Probing structure and microstructure of epitaxial Ni–Mn–Ga films by reciprocal space mapping and pole figure measurements Original Research Article

The crystal structure and complex twinning microstructure of epitaxial Ni–Mn–Ga films on (1 0 0) MgO substrates was studied by X-ray diffraction using 2θ scans, pole figure measurements and reciprocal space mapping (RSM). The orientation distribution of all variants is visualized by RSM, which forms the basis for a better understanding of the crystallographic relation between variants and substrate. Above the martensitic transformation temperature the film consists of single austenite phase with lattice constant a = 5.81 Å at 419 K. At room temperature some epitaxially grown residual austenite with a = 5.79 Å remains at the interface with the substrate, followed by an intermediate layer exhibiting orthorhombic distortion, atrans = 6.05 Å, btrans = 5.87 Å, ctrans = 5.73 Å and a major fraction of 14M (7M) martensite, a = 6.16 Å b = 5.79 Å c = 5.48 Å. The seven-layered modulation of this metastable martensite structure is directly observed by RSM. The intermediate phase observed close to interface indicates the existence of an instable, pre-adaptive martensite phase with a short stacking period.