Atomic-scale studies of cobalt distribution in Co-TiO_2 anatase thin films: Processing, microstructure, and the origin of ferromagnetism

摘要

Using high-resolution, aberration-corrected, scanning transmission electron microscopy and electron-energy-loss spectroscopy we show that in films of single-phase anatase Co: TiO_2, the Co distribution and magnetic properties are strongly dependent both on the overall crystalline quality and postgrowth vacuum annealing process. The Co: TiO_2 films are coherent, epitaxial anatase with no secondary phases or metallic Co. Films of lower crystalline quality reveal a relatively homogeneous Co concentration, while films of higher crystalline quality show a tendency for Co enrichment near the surface region, around grain boundaries, and the substrate interface. Both uniform and nonuniform samples show a notable enhancement in the saturation magnetization with annealing, while the coercive field is considerably higher in the samples with uniform Co distribution. These experiments confirm that films of single-phase anatase Co:TiO_2 with both uniform and nonuniform Co distributions exhibit room-temperature ferromagnetism in the insulating ground state, while the compositional uniformity and film microstructure play a role in the bulk magnetic properties of the material and the mechanisms for ferromagnetic ordering.