Multiscale control of structure and functional properties in the Ni-Mn-Ga shape-memory compounds: from epitaxial thin films to micro and nanostructures

Abstract

The goal of investigation, reported in this thesis can be classified into three levels: Level 1. Multiscale observation of the formation and self-accommodation of the twinning configurations in the martensitic phase of Ni-Mn-Ga epitaxial thin films, micro and nanostructures. Level 2. Investigation of the possible relations between the formation and self-accommodation of the twins and the martensitic transformation processes. Level 3. Looking for the possibilities of controlling the martensitic configurations and the martensitic transformation process in epitaxial films, micro and nanostructures. We investigated the crystallographic relations between the twin boundaries and interfaces from the atomic scale to the microscale and accompanied this investigation with the direct observation of the evolution of the martensitic interfaces vs. temperature. Based on the symmetry relations between the twin variants, we identified the types of twin boundaries and the twinning interfaces, i.e., ridges, valleys, and non-conjugation interfaces. Using the change of modulation direction across the boundary, observed by transmission electron microscopy in Y-type regions, we were able to determine the presence of type I, type II, and modulation twin boundaries with a dominant presence of type II. Beyond these findings, we proposed a transition route originating from the martensitic configuration, highlighting the major role of different martensitic interfaces. The forward transition starts with the heterogeneous formation of twin boundaries at the position of the ridges on the surface of the film and moves towards the substrate. The twin boundaries continue to nucleate and grow until they meet at the other kind of conjugation interfaces, i.e., valleys, or at non-conjugation interfaces, where the growth is hindered. In these regions, the

elastic stress created during the transition is partially stored. This stored energy serves as the driving force for the reverse transition by initiating the nucleation of the austenitic phase. We also investigated the influence of the density of the martensitic interfaces on the critical temperatures of the martensitic transition. It was found that imperfections could serve as the nucleation points, increasing the density of the martensitic interfaces and stored elastic energy and consequently leading to the increase of the transition width. In contrast, the density of the martensitic interfaces as well as the sample roughness shows only a minor effect on the thermal hysteresis of the samples. Nevertheless, by the reduction of the imperfections, annealing plays the paradoxical role of reducing the transition width while increasing the thermal hysteresis.The formation of the twin boundaries in Ni-Mn-Ga epitaxial films was investigated in situ in three different conditions, i.e. temperature induced phase transition (zerofield), temperature induced phase transition assisted by a constant external magnetic field (isofield) and the magnetic field induced phase transition in constant temperature (isothermal). The observations highlight that Y-type twinning gives rise to a relatively sharper transition and lower hysteresis compared to the X-typeconfiguration. Therefore, X and Y-type twins not only show different microstructural and magnetic characteristics, but they also display different characteristics of nucleation and growth during the martensitic transition. Starting from continuous films, micro and nanostructures of epitaxially grown Ni-Mn-Ga were fabricated by means of lithography techniques and reactive ion etching. The critical transition temperatures measured for microstripes, having the lateral size of 3 μm to 100 μm show a minor shift (< 3 K) towards higher temperatures and the thermal hysteresis varies about 2 K upon downscaling. The critical transition temperatures measured for the nanostripes having the lateral size of 350 nm to 700 nm reveal the dominant effect of broadening of the transition width as well as the increase of the transition temperatures over down scaling. The thermal hysteresis also shows a gradual decrease (5-7 K) as function of the lateral size. In addition to the effect of size on the critical transition temperatures, the self-accommodation of the martensitic configurations was found to be influenced by

the lithographically patterning. The fabricated finger-shaped microstructures having the width of < 12 μm , patterned along different orientations with respect to [100] MgO were found to break the equivalency of the X-type twin boundaries by selecting the boundaries, which are relatively parallel to the length of the fingers. This effect was also observed in the nanostripes having the lateral dimension range of 300-550 nm. The effect of shape on the self-accommodation of the martensitic configurations was realized by comparing microstripes and microdisks. The tiny stripes along [100] MgO were found to keep the multiplicity of the X-type twin boundaries while alternate the orientation of the boundaries more frequently as function of reduction of size. Microdisks however, were observed to reduce the multiplicity of the X-type twin boundaries as a function of the lateral size. The observed effects are expected to be mainly due to the impact of lithographically patterning on the number of the nucleation sites, growth and the self-accommodation of the martensitic twin boundaries. It was also reported that different simple post-growth treatments (i.e. post-annealing, magnetic field cooling and the application of a local mechanical stress) open up the possibility to tailor the twinning configuration of FSMA epitaxial thin films. Taking the advantage of growth temperature and post annealing treatment, twinning configuration can be easily tuned from full X-type to mixed X/Y-type with different geometrical distribution, in which controlling the defects and disorder affecting the martensitic transition path play an important role. Mixed X/Y-type microstructure can be modified by applying an external magnetic field while crossing the martensitic transformation, exploiting the additional Zeeman energy term. In this case, with the field applied in the film plane, a predominant Y-type microstructure can be obtained. The application of post-growth stress is suitable for locally transforming X-type to Y-type configuration along the direction of the applied stress. Overall, the multiple approaches used in this thesis shed light into the direct link between the martensitic configuration at the different length scales and the martensitic transition. The present results can be considered a step forward for understanding the transition processes as well as for tuning the characteristics of the transition such as hysteresis and transition

width, by microstructural engineering. The tuning aims at the full exploitation of martensitic Heuslers for the applications requiring cyclic phase transition.