Role of shuffles and atomic disorder in Ni–Mn–Ga

Abstract

We report results of ab initio calculations of the ferromagnetic Heusler alloy Ni 2 MnGa. Particular emphasis is placed on the stability of the low temperature tetragonal structure with c/a = 0.94. This structure cannot be derived from the parent L2 1 structure by a simple homogeneous strain associated with the soft elastic constant C'. In order to stabilize the tetragonal phase, one has to take into account shuffles of atoms, which form a wave-like pattern of atomic displacements with a well defined period (modulation). While the modulation is related to the soft acoustic [ 1 1 0]-TA 2 phonon mode observed in Ni 2 MnGa, we obtain additional atomic shuffles, which are related to acoustic-optical coupling of the phonons in Ni 2 MnGa. In addition, we have simulated an off-stoichiometric system, in which 25% of Mn atoms are replaced by Ni. The energy of this structure also exhibits a local minimum at cla = 0.94. This allows us to conclude that both shuffles and atomic disorder stabilize the c/a = 0.94 structure. In both cases the stability seems to be associated with a dip in the minority-spin density of states (DOS) at the Fermi level, being related to the formation of hybrid states of Ni d and Ga p minority-spin orbitals.