[Truncated abstract] In this dissertation we present a theoretical study of electron transport through domain walls, with a particular focus on conductance properties, in order to understand various transport measurements that have been carried out recently on ferromagnetic nanowires. The starting point for our work is a ballistic treatment of transport through the domain wall. In this case conduction electrons are generally only weakly reflected by the domain wall, and the principal effect is a mixing of transmitted electron spins between up and down states. For small spin-splitting of conductance electrons the latter can be characterized by an appropriate adiabaticity parameter. We then incorporate the effect of spin-dependent scattering in the regions adjacent to the domain wall through a circuit model based on a generalization of the two-resistor theory of Valet and Fert. Within this model we find that the domain wall gives rise to an enhancement of resistance similar to the giant magnetoresistance effect found in ferromagnetic multilayer systems. The effect is largest in the limit of an abrupt wall, for which there is complete mistracking of spin, and decreases with increasing wall width due to the reduction of spin mistracking. For reasonable physical parameter values we find order-of-magnitude agreement with recent experiments. Going beyond the assumption of ballistic transport, we then consider the more realistic case of a domain wall subject to impurity scattering. A scattering matrix formalism is used to calculate conductance through a disordered region with either uniform magnetization or a domain wall. By combining either amplitudes or probabilities we are able to study both coherent and incoherent transport properties. The coherent case corresponds to elastic scattering by static defects, which is dominant at low temperatures, while the incoherent case provides a phenomenological description of the inelastic scattering present in real physical systems at room temperature. It is found that scattering from impurities increases the amount of spin mistracking of electrons travelling through a domain wall. This leads, in the incoherent case, to a reduction of conductance through the domain wall as compared to a uniformly magnetized region. In the coherent case, on the other hand, a reduction of weak localization and spin-reversing reflection amplitudes combine to give a positive contribution to domain wall conductance, which can lead to an overall enhancement of conductance due to the domain wall in the diffusive regime. A reduction of universal conductance fluctuations is found in a coherent disordered domain wall, which can be attributed to a decorrelation between spin-mixing and spin-conserving scattering amplitudes.

Thesis (Ph.D.) - James Cook University, 2002. / Typescript (photocopy). Includes bibliographical references.

Thesis (Ph. D.)--University of Canterbury, 2008. / Typescript (photocopy). Includes bibliographical references. Also available via the World Wide Web.

Thesis (Ph. D.)--University of Alabama at Birmingham, 2007. / Title from first page of PDF file (viewed June 23, 2008). Includes bibliographical references.

Thesis (M. Eng.), Memorial University of Newfoundland, 1998. / Bibliography: leaves 118-122.

Thesis (M.A)--Memorial University of Newfoundland, 1999. / Bibliography: leaves [199]-212.

Thesis (Ph.D.)--University of Alabama at Birmingham, 2010. / Title from PDF title page (viewed on June 28, 2010). Includes bibliographical references.

Thesis (Ph.D.)--Memorial University of Newfoundland, 2003. / Bibliography: leaves 215-232.

