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

We report results of ab initio calculations of the ferromagnetic Heusler alloy Ni2MnGa. 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 L21 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]-TA2 phonon mode observed in Ni2MnGa, we obtain additional atomic shuffles, which are related to acoustic-optical coupling of the phonons in Ni2MnGa. 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 c/a=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.