Study Of The Geometric And Electronic Structures Of Oxide-diluted Magnetic Semiconductors

The study of diluted magnetic semiconductors is very important in spintronics. At present there has been much focus on the high-Tc transition-metal-doped oxides and nitrides. But the origin of room-temperature ferromagnetism has been debated extensively. To explore the various microstructures related to magnetic property is the key to this problem.In this thesis we has briefly reviewed the research history and present status of the diluted magnetic semiconductors and diluted magnetic oxides; The Atomistic Simulation Method, Density Functional Theory (DFT), Discrete Variational Method (DVM) and ADF are introduced in detail.Firstly we utilize the atomistic simulation to study three kinds of typical defect structures in Co-doped TiO2 and Fe-doped SnO2, including their segregation in the two-dimensional surfaces. Finally the electronic and magnetic properties of diluted magnetic oxides nanocrystallites are investigated from first-principles calculations based on DFT.Our calculated results show that three kinds of defect clusters [Co"TiVo¨]×, [ Co'TiCo'TiCo¨i]×and [Co'TiCo'TiVo¨]×mostly favor the non-uniform distributions in Co-doped TiO2 anatase and rutile, which agrees with Co accumulation in the experiments. And the oxygen vacancy prefers to stay near to the substitutional Co ions, in good agreement with the previous first-principles study.As to TiO2 clean surfaces, we find that the most stable surfaces are (011) one in anatase and (110) one in rutile. The atomic relaxations of the rutile (110) surface are in excellent agreement with experimental measurements. Three types of Co defect clusters have different segregation energy in the different surfaces. [ Co'TiCo'TiCo¨i]×has very large segregation energies of 1.77eV, 3.23eV in the anatase (001) and (100) surfaces. The segregation energies of [Co"TiVo¨]×and [Co'TiCo'TiVo¨]×in the anatase (001) surface are 0.49eV and 0.33eV respectively. However, in the rutile (110) surface the segregation energy of [Co"TiVo¨]×is negligible, which fits the experimental observation of Park et al.For the Fe-doped SnO2 systems, the [Fe'SnFe'SnVo¨]×cluster likes to take the non-uniform distributions of (4×4×1) and (4×2×1), which are the origins of the isolated paramagnetic centers. The relatively uniform distributions of (2×2×2), (2×2×4) and (3×3×3) tend to form the magnetic-ordering structures. It can explain the coexistence phenomenon of the Fe3+ paramagnetic (76%) and ferromagnetic (24%) structures.The rutile SnO2 (110) surface is most stable. The [Fe"SnVo¨]×cluster is easy to segregate to the (101) and (110) surface, [Fe'SnFe'SnVo¨]×prefers to be in the bulk instead.Finally we present a density-functional investigation of the structure and magnetic property of TM-doped TiO2 anatase nanocrystallites. The total magnetic moments of the nanoclusters depend on the stoichiometry of oxygen atom. The non-stoichiometric TM:Ti8O38 (TM=Fe, Co, Ni) has much smaller magnetic moments than the stoichiometric TM:Ti8O18.