Investigation of the cobalt distribution in the room temperature ferromagnet TiO2:Co

Recent reports of room temperature ferromagnetism in thin films of ${\mathrm{TiO}}_2$ doped with small amounts of cobalt have excited a great deal of interest due to the possible applications of this ferromagnetic, transparent semiconductor to spintronics devices. Ferromagnetism has been reported for ${\mathrm{TiO}}_2$ grown in both the anatase and rutile phases, although a range of magnetic moments has been reported by different groups. We have grown epitaxial thin films of ${\mathrm{Ti}}_{\text{1−x}}{\mathrm{Co}}_x{\mathrm{O}}_2,$ with x=0.01–0.07 by pulsed laser deposition in both the anatase and rutile forms on single crystal substrates of ${\mathrm{LaAlO}}_3,$ ${\mathrm{SrTiO}}_3,$ and ${\mathrm{Al}}_2{\mathrm{O}}_3.$ These films are ferromagnetic at room temperature with a magnetic moment of ∼1.7 ${\mu }_B/\mathrm{Co}$ in the anatase phase and ∼0.6 ${\mu }_B/\mathrm{Co}$ in the rutile phase. Detailed transmission electron microscopy measurements show a variation in the cobalt distribution in these materials that corresponds to the different magnetic moments. The cobalt does not appear to enter the anatase phase of ${\mathrm{TiO}}_2,$ but rather forms crystallites of cobalt metal on the surface of anatase films. This results in a magnetic moment similar to that of bulk cobalt. In the rutile phase the cobalt has substituted uniformly throughout the lattice. We present the morphology and magnetization of ${\mathrm{TiO}}_2\mathrm{:Co}$ as a function of the ${\mathrm{TiO}}_2$ growth phase and substrate material. Preliminary electrical measurements are also mentioned.