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

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.