In situ studies of the martensitic transformation in epitaxial Ni–Mn–Ga films

The martensitic transformation of epitaxial Ni–Mn–Ga films is investigated with respect to changes of structure, microstructure, magnetic and electronic properties. For this, temperature dependent atomic force microscopy (AFM), X-ray, magnetization and resistivity measurements are performed in situ, during martensitic transformation of a 500 nm thick film. The combination of these methods gives a comprehensive understanding of the martensitic transformation and allows to identify differences of constrained epitaxial films compared to bulk. Experiments show the formation of a twinned, orthorhombic martensite with high uniaxial magnetocrystalline anisotropy from the austenite around room temperature. High resolution AFM micrographs directly reveal how martensite variants grow and show the converging of variants nucleated at different nucleation sites. While most features are in agreement with a first-order transformation, the transformation proceeds continuously to lower temperatures, an effect which can be explained by the constraint from the substrate.