Synthesis And Magnetism Of Diluted Magnetic Semiconductors Zn_1-xCo_xO And Zn_1-xCo_xS

Diluted magnetic semiconductors (DMSs), which integrate the magnetism and semiconducting properties, are very significant from the view of both basic research and practical application. So far, research on diluted magnetic semiconductors is one of the hot topics in the world. In this thesis, transition metallic ions (Co²⁺) were introduced into ZnO and ZnS lattices by chemical co-precipitation method, the magnetic behaviors of Zn_1-xCo_xO (x = 0.01, 0.05, 0.12, 0.15, 0.42) and Zn_1-xCo_xS (x = 0.013, 0.021, 0.065, 0.068) samples and their ferromagnetism origins were also studied. The main content of the thesis is as follows:The research background and application perspectives of DMSs are introduced in Chapter 1. Some of basic properties of DMSs such as lattice structure, magnetic effect, origin of ferromagnetism are also reviewed. To our knowledge, few reports deal with magnetic properties of Co-doped DMS powder samples. Therefore, our study focus on the synthesis and magnetism of the Co-doped ZnO and Co-doped ZnS power samples.The synthesis process of the Zn_1-xCo_xO samples had been studied firstly. It shows that the aging time of the solution and pH value of the coprecipitation are two key factors to synthesize Zn_1-xCo_xO samples. Then a possible mechanism of the reaction is proposed based on coordination chemistry. The results indicate that ferromagnetic behavior at room temperature of Zn_1-xCo_xO samples only presents in low doping content samples, and unexpectedly, those doped samples in high doping content only show paramagnetism at room temperature.The similar phenomenon has been observed in the synthesized Zn_1-xCo_xS samples — only the samples in low doping content show ferromagnetic behavior at room temperature. Moreover, the magnetization of Zn_1-xCo_xS sample is much larger than that of Zn_1-xCo_xO samples with the same doping content.The XRD results indicate that all Zn_1-xCo_xO samples own Wurtzite crystalline structure in low doping content, and all Zn_1-xCo_xS samples are mixture of cubic and hexagonal crystalline. Moreover, those samples with low doping content showferromagnetic behavior at room temperature and no secondary magnetic phase has been found in our measurements. It is suggested that the ferromagnetism of the samples is the intrinsic property of the Co-doped DMSs but not comes from the secondary phase. However, since there are a lot of reports on synthesis and magnetic properties of Co-doped diluted magnetic semiconductors, these results are very different and even more some of them are contradictory, there exist various arguments on origin of the ferromagnetism of Co-doped DMSs. In our paper, a possible origin of the magnetization behaviors of the Zn_1-xCo_xO and Zn_1-xCo_xS samples has been proposed based on super-exchange coupling mechanism between neighboring magnetic metal ions. It can give a reasonable explanation for all of our experimental results.Up to date, most of the reported DMSs films show not only very low Curie temperatures but also much poor reproducibility. On the contrary, our Zn_1-xCo_xO and Zn_1-xCo_xS samples synthesized by the chemical coprecipitation method present room-temperature ferromagnetism as well as good reproducibility, which means their potential wider applications in the future.