Metamagnetic Heusler compounds display a first order structural transformation associated with large changes of magnetization and/or magnetic order. Their properties can be tuned by suitable changes in composition making them a versatile class of multifunctional materials, thanks to the strong interplay between thermal, mechanical and magnetic degrees of freedom 1. In addition, their hierarchically interrelated twin-with-twin microstructure, in the low temperature martensitic phase, and the strong spin-lattice coupling allow to control their magnetic and multifunctional properties from the atomic to the micro-scale by tuning growth conditions and applying external fields 2, 3. In my talk I will report on our recent results on nano/microscale materials obtained by different fabrication methods (e.g. epitaxial thin films, patterned and free-standing structures) 3-6. Thin films and micro/nanostructures are of particular interest not only for the realization of new-concept devices, but also for providing insights into the magnetostructural coupling at the different length scales. The talk will focus on microstructure engineering and microstructure related effects on the martensitic transformation, also in view of the possible exploitation of this class of materials in energy related and smart applications. In particular, by an accurate magnetic and structural investigation at different length-scales, I will demonstrate how growth temperature and simple post-growth treatments, i.e. postannealing at low T, magnetic field cooling and mechanical stress, are suitable to manipulate the twin variant configuration in epitaxial thin films. X-type variants with out-of-plane magnetic easy axis or Y-type variants with in-plane magnetic easy-axis can be selected, as well as their geometrical distribution, in films with mixed X/Y-type microstructure 3. Taking advantage from the possibility to manipulate microstructure we have studied the role of specific martensitic configurations on the martensitic transition path. By advanced magnetic force microscopy imaging in a wide temperature (260–350 K) and magnetic field range (up to 14 T) we have directly observed the nucleation and the self-accommodation of the martensitic twinning configurations under zero-field, isofield and isothermal conditions. We have found that between the two possible twinning configurations, the Y-type, which nucleates first, shows a significantly smaller thermal hysteresis as well as a sharper phase transition with respect to X-type microstructure, for all the three investigated conditions 4. The effects of lateral size, shape and geometry on the properties of patterned epitaxially grown Ni-Mn-Ga films will also be discussed. In particular, I will present the characterization of arrays of microstructures, obtained by means of different lithographic techniques, with lateral sizes down to 200 nm range and having different shapes and orientations with respect to the substrate edges 5. The last part of my talk will be devoted to free-standing nanostructures in view of their possible exploitation as micro/nano actuators exploiting microstructure-controlled actuation mechanisms by the combined application of temperature and magnetic field 6.

References:

1 M. Acet, L. Mañosa, A. Planes, Handbook of Magnetic Materials (Ed: K. H. J. Buschow), Elsevier, Amsterdam 2011, pp. 231–289
2 P. Ranzieri et al., Adv. Mater. 2015, vol. 27, p. 4760
3 M. Takhsha Ghahfarokhi et al. Acta Mater. vol. 187, p. 135–145 (2020)
4 M. Takhsha Ghahfarokhi et al. Acta Mater. vol. 23, 117356 (2021)
5 M. Takhsha Ghahfarokhi, et al. Appl. Mater. Today vol. 23, p. 101058 (2021).
6 M. Campanini et al., Small, vol. 14, p. 1803027 (2018)