Recent advances in SmFe12-based permanent magnets

Abstract

After the rare-earth element crisis in 2011, research interests in the development of rare-earth lean magnets have increased. One of the potential rare-earth lean magnets is SmFe12-based compound with ThMn12 structure because of its high magnetization and Curie temperature. By substituting Fe with Co in SmFe12 compound, high intrinsic properties such as a large magnetization 0Ms = 1.68 T, high anisotropy K ~ 12 T, and high Curie temperature Tc ~ 859 K were obtained [1]. These intrinsic properties are superior to those of Nd2Fe14B. It showed the potential of Sm(Fe-Co)12-based compound for room and elevated temperature applications. Investigations on the intrinsic magnetic properties of SmFe12-based compounds in the thin film form showed that Sm(Fe0.8Co0.2)12 has the potential for the next generation of permanent magnets because of its high 0Ms, K, and Tc. However, unless these intrinsic magnetic properties are not transferred into the extrinsic ones, in particular, a large remanent magnetization and sufficiently large coercivity (µ0Hc > µ0Ms/2 > 0.9 T), SmFe12-based compounds cannot be realized as practical permanent magnets. Since 0Hc is an extrinsic magnetic property and is governed by realizing an optimum microstructure, microstructural control is the key for a high 0Hc. In the thin film, relatively high 0Hc of 1.2 T was achieved by the addition of B [2]. It formed a nanogranular microstructure in which Sm(Fe0.8Co0.2)12 grains are perfectly enveloped by B-rich amorphous grain boundary phase. The difference in the magnetization and anisotropy between Sm(Fe0.8Co0.2)12 grains and grain boundary phase causes the pinning site of the magnetic domain wall motion. In the hot-deformed Sm12Fe74V12Cu2 magnet [3], 0.96 T of 0Hc was obtained in the microstructure with Sm-rich grain boundaries. Based on these examples, relatively high 0Hc can be achieved in the bulk form by the suitable microstructure control. In this talk, we first review the fundamentals of the RFe12 phase. Next, we show the potential of the SmFe12–based phase as the permanent magnet in a model experiment of a thin film and also discuss recent progress for bulk magnets. References : [1] Y. Hirayama, et al., Scr. Mater.138, 62 (2017). [2] H. Sepehri-Amin, et al., Acta Mater.194 227 (2020). [3] A.M. Schönhöbel et al., Acta Mater.200, 652 (2020).