Role of Shuffles and Atomic Disorder in Ni-mn-ga

We report results of ab-initio calculations of the ferromagnetic Heusler alloy Ni-Mn-Ga. 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 stabilise 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 [110]-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 systems, 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 Gap minority-spin orbitals. Ni-Mn-Ga alloys (close to stoichiometric Ni 2 MnGa) are known to exhibit unique magneto-elastic properties. They are ferromagnetic at room temperature (T C ∼ 380 K), and undergo (including the precursor) a two-step martensitic transformation for T M < T C (T M ∼ 200 K) [1]. In the martensitic state of Ni-Mn-Ga the structure consists of differently oriented martensitic domains (twin variants), which are also magnetic domains. This makes the martensitic structure of Ni-Mn-Ga sensible to an external magnetic field, which can induce a redistribution of the martensitic domains in the sample. Those domains with easy magnetic axes are along the field will gain in energy on cost of the domains with less favourable orientation of the magnetisation. Alignment of twin variants by the motion of twin boundaries can result in large macroscopic strain up to 6 % [2, 3]. This effect is used in the magnetic-shape-memory (MSM) technology [4]. The MSM technology is based on the magnetic field induced redistribution of marten-sitic domains in the sample. From a technological point of view, Ni-Mn-Ga is more promising than other materials being presently in commercial use, for example, the well-known material Tb-Dy-Fe (Terfenol-D) which exhibits magnetostrictive strains of about 0.1 …