Engineering the microstructure of ferromagnetic shape memory thin films by post growth treatments

Ferromagnetic shape memory Heuslers show multifunctional properties arising from a strong coupling between magnetic, thermal and mechanical degrees of freedom. Within this class of compounds, Ni2MnGa is a model system, which shows a martensitic phase transformation from a cubic L21phase (austenite) to a lower symmetry phase (martensite) by decreasing temperature.We have investigated the possibility of modifying the martensitic microstructure in Ni-Mn-Ga films with thickness between 75 and 200 nm, grown by r.f. sputtering on MgO(100) or Cr/MgO(100). Films were grown in the temperature range 200 - 400 °C, with different growth parameter (i.e., sputtering rate, sputtering power) and composition Ni54Mn22Ga24. A multiscale magnetic and structural study was performed by 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.The L21austenitic phase grows epitaxial at high temperature both on MgO(100) and Cr/MgO(100). The martensitic phase, which is stable at room temperature, has a monoclinic 7M incommensurated structure. Both the phases are ferromagnetic, but the martensitic phase shows higher magnetocrystalline anisotropy. 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 [1]. Controlling the orientation and organization of X- and Y-type twins in epitaxial films and nanostructures could pave the way towards multifunctional applications such as thermomagnetic actuation [2], magnetic storage [3] and magnetic anisotropy dependent properties in fluids [4].We have focused on microstructure/magnetic pattern engineering by post-growth treatments such as post-annealing, stress, annealing in magnetic field. Through these post-growth treatments, a variety of martensitic patterns (i.e. orientation and spatial organization of the martensitic twin variants) were obtained, demonstrating that it is possible to engineer the magnetic patterns and strongly influence the magnetization processes (Figure 1). The intimate link between magnetic and structural degrees of freedom and the flexible twin-within-twin martensitic structure makes epitaxial Ni-Mn-Ga 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.