Microstructure And Ferromagnetism Of Doped SnO₂ Diluted Magnetic Materials

Transition-metal doped SnO₂ is considered as an important candidate material in the field of diluted magnetic semiconductor due to its excellent physical and chemical properties. Particularly, the discoveries of a giant magnetic moment and high Curie temperature in Co-doped SnO₂ films have inspired tremendous experimental and theoretical investigations. Up to date, the ferromagnetic origin and related physical mechanism in SnO₂-based diluted magnetic material have not been fully understood. In this dissertation, Co-doped SnO₂ films and Co/Zn-doped SnO₂ nanorods are prepared by magnetron sputtering and solvethermal methods, respectively. Through changing experimental conditions, we systematically investigate the microstructure and magnetic properties of the materials. These researches focus on the local structure of the doping element in SnO₂ and discuss the correlations between magnetic properties and the microstructure. Based on the above studies, the ferromagnetic origin as well as the related mechanism is further explained.To clear the ferromagnetic origin, X-ray abosorption fine structure characterization with first-principle calculations are used to investigate the local environment of doped elements in SnO₂. The results show that the doped elements have successfully substituted for Sn elements in SnO₂ lattice without forming secondary phases, which confirms the intrinsic nature of ferromagnetism in doped SnO₂ diluted magnetic materials.Room-temperature ferromagnetism is achieved in insulating Co-SnO₂ diluted magnetic films. The investigations on insulating films efficiently separate the influences of structural defects and free carriers on the magnetism of the materials, which is helpful for clarifying the ferromagnetic mechanism. Sputtering targets, deposition parameters and buffer layer thickness are changed to adjust the microstructure and magnetic properties of Co-SnO₂ films. The experimental results demonstrate the mediation effect of structural defects on the long-term ferromagnetic ordering. Furthermore, various post-annealing processes are used to vary the concentrations of oxygen vacancies and Sn interstitials in Co-SnO₂ films, which confims the crucial roles of donor defects in tuning magnetic properties. Additionally, nitrogen is codoped with Co to create acceptor defects in (Co,N)-codoped SnO₂ films. The dependence of ferromagnetism on nitrogen concentration implies that the acceptor defect is another key factor in mediating magnetism in doped SnO₂ materials. According to the above analyses, the ferromagnetic mechanism in SnO₂-based diluted magnetic materials is reasonabley explained using bound magnetic polaron (BMP) mechanism based on the presence of donor and acceptor defects.To further verify the BMP model in different dimensional SnO₂-based diluted magnetic materials, we have successfully synthesized room–temperature ferromagnetic Co-doped SnO₂ nanorods via solvethermal methods. Magnetic property measurements indicate that the long-range ferromagnetic ordering mediated by structural defects coexists with the antiferromagnetic coupling between adjacent Co ions in Co-doped SnO₂ nanorods. Such result is consistent with the description of BMP model. Moreover, room-temperature ferromagnetism is observed in nonmagnetic Zn element doped SnO₂ nanorods. Combining with microstructure analyses, it is found that the ferromagnetism of the material originates from the Zn doping-induced Sn vacancies. The saturated magnetization is sensitive to the concentration of Sn vacancies, and can be well tunned by modifying the content of Zn dopants. This study not only offers experimental support to the first-principle calculations of d0 mechanism, but also opens a new way to fabricate room-temperature SnO₂-based diluted magnetic materials.