Article
Chemistry, Physical
Yuqing Li, Dan Wu, Mingkun Wang, Zhanjia Wang, Weiqiang Liu, Yuan Teng, Ming Yue, Hongguo Zhang, Weixing Xia
Summary: This paper presents an in-situ observation of the magnetic domain structure and its dynamic evolution of core-shell structure grains in the grain-boundary diffused Nd-Fe-B magnet. It is found that the shell regions have wider domains due to higher anisotropy constant and lower saturation magnetization. The study reveals that the demagnetization process of core-shell structure exhibits a multi-step nucleation process.
JOURNAL OF ALLOYS AND COMPOUNDS
(2022)
Article
Materials Science, Multidisciplinary
Xin Tang, Jiangnan Li, Hossein Sepehri-Amin, Anton Bolyachkin, Andres Martin-Cid, Shintaro Kobayashi, Yoshinori Kotani, Motohiro Suzuki, Asako Terasawa, Yoshihiro Gohda, Tadakatsu Ohkubo, Tetsuya Nakamura, Kazuhiro Hono
Summary: The reduction in magnetization of the intergranular phase of Nd-Fe-B-based magnets leads to a change in the angular dependence of coercivity, resulting in an improved coercivity of the magnets.
NPG ASIA MATERIALS
(2023)
Article
Chemistry, Applied
Weiqiang Liu, Yi Li, Dan Wu, Ming Yue, Zhanjia Wang, Shanshun Zha, Youhao Liu, Xiaofei Yi, Yihui Du
Summary: The high performance Nd-Fe-B sintered magnets were prepared using grain boundary diffusion (GBD) technology through NdH3 and TbH3 nanoparticle diffusion. The core-shell structure formed by heavy rare earth substitution is found to be the main factor of coercivity enhancement, directly influencing the magnetic domain reversal mode.
JOURNAL OF RARE EARTHS
(2021)
Article
Materials Science, Multidisciplinary
Yutaka Matsuura, Keisuke Ishigami, Ryuji Tamura, Tetsuya Nakamura
Summary: The demagnetization processes of Nd-Fe-B sintered magnets and ferrite magnets were studied using XMCD. The average XMCD demagnetization curves were compared with the curves from magnetic measurements. It was found that magnetization reversal in the magnets occurs in two steps, with grains being discretely demagnetized followed by the reversal of adjacent grains to form clusters. The results from XMCD measurements accurately reflect the bulk information when considering the self-demagnetization field and magnetic after-effect.
JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS
(2023)
Article
Chemistry, Physical
Zhehuan Jin, Guangfei Ding, Xiaodong Fan, Shuai Cao, Sining Fan, Zhixiang Wang, Bo Zheng, Shuai Guo, Renjie Chen, Aru Yan, Xincai Liu
Summary: The grain boundary diffusion process of Pr-Al-Cu at various temperatures in sintered Nd-Fe-B magnets and its influence on the magnetic properties were investigated. The results showed that the diffusion process resulted in the formation of shell phases rich in Pr and Al at 700 degrees C, which significantly enhanced the coercivity of the magnets after annealing. The thickness of the shell phase decreased during the annealing process and facilitated the formation of intergranular phases.
JOURNAL OF ALLOYS AND COMPOUNDS
(2022)
Article
Chemistry, Applied
Jinhao Zhu, Guangfei Ding, Lei Jin, Zhehuan Jin, Bo Zheng, Shuai Guo, Renjie Chen, Aru Yan
Summary: This study proposes a grain boundary modification approach using Nd-Ga addition to significantly enhance the coercivity of B-lean Nd-Fe-B sintered magnets. The coercivity is increased from 1.51 to 2.04 T by optimizing the microstructure and adjusting the phase composition of the grain boundary phase. The addition of Nd-Ga forms a continuous thin grain boundary phase and intermetallic Nd6Fe13Ga phases, resulting in improved magnetic properties.
