Martensite enabled magnetic flexibility of shape-memory Heuslers by microstructure engineering

Magnetic shape-memory (MSM) Heuslers show strong coupling between magnetic and structural degrees of freedom, evidencing correlation between magnetic, thermal and mechanical characteristics of the material through a magnetostructural phase transformation. In particular, MSM Heusler thin films are of special interest due to possible integration in smart micro/nanodevices such as sensors, energy harvesters and actuators. Recently, successful epitaxial growth of MSM Heusler thin films on silicon substrates using SrTiO3 buffer layer has facilitated integrating these materials into micro/nanoelectronics and micro/nanomachining technology based on silicon [1]. The ability to control the microstructure, which rules out the magnetic characteristics of these films at different length scales, is the key point for the optimization of their magnetic functional properties. Upon temperature variation, Ni-Mn-Ga films undergo phase transformation between a paramagnetic high-temperature high-symmetry phase (austenite) and a ferromagnetic low-temperature low-symmetry phase (martensite). The low-temperature martensite phase consists either of both or one of the differently oriented hierarchical twinning configurations, where magnetic domain direction is alternatively out of plane and in plane (X-type) or in plane (Y-type) [2,3]. Different arrangement of these twinning configurations gives rise to various magnetic properties. In our previous works, we have reported a number of strategies for engineering these hierarchical twinning assembles in epitaxial Ni-Mn-Ga films, in most of which we succeeded in inducing, aligning, or expanding the portion of Y-type twinning configurations [3-5]. In this study, we report a simple strategy for the reverse process by converting the entire microstructure of the samples into X-type. We show that the twinning configurations of 200-nm Ni-Mn-Ga films, epitaxialy grown on MgO(001) using Cr underlayer, can be permanently converted into X-type by a single-step post-annealing treatment. We evidence that this simple twinning configuration engineering approach is applicable not only to continuous films but also to patterned micrometer structures of Ni-Mn-Ga. Finaly, advanced characterization techniques including temperature dependent scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) enable us analyzing the atomic structure and surface of the samples. The simple microstructure engineering strategy reported in this study facilitates magnetic manipulation of shape-memory Heuslers in miniaturazed functional devices. [1] L. Fink et al. Adv. Funct. Mater. 33 (2023), 2305273. [2] A. Diestel et al. J. Condens. Matter Phys. 25 (2013), 266002-266013. [3] P. Ranzieri, et al. Adv. Mater. 27 (2015), 4760-4766.‏ [4] M. Campanini et al. Small 14 (2018), 1803027. [5] M. Takhsha Ghahfarokhi et al. Acta Mater. 187 (2020), 135-145.