Exchange coupling between soft magnetic ferrite and hard ferromagnetic Sm2Fe17N3 in ferrite/Sm2Fe17N3 composites Exchange coupling between soft magnetic ferrite and hard ferromagnetic Sm2Fe17N3 in fer[.]
Exchange coupling between soft magnetic ferrite and hard ferromagnetic Sm2Fe17N3 in ferrite/Sm2Fe17N3 composites , N Imaoka , E Kakimoto, K Takagi, K Ozaki, M Tada, T Nakagawa, and M Abe Citation: AIP Advances 6, 056022 (2016); doi: 10.1063/1.4944519 View online: http://dx.doi.org/10.1063/1.4944519 View Table of Contents: http://aip.scitation.org/toc/adv/6/5 Published by the American Institute of Physics AIP ADVANCES 6, 056022 (2016) Exchange coupling between soft magnetic ferrite and hard ferromagnetic Sm2Fe17N3 in ferrite/Sm2Fe17N3 composites N Imaoka,1,2,a E Kakimoto,2 K Takagi,1 K Ozaki,1 M Tada,3 T Nakagawa,4 and M Abe3 National Institute of Advanced Industrial Science and Technology (AIST), Nagoya, 463-8560 Japan Asahi Kasei Corporation, Fuji, 416-8501 Japan Tokyo Institute of Technology, Tokyo, 152-8550 Japan Osaka University, Suita, 565-0871 Japan (Presented 12 January 2016; received November 2015; accepted 25 January 2016; published online 14 March 2016) In our previous work, we succeeded in fabricating ferrite/Sm2Fe17N3 composite magnets from explosive-consolidating Sm2Fe17N3 powders (2µm size) which were coated with a continuous iron ferrite layer (50nm thick) in an aqueous solution The magnetization curves had no inflection, which suggests that the soft magnetic ferrite layer is exchange-coupled with the hard ferromagnetic Sm2Fe17N3 particles In this paper, we provide evidence of exchange coupling in ferrite/Sm2Fe17N3 composites by the following means: 1) measurements of recoil permeability, 2) detailed microstructural observation and 3) calculations of the reduction in remanence due to the introduction of a ferrite layer in the Sm2Fe17N3 magnets Our ferrite/Sm2Fe17N3 composite magnets are a novel type of spring magnet in which an insulating soft magnetic phase is exchange-coupled with hard magnetic phase C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4944519] I INTRODUCTION Sm2Fe17N3 compounds are good candidates for high performance magnets because of their high saturation magnetization and strong uniaxial anisotropy.1,2 Since Sm2Fe17N3 compounds decompose into SmN and α-Fe when heated above 873K, it is fairly difficult to obtain fully dense magnets Although several researchers have attempted to prepare fully dense magnets by using shock compression, aerosol deposition, hot-isostatic pressure, compression shearing, cylindrical explosive consolidation and high-pressure current sintering,3–8 the Sm2Fe17N3 magnets produced by those methods have not yet achieved practical application because, other than oxidation resistance, Sm2Fe17N3 magnets not provide any overwhelming advantages, such as high resistivity or low cost processing, over existing sintered rare-earth magnets Thus, at present, their application is limited to resin bonded magnets In permanent magnets which are operated at high frequency in advanced applications such as electrical vehicle magnets, eddy current loss is a serious problem Although Nd2Fe14B magnets are often used in this application, their resistivity is approximately 100 µΩcm at most Thus, a ten-fold increase in resistivity is necessary The resistivity of Sm2Fe17N39 is four times higher than that of Nd2Fe14B, but this is still not sufficient for high frequency applications Our basic strategy is to develop a ferrite/Sm2Fe17N3 composite magnet which provides enhanced resistivity without greatly reducing magnetic properties In our previous paper,9 we successfully fabricated ferrite/Sm2Fe17N3 composite magnets from explosive-consolidating Sm2Fe17N3 powders which a Author to whom correspondence should be addressed E-mail: n-imaoka@aist.go.jp 2158-3226/2016/6(5)/056022/6 6, 056022-1 © Author(s) 2016 056022-2 Imaoka et al AIP Advances 6, 056022 (2016) were coated in advance with a continuous iron ferrite layer in an aqueous solution We estimated the resistivity for fully dense ferrite/Sm2Fe17N3 magnets as being 4000 µΩcm, which is 10 times higher than