Presentation Information
[P2-57]Surface engineering to improve coercivity of Sm2Fe17N3 powder
*Wataru Yamaguchi1, Jian Wang1, Akihide Hosokawa1, Kenta Takagi1, Yusuke Hirayama1 (1. AIST (Japan))
Keywords:
Magnetic anisotropy field,Coercivity,Crystal field,Coating
Various factors contribute to the gap between the magnetic anisotropy field and the actual coercivity in rare-earth magnets. However, it is well-established that the coercivity of both sintered magnets and bonded magnet is strongly influenced by the magnetic properties of the main phase at the grain/particle surface. For Nd2Fe14B sintered magnets, it is a well-accepted fact that coercivity increases by locally enhancing the anisotropy field through the diffusion of elements like Dy and Tb into the grain boundaries.
Sm2Fe17N3 has a very strong uniaxial anisotropy, with the anisotropy field more than three times that of Nd2Fe14B [1], but its actual coercivity is not as high as expected. The Sm2Fe17 alloy, which is the base compound of Sm2Fe17N3, is a ferromagnet with in-plane anisotropy, and it only transitions to uniaxial anisotropy upon nitriding [2]. Additionally, if Sm is replaced with other rare-earth elements, then the nitrides do not exhibit uniaxial anisotropy. This suggests that the strong uniaxial anisotropy of Sm2Fe17N3 is established based on a delicate balance. The uniaxial anisotropy of rare-earth magnets is supported by the crystal field experienced by the 4f electrons of the rare-earth elements. The surface is where the crystal lattice is disrupted, and the crystal field loses the symmetry as the bulk interior, making it highly likely that the uniaxial anisotropy supported by Sm is also locally lost [3].
Based on these insights, a material phase was designed and coated on the surface of Sm2Fe17N3 crystal grains so that the crystal field experienced by Sm at the surface would be similar to that within the bulk interior. Surface oxide free Sm2Fe17 fine powder was prepared by pulverization in a low-oxygen atmosphere, then it was sputter-coated with designed materials with continuous stirring [4], and was subjected to nitriding with N2 gas. The essence of the crystal field is the Coulomb interaction, which is not directly related to magnetic order, so this coating phase was chosen to be a non-magnetic material with the same crystal structure as Sm2Fe17N3, forming a coherent interface with the Sm2Fe17N3 phase. As a result, it was confirmed that when the computed crystal field at the Sm sites in the coating phase has the same sign as that in Sm2Fe17N3, the coercivity increases significantly, while no increase in coercivity is observed when the signs are opposite. Additionally, it was found that the interface between the main phase and the coating phase forms a coherent diffusion buffer layer of about 10 nm in thickness.
[1] T. Iriyama, K. Kobayashi, N. Imaoka, T. Fukuda, IEEE Trans. Magn. 28, 2326 (1992).
[2] Y. Otani, D. P. F. Hurley, H. Sun, and J. M. D. Coey, J. Appl. Phys. 69, 5584 (1991).
[3] T. Yoshioka, and H. Tsuchiura, Appl. Phys. Lett. 112, 162405 (2018).
[4] W. Yamaguchi, R. Soda, and K. Takagi, J. Magn. Magn. Mater. 498, 166101 (2020).
Sm2Fe17N3 has a very strong uniaxial anisotropy, with the anisotropy field more than three times that of Nd2Fe14B [1], but its actual coercivity is not as high as expected. The Sm2Fe17 alloy, which is the base compound of Sm2Fe17N3, is a ferromagnet with in-plane anisotropy, and it only transitions to uniaxial anisotropy upon nitriding [2]. Additionally, if Sm is replaced with other rare-earth elements, then the nitrides do not exhibit uniaxial anisotropy. This suggests that the strong uniaxial anisotropy of Sm2Fe17N3 is established based on a delicate balance. The uniaxial anisotropy of rare-earth magnets is supported by the crystal field experienced by the 4f electrons of the rare-earth elements. The surface is where the crystal lattice is disrupted, and the crystal field loses the symmetry as the bulk interior, making it highly likely that the uniaxial anisotropy supported by Sm is also locally lost [3].
Based on these insights, a material phase was designed and coated on the surface of Sm2Fe17N3 crystal grains so that the crystal field experienced by Sm at the surface would be similar to that within the bulk interior. Surface oxide free Sm2Fe17 fine powder was prepared by pulverization in a low-oxygen atmosphere, then it was sputter-coated with designed materials with continuous stirring [4], and was subjected to nitriding with N2 gas. The essence of the crystal field is the Coulomb interaction, which is not directly related to magnetic order, so this coating phase was chosen to be a non-magnetic material with the same crystal structure as Sm2Fe17N3, forming a coherent interface with the Sm2Fe17N3 phase. As a result, it was confirmed that when the computed crystal field at the Sm sites in the coating phase has the same sign as that in Sm2Fe17N3, the coercivity increases significantly, while no increase in coercivity is observed when the signs are opposite. Additionally, it was found that the interface between the main phase and the coating phase forms a coherent diffusion buffer layer of about 10 nm in thickness.
[1] T. Iriyama, K. Kobayashi, N. Imaoka, T. Fukuda, IEEE Trans. Magn. 28, 2326 (1992).
[2] Y. Otani, D. P. F. Hurley, H. Sun, and J. M. D. Coey, J. Appl. Phys. 69, 5584 (1991).
[3] T. Yoshioka, and H. Tsuchiura, Appl. Phys. Lett. 112, 162405 (2018).
[4] W. Yamaguchi, R. Soda, and K. Takagi, J. Magn. Magn. Mater. 498, 166101 (2020).