Ni-Mn-Ga films and nanodisks: controlling magnetism and functional properties through martensitic microstructure

Magnetic shape memory materials show outstanding and multifunctional properties (e.g. "giant" magnetomechanical, magnetocaloric, barocaloric), originating from the occurrence of both a martensitic transformation and magnetic order. Thin films and nanostructures of these materials have a great potential for different applications, such as micro- or nano-actuators, energy harvesters, valves and solid-state microrefrigerators [1]. We have demonstrated that the microstructure and magnetic properties of Ni-Mn-Ga thin films can be engineered by properly choosing substrate, growth conditions [2] and post-growth treatments. The films have been epitaxially grown on MgO(100) or on Cr/MgO(100) by RF sputtering, with thicknesses up to 200 nm. We have examined the relation between microstructure and magnetization process, simulating magnetization processes in films with different orientation and spatial organization of the martensitic twin variants. The micromagnetic simulations show a good agreement with the experimental results. Starting from the films grown on Cr/MgO, we have also realized Ni-Mn-Ga nanodots (d=160, 650 nm) by polystyrene-nanosphere lithography, and freestanding nanodisks, by subsequently removing the Cr underlayer via chemical etching. The microstructure and magnetic configuration of the nanostructures are influenced by the lateral confinement and release from the substrate. Furthermore, by varying temperature and applying a magnetic field to the free-standing nanodisks, we have obtained important microstructural changes, enabling different actuation modes [3]. [1] A. Backen et al., Adv. Eng. Mater. 14, 696-709 (2012) [2]P. Ranzieri et al., Adv. Mater. 27, 4760 (2015) [3]M. Campanini et al., Small 2018, 1803027