Synthesis And Ferromagnetism Properties Of Diluted Magnetic Oxides

Dilute magnetic oxides have been in the forefront of applied research for various fields, which include nanometerscale optoelectronics, electronic devices, and bio-technology, and so on. In this PhD thesis, we mainly study the topics on origion of room temperature ferromagnetism and modulating the ferromagnetism by using the antiferromagnetic materials (CoO,Co3O4) and different phase Al2O3nanoparticles. We mainly study the effects of vacuum treating, grain size, phases to the room tem-perature ferromagnetism. The major finding and innovations are shown as follow:1. Single-crystalline CoO nanospheres with the different sizes distribution were prepared by a solvothermal method. Magnetic measurements indicate that the vacuum-annealed samples show room temperature ferromagnetism after being post-annealed in vacuum atmosphere. The saturation magnetization of all post-annealed samples monotonically increases as the decrease of nanospheres diameter. The formed oxygen vacancies at the surface of the CoO nanospheres during the vacuum-annealed process account for the observed ferromagnetism.2. Porous hierarchical nanostructures of Co3O4freely standing on Al substrates were controllably and exclusively synthesized by hydrothermal method combined with subsequent calcination. Three typical kinds of hierarchical morphologies, i.e., free-standing nanowalls, nanowires-nanowalls, and nanosheets-nano-wires-nanowalls, were obtained. It has been found that the reaction times play very important roles in determining the final structure of the samples. The mag-netic measurement reveals that as-prepared samples show PM, while va-cuum-annealed samples show FM. The origin of the FM in Co3O4may be in-duced from oxygen vacancies.3. The octahedral CoO which synthesis by a solvothermal method heat treatment at different times, then prepared the CoO/Co3O4core-shell structure, in air or gas mixture of Ar and O2, respectively. The results showed that the core-shell struc- ture was prepared in the air to show the paramagnetic, while the core-shell struc-ture was prepared in Ar and O2gas mixture shows room temperature ferromag-netism, and the saturation magnetization first increases and then decreases with increasing heat treatment time. The origin of ferromagnetism of the CoO/Co3O4core-shell structure is the oxygen defects. We also observed the exchange bias phenomenon in the core-shell structure.4. Amorphous and crystalline Al2O3nanoparticles were synthesized by sol-gel me-thod with post-annealing at different temperatures. Magnetism measurements have indicated that all Al2O3nanoparticles exhibit intrinsic room temperature ferromagnetism, and the saturation magnetism of the samples increases after va-cuum annealing, whereas bulk Al2O3presents paramagnetism. The origin of the ferromagnetism in Al2O3nanoparticles could be attributed to the singly charged oxygen vacancies (F+centers). The variation of the oxygen vacancies relative area and the number of free electron is consistent with the change of saturation magnetization for the samples. Combining with these results, a direct correlation of ferromagnetism with F+centers exchange mechanism is established.