ideally, the entire Fe—Co—N based soft magnetic material is preferably composed of the nitrogen-deficient α' phase.

[0073] The average c-axis lattice parameter (average c-axis length) of the nitrogen-deficient α' phase is greater than 0.289 nm and less than 0.309 nm, taking into account the preferable range of N concentration explained above. The average c-axis lattice parameter of an actual material can be calculated, for example, from XRD measurements.

[0074] When the constituent phases of the actual Fe-Co-N soft magnetic material of the present invention are identified by XRD measurement, if the N concentration is sufficiently low, the difference in lattice constant between the nitrogen-deficient α' phase and the α phase becomes small due to the low N concentration, and the detected diffraction peak may be identified as the α phase. For this reason, in the present invention, when the constituent phases are identified by XRD measurement, any of the following is determined to be a preferable state: a single nitrogen-deficient α' phase, a mixed phase of nitrogen-deficient α' phase and the α phase, or a single α phase.

[0075] As mentioned above, first-principles calculations seek to determine the ground-state wave function of electrons in a material. In other words, this represents the equilibrium state of the material over an infinite time period. However, because real manufacturing processes are carried out within a finite time period that is industrially meaningful, non-equilibrium states and phases may emerge, or unwanted heterogeneous phases may appear.

[0076] As a result of numerous calculations by the present inventors, it can be said that in order to satisfy the above-mentioned requirement of the partial radial distribution function of Co atoms surrounding an N atom, the volume fraction of the α″ phase is preferably less than 10 vol %. The volume fraction of the α″ phase is more preferably 5 vol % or less, and even more preferably 3 vol % or less. As mentioned above, by performing precise measurements of the crystal structure of real substances, the α″ phase and the α' phase can be distinguished from each other based on the presence or absence of a superlattice peak resulting from the regular arrangement of N atoms.

[0077] In conventional Fe-N alloy materials, from the viewpoint of magnetic properties, it is considered undesirable to leave a γ phase (austenite phase) or to create / form a γ' phase (Fe4N phase) or an ε phase (Fe3N phase). The same is true for the Fe-Co-N soft magnetic material of the present invention, where the remaining γ phase or the creation / formation of a γ' phase (here, the (Fe,Co)4N phase) or an ε phase (here, the (Fe,Co)3N phase) is undesirable. However, from a practical viewpoint, a total of 5% by volume or less of the γ phase, γ' phase, and ε phase is acceptable. The total amount of these heterogeneous phases is more preferably 3% by volume or less, and even more preferably 1% by volume or less.

[0078] Methods for reducing the γ, γ', and ε phases are similar to those of conventional techniques, such as sub-zero treatment and nitriding process innovations. Unfortunately, a process for actively generating / forming the α' phase while avoiding the α" phase has not yet been established. However, a possible solution may be aging treatment in a temperature range that facilitates the migration / diffusion of N atoms while suppressing the migration / diffusion of metal atoms.

[0079] Substitutable Components: In the Fe-Co-N based soft magnetic material of the present invention, a portion of the Fe and / or Co may be substituted with 0 atomic % to 1 atomic % of Ni, 0 atomic % to 1 atomic % of Mn, 0 atomic % to 1 atomic % of Cu, and 0 atomic % to 1 atomic % of Cr. These Ni, Mn, Cu, and Cr components have the advantage of reducing material costs without significantly degrading magnetic properties. On the other hand, if the content of Ni, Mn, Cu, or Cr exceeds 1 atomic %, the degree of degradation of magnetic properties increases.

[0080] Furthermore, in the Fe-Co-N based soft magnetic material of the present invention, part of the N may be substituted with C, O (oxygen), or B (boron). Substitution of appropriate amounts of these C, O, and B components can contribute to reducing iron loss Pi without significantly deteriorating the magnetization properties.

[0081] In the case of C substitution, it is preferable that the C concentration is less than the N concentration, and the sum of the C concentration and the N concentration satisfies [N] in the above-mentioned preferable relational expression (2) of N concentration. Similarly, in the case of O substitution and B substitution, it is preferable that the O concentration and the B concentration are each 10% or less of the N concentration, and the sum of the O concentration and / or the B concentration and the N concentration satisfies [N] in the relational expression (2).

