Magnetic shape memory materials show remarkable multifunctional properties (e.g. magnetomechanical, magnetocaloric) arising from the presence of a martensitic transformation and magnetic states . Low-dimensional materials, mainly thin films, have recently attracted a great interest for their potential in smart and energy applications (e.g. microactuators, valves, solid-state microrefrigerators) . With respect to bulk materials, thin films offer the possibility to tune properties by exploiting the epitaxial growth on suitable substrates and underlayers [3, 4].
The present talk will be focused on NiMnGa thin films (10- 200 nm) grown on Cr underlayer, and on- substrate and free-standing nanodisks obtained by large-area lithographic techniques. 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 AGFM and SQUID magnetometry and susceptometry.
We have found that microstructure and the consequent functional properties of thin films can be finely tuned by and an appropriate choice of substrates, thickness and growth parameters, including temperature and stress applied during growth. Laterally confined nanostructures show peculiar microstructural features, but maintain the martensitic and magnetic properties that are the basis of the multifunctional exploitation of thin films. Possible actuation mechanisms obtained by the combined application of temperature and magnetic field in free-standing nanodisks will also be presented.
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4. P. Ranzieri et al., Adv. Mater. 32 4760 (2015).