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

Abstract

Magnetic shapememoryHeuslers have a greatpotential for their exploitation in next-generation 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 thesematerials in applications. UsingNi-Mn-Ga seven-modulated epitaxial thin films as amodel system, we investigated the possible links between the phase transition and the details of the twin variants configuration in themartensitic phase. We explored the crystallographic relations between themartensitic variants from the atomic-scale to the micro-scale through high-resolution techniques and combined this information with the direct observation of the evolution of martensitic twin variants vs. temperature. Based on our multiscale investigation, we propose a route for the martensitic phase transition, in which the interfaces between different colonies of twins play the major role of initiators for both the forward and reverse phase transition. Linking the martensitic transition to the martensitic configuration sheds light onto the possible mechanisms influencing the transition and paves the way towards microstructure engineering for the full exploitation of shape memory Heuslers in different applications.