Franca Albertini

Abstract:

Magnetic shape memory Heuslers are an important class of ferroic materials for next-generation remote actuation and energy conversion (i.e. solid-state cooling and energy harvesting), arising from giant multifunctional effects (e.g. thermo/magneto-mechanical, magneto/elasto-caloric) that can be driven by external stimuli (i.e magnetic field, temperature, pressure and stress) [1]. Low-dimensional materials, mainly thin films, have recently attracted much interest for their great potential in applications (e.g. microactuators, solid-state microrefrigerators, microvalves) [2]. With respect to the bulk, they offer the further possibility of tuning properties by exploiting the epitaxial growth on suitable substrates and underlayers. Patterned films and 2D nanostructures are nowadays a vast and almost unexplored field.
This talk is focused on microstructure engineering of continuous and patterned NiMnGa thin films, and free standing nanodisks. By a thorough multiscale magnetic and structural study ,we will show that martensitic microstructure is sensitive to size confinement and can also be easily tuned by tuning growth parameters and performing suitable post-growth treatments (magnetic field, T, stress) [3, 4]. Microstructure engineering can be exploited for the optimization of the multifunctional properties. As an example, we demonstrate the possible actuation of free standing nanodisks by the combined application of temperature and magnetic fields, giving rise to areal strain (up to 5.5%) whose intensity and sign is ruled by a martensitic microstructure [5]. On the other hand, such "microstructure flexibility" makes magnetic shape memory materials a unique system, among magnetic materials, for the "magnetic flexibility"; magnetism can be easily manipulated at the different length-scales by taking advantage of martensitic microstructure and strong spin-lattice coupling.

[1] M. Acet, et al., Handbook of Magnetic Materials vol. 19, Elsevier, Amsterdam, 2011.
[2] A. Backen et al., Adv. Eng. Mat. 14,(2012) 696.
[3] P. Ranzieri et al., Acta Mater. 61 (2013) 263.
[4] P. Ranzieri et al., Adv. Mater. 32, (2015) 4760.
[5] M. Campanini et al. Small 14 (2018) 1803027.