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 shape memory Heuslers have a great potential 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 these materials in applications. Using Ni-Mn-Ga seven-modulated epitaxial thin films as a model system, we investigated the possible links between the phase transition and the details of the twin variants configuration in the martensitic phase. We explored the crystallographic relations between the martensitic 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.

Dewetting Process in Ni-Mn-Ga Shape-Memory Heusler: Effects on Morphology, Stoichiometry and Magnetic Properties

“…Figure 1 summarizes the morphology and the magnetic characteristics of the pristine sample. The AFM images in Figure 1a demonstrate the presence of X-type twins that are characterized by different orientations of the twinning planes (i.e., at 45 • degrees to the MgO(001) substrate) [42][43][44][45][46][47]. The corresponding out-of-plane contribution of the magnetic domains can be visualized in the MFM images as the dark and the bright contrasts (Figure 1b) [42][43][44][45][46][47][48][49].…”

“…The AFM images in Figure 1a demonstrate the presence of X-type twins that are characterized by different orientations of the twinning planes (i.e., at 45 • degrees to the MgO(001) substrate) [42][43][44][45][46][47]. The corresponding out-of-plane contribution of the magnetic domains can be visualized in the MFM images as the dark and the bright contrasts (Figure 1b) [42][43][44][45][46][47][48][49]. When cooling from 400 K, the thermomagnetic curves of the pristine sample (Figure 1c) show an abrupt rise in the susceptibility at around 344 K, which is ascribed to crossing the austenitic Curie temperature of the sample [50], where it transforms from the paramagnetic austenitic phase to ferromagnetic austenitic phase.…”

Magnetic Shape-Memory Heuslers Turn to Bio: Cytocompatibility of Ni–Mn–Ga Films and Biomedical Perspective

“…Coherently, the high-resolution backscattered electron (BSE) image in Figure d shows the dominant presence of Y-type twining configurations and the small regions of X-type twining configurations, some of which are labeled in the figure. X-type and Y-type are characterized by different orientations of the twins (i.e., at 45° and 90° degrees to the MgO(001) substrate, respectively) in the martensitic phase. − As a consequence, they are also characterized by different magnetic anisotropy contributions, all in-plane for the Y-type and both in-plane and out-of-plane for the X-type. The blurred regions in Figure d are expected to be due to the material undergoing the martensitic transition at room temperature.…”

“…The blurred regions in Figure d are expected to be due to the material undergoing the martensitic transition at room temperature. Comprehensive structural and microstructural characterizations can be found in references. ,− …”