Understanding the Phase Transitions of the Ni2MnGa Magnetic Shape Memory System from First Principles

The free energies of the austenite, the (modulated) premartensite and the unmodulated martensite of Ni2MnGa are determined using density functional theory and including quasiharmonic phonons and fixed-spin-moment magnons. This approach very well reproduces the complete phase sequence (martensite <-> premartensite <-> austenite) of stoichiometric Ni2MnGa as a function of temperature. By analyzing the relevant free energy contributions, we also understand the delicate interplay of phonons and magnons driving both phase transitions.