Presentation Information
[P1-21]Coercivity enhancement of HDDR anisotropic Nd-Fe-B magnetic powder
*Jingzhi Han1, Hui-Dong Qian1, Peng Shen1, Tao Zhu1, Qing Xu1, Qiang Gao1, Shunquan Liu1, Weixing Xia3, Shuai Guo3, Wenyun Yang1, Jinbo Yang1,2, Yingchang Yang1 (1. Institute of Condensed Matter and Material Physics, School of Physics, Peking University (China), 2. State Key Laboratory for Mesoscopic Physics and School of Physics, Peking University (China), 3. Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences (China))
Keywords:
HDDR
Although the grain size of HDDR treated Nd-Fe-B alloy is comparable to that of Nd2Fe14B monodomain particles, its coercivity is only about 13 kOe, much smaller than the anisotropic field of Nd2Fe14B. The coercivity of rare earth permanent magnet materials is sensitive to microstructure, which is not only related to the size of the grains, but also to the structure and magnetism of the grain boundaries. Therefore, by refining the grain and regulating the structure and magnetism of grain boundaries, the coercivity of HDDR magnetic powder can be improved. However, to date, it has not been possible to prepare HDDR anisotropic Nd-Fe-B magnetic powder with a coercivity exceeding 20 kOe. This study used a combination of refining grain size and modifying the structure and magnetic properties of grain boundary layer to increase the coercivity of HDDR magnetic powder to 20 kOe for the first time.
We first refined the grain size of the HDDR treated Nd-Fe-B alloy to 250 nm and achieved a coercivity of 18 kOe. Then, through further diffusion of Pr68Cu32 alloy powder, the coercivity of the material was increased from 18 kOe to 20 kOe. Microstructural analysis revealed that as the diffusion amount of Pr-Cu alloy increases, the Ms and Mr of HDDR magnetic powder monotonically decrease, and the coercivity monotonically increases. At an addition amount of 15 wt%, the coercivity reaches its maximum value (20 kOe). Subsequent microstructural observations indicated that the grain boundary thickness significantly widens after diffusion of Pr-Cu alloy, and the content of ferromagnetic elements in the grain boundary phase decreases significantly.
In order to analyze the reasons for the increase in coercivity, we investigated the magnetic domain of the sample after diffusion and its domain evolution under different external magnetic field using a Lorentz magnetic electron microscope. It was found that when the sample is in a thermally demagnetized state, there exists obvious domain walls inside some grains, indicating the presence of a certain number of multi domain grains in the HDDR treated grains. As the external magnetic field increases, the domain walls move towards the grain boundaries; When the external magnetic field reaches a certain value, these domain walls move to the grain boundaries and are firmly nailed; With the further increase of the magnetic field, it completely disappears, indicating that the sample has reached complete saturation. At the same time, it is also found that as the reverse magnetic field increases, domain walls gradually appear within the grain. With the further increase of the reverse magnetic field, the domain walls begin to move from the grain boundary and enter the grain boundary before gradually disappearing. The above results suggested that the coercive force mechanism of HDDR magnetic powder after diffusion treatment is different from both the nucleation mechanism of Nd-Fe-B sintered magnets and the pinning mechanism of melt-spun Nd-Fe-B magnetic powder. Its specific coercive force mechanism should be closely related to grain size and grain boundary layer’s structure and magnetic properties.
Reference:
[1] H. Sepehri-Amin, T. Ohkubo, T. Nishiuchi, S. Hirosawa, and K. Hono, Scripta Mater. 2010, 63: 1124-1127.
[2] Y.F. Zhang, J.Z. Han, F.M. Wan, S.Q. Liu, H.D. Tian, X.D. Zhang, C.S. Wang, J.B. Yang, and Y.C. Yang, Scripta Mater. 2016, 110: 57-60.
[3] Z. Lin, J.Z. Han, M.Y. Xing, S.Q. Liu, R. Wu, C.S. Wang, Y. Zhang, Y.C. Yang, and J.B. Yang, Appl. Phys. Lett. 2012, 100: 052409
We first refined the grain size of the HDDR treated Nd-Fe-B alloy to 250 nm and achieved a coercivity of 18 kOe. Then, through further diffusion of Pr68Cu32 alloy powder, the coercivity of the material was increased from 18 kOe to 20 kOe. Microstructural analysis revealed that as the diffusion amount of Pr-Cu alloy increases, the Ms and Mr of HDDR magnetic powder monotonically decrease, and the coercivity monotonically increases. At an addition amount of 15 wt%, the coercivity reaches its maximum value (20 kOe). Subsequent microstructural observations indicated that the grain boundary thickness significantly widens after diffusion of Pr-Cu alloy, and the content of ferromagnetic elements in the grain boundary phase decreases significantly.
In order to analyze the reasons for the increase in coercivity, we investigated the magnetic domain of the sample after diffusion and its domain evolution under different external magnetic field using a Lorentz magnetic electron microscope. It was found that when the sample is in a thermally demagnetized state, there exists obvious domain walls inside some grains, indicating the presence of a certain number of multi domain grains in the HDDR treated grains. As the external magnetic field increases, the domain walls move towards the grain boundaries; When the external magnetic field reaches a certain value, these domain walls move to the grain boundaries and are firmly nailed; With the further increase of the magnetic field, it completely disappears, indicating that the sample has reached complete saturation. At the same time, it is also found that as the reverse magnetic field increases, domain walls gradually appear within the grain. With the further increase of the reverse magnetic field, the domain walls begin to move from the grain boundary and enter the grain boundary before gradually disappearing. The above results suggested that the coercive force mechanism of HDDR magnetic powder after diffusion treatment is different from both the nucleation mechanism of Nd-Fe-B sintered magnets and the pinning mechanism of melt-spun Nd-Fe-B magnetic powder. Its specific coercive force mechanism should be closely related to grain size and grain boundary layer’s structure and magnetic properties.
Reference:
[1] H. Sepehri-Amin, T. Ohkubo, T. Nishiuchi, S. Hirosawa, and K. Hono, Scripta Mater. 2010, 63: 1124-1127.
[2] Y.F. Zhang, J.Z. Han, F.M. Wan, S.Q. Liu, H.D. Tian, X.D. Zhang, C.S. Wang, J.B. Yang, and Y.C. Yang, Scripta Mater. 2016, 110: 57-60.
[3] Z. Lin, J.Z. Han, M.Y. Xing, S.Q. Liu, R. Wu, C.S. Wang, Y. Zhang, Y.C. Yang, and J.B. Yang, Appl. Phys. Lett. 2012, 100: 052409