Proton-irradiation-induced structural and magnetic property changes in Ni–Mn–Ga high-temperature shape memory films

Ni_51.90Mn_28.86Ga_19.24 alloy films were irradiated by 120 keV protons at different fluences to investigate the effect of irradiation on their crystal structure, phase-transition temperature, and magnetic properties. The results show not only that a nanoscale amorphous layer may have been induced on the surface, but also that non-modulated (NM) martensite was produced. Moreover, differential scanning calorimetry peaks of the martensitic and austenitic transition temperatures are shifted to higher temperatures after irradiation; in particular, two endothermic and two exothermic peaks were observed for a proton fluence of 2.0 × 10¹⁶/cm². Further, the formation of NM martensite could produce an extra shoulder on the M–T curves of the irradiated films. Regarding the magnetic properties, a gradual decrease in the Curie temperature T_C can be caused by irradiation at a higher proton fluence. The changes in the saturation magnetization M_s with the proton fluence seem to be related to the reduction in T_C.