First-principles Investigation On Oxide-based Dilute Magnetic Semiconductors Doped With Nonmagnetic Elements

Dilute magnetic semiconductors with half-metallic ferromagnetism is the key martial for application in spintronic devices. Most previous attention to DMS has been focused on magnetic cation (3d or4f elements) doing conventional semiconductors and wide gap oxides. People have made a great effort to study the photocatalysis and photoelectron, but to neglect the magnetism induced by doping negative ion in host material. Therefore, in the field of dilute magnetic semiconductors, it is necessary to probe into the possibility of preparing ferromagnetic semiconductors by doing non-magnetic elements in wide gap oxides semiconductors. In this paper, we investigate the electronic structure and magnetic properties in oxides doped with non-magnetic elements, and explore the origin of magnetism in these systems. This paper is organized as follows:In chapter1, we introduce conceptions of spintronics and dilute magnetic semiconductor, depict the development of the DMS and its essential properties, expatiate on various mechanism of the magnetic coupling, and summarize the uptodate progression in SnO2-, CeO2-and Ga2O3-based oxide DMSs.In chapter2, we introduce computing method based on first-principles density functional theory calculations as implemented in the Vienna ab initio Simulation Package (VASP).In chapter3, the electronic and magnetic properties in SnO2doped with non-magnetic elements N and Ag are investigated. Calculations predict that the spin-polarized state, with a magnetic moment of about1.0μB per Nitrogen-dopant, is more favorable in energy (about478meV) than the non-spin polarized state. The magnetic moment mainly arises from p orbital of Nitrogen which substitutes the Oxygen atom, with a little contribution from the Oxygen atoms surrounding Nitrogen atom. Furthermore, the coupling between different Nitrogen is discussed, and the results show that Nitrogen impurities not only couple antiferromagnetically but also ferromagnetically. As for Ag-doped SnO2, calculations demonstrate that Ag-doping introduces spin-polarization in SnO2and gives rise to a local magnetic moment of1.0μB per substitutional silver ion. The hole-mediated ferromagnetic coupling between two Ag ions in this material is possibly ascribed to a p-d hopping interaction between O and Ag ion. Oxygen vacancy (V0) plays an important role in determining the magnetic properties of Ag-doped SnO2system. The Vo enhances stability of spin-polarized sate for the case of single Ag doped system, and imposes intricate effect on a pair of Ag doped configurations. The ferromagnetic coupling between two Ag ions is possibly reinforced if Vo is sufficiently far away from them. The result indicates that Ag-doped SnO2is a promising candidate for applications in spintronic devices.In chapter4, the electronic structures and magnetic properties in Carbon-doped CeO2have been investigated. First, the formation energies of N-and C-doped CeO2are estimated. Under the Ce-rich condition, N is preferentially incorporated into CeO2and substitutes oxygen, while N cannot easily doped into the CeO2matrix. The substituental N and C contribute a magnetic moment of1.00and2.00μB per dopant, respectively, which mainly stems from Hund’s rule coupling. The magnetic moment mainly arises from N or C, with a little contribution from the Oxygen atoms surrounding it. For the case of N-doped CeO2, the half-metal ferromagnetic ground state is predicted by GGA, LSDA, GGA+U and LSDA+U. The predicted ferromagnetism by LSDA attributes to the hole-mediated long-range double exchange mechanism. To establish the collective ferromagnetism, the minimum percolation concentration must be larger than4.6%. Half-metallic characteristics in C-doped CeO2can be attributed to the collective effects of the p-p, p-d, and p-f hybridizations between C and neighboring O or Ce atoms. Half-metallic ferromagnetism in N and C-doped CeO2make it possible to be an ideal material for spintronic devices.In chapter5, the electronic and magnetic properties of nitrogen-doped monoclinic β-phase gallium oxide are investigated. The substituental N prefers to occupy a high symmetric O site in β-Ga2O3crystal. Calculations predict that the spin-polarized state is stable with a magnetic moment of about1.0μB per nitrogen-dopant. Similar to p-d hybridization, p-p exchange mechanism stabilize the short-range ferromagnetism. Results also reveal experimentally observed red-shift should be N-2p gap states to band transition. Calculations predict that Ni-doped β-Ga2O3is predicted to be robust half-metallic ferromagnet. When one Ni atom occupies the octahedral or tetrahedral site, each Ni-impurity give rise to1.0or3.0μB. The structure in which one Ni substitutes the octahedral site is most stable. The double exchange mechanism stablizes the ferromagnetic ground state with an ordering temperature above room-temperature.