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

Abstract

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.