Structure And Magnetic Properties Of Cu-doped TiO₂ Powders

Diluted magnetic semiconductors (DMSs), especially those having room-temperature ferromagnetism (RTFM), have attracted great interest in the last years due to their potential applications in spintronic devices. TiO₂-based DMSs have become a very attractive subject due to the possibility of RTFM in such wide-based gap oxides. In this paper, Cu-doped Anatase and rutile TiO₂ nanoparticles were synthesized by the sol-gel method. Their structure and magnetic properties were investigated systematically.In the air, the dried gels were annealed at 450oC and 800oC for 2h to get the TiO₂ nanoparticles of the anatase and rutile, respectively. Analysis of the structure indicate that the Cu-doped TiO₂ system is propitious to conversion from the anatase to the rutile, and is easy to separate out CuO when sintered temperature is elevated. Analysis of the X-ray photoelectron spectroscopy spectra show that Ti ion is Ti4+, and Cu ion which substitutes Ti ion is Cu¹⁺.Analysis of the electronic band structure show that the shallow donor impurity level forms at the bottom of conduction band due to the oxygen vacancy in pure TiO₂. Besides the shallow donor impurity level due to the oxygen vacancy, the impurity level also forms due to the hybridization of the Cu 3d with the O 2p orbits in Cu-doped TiO₂.The magnetic property of the anatase sample is similar to that of the rutile sample. Both of them exhibit room-temperature ferromagnetism. And with the Cu ion concentration increasing, the saturation magnetization increases at first and then decreases. In compare with the anatase, the saturation magnetization of the rutile is weaker due to less the Cu-doped concentration and smaller the specific surface area.To investigate the origin of the ferromagnetism, the anatase samples were processed under different conditions. First, the anatase samples annealinged in different atmosphere show that the magnetism is enhanced at the poor-oxygen condition which can create more oxygen vacancies, whereas at the rich-oxygen condition the magnetism is weakened. The anatase samples underwent with different external pressure show that the magnetism of samples is enhanced with increasing pressure or Cu-doped concentration. The F-center model can be used to explain the above phenomena. So we think that the origin of the Cu-doped TiO₂ system of the ferromagnetism should use F-center model to explain.