Theoretical design of tetragonal rare-earth-free alloys with high magnetisation and high magnetic anisotropy

Abstract

Tetragonal alloys, such as D022-Mn3Ga, are potential candidates for rare-earth free permanent magnets due to their high Curie temperature and uniaxial magnetic anisotropy. For high-performance permanent magnets, high saturation magnetisation is necessary. However, the saturation magnetisation of D022-Mn3Ga is small due to ferrimagnetic ordering. We investigated the possibility of developing ferromagnetic Heusler alloys with high magnetic anisotropy and saturation magnetisation using the first-principles calculation. We focused on the effects of Fe substitution for Mn in D022-Mn3Ga as well as the consequent volume expansion; the ferromagnetic tetragonal XA phase is stabilized in Fe2MnGa by an 8% volume expansion. This tetragonal XA-Fe2MnGa has desirable properties for a high-performance permanent magnet, such as high magnetisation (1350 emu cc−1), perpendicular magnetic anisotropy (2.12 MJ m−3), and Curie temperature (1047 K). In addition, the substitution of Sn and increasing the Ga composition in the Fe2MnGa alloy results in volume expansion, which stabilizes the ferromagnetic tetragonal XA phase.