JOURNAL OF RARE EARTHS
(2022)
Article
Chemistry, Physical
Enxiang Yang, Weiyang Jin, Chao Man, Xiaolian Liu, Song Fu, Lan Bo, Shuainan Xu, Luyao Zhao, Zhi Li, Zhen Shi, Menglin Yu, Lizhong Zhao, Xuefeng Zhang
Summary: Grain boundary diffusion process has been applied for enhancing coercivity of Nd-Fe-B magnets by forming core-shell structure with less Tb consumption. However, excessively thick and asymmetric Tb-rich shells near the shallow diffused surface have minor contribution to coercivity enhancement and consume more Tb resources.
JOURNAL OF ALLOYS AND COMPOUNDS
(2023)
Article
Materials Science, Multidisciplinary
A. Martin-Cid, T. Kawahara, S. Kobayashi, K. Toyoki, D. Billington, Y. Kotani, H. Kubo, Y. Une, T. Iriyama, M. Sagawa, T. Nakamura
Summary: The demagnetization process of Nd-Fe-B-based magnets produced using different manufacturing techniques was studied using soft X-ray magnetic circular dichroism microscopy. The study found that the coercivity of the HDDR processed magnet is lower than the magnets produced with N2 jet-milled powder, even though the grain size is smaller. Additionally, it was observed that in the HDDR processed magnet, nucleation of individual grains occurs before the reversed magnetic domains propagate.
JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS
(2021)
Article
Chemistry, Physical
Feifei Li, Jiajie Li, Sajjad Ur Rehman, Lili Zhang, Yikuan Hu, Munan Yang, Xiaoqiang Yu, Shuwei Zhong, Tongxiang Liang
Summary: By utilizing composite powders for grain boundary diffusion, a significant improvement in coercivity of magnets can be achieved, while optimizing the microstructure can enhance the temperature coefficient of coercivity. Improved Pr-Al-rich grain boundary phases contribute to excellent diffusion efficiency and Dy diffusion depth.
JOURNAL OF ALLOYS AND COMPOUNDS
(2022)
Article
Physics, Multidisciplinary
Jin-Hao Zhu, Lei Jin, Zhe-Huan Jin, Guang-Fei Ding, Bo Zheng, Shuai Guo, Ren-Jie Chen, A-Ru Yan
Summary: The study investigates the effects of post-sinter annealing on the microstructure and magnetic properties of Nd-Fe-B magnets with different Nd-Ga intergranular additions. The results show that the coercivity and remanence of the magnets change differently after annealing depending on the Nd-Ga concentrations. This is attributed to the evolution of the grain boundary structure after the annealing process.
Article
Chemistry, Physical
Q. Feng, Y. L. Huang, H. F. Li, J. Y. Yu, C. Y. Wang, Z. J. Wu, Y. F. Yao, Y. H. Hou, W. Li, L. Ma, H. B. Yu
Summary: In this study, a DyCr film was prepared on the surface of Nd-Fe-B magnets through magnetron sputtering, and then subjected to grain boundary diffusion (GBD) to improve magnetic properties and enhance corrosion resistance. The results showed that the residual Cr-rich film acted as a protective coating to enhance corrosion resistance, and Cr element in the grain boundary phase improved the electrochemical potential. The coercivity was significantly improved, attributed to the formed Dy-rich shell and enhanced distribution of grain boundary phase.
JOURNAL OF ALLOYS AND COMPOUNDS
(2023)
Article
Chemistry, Physical
Masaru Itakura, Koichi Sugiyama, Hiroshi Akamine, Minoru Nishida, Michihide Nakamura, Tetsuhiko Mizoguchi, Takahiko Iriyama
Summary: The microstructures of pressless processed Nd-Fe-B sintered magnets with and without Zr addition were investigated in this study. It was found that the addition of a small amount of Zr effectively suppressed abnormal grain growth and promoted a more homogeneous distribution of the liquid phase.
JOURNAL OF ALLOYS AND COMPOUNDS
(2021)
Article
Chemistry, Physical
Sangen Luo, Munan Yang, Zaiping Xu, Tongyun Zhao, Sajjad Ur Rehman, Xi Yu, Shuwei Zhong, Haitao Wang, Chaoxiang Jin, Qiang Ma, Bin Yang
Summary: This paper investigates the effect of grain size in the diffusion matrix on the grain boundary diffusion mechanism of Tb element. The results show that with only 0.75 wt. % TbF3 diffusion, the intrinsic coercivity increases significantly for the magnet with finer average grain size compared to the coarse-grain magnet. The increase in intrinsic coercivity is attributed to the increase in defect density and the higher grain orientation caused by grain refinement, which leads to improved diffusion efficiency and magnetic properties.
