Epitaxial NiMnGa thin films: microstructure and magnetic engineering by post-growth treatments

Ferromagnetic shape memory Heuslers, such as NiMnGa, show multifunctional properties arising from a strong coupling between magnetic, thermal and mechanical degrees of freedom. Thin films have recently attracted much interest for their possible integration in microsystems for the realization of new-concept devices (e.g. microactuators, energy harvesters, solid-state microrefrigerators). Ni2MnGa shows a martensitic phase transformation from a cubic L21 phase (austenite) to a lower symmetry phase (martensite), by decreasing T. In the presently investigated composition (i.e. Ni54Mn22Ga24) the martensitic phase is ferromagnetic with a monoclinic 7M incommensurated structure. The martensitic microstructure is made of complex arrays of differently oriented hierarchical twin microstructures, i.e. X-type and Y-type, where the easy-magnetization directions are respectively out-of-plane and in-plane. In a previous paper we showed that growth conditions (including stress applied during growth) strongly influence the martensitic microstructure of films grown on a Cr underlayer [1]. In the present study we focus on microstructure engineering by post-growth treatments of films directly grown on MgO. Ni54Mn22Ga24 thin films of thickness 200 nm were epitaxially grown on MgO (001) in the range T=200-400 °C by rf sputtering. A multiscale thorough magnetic and structural study was performed be means of several techniques. AFM, SEM, TEM, XRD were used for characterizing microstructure, while the magnetic properties were studied by MFM, Lorentz microscopy, AGFM and SQUID magnetometry. By applying T, magnetic fields and stress to the substrate after growth, a variety of martensitic patterns was obtained, demonstrating that it is possible to engineer the magnetization patterns and strongly influence the magnetization processes. The intimate link between magnetic and structural degrees of freedom and the flexible twin-within-twin martensitic structure makes epitaxial NiMnGa films a unique platform for the precise control of the magnetic configuration from the atomic to the macro-scale also by means of easy and suitable post-growth treatments.