Martensitic transformation and microstructure of sputter-deposited Ni-Mn-Ga films

The martensitic transformation and microstructure of Ni-Mn-Ga films deposited on an alumina substrate and annealed at 1073 K for 36 ks are studied. Electrical resistivity and calorimetry measurements reveal a non-monotonous thickness dependence of the martensitic start temperature, T_ms, at submicron film thickness. Focused Ion Beam (FIB) and standard SEM techniques are used to clarify the film microstructure. A martensitic morphology of films is confirmed by the FIB imaging to be a laminated twin structure aligned almost parallel to the film plane in each crystallite as a consequence of {110}-type crystallographic texture. A thermodynamic model based on the Landau formalism taking into account the substructure of the film and the elastic interaction between film and substrate describes the essential features of the thickness dependence of T_ms.