The precise control of ferromagnetic domain wall formation opens up exciting avenues of research and potential application in spintronics ? the manipulation of charge carriers via their spin properties. Recent experiments on Cobalt-Platinum multilayers containing artificially created domains provide the motivation for this work. In this thesis the electronic structure of CoPt multilayers are calculated by an ab initio multiple scattering method, and attempts are made at replicating the systems used in experiments, including lattice relaxations and the effects of substitutional alloying. The magnetic reversal process in Pt/Co/Pt trilayer systems is studied in the framework of micromagnetics, in which effects such as exchange, magnetocrystalline anisotropy and the demagnetising field are treated phenomenologically. The results are compared to recent experiments and the switching mechanism can be understood in terms of domain growth and domain wall nucleation. A ballistic transport framework is outlined in terms of a tight binding Green function method. The domain wall is modelled as a change in the local spin reference frame. The method is applied to Cobalt Platinum trilayers, and it is found that the resulting domain wall resistances are sensitive to the details of the Fermi energy bands. Subsequently, the angular dependence of domain wall resistivity in Pt/Co/Pt systems is studied by a model based on the anisotropic resistivity tensor that is expected in a domain wall. The results are used to extract resistivity parameters from experimental data, and a positive domain wall resistivity is identified, whilst resulting arguments provide supporting evidence for the Levy-Zhang theory of domain wall resistance. Finally, recent experiments on the dilute magnetic semiconductor (Ga,Mn)As have provided evidence for a negative intrinsic domain wall resistance in this material. By applying a similar model to that used on the CoPt systems, it is shown that the anomalous magnetoresistance effect can also provide a significant negative contribution to the measured resistance via induced eddy current at the domain wall boundary.

Liu, Yang = 軌道簡并性對強關聯系統的磁性的研究 / 劉陽. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2009. / Includes bibliographical references (p. 49-51). / Abstracts in English and Chinese. / Liu, Yang = Gui dao jian bing xing dui qiang guan lian xi tong de ci xing de yan jiu / Liu Yang. / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Phenomena of Magnetism --- p.1 / Chapter 1.1.1 --- Magnetic susceptibility and classification of magnetic phases --- p.2 / Chapter 1.1.2 --- Paramagnetism and Diamagnetism --- p.3 / Chapter 1.1.3 --- "Ferromagnetism, antiferromagnetism, and ferrimagnetism" --- p.3 / Chapter 1.2 --- Models describing the magnetic system --- p.5 / Chapter 1.2.1 --- The quantum mechanical origin of the interaction between magnetic moments --- p.5 / Chapter 1.2.2 --- Heisenberg model --- p.6 / Chapter 1.2.3 --- Hubbard model --- p.7 / Chapter 1.2.4 --- Background of ferromagnetism in Hubbard model --- p.8 / Chapter 1.3 --- Two-orbital Hubbard model --- p.9 / Chapter 2 --- Low energy effective theory and perturbation method --- p.14 / Chapter 2.1 --- Perturbation by diagrams --- p.14 / Chapter 2.2 --- Illustration of the basic idea: from Hubbard model to Heisenberg model --- p.15 / Chapter 2.2.1 --- Low energy space and intermediate basis --- p.16 / Chapter 2.2.2 --- Diagrams presenting the effective coupling --- p.17 / Chapter 2.2.3 --- Operator form --- p.18 / Chapter 2.3 --- Effective Hamiltonian of quarter filled two-orbital Hubbard model in the large-U limit --- p.19 / Chapter 2.3.1 --- Low energy space and intermediate basis --- p.19 / Chapter 2.3.2 --- Diagrams presenting the effective coupling --- p.20 / Chapter 2.3.3 --- Operator form --- p.23 / Chapter 2.4 --- Two-site problem of two-orbital Hubbard model --- p.25 / Chapter 2.4.1 --- Exact solution --- p.25 / Chapter 3 --- ED method and Numerical results --- p.29 / Chapter 3.1 --- ED method --- p.29 / Chapter 3.1.1 --- Construction of Hilbert space of the many-body model --- p.30 / Chapter 3.1.2 --- Lanczos iterative algorithm --- p.30 / Chapter 3.2 --- Spin structure factor --- p.31 / Chapter 3.2.1 --- Spin structure factor changes with various filling --- p.32 / Chapter 3.2.2 --- Spin structure factor changes with various J --- p.34 / Chapter 3.2.3 --- Spin structure factor changes with various U' --- p.34 / Chapter 3.3 --- Ground state spin magnetization --- p.36 / Chapter 4 --- Quantum phase transition in 1D Half-filled Asymmetric Hubbard model --- p.38 / Chapter 4.1 --- Introduction --- p.38 / Chapter 4.2 --- Spin stiffness --- p.41 / Chapter 4.3 --- Numerical result --- p.43 / Chapter 4.4 --- Summary --- p.46 / Chapter 5 --- Summary --- p.48 / Bibliography --- p.49 / Chapter A --- Conditions of spin and orbital rotational invariant --- p.52 / Chapter A.l --- condition of spin rotational invariant --- p.52 / Chapter A.2 --- conditions of orbital rotational invariant --- p.54 / Chapter B --- Detailed calculation of effective Hamiltonian of two-orbital Hubbard model --- p.57 / Chapter C --- Perturbation of the quartic equation --- p.89

Graduation date: 2003

Self-organized GaN nanostructures have been accomplished with lattice-(mis)matched using MOCVD. A lattice mismatched system (i.e. GaN nanostructure/ AlN) was utilized with S-K mode mechanism, whereas, metallic droplet method (i.e. Vapor-Liquid-Solid method) was employed in the lattice matched system (i.e. GaN nanostructure / AlGaN). The nanostructure size is adjustable by changing growth parameters (height: 2 ~ 15nm and diameter: 10 ~ 100nm). It has been found that the photon emission energy is tunable relative to the nanostructure size, and smaller nanostructures have larger photon energy. However, a numerical modeling was performed to investigate the relationship between quantum confinement (and/or piezoelectric polarization) and the dot size. For dot height < 4.1nm, the confinement effect is larger than the piezoelectric effect, otherwise the piezoelectric effect is more dominant. In addition, GaN nanostructures grown on Al0.15Ga0.85N have smaller lattice mismatch (less than 0.5%) than the GaN nanostructures grown on AlN. Therefore, the quantum confinement in a GaN/Al0.15Ga0.85N system is more dominant in determining photon emission energy than in a GaN/AlN system. The nanostructure advantages of quantum confinement and high thermal stability have been studied for the achievement of room temperature ferromagnetism using TM (transition metal; Mn or Fe). The transition metal (Mn or Fe) enhances nucleation of islands, resulting in size and density improvements. The magnetization measurements revealed magnetic properties of ferromagnetic nanostructure. Especially, room temperature ferromagnetism was observed in GaFeN nanostructures, which can contribute to ferromagnetic semiconductors operating above room temperature.

Thesis (Ph. D.)--University of Texas at Austin, 2003. / Supervisor: Linda E. Reichl. Vita. Includes bibliographical references. Also available from UMI.

Not available / text

No description available.

No description available.

No description available.

No description available.