Phase stability and magnetic properties of hexagonal and cubic Fe2MnGe

•Origin of the large saturate moment at 5 K in Fe2MnGe DO19 phase was determined.•Phase stability of cubic and hexagonal Fe2MnGe was investigated.•“Why Fe2MnGe DO19 phase is stable at room temperature” was discussed.•Information on electronic structure of Fe2MnGe DO19 phase was presented. The electronic structure, phase stability and magnetic properties of cubic and hexagonal type Fe2MnGe were investigated and compared with the well-known Heusler alloy Fe2MnSi. Three different structures L21, DO3 and DO19 were considered for them. In both alloys the cubic phase (L21 and DO3) has a lower total energy comparing with the hexagonal DO19 phase at 0 K. But in Fe2MnGe the energy difference between the DO19 and L21 phases is much smaller than Fe2MnSi. The DO3 disorder can lower the stability of the cubic phase further and make the total energies of hexagonal and cubic phases closer, this can help the DO19 phase gain stability. Finally, in Fe2MnGe, the energy difference is 0.12 eV/f.u., much smaller than the 0.41 eV/f.u. in Fe2MnSi. That is the possible reason why Fe2MnGe DO19 phase can be synthesized experimentally and Fe2MnSi cannot. L21 type Fe2MnGe is a half-metal with 100% spin polarization and has a total spin moment of 3μB/f.u. The spin polarization ratio of DO3 phase is also high. But for the DO19 hexagonal phase, a normal ferromagnetic metal character is observed. The calculated total moment for DO19 type Fe2MnGe is larger than 6μB/f.u. This total moment comes mainly from the large and parallel coupled partial spin moments of Fe and Mn.