Dependence of texture of magnetite thin films on different substrates and orientations

The knowledge about microstructure and grain morphology of magnetite materials is very important in order to understand the partially puzzling magnetic properties. We have, therefore, investigated a variety of magnetite samples concerning details of their respective microstructures. (001)- and (111)-oriented magnetite thin films were grown on MgO substrates (film thickness 200 nm) by means of oxygen-plasma-assisted molecular beam epitaxy and by laser-ablation. Further samples investigated in this study are electroplated magnetite thin films on Si/copper substrates and films from biogenic magnetite. The achieved grain orientations are analyzed by means of the electron backscatter diffraction (EBSD) technique. The EBSD technique enables the crystallographic orientation of individual grains to be determined with a high spatial resolution of up to 40 nm on such ceramic materials. A high image quality of the recorded Kikuchi patterns was achieved enabling multi-phase scans (e.g., Fe₃O₄, MgO, γ-Fe₂O₃) to be performed. The facets of individual grains, which exhibit sizes of several tens of nanometers in the case of electroplated and biogenic magnetite, are analyzed in detail. Furthermore, the (001)-surface of the films grown by molecular beam epitaxy after a short annealing in air (1 min, 250 °C) is characterised by the presence of tiny (diameter 100-200 nm) misoriented islands, which may have an influence on the anti-ferromagnetic coupling within the film. The properties of these interfaces are studied in detail, including magnetic force microscopy (MFM) measurements in applied magnetic fields. In the (111)-oriented films and the laser-ablated films, such defects are found to be absent, and the films show a very homogeneously oriented surface.