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

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 (90-655 nm) were systematically investigated. XRD analyses reveal that the film exhibit austenitic phase with L2(1) 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 similar to 90 nm which could be due to small grain size of these films. For thickness greater than 153 nm, 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 (S-M). Nanoindentation studies reveal the higher values of hardness of 7.2 GPa and elastic modulus of 190 GPa for the film thickness of 153 nm. The value of refrigeration capacity (RC) which is an important figure of merit has been found to be 155.04 mJ/cm(3). Maximum exchange bias of 0.0096 T and large magnetic entropy change Delta S-M = 15.2 mJ/cm(3) K (field 2 T) at martensitic transition has been obtained for film thickness of 655 nm which makes them useful for microelectromechanical systems (MEMS) applications. (C) 2015 Elsevier B.V. All rights reserved.