Effect of low annealing temperature on the structure and chemical order of Ni2MnGa thin films

Low temperature annealing effect on the structure, residual stress and local chemical order of Ni2MnGa alloys thin films were investigated by atomic force microscopy, x-ray diffraction and x-ray stress analyses as well as extended x-ray absorption fine structure. Thin films were prepared by sputtering on GaAs substrates maintained at room temperature and 300 °C during deposition. The samples prepared at 300 °C were subsequently annealed at 150 °C and 300 °C for one and half hour. X-ray diffraction measurements indicated both a development of crystalline texture in which austenite (110) crystalline planes are preferentially aligned parallel to the film plane as well as compressive residual stress, mainly induced by the accommodation of the film on the substrate surface and by heating-cooling in the growth process. Atomic force microscopy analyses reveal a surface morphology dominated by a topography of hillocks and peaked massifs, whose roughness increases by about twenty times with the increase in the crystalline texture induced by annealing at 300 °C of the samples. Interestingly, extended x-ray absorption fine structure analysis performed at Mn K-edge indicate that residual stress is preserved even after significant increase of the structural and chemical ordering induced by the low temperature annealing.