Martensite-enabled magnetic flexibility: The effects of post-growth treatments in magnetic-shape-memory Heusler thin films

Magnetic-shape-memory Heusler thin films have a great potential for new-concept integrated devices, such as microactuators, energy harvesters and solid-state microrefrigerators, thanks to the intimate coupling between structure and magnetism. The control of microstructure in their martensitic phase is crucial for their full exploitation. Here, by an accurate magnetic and structural investigation at different length-scales, we demonstrate how growth temperature and simple post-growth treatments, i.e. post-annealing at low T, magnetic field cooling and mechanical stress, are suitable to manipulate the twin variant configuration in epitaxial thin films. X-type variants with out-of-plane magnetic easy axis or Y-type variants with in-plane magnetic easy-axis can be selected, as well as their geometrical distribution in films with mixed X/Y-type microstructure. The mechanisms behind the overall and local manipulation of microstructure are discussed by taking into account the role of microstructural defects, disorder and external fields on the martensitic transformation path. Our findings provide a platform for a controlled manipulation of microstructure and magnetism in magnetic-shape-memory thin films, which opens up a window towards engineering smart magnetic materials for multiple purposes. This remarkable "magnetic flexibility" makes magnetic-shape-memory alloys a unique class, among magnetic materials, for the easy tuning of the magnetic configuration at different length scales.