Effect of temperature and compositional changes on the phonon properties of Ni-Mn-Ga shape memory alloys

Abstract

We report on the vibrational properties of the ferromagnetic shape memory alloy system Ni-Mn-Ga in its stoichiometric Ni${}_{2}$MnGa and off-stoichiometric Ni${}_{49}$Mn${}_{32}$Ga${}_{19}$ compositions. Elastic and inelastic neutron scattering measurements at different temperatures are presented with a focus on the austenite phase and compared to first-principles calculations. The overall behavior of the full phonon dispersion is similar for both compositions with remarkable exceptions for the TA${}_{2}$[$\ensuremath{\xi}\ensuremath{\xi}0$] acoustic branch and optical phonon branches. Less dispersion is found in the optical phonons for Ni${}_{49}$Mn${}_{32}$Ga${}_{19}$ in the whole reciprocal space when compared to Ni${}_{2}$MnGa and is explained by the occupation of regular Ga sites by excess Mn atoms. A pronounced softening in the TA${}_{2}$[$\ensuremath{\xi}\ensuremath{\xi}0$] phonon branch within the austenite phase is observed in both samples when approaching the martensitic transition. Its location in reciprocal space reveals the martensitic transition mechanism. The austenite $L$2${}_{1}$ structure transforms to the tetragonal modulated martensite structure by shuffling (110) planes in the [1$\overline{1}$0] direction, similarly to what has been observed at the martensitic transitions of the ${d}^{1}$ and ${d}^{2}$ transition metals. Whereas the temperature dependence of the softening of the TA${}_{2}$[$\ensuremath{\xi}\ensuremath{\xi}0$] phonons in the stoichiometric sample coincides perfectly with the magnetic and structural transitions, this is not the case for the off-stoichiometric sample. Here the relation between the magnetic ordering and the vibrational properties is still an open question.