Following the martensitic configuration footprints in the martensitic transition route of shape memory Heusler films

Magnetic shape memory Heuslers have a great potential for being exploited into the next generation of cooling devices and actuating systems due to their "giant" caloric and thermo/magnetomechanical effects, arising from the combination of magnetic order and a martensitic transition. Thermal hysteresis, broad transition range, and twinning stress are among the major obstacles preventing the full exploitation of these materials. In order to find possible solutions to overcome these unfavorable 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 [1-4]. 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 observations. In the present study, we directly visualize the crystallography of seven-fold modulated (7M) Ni-Mn-Ga epitaxial films, the symmetry relations between the twin boundaries [5], and the interfaces [6] between colonies of different twin boundaries in the martensitic phase by means of different transmission electron microscopy (TEM) techniques. We combine our direct observations through TEM techniques to atomic force microscopy (AFM) topography imaging vs. temperature [7]. We propose a route for the martensitic forward and reverse transitions, highlighting the major role played by the different martensitic interfaces [7]. The present results represent a step forward in the understanding of the transition processes, and pave the way to the possibility of tuning the characteristics of the transition, e.g. hysteresis and transition width, by microstructure engineering aimed at the full exploitation of martensitic Heuslers for applications requiring cyclic phase transition.