Magnetic shape memory Ni-Mn-Ga films: stress effect on microstructure and magnetisation process

Magnetic shape memory materials show remarkable multifunctional properties (e.g. "giant" magnetomechanical, magnetocaloric, barocaloric), which arise from the presence of a martensitic transformation and magnetically ordered states [1-3]. Thin films of these materials have recently attracted much interest for their great potential in applications, such as microactuators, valves and solid-state microrefrigerators [4]. We have recently demonstrated that a huge and reversible magnetically induced reorientation of twin variants (MIR) can be achieved in 200 nm Ni-Mn-Ga films. This is possible when the proper microstructure is obtained: growth conditions and a stress applied on the substrate enable the proper microstructure, where differently twinned martensitic regions are aligned anisotropically (Fig. 1); this configuration enables the occurrence of record values of anisotropic MIR effect [5]. The films were epitaxially grown on Cr/MgO at high temperature (i.e., in the austenitic phase) by r.f. sputtering. Their characterisation was carried out by different techniques, thus realizing a multi-scale structural and magnetic study. We here focus on the stress effect on the microstructure and magnetisation process. The MIR effect can be in fact controlled by applying a stress to the film constrained to the substrate, and the stress effect on microstructural and magnetic pattern investigated by AFM/MFM (Fig. 1). The occurrence of the MIR effect in the stressed sample has been monitored by magnetometry, measuring magnetisation curves with field applied along different directions of the substrate crystal (Fig. 1). Reversible and irreversible contributions to the magnetization process during first magnetisation and magnetisation reversal have been also investigated. [1]M. Acet et al., Chapter 4, 231-289, Handbook of Magnetic Materials 19 (ed Buschow KHJ), Amsterdam (The Netherlands), Elsevier [2] J. Liu et al., Nature Mat. 11, 620-626 (2011) [3] L. Manosa et al., Nature Mat. 9, 478-481 (2010) [4] A. Backen et al., Adv. Eng. Mater. 14, 696-709 (2012) [5] P. Ranzieri et al., Adv. Mater. 27 (2015) 4760