JOURNAL OF ALLOYS AND COMPOUNDS
(2023)
Article
Chemistry, Applied
Jinhao Zhu, Guangfei Ding, Bo Zheng, Haichen Wu, Lei Jin, Zhehuan Jin, Shuai Guo, Renjie Chen, Aru Yan
Summary: By adding Pr-Cu-Ti alloy powders, the grain boundary and local composition system of Nd-Fe-B magnets can be optimized, leading to an increase in coercivity. The addition of the alloy induces the generation of thin grain boundary phases, promoting intergrain exchange decoupling and increasing the coercivity. However, excess addition results in structural defects and further improvement in microstructure is needed.
JOURNAL OF RARE EARTHS
(2022)
Article
Chemistry, Physical
Mario Schoenfeldt, Urban Rohrmann, Philipp Schreyer, Mahmudul Hasan, Konrad Opelt, Juergen Gassmann, Anke Weidenkaff, Oliver Gutfleisch
Summary: The effects of multiple recycling of scrap magnets from magnetic resonance tomography devices were investigated. The changes in different material properties were studied, including chemical composition, impurity content, particle size, magnetic properties, microstructure, and degree of alignment. The recycling process led to a decrease in texture, orientation, and magnetic properties, as well as an increase in impurities and particle size. The addition of Nd hydride improved the properties, with 4 wt.% Nd hydride fully restoring the density of the recycled magnets. The recycled magnets met the specification of primary magnets and outperformed them in terms of sustainability and production cost reduction.
JOURNAL OF ALLOYS AND COMPOUNDS
(2023)
Editorial Material
Materials Science, Multidisciplinary
Satoshi Hirosawa, Kazuhiro Hono
SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS
(2022)
Article
Nanoscience & Nanotechnology
Ruma Mandal, Ivan Kurniawan, Ippei Suzuki, Zhenchao Wen, Yoshio Miura, Takahide Kubota, Koki Takanashi, Tadakatsu Ohkubo, Kazuhiro Hono, Yukiko K. Takahashi
Summary: In this study, a high-quality half-metallic epitaxial nano-scale thick NiMnSb alloy film grown on a Cr/Ag-buffered MgO (001) single crystalline substrate was reported. The highly ordered C1(b) structured nano-scale thick NiMnSb film deposited at 573 K exhibited ultralow Gilbert damping, high anisotropic field, and saturation magnetization. The analysis of theoretical and experimental data revealed a strong correlation between crystal ordering and magnetic parameters, as well as sample quality. The negative anisotropic magnetoresistance ratio demonstrated the robust bulk half-metallicity. This unique combination of highly ordered nano-scale half-metallic NiMnSb alloy with ultralow damping can be advantageous for spintronic device applications.
ACS APPLIED NANO MATERIALS
(2022)
Article
Physics, Applied
Tomohiro Ichinose, Tatsuya Yamamoto, Jun Uzuhashi, Takayuki Nozaki, Tadakatsu Ohkubo, Kay Yakushiji, Shingo Tamaru, Kazuhiro Hono, Shinji Yuasa
Summary: Thermally robust perpendicular magnetic anisotropy and voltage-controlled magnetic anisotropy effects were achieved in magnetic tunnel junctions with ultrathin CoFeB films by utilizing Ta/Mo layered adhesion structures.
JOURNAL OF APPLIED PHYSICS
(2022)
Article
Physics, Condensed Matter
Akira Uedono, Hideki Sakurai, Jun Uzuhashi, Tetsuo Narita, Kacper Sierakowski, Shoji Ishibashi, Shigefusa F. Chichibu, Michal Bockowski, Jun Suda, Tadakatsu Ohkubo, Nobuyuki Ikarashi, Kazuhiro Hono, Tetsu Kachi
Summary: In this study, the annealing behaviors of defects and the diffusion characteristics of Mg in ion-implanted GaN were investigated. It was found that high concentration of Mg doping caused Mg diffusion into the bulk and reaction with vacancies, while nitrogen implantation suppressed the Mg diffusion and enhanced the donor-acceptor pair emission, which is of great significance for improving the performance of GaN materials.
PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS
(2022)
Article
Nanoscience & Nanotechnology
Kazuho Okada, Akinobu Shibata, Taisuke Sasaki, Hisashi Matsumiya, Kazuhiro Hono, Nobuhiro Tsuji
Summary: This study aimed to improve the resistance against hydrogen embrittlement by increasing the concentration of carbon segregated at prior austenite grain boundary (PAGB), XPAGB, in low-carbon martensitic steels. The specimens with and without carbon segregation treatment (Non-seg and Seg specimens, respectively) had similar microstructures, except for higher XPAGB in the Seg specimen. The Seg specimen exhibited higher maximum stress and smaller fraction of intergranular fracture surface under hydrogen-charged conditions, indicating that segregated carbon suppressed hydrogen accumulation and increased cohesive energy of PAGB.
SCRIPTA MATERIALIA
(2023)
Article
Materials Science, Multidisciplinary
S. Kobayashi, Y. K. Takahashi, K. Ishigami, Y. Kotani, M. Suzuki, T. Yoshioka, H. Tsuchiura, H. Sepehri-Amin, T. Ohkubo, S. Hirosawa, T. Nakamura
Summary: This study focused on the effects of Zr substitution for Sm in (Sm1-xZrx)(Fe0.8Co0.2)(12) single-crystalline films on their structural and magnetic properties. The results showed that Zr selectively occupied the Sm site without disturbing the Fe sublattices. Electron transfer from Zr to the Fe band increased the magnetic moment and magnetic interactions in the Fe8f site, leading to magnetization enhancement in (Sm1-xZrx)(Fe0.8Co0.2)(12) single-crystalline films.
Article
Microscopy
Sujin Lee, Atsuko Sato, Takehiro Tamaoka, Kunio Yubuta, Mitsunari Auchi, Taisuke Sasaki, Tadakatsu Ohkubo, Kazuhiro Hono, Yasukazu Murakami
Summary: This paper proposes a method using electron holography to provide a phase image of the demagnetization field (H-d) within a thin-foil permanent magnet. The observation of H-d remains challenging because electron holography can only image the magnetic flux density (B), which is a combination of contributions from magnetization (M), stray magnetic field (H-s) outside the specimen, and H-d inside the specimen. The phase map approximating H-d, determined from electron holography observation of a Nd2Fe14B single-crystalline specimen, shows good agreement with micromagnetic theory's prediction. This method can be applied to study the coercivity mechanism of permanent magnets, which is sensitive to the demagnetization field.
Article
Materials Science, Multidisciplinary
Z. Wang, T. T. Sasaki, Y. Une, T. Ohkubo, K. Hono
Summary: Record-breaking high coercivity is achieved in Dy-free sintered magnets by separately diffusing Dy70Cu30 and Pr68Cu32 eutectic alloys. The low Dy content sample exhibits a high coercivity of 2.8 T and an excellent temperature coefficient of coercivity. The unique presence of a uniform Dy-rich shell and enriched Pr near the surface of Nd2Fe14B grains strengthens the anisotropy field, resulting in the high coercivity without alloying Dy to the initial magnet.
Article
Physics, Applied
Amran Mahfudh Yatmeidhy, Yoshihiro Gohda
Summary: We investigate the microscopic origin of strain-induced changes in the magnetocrystalline anisotropy energy of Co2FeSi, Co2MnSi, and Fe3Si Heusler alloys using first-principles electron theory. The anisotropy modulation in Co2FeSi and Co2MnSi is dominated by the quadrupole moment of Co minority-spin states under strain within the (001) plane, leading to giant magnetoelectric couplings in multiferroic heterointerfaces containing these compounds. On the other hand, the strain-induced anisotropy modulation in Fe3Si has mixed contributing factors including the anisotropy term of the orbital magnetic moment and the quadrupole term.