Current state-of-the-art plasma thrusters are limited in power density and thrust density by power losses to plasma-facing walls and electrodes. In the case of Hall effect thrusters, power deposition to the discharge channel walls and anode negatively impact the efficiency of the thruster and limit the attainable power density and thrust density. The current work aims to recreate thruster-relevant wall-interaction physics in a quiescent plasma and investigate them using electrostatic probes, in order to inform the development of the next generation of high-power-density / high-thrust-density propulsion devices. Thruster plasma-wall interactions are complicated by the occurrence of the plasma sheath, a thin boundary layer that forms between a plasma and its bounding wall where electrostatic forces dominate. Sheaths have been recognized since the seminal work of Langmuir in the early 1900’s, and the theory of sheaths has been greatly developed to the present day. The theories are scalable across a wide range of plasma parameters, but due to the difficulty of obtaining experimental measurements of plasma properties in the sheath region, there is little experimental data available to directly support the theoretical development. Sheaths are difficult to measure in situ in thrusters due to the small physical length scale of the sheath (order of micrometers in thruster plasmas) and the harsh plasma environment of the thruster. Any sufficiently small probe will melt, and available optical plasma diagnostics do not have the sensitivity and/or spatial resolution to resolve the sheath region. The goal of the current work is to experimentally characterize plasma sheaths xxvi in a low-density plasma that yields centimeter-thick sheath layers. By generating thick sheaths, spatially-resolved data can obtained using electrostatic probes. The investigation focuses on the effects of electron emission from the wall and several factors that influence it, including wall material, wall temperature, wall surface roughness and topology, as well as the scaling of sheaths from the low-density plasma environment towards thruster conditions. The effects of electron emission and wall material are found to agree with classical fluid and kinetic theory extended from literature. In conditions of very strong emission from the wall, evidence is found for a full transition in sheath polarities rather than a non-monotonic structure. Wall temperature is observed to have no effect on the sheath over boron nitride walls independent of outgassing on initial heat-up, for sub-thermionic temperatures. Wall roughness is observed to postpone the effects of electron emission to higher plasma temperatures, indicating that the rough wall impairs the wall’s overall capacity to emit electrons. Reductions in electron yield are not inconsistent with a diffuse-emission geometric trapping model. Collectively, the experimental data provide an improved grounding for thruster modeling and design.Current state-of-the-art plasma thrusters are limited in power density and thrust density by power losses to plasma-facing walls and electrodes. In the case of Hall effect thrusters, power deposition to the discharge channel walls and anode negatively impact the efficiency of the thruster and limit the attainable power density and thrust density. The current work aims to recreate thruster-relevant wall-interaction physics in a quiescent plasma and investigate them using electrostatic probes, in order to inform the development of the next generation of high-power-density / high-thrust-density propulsion devices. Thruster plasma-wall interactions are complicated by the occurrence of the plasma sheath, a thin boundary layer that forms between a plasma and its bounding wall where electrostatic forces dominate. Sheaths have been recognized since the seminal work of Langmuir in the early 1900’s, and the theory of sheaths has been greatly developed to the present day. The theories are scalable across a wide range of plasma parameters, but due to the difficulty of obtaining experimental measurements of plasma properties in the sheath region, there is little experimental data available to directly support the theoretical development. Sheaths are difficult to measure in situ in thrusters due to the small physical length scale of the sheath (order of micrometers in thruster plasmas) and the harsh plasma environment of the thruster. Any sufficiently small probe will melt, and available optical plasma diagnostics do not have the sensitivity and/or spatial resolution to resolve the sheath region. The goal of the current work is to experimentally characterize plasma sheaths xxvi in a low-density plasma that yields centimeter-thick sheath layers. By generating thick sheaths, spatially-resolved data can obtained using electrostatic probes. The investigation focuses on the effects of electron emission from the wall and several factors that influence it, including wall material, wall temperature, wall surface roughness and topology, as well as the scaling of sheaths from the low-density plasma environment towards thruster conditions. The effects of electron emission and wall material are found to agree with classical fluid and kinetic theory extended from literature. In conditions of very strong emission from the wall, evidence is found for a full transition in sheath polarities rather than a non-monotonic structure. Wall temperature is observed to have no effect on the sheath over boron nitride walls independent of outgassing on initial heat-up, for sub-thermionic temperatures. Wall roughness is observed to postpone the effects of electron emission to higher plasma temperatures, indicating that the rough wall impairs the wall’s overall capacity to emit electrons. Reductions in electron yield are not inconsistent with a diffuse-emission geometric trapping model. Collectively, the experimental data provide an improved grounding for thruster modeling and design.

Made available in DSpace on 2014-12-17T14:48:18Z (GMT). No. of bitstreams: 1 FabianaAS_DISSERT.pdf: 5525244 bytes, checksum: b567af14ad5856197ebf8368098dd7fb (MD5) Previous issue date: 2013-12-16 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / It is presented the analysis of a retaining wall designed for the basement of a residential building, located in Natal/RN, which consists in a spaced pile wall, anchored by tiebacks, in sand. This structure was instrumented in order to measure the wall s horizontal movements and the load distribution throughout the anchor fixed length. The horizontal movements were measured with an inclinometer, and the loads in the anchors were measured with strain gages, installed in three places throughout the anchor fixed length. Measurements for displacement were done right after the implementation of each stage of the building and right after the conclusion of the building, and the measurements for loads in the anchors were done during the performance test, at the moment of the locking off and, also, right after the conclusion of the building. From the data of displacement were obtained velocity and acceleration data of wall. It was found that the time elapsed on braced installation was decisive in the magnitude of the displacements. The maximum horizontal displacement of wall ranged between 0,18 and 0,66% of the final depth of excavation. The loads in the anchors strongly reduced to approximately half the anchor fixed length, followed an exponential distribution. Furthermore, it was found that there was a loss of load in the anchors over time, reaching 50% loss in one of them / O presente trabalho apresenta um estudo sobre uma estrutura de conten??o projetada para constru??o do subsolo de um edif?cio residencial multifamiliar, localizado na cidade de Natal/RN, que consiste em uma cortina de estacas escavadas ancorada por tirantes, em areia. Essa estrutura foi instrumentada a fim de se medirem os deslocamentos horizontais da cortina e a distribui??o de cargas ao longo do trecho ancorado dos tirantes. Os deslocamentos horizontais foram medidos atrav?s de inclin?metro, e as cargas nos tirantes foram obtidas com uso de extens?metros el?tricos de resist?ncia instalados em tr?s pontos ao longo do trecho ancorado. As medi??es de deslocamento foram feitas ap?s cada etapa de execu??o da cortina e ap?s a conclus?o da obra, enquanto as medi??es de carga nos tirantes foram feitas durante os ensaios de recebimento, no momento da incorpora??o e tamb?m ap?s a conclus?o da obra. A partir dos dados de deslocamento foram obtidos dados de velocidade e acelera??o da cortina. Constatou-se que o tempo gasto na instala??o do escoramento foi decisivo na magnitude dos deslocamentos. O deslocamento horizontal m?ximo final da cortina variou entre 0,18 e 0,66% da profundidade final de escava??o. As cargas nos tirantes sofrem uma redu??o muito acentuada at? aproximadamente a metade do trecho ancorado, seguindo uma distribui??o exponencial. Al?m disso, constatou-se que houve perda de carga nos tirantes ao longo do tempo, chegando a 50% de perda em um deles