Structure And Magnetic Properties Of （Fe, Cu） Codoped TiO₂ Diluted Magnetic Semiconductors

Due to the magnetic and semiconductor properties of diluted magnetic semiconductors, they have very important research and application value in various fields of spintronics and microelectronics. However, early their Curie temperature was generally lower than room temperature, so that they could not be applied to room temperature environment. With the further development of the theory and experiment of diluted magnetic semiconductors, many researches on room-temperature ferromagnetism of diluted magnetic semiconductors have been reported in the last decade. But there are still a lot of controversy on their magnetic origin and generation mechanism. Hence, the research on dilute magnetic semiconductors is of great significance in the theory, experiment and future practical application.In this work, (Fe, Cu)-codoped TiO2 powders were prepared by sol gel method. XRD analyses show that the synthesized powders belong to tetragonal anatase TiO2 structure. The diffraction peaks of metal Fe, Cu and their oxides were not found in the resolution range of the XRD instrument. The results of SEM show that the samples composed by many small particles, particle size is mainly distributed between 20～ 40nm and most of the particles are similar to spherical, some other particles are irregular.57Fe Mossbauer spectra results indicate that the iron ions in the samples have substituted for Ti4+ ions in the form of Fe3+ into the TiO2 lattice. VSM measurements show that all (Fe, Cu) codoped TiO2 samples exhibit room temperature ferromagnetism. Compared with (0 at.% Cu,2 at.%Fe) doped TiO2, the saturation magnetization of (1 at.% Cu,2 at.% Fe) doped TiO2 is slightly increased, but the difference is not significant. And the saturation magnetization of (3 at.% Cu,2 at.% Fe) doped sample is stronger than their ferromagnetism. The coercivity of all the doped samples is close to about 210Oe. Based on the experimental results, we have analyzed the magnetic origin of the prepared samples, and discussed the magnetic generation mechanism of the system.In this work, (Fe. Cu)-codoped TiO2 powders were prepared by mechanical alloying. XRD patterns indicate that unmilled TiO2 is made up of a mixture of rutile and anatase. After high energy ball milling, TiO2 shows a rutile structure and the diffraction peaks of metal Fe, Cu and their oxides were not detected in the samples. 57Fe Mossbauer measurements also did not find the magnetic six line peaks of metallic iron and their oxides in the ball milling samples. XPS spectra further support the results of XRD and Mossbauer spectra, which suggest that Fe and Cu atoms have been doped into the TiO2 lattice. VSM shows that all milled samples exhibit room temperature ferromagnetism. From the results of XRD, MS and XPS, we can know the ferromagnetism is not from second magnetic phases (such as Fe or its oxides) but from the (Fe, Cu)-codoped TiO2 matrices. When doped with copper content≤3.076 at.%, the saturation magnetization of the milled samples increased with the increase concentration of copper. When doped with copper concentration increased to 5.538 at.%, the saturation magnetization of the sample was significantly reduced, which could due to the anti ferromagnetic interaction over ferromagnetic interaction, leading to a decrease of magnetization. After air annealing, the saturation magnetization of annealed samples was decreased, which indicated that oxygen vacancy defects have an important effect on the magnetic properties of the samples in the system. The oxygen vacancies in the TiO2 lattice were filled with oxygen atoms, so the oxygen vacancy concentration in the air annealed samples was decreased, which resulting in the reduction of the magnetization. In the (Fe, Cu)-codoped TiO2 system, the oxygen vacancies play an important role in the origin of ferromagnetism.