Vibrational properties of Ni-Mn-Ga shape memory alloy in the martensite phases

Studying the phonon dispersion of the ferromagnetic shape memory alloy system Ni-Mn-Ga gives insight into the mechanism of the martensite transition and the forces driving the transition. Transformation of austenite single crystals under uniaxial stress results in the coexistence of two martensitic variants with perpendicular modulation vector. Here we report on inelastic neutron scattering studies of martensite crystals with off-stoichiometric compositions, varying from non-modulated (NM) to five- (5M) and seven- (7M) layer modulated martensite phases. Both the 5M and 7M crystals show fully commensurate satellite peaks along [ζ ζ 0], corresponding to the five- and seven-layer modulation. These superstructure peaks become 0-points of the modulated structure. Due to the coexistence of two variants within the (001) plane, both new acoustic phonons reflecting the modulation vector [ζ ζ 0] and acoustic TA2[ζ ζ 0] phonons corresponding to the non-modulated direction are observed. The latter display a pronounced softening around ζ = 0.2-0.4 when approaching the martensite-austenite transition from above and below, i.e. this soft mode has lowest frequency at the transition temperature. Overall the phonon dispersion of the austenite and martensite phase resemble each other very much.