APPLIED PHYSICS EXPRESS
(2023)
Article
Physics, Applied
Soma Nishino, Yoshihiro Gohda
Summary: The stable crystal structures of SmFe12-based permanent magnets, which are expected to have crystalline grain-boundary subphases, are examined by first-principles thermodynamics. A free-energy landscape of ternary Sm-Cu-Fe is constructed, overcoming the dynamical instability problem, to identify stable crystal structures dependent on temperatures and compositions. Cu-rich B2 Sm-Cu is substantially stabilized by both phonons and configurational entropy, and Fe atoms in the Cu sublattice of B27 SmCu contribute to the phase stabilization. These findings expand the stable composition region of nonmagnetic SmCu-based intermetallics by revealing the existence of B2-B27 two-phase equilibria.
JAPANESE JOURNAL OF APPLIED PHYSICS
(2023)
Article
Chemistry, Physical
Z. H. Kautsar, H. Sepehri-Amin, Xin Tang, R. Iguchi, K. Uchida, T. Ohkubo, K. Hono
Summary: The resistivities of anisotropic Nd-Fe-B-based magnets were increased in parallel and perpendicular directions to the c-axis by hot-deforming Nd-Fe-B melt-spun flakes coated with DyF3 through electrophoretic deposition. As a result, the operating temperature under high-frequency AC magnetic field decreased by 20°C due to the reduction of eddy current loss. The formation of a resistive layer composed of NdF3 at the interface of original ribbons in the hot-deformed magnet was observed. The excess Dy diffused into the original ribbon flakes and formed a Dy-rich shell in Nd2Fe14B grains, resulting in a high coercivity.
JOURNAL OF ALLOYS AND COMPOUNDS
(2023)
Article
Physics, Applied
Shunsuke Tsuna, Yoshihiro Gohda
Summary: First-principles investigations reveal that the presence of B in neodymium-magnet compounds leads to acoustic phonon hardening. Substituting B with other light elements results in a decrease in phonon frequencies with increasing atomic number. However, imaginary phonons in Nd2Fe14F are attributed to electronic states and atomic configurations, rather than the heavier mass of F. Additionally, the study shows that magnetic disordering reduces the softening of phonons in paramagnetic Nd2Fe14B compared to ferromagnetic Nd2Fe14B, suggesting that phonon effects decrease the Curie temperature by stabilizing the paramagnetic state.
JOURNAL OF APPLIED PHYSICS
(2023)
Article
Instruments & Instrumentation
Hiroyuki Ohsumi, Yoshinori Fujikawa, Lihua Liu, Yoshinori Kotani, Tetsuya Nakamura
Summary: A method of post-acquisition upsampling is developed to improve the spatial resolution of scanning x-ray microscopy. This method is applicable when the probe beam size is significant compared to the pixels of the scan grid. By solving a stochastic inverse problem at a higher resolution, the unconvoluted spatial variation in a photoresponse is estimated and the spatial cutoff frequency is increased due to reduced noise. The proposed method is validated using raster micrographs of x-ray absorption and demonstrates improved spatial resolution.
REVIEW OF SCIENTIFIC INSTRUMENTS
(2023)
Article
Microscopy
Jun Uzuhashi, Tadakatsu Ohkubo, Kazuhiro Hono
Summary: Atom probe tomography (APT) is a widely used technique for analyzing the microstructure of alloys and devices. In this study, a method for automatically fabricating the desired tip shape using a script-controlled FIB-SEM system was developed. The effect of intentionally changing the tip shape on APT data was also discussed.
Article
Chemistry, Physical
Satoru Enomoto, Sonju Kou, Taichi Abe, Yoshihiro Gohda
Summary: Computational examination of the phase equilibria in the Sm-Fe-Cu ternary system reveals the possibility of liquid-phase sintering producing nonmagnetic grain-boundary subphases in SmFe12-based magnets. The B2 Sm-Cu-Fe is identified as a potential nonmagnetic subphase, with attractive Cu-Fe interaction observed within the B2 sublattice. The equilibrium between the liquid phase of Sm-Cu and SmFe12 is also verified. The findings provide insights into the electron theory and suggest a feasible approach for liquid-phase sintering of SmFe12-based magnets.
JOURNAL OF ALLOYS AND COMPOUNDS
(2023)
