Magnetization process and martensitic microstructure of Ni-Mn-Ga epitaxial films

Magnetic shape memory materials show outstanding and multifunctional properties (e.g. magnetomechanical, magnetocaloric, magnetoresistive), originating from the occurrence of both a martensitic transformation and magnetically ordered states [1]. Thin films of these materials have a great potential for different applications, such as microactuators, valves and solid-state microrefrigerators [2]. We have demonstrated that a huge and reversible magnetization jump can be achieved in 200 nm epitaxial Ni-Mn-Ga films [3, Figure 1]. This is possible when the proper microstructure is obtained: growth conditions and a stress applied to the substrate enable the proper microstructure, where differently twinned martensitic regions are aligned anisotropically. In addition, we have found that lateral confinement in patterned thin films is able to influence the twin variants configuration [4]. We here examine the correlation between martensitic microstructure and magnetization process in Ni-Mn-Ga films epitaxially grown on MgO(100) or Cr/MgO(100) by r.f. sputtering. We have obtained a variety of martensitic patterns by modifying the growth conditions, applying a stress during or after growth and annealing the films. The patterns are different in orientation and spatial organization of the martensitic twin variants and give rise to distinctive magnetization processes, with magnetization jumps of variable intensity along different crystallographic directions. The occurrence of magnetization jumps is typically attributed to the magnetically induced reorientation of twin variants, similarly to what occurs in bulk materials [1 and references therein]. We have instead simulated magnetization processes of purely magnetic origin in films with different martensitic patterns by the OOMMF code [5]. The micromagnetic simulations demonstrate that magnetization jumps of purely magnetic origin and with variable intensity can occur in the first quadrant of the (M,H) diagram, showing a good agreement with the experimental results (Figure 1). Figure 1. Experimental (left) and simulated (right) hysteresis curves, obtained applying a magnetic field along different directions of the MgO substrate [1] M. Acet, et al., Handbook of Magnetic Materials vol. 19, Elsevier, Amsterdam, 2001. [2] A. Backen et al., Adv. Eng. Mat. 14, 696 (2012). [3] P. Ranzieri et al., Adv. Mater. 27, 4760 (2015). [4] M. Campanini et al., under review. [5] M. J. Donahue and D. G. Porter, OOMMF User's Guide, Version 1.0: Interagency Report (National Institute of Standards and Technology, Gaithersburg, MD, 1999).