Engineering Hierarchical Martensite Twins in Epitaxial Magnetic Shape Memory Films by Tuning Growth Conditions and Post-Growth Treatments

Ferromagnetic shape memory alloys (FSMA) such as NiMnGa thin films show a strong coupling between magnetic and structural degrees of freedom, which makes them a promising candidate for smart micro and nano-device applications [1]. The ability to control the microstructure at different length scales is of particular interest for the magnetic field induced strain applications. In low-temperature ferromagnetic phase, NiMnGa film consists of differently oriented twins [2,3]. Magnetic properties can be tuned by engineering these twin microstructures [4]. In the present study, NiMnGa films (75-200nm) were epitaxially grown on MgO (100) at 200- 380°C using RF sputtering technique. The deposition rate was 3.83 to 6.03nm/min. Morphology, composition, and microstructural characterizations were performed using AFM, SEM, EDS, XRD, and TEM. Magnetic configuration and behavior were studied by MFM, AGFM, and SQUID. Samples were post-treated by annealing, mechanical stress, and magnetic field cooling. We found that hierarchical configuration of twins and magnetic properties of substrateconstrained films can be manipulated by growth temperature, post-heating, mechanical stress, and field cooling.