Domain model for the magnetic shape-memory effect in non-modulated tetragonal Ni-Mn-Ga

Abstract

We employ a general phenomenological domain theory for ferromagnetic shape memory (FSM) materials to model equilibrium domain states in martensites with tetragonal twin variants having axes ratios c/a > 1 and easy-plane magnetic anisotropy. The model is evaluated for realistic material parameters of non-modulated (NM) Ni2MnGa alloys. Phase diagrams under competing external stresses and magnetic fields are presented that show regions of fully magnetized single-variant states and various multiphase regions. The theoretical elastic stresses that block single-variant states are found to be much smaller in NM than in the 5M structure of Ni2MnGa alloys.