Room-temperature ferromagnetism in CoxTi(1-x)O2 powders made by sol-gel method: a NMR study

Co-doped TiO₂ system is one of the most promising candidates for room-temperature ferromagnetic semiconductors. However, the origin on its room-temperature ferromagnetism has not been totally understood. In this paper, Co_0.04Ti_0.96 O₂ powders were fabricated by sol-gel method. The structure and magnetic properties were investigated at different annealing conditions systematically with emphasis on the influence of oxygen pressure. Pure anatase structure was acquired for all samples annealed at 450°C for 1 h except the one annealed in oxygen flow. The samples annealed in air exhibit evident room-temperature ferromagnetism with small magnetic moment of 0.029 μ_B per Co atom, while the samples annealed in vacuum have strong ferromagnetism with larger magnetic moment of 1.18 μ_B per Co atom. Zero field spin echo nuclear magnetic resonance spectrum of ⁵⁹Co detected the existence of Co clusters in the latter samples, implying that the strong ferromagnetism in these samples annealed in vacuum comes from the Co clusters and is not the intrinsic property of the Co_0.04Ti_0.96O₂ materials.