Martensitic phase transformation of magnetron sputtered nanostructured Ni–Mn–In ferromagnetic shape memory alloy thin films

Abstract

In this paper, we report the influence of film thickness on the first order martensitic phase transformation of Ni–Mn–In ferromagnetic shape memory alloy (FSMA) thin films synthesized by magnetron sputtering. The structural, magnetic, electrical and mechanical properties of these films of various thicknesses (90–655nm) were systematically investigated. XRD analyses reveal that the film exhibit austenitic phase with L21 structure at room temperature. The grain size and crystallization extent increase with increase in film thickness. The temperature dependent magnetization and electrical measurements demonstrated the absence of phase transformation in the films with lower thickness ∼90nm which could be due to small grain size of these films. For thickness greater than 153nm, the films show first order martensitic phase transition with thermal hysteresis width, which increases with further increase in film thickness. The field dependent magnetization curves also show the increase in saturation magnetization (SM). Nanoindentation studies reveal the higher values of hardness of 7.2GPa and elastic modulus of 190GPa for the film thickness of 153nm. The value of refrigeration capacity (RC) which is an important figure of merit has been found to be 155.04mJ/cm3. Maximum exchange bias of 0.0096T and large magnetic entropy change ΔSM=15.2mJ/cm3K (field 2T) at martensitic transition has been obtained for film thickness of 655nm which makes them useful for microelectromechanical systems (MEMS) applications.