Challenges toward development of rear-earth free FeCo based permanent magnet

Among the various permanent magnets reported to date, FeNdB exhibits the highest magnetic performance, but reducing the use of the rare-earth elements is necessary to avoid exhaustion of rare-earth elements. The key to improve the performance of permanent magnets is to increase magnetization and coercivity. FeCo has a very large magnetization, but has a very small coercivity caused by the extremely small magnetic anisotropy due to the cubic crystal structure, so that FeCo has long been unsuitable for permanent magnets. However, recent reports have revealed that the transformation of the crystal structure from cubic to tetragonal causes large magnetic anisotropy. In 2017, the author reported that a large coercivity can be obtained by fabricating tetragonal FeCo nanodots. This article reviews recent progress of rear-earth free permanent magnets especially for the tetragonal FeCo based alloys. (author)