Fabrication and Magnetic Properties of Co-Doped TiO₂ Powders Studied by Nuclear Magnetic Resonance

Co_0.04Ti_0.96O₂ powders are fabricated by sol-gel method. The structure and magnetic properties are investigated under different annealing conditions systematically with emphasis on the influence of oxygen pressure. Pure anatase structure was acquired for all the samples annealed at 450°C for one hour. The samples annealed in air exhibit evident room-temperature ferromagnetism (RTFM) with a small magnetic moment of 0.029μ_B per Co atom and coercivity H_c of 26Oe, while the samples annealed in vacuum have strong RTFM with a larger magnetic moment of 1.18μ_B per Co atom and H_c of 430Oe. The zero-field spin echo nuclear magnetic resonance spectrum of ⁵⁹Co is obtained to prove the existence of Co clusters in the latter samples, implying that the Co clusters are responsible for the strong RTFM in the samples annealed in vacuum. No Co cluster could be observed using both XPS and NMR techniques in the samples annealed in air, implying that the RTFM found in these samples is intrinsic.