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

Following the Martensitic Configuration Footprints in the Transition Route of Ni-Mn-Ga Magnetic Shape Memory Films: Insight into the Role of Twin Boundaries and Interfaces

“…Atomic and magnetic force microscopy (Veeco Dimension 3100, CA, USA) imaging (supplementary material Figure S1) confirmed the X-type microstructure for the as-grown samples [26]. For sample #1, Y-type configuration stripes were induced by mechanical stress [7]. The details of X-and Y-microstructures are reported in Section 2.2.…”

“…The first sample (#1) was grown directly on (100) MgO followed by post-annealing (3600 s at 623 K in 10 −3 mPa) and local mechanical stress (normal to the substrate, straining the film along the edge of MgO). The applied stress value was estimated to be >20 MPa, as described in [7], where complementary and detailed results on this sample can be found. The second sample (#2) was grown on 50 nm Cr underlayer, and in turn grown on the (100) MgO substrate.…”

“…By exploiting epitaxial growth on different substrates, suitable growth conditions (including stress), and geometrical parameters (e.g., thickness) high quality and suitably oriented epitaxial films can be obtained. They allow accurate studies of structure and magnetism at the different length scales [7][8][9]. Despite their different martensitic configuration with respect to bulk materials, thin films also provide a suitable platform to study the role of intrinsic and extrinsic properties in the martensitic transformation process and formulate models that can be extended to bulk materials [10,11].…”

“…Among the obstacles are the thermal and magnetic hysteresis [17], broadness of the transition [18], and twinning stress [19], which are affected by several parameters, e.g., composition, chemical order, crystal structure, geometric compatibility of the martensite and austenite [20], dynamics of the transition [21], internal stress, and crystal defects. The two latter parameters are strongly linked to the configuration of the twin variants in the martensitic phase, which covers a long range from nano-to macro-scale [7,22,23].…”

“…Therefore, in order to find possible solutions to overcome these obstacles, it is necessary to gain a comprehensive view of the configuration of the twin variants at the different length scales and its evolution upon martensitic phase transition. In the literature, there are a few works focused on the crystallographic structures and the martensitic configurations of epitaxial Ni-Mn-Ga thin films through experiments and models [7][8][9][10][24][25][26]. However, the knowledge about the multiscale hierarchical self-accommodation of the twin variants in the martensitic phase and its possible links to the transition route is still limited, mainly due to lack of direct multiscale observation.…”