Research interest in rare-earth lean SmFe12-based compounds has been revived due to their excellent intrinsic hard magnetic properties making them potential for new permanent magnet materials that are not dependent on scarce elements such as Dy 1,2. In order to realize these materials for practical applications, the main challenge is development of anisotropic SmFe12-based sintered magnet with a large coercivity and high remanent magnetization.
We carried out machine-learning on a dataset extracted from literature on Sm(Fe,X)12-based alloys to understand the most influential parameters for coercivity. It was found addition of V to the alloy composition is the most influential to coercivity 3. The prediction from machine learning was experimentally validated by developing SmFe11Ti and SmFe10TiV melt-spun ribbons where coercivity increased from 0.5 T to 1.1 T upon addition of V. Detailed microstructure characterizations revealed enhanced coercivity is originated from formation of Sm-rich intergranular phase enveloping SmFe12-based grains. The formation of Sm-rich intergranular phase acts as pinning cites against magnetic domain wall propagation resulting in an enhanced coercivity in the V-containing magnet. Motivated by this study, we have developed anisotropic Sm(Fe,Ti,V)12-based sintered magnet via conventional powder processing including the development of jet-milled powders, preparation of green compact, and liquid sintering process 4,5. The sintered magnets showed coercivity of μ0Hc=0.6-1.0 T and remanent magnetization of μ0Mr = 0.6-0.8 T depending on Ti content (Fig. 1). Detailed multi-scale microstructure characterizations showed the origin of realizing a coercivity of above 0.6 T is the formation of Sm-rich intergranular phase 4,5. However, {101} twins were also observed in the microstructure of the magnets as shown in Fig. 2 (a). The origin of the twins is induced stress in the powders during the jet-milling process. MOKE observations showed that the magnetization reversal can start from twinned grains which can be detrimental for coercivity (Fig. 2).
However, micromagnetic simulations showed that the existence of intergranular phase isolating individual grains can overcome the detrimental effect of twins to coercivity by preventing the propagation of reversed domains to the neighboring grains and hence resulting in a large coercivity. Based on our detailed microstructure investigations and micromagnetic simulations, we will discuss how an optimum microstructure can be realized in the anisotropic SmFe12-based magnets which results in a large μ0Hc and μ0Mr.
References:
1 K. Ohashi, Y. Tawara, R. Osugi, M. Shimao, J. Appl. Phys. 64 (1988) 5714-5716.
2 P. Tozman, H. Sepehri-Amin, Y.K. Takahashi, S. Hirosawa, K. Hono, Acta Mater. 153 (2018) 354.
3 Xin Tang, J Li, AK Srinithi, H Sepehri-Amin, T Ohkubo, K Hono, Scripta Mater. 200 (2021) 113925.
4 J.S. Zhang, Xin Tang, H. Sepehri-Amin, A.K. Srinithi, T. Ohkubo, K. Hono, Acta Mater. 217 (2021) 117161.
5 J.S. Zhang, Xin Tang, A. Bolyachkin, A.K. Srinithi, T. Ohkubo, H. Sepehri-Amin, K. Hono, Submitted.