[0082] Inevitable impurities: When manufacturing alloy materials industrially, the inclusion of some impurities is unavoidable. In other words, it is reasonable for the Fe-Co-N-based soft magnetic material of the present invention to contain unavoidable impurities. Examples of unavoidable impurities include Si (silicon), P (phosphorus), S (sulfur), and H (hydrogen). The total content of unavoidable impurities is preferably 3 atomic % or less, more preferably 2 atomic % or less, and even more preferably 1 atomic % or less, within a range that does not significantly adversely affect the magnetic properties.

[0083] (Products to which the Fe-Co-N based soft magnetic material of the present invention can be applied) There are no particular limitations on the products to which the Fe-Co-N based soft magnetic material of the present invention can be applied, but from the perspective of nitriding treatment during production, it is preferable that the material have a plate shape that allows N atoms to easily penetrate and diffuse. When applied to a soft magnetic plate, it can be suitably used as a plate material for laminated iron cores. When applied to a soft magnetic plate for a laminated iron core, from the perspectives of production cost and ease of use, the thickness of the plate material is preferably 0.01 mm or more and 3 mm or less, and more preferably 0.01 mm or more and 1 mm or less.

[0084] The present invention will be explained in more detail below by showing some experimental examples, but the present invention is not limited to the configurations and structures described in these experimental examples.

[0085] [Experimental Example 1] In Experimental Example 1, an experiment was carried out to confirm the consistency between the results of fabricating and measuring an actual substance and the first-principles calculations using a supercell.

[0086] (Preparation of Samples 1 and 2 of Fe-Co-N-based soft magnetic material) As the starting material, an Fe-Co alloy plate with a composition of Fe-20 atomic % Co and a thickness of 0.1 mm was prepared. The prepared Fe-Co alloy plate was subjected to nitriding heat treatment at 600°C in an NH3 (ammonia) gas atmosphere. At this time, the NH3 gas concentration and holding time were adjusted to prepare two types of actual material samples with different N concentrations. After the nitriding heat treatment, the plate was subjected to rapid cooling by water cooling, ultra-subzero treatment, and tempering treatment in that order.

[0087] The N concentrations of the prepared real material samples 1 and 2 were investigated by precise mass measurement and elemental analysis using an electron probe microanalyzer (EPMA, manufactured by JEOL Ltd., JXA-8530F). As a result, the N concentration of real material sample 1 was measured to be approximately 0.8 atomic %, and the N concentration of real material sample 2 was measured to be approximately 1.5 atomic %.

[0088] Next, the magnetic properties of the prepared real material samples 1 and 2 were measured using a vibrating sample magnetometer (Riken Denshi Co., Ltd., BHV-525H) under conditions of a magnetic field of 1.6 MA / m and a temperature of 20°C. In addition, supercell samples 1 and 2 (102Fe-26Co-1N supercell and 102Fe-26Co-2N supercell) were constructed to correspond to real material samples 1 and 2, and the magnetic properties of supercell samples 1 and 2 were calculated using first-principles density functional theory. The measurement and calculation results are shown in Table 3.

[0089]

[0090] As shown in Table 3, for real substance samples 1 and 2 and supercell samples 1 and 2, the difference between the experimental and calculated Ms values ​​is sufficiently small, confirming that the first-principles calculations performed in this invention accurately represent the properties of the real substances.

[0091] [Experimental Example 2] (Investigation of the effect of metal element substitution in Fe-Co-N based soft magnetic materials) A supercell was constructed using 122Fe-6Co as the base Fe-Co alloy, with a Co concentration of approximately 4.7 atomic %. Next, a supercell was constructed using 122Fe-6Co-4N, with an N concentration of approximately 3.0 atomic %.

[0092] The a-axis and c-axis lattice constants and magnetic properties of these supercells were calculated using first-principles density functional theory. The results for the 122Fe-6Co supercell were "a-axis length = 0.567 nm," "c-axis length = 0.567 nm," and "Ms = 223.3 emu / g." The results for the 122Fe-6Co-4N supercell were "a-axis length = 0.568 nm," "c-axis length = 0.583 nm," and "Ms = 228.8 emu / g." These results confirm that nitriding the base Fe-Co alloy changes its crystal structure from "bcc" to "bct," improving its magnetic properties.

[0093] Next, we constructed supercells by substituting some of the Fe atoms in the nitrided 122Fe-6Co-4N supercell with other metal atoms: a 121Fe-1Ni-6Co-4N supercell containing approximately 0.8 atomic percent Ni, a 121Fe-1Mn-6Co-4N supercell containing approximately 0.8 atomic percent Mn, a 121Fe-1Cu-6Co-4N supercell containing approximately 0.8 atomic percent Cu, and a 121Fe-1Cr-6Co-4N supercell containing approximately 0.8 atomic percent Cr.

