Preparation And Magnetism Of TiO₂ Diluted Magnetic Semiconductors Nanoparticles

In recent years, dilute ferromagnetic oxides and nitrides, which are wide-gap semiconductors with Curie temperatures Tc well in excess of room temperature, have become the most interesting new magnetic materials and pop topic in the fields of spin-electronic semiconductors, material physics, condensed matter physics and so on. Because of this kind of materials'potential application for magneto-optic and spin-electronic devices, they will still be the most attractive and important researching work in the next few years. Our research work concerning about transitional metal ions doped TiO₂ emerge as the time require.The nanoparticles samples of transitional metal ions doped TiO₂ were synthesized by sol-gel method. The structure and magnetic properties of our samples were analyzed in detail by using some related equipments. The origin of magnetic properties was discussed. The main points as follows:Firstly, the preparation process was widely explored, and then we successfully synthesized V and Ni ions doped TiO₂ nanoparticles by sol-gel method, respectively. The structure properties, particle size of Ti_1-xV_xO₂ (x=0～0.16) and Ti_1-xNi_xO₂ (x=0.01～0.08) samples, doped with different concentration and sintered at different temperatures, were characterized through X-ray diffraction (XRD), transmission electron microscopy (TEM), scan electron microscopy (SEM). For the Ti_1-xV_xO₂ (x=0～0.16) system, when x≦0.1, the samples are pure structure of TiO₂, while secondary phase was detected as V₂O₅ in x=0.16 sample. The particle sizes were 20-50nm approximately. For Ti_1-xNi_xO₂ (x=0.01～0.08) system, we didn't find the second phase in the doped concentration range. The particle sizes were about 20nm.Secondly, the magnetic properties of Ti_1-xV_xO₂ (x=0～0.16) system were characterized by physical property measurement system (PPMS, Quantum Design). The magnetization M dependence of magnetic field H with doped different concentrations, M dependence of H and temperature T respectively with sintered in different temperatures of the Ti_0.96V_0.04O₂ samples were presented. The saturated magnetization increased as the doped concentration increasing. There is a competence between ferromagnetic and antiferromagnetic coupling in the samples we fabricated. The magnetic properties of Ti_0.92V_0.08O₂ samples sintered in air and argon distinctly were compared. The sample sintered in argon with high temperature seems in favor of forming ferromagnetic order.Thirdly, the magnetic properties of Ti_1-xNi_xO₂ (x=0.01～0.08) system were characterized by PPMS. The magnetization M dependence of magnetic field H as samples doped with different concentrations was obtained. Especially, the magnetic properties of Ti_0.99Ni_0.01O₂ samples sintered at different temperatures were analyzed in detail with testing M dependence H and T respectively. The saturated magnetization reduced as the doped concentration increasing. And also the magnetic properties of the Ti_0.99Ni_0.01O₂ samples sintered at the same temperature in air and argon distinctly were compared. The sample sintered in air with lower temperature seems in favor of forming ferromagnetic order.Lastly, based on the structure analysis and magnetic properties testing results, together with preparation process, we discussed the origin of magnetic behaviors in TiO₂ doped with transitional metal ions. The XRD data of our samples showed that there was no secondary phase detected, so we ascribed the ferromagnetic order to be intrinsic. But it is inconsistent with many different explanations in the origin of the sample's magnetism. Based on our studies, a possible explanation for the magnetization behaviors of our sample's magnetism is presented according the bound magnetic polaron theory. In our samples, the bound magnetic polarons formed by magnetic ions and oxygen vacancies around them. The ferromagnetic interactions will occur in overlapped bound magnetic polarons. The magnetic order in our samples was ascribed to ferromagnetic interaction among the bound magnetic polarons. When the sintered temperature or doped concentration varied, the magnetic order would be enhanced or reduced because of the competence between ferromagnetic among polarons and antiferromagnetic interactions in the magnetic ions, or no ferromagnetic interaction among polarons due to be isolated.