The evolution of martensitic transformation in Ni-Mn-Ga/Al₂O₃ polycrystalline 100-nm – 2-μm films with Ni- and Ga-excess

Polycrystalline Ni₅₄Mn₁₈Ga₂₈ films with thicknesses ranging from 100-nm to 2-μm were fabricated by deposition on alumina ceramic substrates; bulk and surface properties were then characterized by structural and magnetic methods. The evolution of martensitic transformation of the films was studied. Both martensitic transformation and Curie temperature were found to be above room temperature, with the former overlapping the latter when films thicker than 100-nm were used. Static magnetic properties revealed the presence of strong internal stresses in the films, thus influencing coercive force value which changes with the increase in thickness. A distinct presence of martensitic twinning at room temperature was revealed, which started with the 400-nm film and increased with the 1- and 2-μm films. A maze-like domain structure (without correlation to the surface features), was found in the 1- and 2-μm films. A strong dependence between crystallites and magnetic domains was discovered in films ranging from 100- to 400-nm. A nonlinear dependence of the domain width on the film thickness was also observed, which converges with the coercive force dependence.