the estimated resistivity of fully dense Sm2Fe17N3 magnets, while their remanence Br and coercivity HcJ were only slightly reduced in comparison with sample Sm2Fe17N3 magnets The magnetization curves had no inflection, which suggests that the soft magnetic ferrite layer is exchange-coupled with the hard ferromagnetic Sm2Fe17N3 particles Our ferrite/Sm2Fe17N3 composites consist of coarse hard magnetic particles (diameter: 2µm, not nano-sized) which have a comparative thick (50nm) soft magnetic layer In the well-known exchange spring magnet, the optimal thicknesses of the hard magnetic phase is 2δ (δ : domain wall thickness) and that of soft magnetic phase is 2δs These thickness values are given by δ = π (Ah/Kh)1/2 and δs = π (As/2Kh)1/2, where Ah and As are the exchange stiffness constants of the hard and soft magnetic phases, respectively, and Kh is the magnetocrystalline anisotropy constant of the hard magnetic phase The values of 2δ and 2δs are almost equal to each other10; therefore, these optimal thicknesses not exceed a few 10nm, because δ of rare-earth magnet material is about 4-9nm.11 Similar results for these optimal thicknesses have also been reported based on the results of three-dimensional computer simulations.12 Exchange-coupling composites having a “large hard magnetic core and thick soft magnetic layer (LHATS)” have been designed not only by the present authors, who used ferrite/Sm2Fe17N3, but also by several other researchers who used, for instance, α-Fe/Sm-Co or Nd-Fe-B.13,14 LHATS magnets have a number of merits, in that the large hard core aligns along the direction of an external magnetic field and the thick soft layer results in a large volume fraction of the soft magnetic phase Nevertheless, these designs are in principle very difficult to realize due to the thin 2δs and the fine critical diameter d for a single domain particle of the soft magnetic phase II EXPERIMENT The Sm2Fe17N3 powders (particle size: 2µm) were provided by Sumitomo Metal Mining Co., Ltd 15 The Sm2Fe17N3 powders were coated with an iron ferrite (Fe3O4-γFe2O3 intermediate) layer by ferrite plating, which is an aqueous process.16 After etching the surfaces of the Sm2Fe17N3 powders with an acidic solution, the powders were dispersed in distilled water, to which a FeCl2 reaction solution (pH up to 4) and a pH adjusting solution of KOH (pH up to 14) were added with continuous stirring under an air atmosphere.17 Ferrite/Sm2Fe17N3 and Sm2Fe17N3 powder compacting (PC) magnets were then formed at less than 1GPa in a field of 15 kOe by die pressing The magnets were compacted by an explosive consolidation (EC) technique using water as the transmitting medium18 with no external magnetic field throughout the preparation process Conventional magnetization measurements for variable demagnetization and recoil curves were performed using a vibrating-sample magnetometer (VSM) Resistivity (ρ) was measured by the four-probe method III RESULTS AND DISCUSSION The volume ratio of ferrite to Sm2Fe17N3 in ferrite/Sm2Fe17N3 EC magnets was estimated as 15:85 based on the results of SEM observations (e.g., Fig 1(B)) and the densities of the magnets TEM observation (Fig 1(A)) revealed that the ferrite coating phase had a grain size of about 10nm Fig 1(D) shows a SEM photograph of iron ferrite nano-powders (IFNP) prepared by the above-mentioned aqueous process without Sm2Fe17N3 powders The particle size of these IFNP is within a range of 5-50nm The crystalline structure of the IFNP, which corresponds to that of the ferrite coating phase in our magnets, is of the cubic spinel-type with no impurity phase, as revealed by XRD analysis The lattice constant is smaller than that of magnetite (Fe3O4), since the iron ferrite prepared by ferrite plating becomes an intermediate between magnetite (Fe3O4) and maghemite (γ-Fe2O3).9,16 Figure 2(A) shows a comparison of the demagnetization and recoil curves for Sm2Fe17N3 and ferrite/Sm2Fe17N3 EC magnets These curves were all measured after saturation by a 60 kOe pulse field The curves gave a recoil permeability (µr) of 1.31-1.35 for ferrite/Sm2Fe17N3, which was 056022-3 Imaoka et al AIP