Study Of The Magnetic Behavior And Microstructure Of Nano Oxides

Dilute Magnetic Semiconductor is an important material for the production of spintronic and related devices since it manipulates both spin and charge of the carriers. The advantage of dilute magnetic semiconductor material based device lies in the high speed data processing, non-volatile semiconductor performance, low energy consumption, etc. Preparation of room temperature ferromagnetic semiconductor has become one of the hot test topics in the field of semiconductor spintronics, and provides the realistic foundation for the realization of the alternatives to traditional semiconductor device. But until now, the origin of ferromagnetism in the material is still an unsolved problem. Especially in recent years, ferromagnetism is also obseved in many undoped and nonmagnetic doping semiconductors and oxides. This further increases the complexity of the origin of magnetism. Understanding the origin of room temperature ferromagnetism has vital significance for the preparation of magnetic semiconductors and its application in spintronics, and is important for the further realization of controllable ferromagnetism.This paper first introduced the research progress of magnetic semiconductors and oxides, and summarized the different aspects of the origin of the magnetism. By careful designing the experiment schedule, we selected a series of typical nano oxide materials, and studied the origin of the magnetism of these undoped materials. Positron annihilation measurements were performed to investigate the microstructues of these nano oxides. The interfacial defects in these oxides were studied in more details. By combining the change of magnetic behavior and defect properties during high temperature annealing, we can find a direct link between magnetic properties and defects. The main research contents in this thesis are as follows:1. Undoped MgO nanocrystals were compressed into pellects, and then annealed in air for2hours from100to1400?. XRD measurements give the information of the crystal quality of nanomaterials. It was found that the crystallinity of material was improved after high temperature annealing, and the grain size increases gradually. The interfacial defects were studied by positron annihilation lifetime and Doppler Broadening measurements. There are a lot of monovacancies and vacancy clusters in the interface area observed by positron lifetime measurements, and there is also a small amount of larger pores which exit in the intergranular region. After high temperature annealing, these pores can maintain stable, while the monovacancies and vacancy clusters are gradually removed with the increase of annealing temperature. After1400?high temperature annealing, the interfacial defects were basically eliminated. Doppler broadening measurement also shows the same defect recovery process, and the correlation between the Doppler broadening S and W parameters (S versus W curves) shows that defect style has no change in the process of annealing. Magnetic measurement results show that the undoped MgO nanocrystals exhibits room temperature ferromagnetism, after1000?annealing the magnetism is weakened, but hysteresis loop can still be seen. After1400?annealing the magnetism disappears. This is quite consistent with the recovery process of the interfacial defects. It is determined that the interface defects have a connection with the magnetism. However, the MgO nanocrystals grain size also increases at the same time in the process of annealing, so we still cannot rule out the impact of the grain size effect on the magnetism. In another sample, the CeO2nanocrystals, we also got similar results. XRD measurement shows that the main diffraction peak was sharper with the increase of annealing temperature. The calculated grain size increases with annealing temperature. Positron annihilation measurements show that the interfacial defects gradually eliminate in the process of annealing, which also coincides with the variation of the saturated magnetic moment vursus annealing temperature. It is concluded that the ferromagnetism in CeO2nanocrystals is closely related with the interfacial defects.2. High purity ZrO2nanocrystals were also annealed at different temperatures for2hours. XRD characterization reveals that ZrO2nanograin grows slowly in the annealing process. There is nearly no grain growth was observed after annealing below1000?. This is also confirmed by TEM observation. Positron annihilation measurements display that the interfacial defects begin to recover above500"C and basically are eliminated after1200?annealing. Once again we found that the change of ferromagnetism has good correspondance with the interfacial vacancy defects in ZrO2nanocrystals. The ferromagnetism is evident in the samples after annealing at100"C and500?. After annealing at700?? the magnetism becomes significantly weaker, and at1000?the magnetism disappears. Comparing with the previous research results, after excluding the influence of grain size, we believe that the ferromag netism is directly related to the interfacial defects. In addition, we also observed the consistency of the room temperature ferromagnetism and the defect recovery process in NiO nanocrystals. After1000?annealing, the interfacial defects and magnetic disappear at the same time. XRD measurement shows that NiO grain size increases in the process of annealing. After1000?annealing the grain size increases from30nm to60nm. However, in other NiO oxides with partice size of400nm, before high temperature treatment, we still observed the room temperature ferromagnetism, which shows that the controling factor of the magnetism is mainly the interfacial defects, rather than the grain size effects.3. We also studied the Co doped ZnO nanocrystals with different proportion of Co. They were prepared by solid-stae reaction. Positron annihilation measurements reveal that the interfacial defects are eliminated after annealing above1000?. The magnetic measurements show room temperature ferromagnetism in the low doped ZnO even after after1000?annealing. While for the ZnO with doping content higher than2at.%, ferromagnetism is not observed. XRD measurement shows new phase of CoO in Co doped ZnO sample with high content after annealing at1000?, which suggests that magnetism in Co doped ZnO is no longer related with defects, probably is due to Co doping effect. When the Co content is high, due to the limitation of Co solubility in ZnO, some Co exists in the form of CoO, which leads to the disappearance of the ferromagnetism.