Recent Progress in Research on Sm-Fe System Rare-earth Permanent Magnets

Neodymium magnets (Nd2Fe14B), which are used in the drive motors of electric vehicles, pose high geopolitical risks, so there is active development of alternative magnets in Japan. Sm-Fe compounds have superior intrinsic magnetic properties to Nd2Fe14B compound and are therefore considered to be the leading candidates for the alternative magnets. Sm-Fe compounds are classified into Th2Zn17-type, TbCu7-type, and ThMn12-type based on differences in crystal structure, and have characteristics such as anisotropic magnetic field, saturation magnetization, and Curie temperature that exceed those of Nd2Fe14B compound. However, these Sm-Fe compounds have issues that neodymium magnets do not have, such as difficulty in sintering due to thermal decomposition and the difficulty in synthesizing single crystal powder. Recently, various new metallurgical technologies have been developed to address these issues. For example, Th2Zn17-type Sm2Fe17N3 sintered magnets that do not degrade in magnetic properties have been reported using low-oxygen powder metallurgy and novel liquid phase sintering techniques. Moreover, it was reported that single crystal powder of metastable TbCu7-type Sm-Fe, for which only polycrystalline powder was conventionally available, can be produced by reduction diffusion technique and the thermal plasma technique. In addition, Sm-Fe alloys could not achieve uniaxial anisotropy unless they were nitrided, but nitrogen-free Sm-Fe magnets have currently been invented.