2004/06/05 by A. T. Zayak, Zayak, A. T., P. Entel +1
Physics and Astronomy · #FOS: Physical sciences #Materials Science (cond-mat.mtrl-sci) #cond-mat.mtrl-sci
paper · pdf · doi:10.48550/arxiv.cond-mat/0406139
4 pages, 4 figures
arxiv created 2004/06/05 · arxiv updated 2009/12/01
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 Ga-p minority-spin orbitals.