Intrinsic instability to martensite phases in ferromagnetic shape memory alloy Ni2MnGa : Quasiparticle self-consistent GW investigation

Abstract

We investigated the electronic structure of a ferromagnetic shape memory alloy ${\mathrm{Ni}}_{2}\mathrm{MnGa}$ utilizing an advanced approach, quasiparticle self-consistent $GW$, which takes account of electron localization effects without empirical parameters. The Ni ${e}_{g}$ orbitals in the cubic phase, which lead to martensite phase transition, were found to locate on the Fermi level, implying a clear definitive origin of band Jahn-Teller (JT) effect in comparison with the results obtained by the density functional approach of generalized gradient approximation. From the analysis of generalized susceptibility in the cubic phase, the instabilities responsible for the modulated structures of 10M, 14M, and 6M were found to be an intrinsic property in the electronic states. These states may stabilize the modulated one, accompanied by tetragonal local JT distortions. Their property of Fermi surface nesting sensitively depends on a subtle change in the magnetic moment, corresponding to the experimental fact that the modulated structure appears depending on temperature and the composition of the magnetic element. The secondary nesting vector along [110] direction was discussed in relation to a modulation alignment of the nanotwin boundary.