To shed more light on microscopic mechanism of the long-range magnetic coupling in 3d metal doped oxide semiconductors we studied the influence of oxygen vacancies (Vo) on magnetic properties of Co-implanted TiO₂. First, magnetically anisotropic nanocomposite samples, TiO_(2-y):Co, based on the dispersion of cobalt nanoparticles in the implanted region, or the isotropic semiconducting samples, Co_xTi_(1-x)O_(2-y), doped homogeneously with magnetic Co²⁺ ions have been obtained by the high-fluence implantation with 40 keV cobalt ions into (100)- or (001)-oriented single crystalline substrates of rutile TiO₂. Then, we developed the original technique of the oxygen vacancies migration (or injection) under the applied DC electric field. Fast radiation-accelerated electro-migration of the oxygen vacancies into the selected region of the Co-implanted TiO₂ sample was visually observed. Besides, EDS element mapping in the sample plane have shown that there is no migration of the Co dopant under DC electric field. At last, we studied in detail the influence of Vo density on the room-temperature magnetic hysteresis loops and thermo-magnetic curves of the Co-implanted samples. It was found that manipulations with Vo density by electro-migration do not cause any change in ferromagnetic response of the nanocomposite samples of TiO₂:Co. On the contrary, the saturation magnetic moment (M_S) in the homogeneously doped samples enhances by 3-4 times in the region enriched with Vo, while it sharply diminishes in the region of sample depleted with Vo. Thus, our experiments give a strong indication for relevance of the Vo-mediated (so-called F-center) mechanism of ferromagnetism in the diluted magnetic oxide semiconductor Co_xTi_(1-x)O_(2-y).

The work was supported by RFBR, grant 10-02-01130-a, and by Russian Federal Agency on Education, contract P902.