Phase transformation in Ni–Mn–Sn ferromagnetic shape memory alloy thin films induced by dense ionization

Abstract

The effects of 200 MeV Au ions irradiation on the structural and magnetic properties of Ni–Mn–Sn ferromagnetic shape memory alloy (FSMA) thin films have been systematically investigated. In order to understand the role of initial microstructure and phase of the film with respect to high energy irradiation, the two types of Ni–Mn–Sn FSMA films having different phases at room temperature were irradiated, one in martensite phase (Ni58.9Mn28.0Sn13.1) and other in austenite phase (Ni50Mn35.6Sn14.4). Transmission electron microscope (TEM) and scanning electron microscope (SEM) images along with the diffraction patterns of X-rays and electrons confirm that martensite phase transforms to austenite phase at a fluence of 6×1012 ions/cm2 and a complete amorphization occurs at a fluence of 3×1013 ions/cm2, whereas ion irradiation has a minimal effect on the austenitic structure (Ni50Mn35.6Sn14.4). Thermo-magnetic measurements also support the above mentioned behaviour of Ni–Mn–Sn FSMA films with increasing fluence of 200 MeV Au ions. The results are explained on the basis of thermal spike model considering the core and halo regions of ion tracks in FSMA materials.