First-principles investigation of phonon softenings and lattice instabilities in the shape-memory system Ni2MnGa -: art. no. 132402

Ferromagnetic Ni2MnGa has unique magnetoelastic properties. These are investigated by detailed computational studies of the phonon-dispersion curves for the non-modulated cubic L2(1) and tetragonal structures. For the L2(1) structure, a complete softening of the transverse-acoustic mode has been found around the wave vector q=[1/3,1/3,0](2pi/a). The softening of this TA(2) phonon mode leads to the premartensitic modulated superstructure observed experimentally. Further phonon anomalies, related to other structural transformations in Ni2MnGa, have also been found and examined. These anomalies appear to be due to the coupling of particular acoustic-phonon modes and optical modes derived from Ni.