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

We report on the vibrational properties of the ferromagnetic shape memory alloy system Ni-Mn-Ga in its stoichiometric Ni2MnGa and off-stoichiometric Ni49Mn32Ga19 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)[xi xi 0] acoustic branch and optical phonon branches. Less dispersion is found in the optical phonons for Ni49Mn32Ga19 in the whole reciprocal space when compared to Ni2MnGa and is explained by the occupation of regular Ga sites by excess Mn atoms. A pronounced softening in the TA(2)[xi 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 L2(1) structure transforms to the tetragonal modulated martensite structure by shuffling (110) planes in the [1 $(1) over bar $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)[xi 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.