Investigations on the nature of observed ferromagnetism and possible spin polarization in Co-doped anatase TiO2 thin films

Abstract

High-quality epitaxial thin films of Co-doped anatase TiO2 (Co:TiO2) were grown epitaxially on SrTiO3 (001) substrates by using pulsed laser deposition with in-situ reflection high-energy electron diffraction. The oxygen partial pressure, PO2, during the growth was systematically varied. As PO2 decreased, the growth behavior altered from a two-dimensional layer-by-layer-like growth to a three-dimensional island-like pattern. Electrical conductivity and saturation magnetization increased, seemingly consistent with the picture of carrier-induced ferromagnetism. However, we also found that the spatial distribution of Co ions became highly nonuniform and the chemical state of Co ions changed from ionic to metallic. All of these PO2 dependences, even including the transport and the magnetic properties, can be explained in terms of the formation of cobalt clusters, whose existence was clearly demonstrated by transmission–electron–microscope studies. Our work clearly indicates that the cobalt clustering will result in the room-temperature ferromagnetism observed in our Co:TiO2 films. To check the possible spin polarization of carriers in Co:TiO2 films, we also fabricated a heterojunction composed of a ferromagnetic Co:TiO2, an insulating SrTiO3, and a ferromagnetic half-metallic (La,Ba)MnO3 layer. When the magnetic field was varied, we could not observe any changes in its I–V characteristic curves, which suggests that there might be little spin-polarization effect in the anatase Co:TiO2 layer.