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

Magnetic-shape-memory Heuslers are an important class of ferroic materials, constantly opening new fields of research and application (e.g. magnetic actuation, energy harvesting, ferroic cooling) arising from the strong interplay between magnetism and structure. The exploitation of thin films opens up barely new perspectives in the realization of micro/nanomachines, memories and smart devices, taking advantage also of their possible integration in microsystems. The control of microstructure is a crucial goal for the full exploitation of their functional properties (e.g. shape memory, magneto-mechanical) since the twin variants configuration plays a major role. Few recent works have been addressed to microstructure engineering, mainly exploiting the epitaxial growth on different substrates and varying film thickness and growth parameters. In previous papers we have shown the correlation between structure, microstructure and magnetic properties at the different legth-scales of substrate-constrained Ni-Mn-Ga films grown on Cr/MgO(100) [1]. More recently, we have studied the effects of size confinement and reported the possible thermo-magnetic actuation of free-standing Ni-Mn-Ga nano-disks, enabled by the peculiar martensitic microstructure [2] In the present talk we present an important step forward [3]. By means of a thorough multiscale magnetic and structural study, we demonstrate that a variety of simple post-growth treatments, i.e. post-annealing at low T (350°C), magnetic field cooling and mechanical stress, open up the possibility to tailor the twin variant configuration of epitaxial thin films. In particular, depending on the selected treatment, it is possible to tune the type of variant, i.e. X-type with out-of-plane magnetic easy axis or Y type with in-plane magnetic easy-axis, and their geometrical distribution. The mechanisms underlying the overall and local manipulation of microstructure are discussed by taking into account the role of microstructural defects and disorder together with the influence of external fields on the martensitic transformation path. Our findings definitely provide a platform to easily control and manipulate microstructural and magnetic patterns in magnetic shape memory thin films, paving the way to their full exploitation in smart applications. In a broader context, this remarkable "magnetic flexibility", enabled by the peculiarity of martensitic microstructure, makes magnetic shape memory Heusler thin films, a unique class, among magnetic materials, for the easy manipulation of their magnetic configuration by simple after-growth treatments.