Spontaneous formation of suboxidic coordination around Co in ferromagnetic rutile Ti_0.95Co_0.05O₂ film

To evaluate local atomic structures around Co in high temperature diluted ferromagnetic semiconductor Co-doped TiO₂, x-ray fluorescence holography and x-ray absorption fine structure experiments were carried out on rutile paramagnetic Ti_0.99Co_0.01O₂ and ferromagnetic Ti_0.95Co_0.05O₂ films. The Co atoms in the Ti_0.99Co_0.01O₂ simply substituted for Ti sites in the rutile structure, whereas a suboxidic arrangement of CoO₂Ti₄ formed around Co in the Ti_0.95Co_0.05O₂ films. A theoretical investigation based on a series of first-principles calculations indicated the stability of the aggregated suboxidic clusters in the rutile TiO₂, supporting our hypothesis for the formation of suboxidic coordination in the highly Co-doped sample. The suboxidic coordination may be the source of strong exchange interaction, resulting in the high Curie temperature in Co-doped TiO₂.