Experimental Studies On Magnetism Of TiO₂-Based Oxide Semiconductor

In recent years, studies have attracted a lot of attentions on the ferromagnetism (FM)of transition-metal-doped semiconducting oxide that is a sort of candidates for dilute magnetic semiconductors because it is believed to be a most promising spintronics materials. Among TM-doped oxides, studies have received much attention on the FM of TM-doped TiO2. In this paper, we report experimentally on the FM in undoped, as well as V and Cr-doped TiO2, and the studies emphasis on the the room-temperature ferromagnetism (RTFM) and its origin.Anatase and rutile TiO2?δnanoparticles are synthesized by the sol-gel method, and anatase TiO2 ?δfilms are prepared by radio frequency (RF) magnetron sputtering. The M-H and ZFC/FC curves indicate that reduced TiO2?δnanoparticles exhibit RTFM that becomes stronger with intensifying reducing atmosphere under the same annealing time, or prolonging annealing time under the same reducing atmosphere. Analysis of tight scan Ti 2p XPS indicates that Ti ions are all in the Ti4+ state, and that Ti3+ or Ti2+ ions don't exist in all samples. Analysis of XRD, tight scan O 1s XPS spectra, and UV-vis absorption spectra of TiO2?δnanoparticles (and UV-vis transmission spectra of TiO2?δfilms) indicates that the concentration of oxygen vacancies increases with intensifying reducing atmosphere under the same annealing time, or prolonging annealing time under the same reducing atmosphere. Based on the above discussion, we think that the FM in undoped TiO2?δnanoparticles stems from oxygen vacancies. The mechanism of FM can't be the exchange interaction between Ti2+ and Ti3+ ions but the molecular orbital model proposed by Coey et al.Rutile Ti0.94V0.06O2 powders are synthesized by the sol-gel and standard solid-state reaction methods, and rutile Ti0.94V0.06O2 powders with additional Nb, Al or Cu dopants are synthesized by the standard solid-state reaction method. Magnetic measurements indicate that the rutile Ti0.94V0.06O2 powders unannealed in reducing atmosphere exhibit remarkable RTFM. However, there isn't antiferromagnetic coupling between V ions. FM increases slightly after being annealed in Ar atmosphere. The FM in rutile Ti0.94V0.06O2 powders may mainly originate from V ions doped in matrix, and oxygen vacancies also contribute to the FM. The FM largely decreases, and the conductivity increases in rutile Ti0.94V0.06O2 powders after additional electronic or hole doping (Nb5+ or Al3+ doping), which is consistent with a theoretical prediction that the V-doped TiO2 has the ferromagnetic ground state if there are no other carrier native defects or additional dopants. Though both FM and resistivity decrease after additional Cu doping in rutile Ti0.94V0.06O2 powders, additional 1%Cu doping has moderate RTFM and conductivity.Cr-doped TiO2 powders are synthesized by the sol-gel method, and exhibit strong paramagnetism (PM) and weak FM. PM becomes strong with increasing the concentration of Cr dopants, and FM behaves as an increase in the saturation magnetization Ms and a decrease in the coercive force Hc with increasing the concentration of Cr dopants or after being annealed in Ar atmosphere. Analysis on tight scan Cr 2p XPS spectra indicates that Cr exists in the +3 state in Cr-doped TiO2 powders. Analysis of EPR spectra indicates that two kinds of Cr3+ ions, isolations and clusters, exist in Cr-doped TiO2 powders. The PM and FM in Cr-doped TiO2 powders originate from Cr3+ ions and structural defects introduced by doping Cr, respectively.