Structure and magnetic properties of Co doped anatase TiO2 particles

Co$_{0.028}$Ti$_{0.972}$O$_{2-\delta }$was synthesized via sol-gel method. It's annealed at 600$^{\circ}$C for one hour in air. The X-Rays diffraction study confirmed that samples are of anatase structure and no detectable cobalt clusters or other impurities are observed. The magnetic properties are characterized by SQUID in a broad temperature range 5K $\sim $ 300K, and no ferromagnetic property was observed. The magnetization verse temperature curve fits well with the Curie-Weiss law and the extracted atomic effective moment suggests that the Co(III) is in high spin state. These paramagnetic particles can be turned into ferromagnetic phase with a Curie temperature above 300K after heat treatment in a mixture of H$_{2}$ and Ar gases at 600$^{\circ}$C for one hour. The origin of ferromagnetism has been studied by XPS, SQUID, XRD and TEM.