Advances 6, 056022 (2016) FIG (A) TEM image and (B) SEM image of ferrite/Sm2Fe17N3 EC magnets, and (C) electron diffraction pattern of area encircled by dots in Fig 1(A) Miller indices are shown based on the cubic spinel structure (D) SEM image of IFNP A ferrite (black boundaries) layer surrounds almost the entire surface of the Sm2Fe17N3 (gray grains) powders in Fig 1(B) slightly higher than that (1.23-1.28) for Sm2Fe17N3, as show in Fig 2(B) Here, recoil permeability was calculated by µr = + 4π(Md − M0) / Hd, using magnetization Md under the reverse field Hd (Hd: negative value), and magnetization M0 under the zero field on each of the recoil curves The results described above prove that exchange coupling acts between the Sm2Fe17N3 core and ferrite coating layer in ferrite/Sm2Fe17N3 EC magnets.17 The detailed TEM observation results shown in Fig 3(B) reveal that the Sm2Fe17N3 main phase is directly coupled to the ferrite phase Namely, there is no phase between the main phase and the coating phase This is because the amorphous surface oxide layers (thickness: 10nm) observed in the Sm2Fe17N3 powders8,19 were removed by etching the surfaces of the Sm2Fe17N3 particles prior to coating the powders with ferrite.17 This enabled exchange coupling between the ferrite and Sm2Fe17N3 phases Table I gives the magnetic properties of the Sm2Fe17N3 and ferrite/Sm2Fe17N3 magnets in this work Introducing the ferrite coating phase caused only slight degradation of the remanence of the EC magnets, from 6.24 kG (Sm2Fe17N3) to 5.98 kG (ferrite/Sm2Fe17N3), or a reduction ratio ∆Br of about 4.2% This value was very small in spite of the introduction of a large amount of 15 vol% ferrite phase in the Sm2Fe17N3 EC magnets The models used to calculate ∆Br are shown in Fig 3(A) The details of the above estimation are as follows FIG (A) Demagnetization and recoil curves and (B) recoil permeabilities µ r for (a) ferrite/Sm2Fe17N3 and (b) Sm2Fe17N3 EC magnets 056022-4 Imaoka et al AIP Advances 6, 056022 (2016) FIG (A) Models used in calculation of ∆Br: (i) Ferrite phase covers the etched surface of Sm2Fe17N3 powders and (ii) ferrite phase simply covers the Sm2Fe17N3 starting powders with a surface oxide layer (B) TEM image of interface between Sm2Fe17N3 main phase and ferrite boundary phase of ferrite/Sm2Fe17N3 EC magnets This result supports the assumption in model (i) for calculation of ∆Br According to the Stoner-Wohlforth theory, the remanence of isotropic single domain particles is a half value of saturated magnetization 4πMs We conjecture that the reduction ratio of remanence ∆Br of the ferrite/Sm2Fe17N3 to Sm2Fe17N3 EC magnets was in proportion to the reduction ratio of this intrinsic 4πMs Introducing the ferrite coating phase degraded the observed remanence of the EC magnets by approximately ∆Br obs.= 4.2% First, therefore, we performed a calculation assuming that the ferrite phase simply covered the Sm2Fe17N3 starting powder (see Fig 3(A)(ii)) In this case, the reduction ratio (∆Br calc.1[%]) of Br is {MN(1 − vSO)vN + MOvO}η f ∆Br calc.1 = − × 100 (1) MN(1 − vSO)η S where, 4πMN (=15.7 kG) and 4πMO (= 6.0 kG) are the 4πMs of Sm2Fe17N3 and ferrite, νSO (= 0.03) is the volume fraction of the surface oxide layer of the Sm2Fe17N3 particle, η S and η f are the packing fractions of the Sm2Fe17N3 and ferrite/Sm2Fe17N3 EC magnets and the volume ratio of ferrite to Sm2Fe17N3 in ferrite/Sm2Fe17N3 is vO : vN From formula (1), ∆Br calc.1 is estimated as being approximately about 7.1% However, this value is inconsistent with ∆Br obs Next, we assumed that the surface oxide layers of the Sm2Fe17N3 particles in the ferrite/ Sm2Fe17N3 EC magnets were removed by the etching step in the ferrite plating procedure,17 as shown Fig 3(A)(i) Based on this assumption, formula (1) is modified to the following formula: (MNvN + MOvO)η f ∆Br calc.2 = − × 100 (2) MN(1 − vSO)η S The corrected calculation value ∆Br calc.2 equaled approximately 4.4% Thus, ∆Br calc.2 was essentially equal to ∆Br obs , if it is assumed that the surface oxide layers of the Sm2Fe17N3 particle were removed in the ferrite plating process TABLE I Magnetic