| Diluted magnetic semiconductors (DMSs) are new functional materials formed by doping a few percentages of magnetic ions in nonmagnetic semiconductors, which has wide prospect in the field of information in future society. As the application material carriers of Semiconductor Spintronics, DMS is expected to manipulate the combination of the free degree of charge and spin at the same time.Zinc oxide (ZnO) is considered as a well kind of candidate for the best host materials, which is direct and wide bandgap semiconductor and has excellent characteristics of electric conduction, optics, photoluminescence, pressure sensitivity, and gas-sensitivity. And some transition metal ions such as Co2+and Mn2+have very high solubility in ZnO. However, ZnO-based DMSs have the following problems, which restricts their application:(1) Transition metal ions doped ZnO are easy to produce impurity phase, such as ions clusters and oxides of transition metal;(2) The Curie temperature of ZnO-based DMSs is often lower than room temperature;(3) The recurrence rate of fabrication of ZnO-based DMSs which have intrinsic room temperature ferromagnetism is very low;(4) There is no unification theory to explain the intrinsic ferromagnetism in DMSs. The problems, especially the last one, embarrass the application of ZnO-based DMSs.In this work, the author synthesized different series nano-crystal films and powders with Co, Mn, Fe and Ni doping ZnO by a sol-gel method. The structure, phase composition, morphology and magnetic property of the samples were particular investigated by X-ray diffraction spectrometry, transmission electron microscopy, ultraviolet-visible spectrometry. Fourier transform infrared spectrometry, scanning electron microscopy and vibrating sample magnetometer. Based on the result of experiment, the author used an improved Ising model and Monte Carlo technique to simulate the mechanism of ZnO-based DMSs.The experimental research indicates some important evidences as follows. Firstly, the indirect ferromagnetic exchange of Zn1-xCoxO samples with the intrinsic ferromagnetism at room temperature is mediated by n-type bondage carriers that are trapped by oxygen vacancy and by delocalized donor impurity. The influence of the donor impurity on the ferromagnetism may give prominence while both the donor’s concentration and the crystal integrality all arrive to a high level. Secondly, the samples of Zn1-xMnxO(x=0.005to0.05) nano-crystal powder sintered at500℃to750℃have intrinsic ferromagnetism at room temperature. But the ferromagnetism is much weaker than that of Zn1-xCoxO at the same condition. The samples’ferromagnetism increases markedly when the concentration of accepter Na reaches at1Na/Mn to1.4Na/Mn. It proves that p-type bondage carriers mediate the intrinsic ferromagnetic exchange interaction. Thirdly, the samples of Zn1-xFexO (x=0.01to0.05) nano-crystal powder sintered at500℃to1000℃have intrinsic ferromagnetism at room temperature, which is similar to that of Zn1-xMnxO. The medium of ferromagnetic exchange interaction is a small quantity of uncompensated bound holes. Fourthly, as the solubility of Ni is very low in ZnO, the samples of Zn1-xNixO (x=0.005to0.015) nano-crystal powder sintered at500℃have intrinsic ferromagnetism at room temperature. The ferromagnetism is regulated by bound electrons. The magnetism parameter of all samples is out of proportion to the concentration of magnetic ions, which indicates that there is antiferromagnetic exchange interaction induced by O2+in all samples, and all of the films have no ferromagnetism at room temperature, which shows that the surface and interface hinder the forming of magnetic order.The dissertation introduces Ising model to study the magnetic mechanism of diluted magnetic semiconductors for the first time. The model has been improved to combine with experiment result that contains four interactions as follows:(1) the direct exchange interaction. If there is no any cluster, the term equals zero;(2) the super-exchange interaction induced by O2+;(3) the indirect exchange interaction mediated by carries, in which the long range indirect exchange interaction of RKKY model and the cant spin of Zener model are cited;(4) the anisotropy crystal field effect accounting the effect of the concentrations and distributions of magnetic ions and intrinsic defects. The author sets up adjustable parameters for above-mentioned interactions, discusses weights of every parameter with Monte Carlo simulation technique, and confirms parameters matching with experimental results. Finally, the author studies the magnetic properties of Zn1-xMnxO nano-films with the improved Ising model, and gets accordant results with experiments. |
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