Magnetic shape memory free-standing nanodisks: actuation mechanisms and possible applications

Magnetic shape memory materials display multifunctional properties (e.g. magnetomechanical, magnetocaloric, magnetoresistive...) arising from the presence of a martensitic transformation and magnetic states [1]. Low-dimensional materials, mainly thin films, have recently attracted much interest for their great potential in novel applications (e.g. microactuators, energy harvesters, solid- state microrefrigerators) [2]. We have shown that in epitaxial thin films the magnetic and structural properties can be optimized at the different length-scales by an appropriate choice of substrates/underlayers, thickness and growth parameters, including temperature and stress applied during growth [3]. In the present talk we will focus on patterned structures and free-standing nanodisks. Patterned thin films were obtained by polystyrene-nanosphere lithography of epitaxial NiMnGa- based thin films grown by sputtering r.f. on MgO substrates with a Cr underlayer. Free-standing nanodisks (d=160, 650 nm) were subsequently obtained by removing the Cr underlayer by a selective chemical etching. A multiscale structural and magnetic study was performed by means of electron microscopy (HREM, STEM-HAADF, electron diffraction, Lorentz microscopy), X-ray diffraction, AFM/MFM, and SQUID magnetometry. Patterned thin films maintain the same macroscopic martensitic and magnetic properties of continuous thin films (e.g. martensitic transformation temperature, crystalline structures, magnetization loops). On the other hand, their microstructural and magnetic configurations are influenced by lateral confinement and release from substrate. Remarkably, the combined application of temperature and field to free-standing nanodisks gives rise to substantial microstructural changes, enabling different actuation modes. Areal variation of the order of some percent, and tunable in intensity and sign by the application of T and magnetic fields have been obtained These features, arising from the combination of ferromagnetic and martensitic properties, pave the way to the realization of ferromagnetic shape memory nanoactuators. Possible new-concept biomedical applications will also be discussed. [1] M. Acet, et al., Handbook of Magnetic Materials vol. 19, Elsevier, Amsterdam, 2001 [2] A. Backen et al., Adv. Eng. Mat. 14,(2012) 696 [3] P. Ranzieri et al., Acta Mater. 61 (2013) 263,P. Ranzieri et al., Adv. Mater. 32, (2015) 4760