[0094] For supercells in which some of the Fe atoms are replaced with other metal atoms, the a-axis and c-axis lattice constants and magnetic properties were calculated using the first-principles density functional method, as described above. The results are shown in Table 4. Table 4 also lists the calculation results for 122Fe-6Co and 122Fe-6Co-4N supercells.

[0095]

[0096] Ni, Mn, Cu, and Cr atoms are transition metals that belong to the same period as Fe and Co atoms, so it can be seen that the a-axis length and c-axis length are hardly affected by the partial substitution of Fe atoms. On the other hand, regarding the magnetic properties, although a decrease in Ms is observed with the substitution of Mn, Cu, and Cr atoms in 122Fe-6Co-4N, it is confirmed that Ms is still sufficiently higher than that of unnitrided 122Fe-6Co.

[0097] The above-described embodiments and experiments have been described to aid in understanding the present invention, and the present invention is not limited to the specific configurations described. For example, it is possible to replace part of the configuration of the embodiments with configurations that are within the technical common sense of those skilled in the art, and it is also possible to add configurations that are within the technical common sense of those skilled in the art to the configuration of the embodiments. In other words, it is possible to delete, replace, or add part of the configurations of the embodiments and experiments in this specification, as long as they do not deviate from the technical concept of the invention.

[0098] 101...unit cell of Fe-Co alloy, 102...metal atom site.

Claims

1. A soft magnetic material comprising Fe, Co, and N, wherein the concentration of Co expressed as [Co] (unit: atomic %) is 0.4 ≦ [Co] ≦ 25, and the concentration of N expressed as [N] (unit: atomic %) is 0 < 1.5 - 0.15 × [Co] ≦ [N] ≦ 7 - 0.2 × [Co], with the remainder consisting of Fe and unavoidable impurities, and wherein, when a partial radial distribution function showing the relationship between the interatomic distance around an N atom in the soft magnetic material and the abundance rate of Co atoms is calculated, the integral of the abundance rate of Co atoms where the interatomic distance is 0.26 nm or less is 40% or less of the integral of the abundance rate of Co atoms where the interatomic distance is up to 1 nm.

2. An Fe-Co-N based soft magnetic material according to claim 1, wherein the integral of the abundance rate of Co atoms in the interatomic distance range of 0.26 nm or less in the partial radial distribution function is smaller than the integral of the abundance rate of Co atoms in the interatomic distance range of more than 0.26 nm to 0.50 nm.

3. The Fe-Co-N soft magnetic material according to claim 1, wherein, when the constituent phases of said soft magnetic material are identified by X-ray diffraction measurement, the α" phase is less than 10% by volume, the total of the γ phase, γ' phase and ε phase is 5% by volume or less, and the remainder is nitrogen-deficient α' phase and / or α phase.

4. The Fe-Co-N soft magnetic material according to claim 2, wherein, when the constituent phases of said soft magnetic material are identified by X-ray diffraction measurement, the α" phase is less than 10% by volume, the total of the γ phase, γ' phase and ε phase is 5% by volume or less, and the remainder is nitrogen-deficient α' phase and / or α phase.

5. The Fe-Co-N based soft magnetic material according to claim 3 or 4, wherein the average lattice constant of the c-axis of the nitrogen-deficient α' phase is greater than 0.289 nm and not greater than 0.309 nm.

6. An Fe-Co-N based soft magnetic material according to any one of claims 1 to 4, characterized in that the Fe and / or the Co in the soft magnetic material is partially substituted with 1 atomic % or less of Ni, 1 atomic % or less of Mn, 1 atomic % or less of Cu, and 1 atomic % or less of Cr.

7. An Fe-Co-N based soft magnetic material according to claim 5, characterized in that the Fe and / or the Co in the soft magnetic material is partially substituted with 1 atomic % or less of Ni, 1 atomic % or less of Mn, 1 atomic % or less of Cu, and 1 atomic % or less of Cr.

8. An Fe-Co-N based soft magnetic material according to any one of claims 1 to 4, characterized in that less than half of the N in the soft magnetic material is substituted with C.

9. The Fe-Co-N based soft magnetic material according to claim 5, wherein less than half of the N in the soft magnetic material is substituted with C.

10. The Fe-Co-N based soft magnetic material according to claim 6, wherein less than half of the N in the soft magnetic material is substituted with C.

Images