Martensitic transformation in polycrystalline substrate-constrained and freestanding Ni-Mn-Ga films with Ni and Ga excess

Highlights: • Martensitic transformation in Ni-Mn-Ga/Al2O3 and freestanding films. • Isotropic behaviour of in plane magnetic properties of the films. • Two phase behaviour of magnetic properties in the freestanding film. • Difference in magnetization values of films above the room temperature. • Maze-like surface magnetic domain structure with domain width of about 0.9 μm. Martensitic transformation (MT) and magnetic properties were investigated in a polycrystalline Ni-Mn-Ga freestanding and a sputter deposited on Al₂O₃ substrate films with excess Ni and Ga. Both films had a 2-μm thickness and MT above the room temperature. The MT region was wider in the freestanding film. The films demonstrated the isotropic in-plane behaviour of magnetic properties. A step-like behaviour was observed in the magnetization reversal process in the freestanding film because there were second-phase impurities. In the substrate-constrained film, only broadening in magnetization loops because of internal stresses was observed. The easy magnetization axis oriented normally to the film plane of the freestanding film but had a preferred orientation in the film plane of the substrate-constrained film. Both films exhibited a maze-like surface magnetic domain structure with the domain width of approximately 0.9 μm with different stripe structures, which confirms the presence of out-of-plane anisotropy. Based on these results, the effects of the substrate on the MT, structural and magnetic properties and the type of internal stresses were discussed.