Ni-Mn-Ga shape memory thin films: controlling the twin microstructure and modeling the magnetisation process

We here examine the correlation between martensitic microstructure and magnetization process in Ni-Mn-Ga films epitaxially grown on MgO(100) or Cr/MgO(100) by r.f. sputtering. We have obtained a variety of martensitic patterns by modifying the growth conditions, applying a stress during or after growth and annealing the films. The patterns are different in orientation and spatial organization of the martensitic twin variants and give rise to distinctive magnetization processes, with magnetization jumps of variable intensity along different crystallographic directions. The occurrence of magnetization jumps is typically attributed to the magnetically induced reorientation of twin variants, similarly to what occurs in bulk materials. We have instead simulated magnetization processes of purely magnetic origin in films with different martensitic patterns by the OOMMF code. The micromagnetic simulations demonstrate that magnetization jumps of purely magnetic origin and with variable intensity can occur in the first quadrant of the (M,H) diagram, showing a good agreement with the experimental results.