properties, electrical resistivities (ρ) and packing fractions (η) of isotropic EC and anisotropic PC magnets studied in this work Br (kG) H cJ (kOe) (BH)max (MGOe) ρ (µΩcm) η (vol%) Ferrite/Sm2Fe17N3 EC magnet Sm2Fe17N3 EC magnet 5.98 6.24 7.73 8.00 6.66 7.57 4800 690 94 92 Ferrite/Sm2Fe17N3 exchange coupled PC magnet Sm2Fe17N3-iron ferrite mixed PC magnet 7.51 7.17 9.90 9.42 12.8 9.42 7500 2300 73 73 056022-5 Imaoka et al AIP Advances 6, 056022 (2016) FIG Demagnetization curves for (a) ferrite/Sm2Fe17N3 and (b) Sm2Fe17N3-iron ferrite mixed PC magnets From the calculation by formula (2), almost no surface oxide layer from the Sm2Fe17N3 starting powder existed between the ferrite phase and the Sm2Fe17N3 phase, revealing that the bond by the exchange interaction functioned strongly Figure shows a comparison of the demagnetization curves for a) ferrite/Sm2Fe17N3 PC magnet and b) Sm2Fe17N3-iron ferrite mixed PC magnet The starting mixed powders of the PC magnets in Fig were fabricated by mixing Sm2Fe17N3 powders (particle size: µm) and IFNP in an agate mortar with hexane These PC magnets contain 15 vol% soft ferrite and have densities of a) 5.31 and b) 5.30 g/cm3, respectively The demagnetization curve of a) has no inflection, which suggested that the ferrite layer is exchange-coupled with the Sm2Fe17N3 particles The curve of b) has an inflection point at kOe, which indicates that no exchange coupling occurred in this magnet As shown in Table I, these PC magnets exhibit the resistivities of a) 7500 and b) 2300 µΩcm, respectively Iron ferrite powders are present only between the grains of the Sm2Fe17N3 powders in the Sm2Fe17N3-magnetite mixed PC magnet, and did not form a coating on the Sm2Fe17N3 particles Therefore, the electric current permeates through the boundary formed by only the Sm2Fe17N3 particles, and electrical resistivity is remarkably smaller compared with that of the ferrite/Sm2Fe17N3 PC magnet, in which the Sm2Fe17N3 particle surface is almost completely coated with a ferrite layer (see Fig 1(B)) Thus, the soft ferrite layer causes extreme deterioration of the magnetic properties of composite magnets without exchange coupling between a ferrite and a Sm2Fe17N3 phase, while providing only slight improvement in electrical resistivity Both δ and d of iron ferrite are about 100-200nm,20,21 which is larger than those of other typical metallic soft magnetic materials It is therefore thought that magnetic reversal does not occur easily from the thick soft layer (50nm) in our exchange-coupling LHATS magnets, even under a high demagnetization field of several kilo-oersted As a result of this feature, or magnet retains exchange-spring characteristics on the demagnetization curves The ferrite layer in our ferrite/Sm2Fe17N3 composite magnet retains magnetic exchange coupling among Sm2Fe17N3 grains, and yet suppresses intergrain electrical coupling, thereby increasing resistivity IV CONCLUSION Ferrite/Sm2Fe17N3 composite magnets were prepared by a novel process with the aim of realizing enhanced resistivity, and their exchange-spring behavior was evidenced by recoil permeability 056022-6 Imaoka et al AIP Advances 6, 056022 (2016) measurements The Sm2Fe17N3 main phase is directly coupled to the ferrite coating phase, which was also clearly evidenced by TEM observations in combination with calculation of the reduction ratio of remanance Due to the spring-exchange interaction, the resistivity of the soft ferrite layer can be increased with no serious reduction of 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(2016) Exchange coupling between soft magnetic ferrite and hard ferromagnetic Sm2Fe17N3 in ferrite/ Sm2Fe17N3 composites N Imaoka,1,2,a E Kakimoto,2 K Takagi,1 K Ozaki,1 M Tada,3 T Nakagawa,4 and. .. coating the powders with ferrite. 17 This enabled exchange coupling between the ferrite and Sm2Fe17N3 phases Table I gives the magnetic properties of the Sm2Fe17N3 and ferrite/ Sm2Fe17N3 magnets in. .. retains exchange- spring characteristics on the demagnetization curves The ferrite layer in our ferrite/ Sm2Fe17N3 composite magnet retains magnetic exchange coupling among Sm2Fe17N3 grains, and