Ferromagnetic-Shape-Memory Heusler Microstructures: The Geometry and Size Effects

Ferromagnetic shape memory (FSM) Heuslers such as Ni-Mn-Ga belong to a class of smart materials, showing a strong coupling between magnetic and structural degrees of freedom, thus giving rise to an evident correlation between magnetic, thermal and mechanical properties [1]. In particular, FSM Heusler thin films are of special interest due to possible integration into micro- and nanoscale thermo/magnetomechanical devices for solid-state cooling, energy harvesting, and sensing/actuation [2,3]. However, so far, there have been only few systematic investigations of the effect of lateral confinement on FSM phenomenology [4-6] and the knowledge about their main properties, such as martensitic phase transition and martensitic twin structure as a function of size is still limited. By varying the temperature, Ni-Mn-Ga encounters reversible first order phase transformation between a high temperature cubic austenitic and a low temperature, low symmetry martensitic phase, where martensitic cells form arrays of hierarchical patterns, called twin variants. The relative orientations of the twin variants alternate quasi-periodically across their borders, which are called twin boundaries [7]. In epitaxial Ni-Mn-Ga thin films, the twin boundaries can be described by their orientations with respect to the substrate. There are typically six total orientations of possible twinning configurations along {110} planes of the cubic austenitic cells, the so-called X-type twinning configurations refer to the four equivalent configurations that are 45⁰ inclined to the substrate plane [8 and the references there in]. The present work makes a substantial step forward. It focuses on patterned epitaxial thin films that are the most homogenous platforms for a systematic investigation of the effects of lateral confinement on FSM Heusler properties. By combining direct resistivity measurements, structural and magnetic studies at different temperatures and different length-scales, we deepen the effects of size and geometry in a variety of Ni-Mn-Ga microstructures obtained by UV lithography. The fabricated microstructures have different lateral size (3 to 100 µm), different shapes (i.e. squares, disks, rings, radial stripes, straight and L-stripes, rings, square-bands) and various orientations with respect to the crystallographic axes. The key intrinsic properties of the material were confirmed to be stable after the microfabrication process. The critical martensitic transition temperatures showed a minor shift (< 3 K) towards higher temperatures and the thermal hysteresis varied only about 2 K. Noticeably, reduction of size and geometry were found to have a strong impact on the twinning configuration. In particular, we obtained a selective formation of the X-type twin boundaries along [110] MgO and [1-10] MgO, by patterning the material into radial stripes, demonstrating the possibility of engineering martensitic twins by means of size and geometry. The present results give an indispensable insight into the correlation between the lateral confinement and the martensitic phenomenology of FSM Heuslers. They also shed light on the possibility of exploiting size and geometry for tuning the martensitic twins in conventional and FSM materials, which play a major role in their thermo/magnetomechanical performances. [1] K. Ullakko, J. K. Huang, C. Kantner, R. C. O’handley, V. V. Kokorin, Appl. Phys. Lett. 1996, 69, 1966. [2] F. Bruederlin, L. Bumke, C. Chluba, H. Ossmer, E. Quandt, M. Kohl, Energy Technol. 2018, 6, 1588. [3] M. Kohl, M. Gueltig, V. Pinneker, R. Yin, F. Wendler, B. Krevet, Micromachines 2014, 5, 1135. [4] T. Gottschall, D. Benke, M. Fries, A. Taubel, I. A. Radulov, K. P. Skokov, O. Gutfleisch, Adv. Funct. Mater. 2017, 27, 1606735. [5] F. Lambrecht, C. Lay, I. R. Aseguinolaza, V. Chernenko, M. Kohl, Shap. Mem. Superelasticity 2016, 2, 347. [6] N. Ozdemir, I. Karaman, N. A. Mara, Y. I. Chumlyakov, H. E. Karaca, Acta Mater. 2012, 60, 5670. [7] A. G. Khachaturyan S. M. Shapiro, S. Semenovskaya, Phys. Rev. B 1991, 43, 10832. [8] M. Takhsha Ghahfarokhi, L. Nasi, F. Casoli, S. Fabbrici, G. Trevisi, R. Cabassi, F. Albertini, Materials 2020, 13, 2103.