Tuning magnetism and functional properties in ferromagnetic shape memory films and nanodisks

Magnetic shape memory alloys have a great potential for micro- and nano-actuators and energy harvesters, thanks their outstanding magnetomechanical properties. We will show that the microstructure and magnetic properties of Ni-Mn-Ga thin films can be engineered by properly choosing the substrate and growth conditions. The films were epitaxially grown on Cr/MgO(100) by r.f. sputtering, with thicknesses up to 200 nm. We then 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. We have also examined the relation between microstructure and magnetization process, simulating magnetization processes in systems with different orientation and spatial organization of the martensitic twin variants. The micromagnetic simulations show a good agreement with the experimental results.