Model for temperature dependence of field-induced strain in ferromagnetic shape memory alloys

Magnetic field-induced twin-boundary motion in Ni–Mn–Ga and other magnetic shape memory alloys is typically observed over a limited range of temperature, 10–120 K below the martensite transformation. Twin-boundary motion requires shuffling of atoms between two stable minima in {1 0 1} planes, a process that occurs only above a threshold yield stress. A model is proposed to account for this phenomenon. It is based on the magnetic torque and thermal activation acting on the atoms about a twin dislocation that separates the two stable twin variants. Field-induced twin-boundary motion is predicted to occur when the thermally activated, temperature-dependent torque exceeds the twin-boundary yield stress (itself, temperature dependent). Activation energies of order (2.4 to 5) × 10−21 J/atom (0.15–0.3 eV) give a good agreement with observations.