Low temperature magnetic force microscopy studies of superconducting niobium films have been undertaken with the goal of studying the interplay between artificial pinning centers and magnetic vortices. / Measurements were performed using a custom built low temperature magnetic force microscope, capable of operation at temperatures ranging from 4.2 K to room temperature. Special attention has been paid to optimizing the instrumentation through a detailed study of the noise characteristics, with particular emphasis placed on achieving a large signal-to-noise ratio and corresponding high force gradient sensitivity. / Magnetic force spectroscopy data has been used to deduce the critical temperature of the superconducting samples, based upon the repulsive Meissner interaction between the magnetic tip and the sample. Images of vortices as a function of applied magnetic field demonstrate the expected linear relation between vortex density and field strength, and confirms that only single vortices, each carrying one flux quantum, are observed. Two different methods are put forward to determine the magnetic penetration depth; one using magnetic force spectroscopy, the other using constant height imaging of vortices. Images of vortices as a function of temperature demonstrate that as temperatures rise, vortices become more easily depinned during the scanning process through interactions with the magnetic field of the tip. Dissipation images of vortices suggest eddy current damping as well as vortex motion within potential wells as major sources of energy loss. Studies on a patterned niobium film show that only interstitial vortices are easily detectable by MFM, but that a strong tip influence results in significant tip induced motion of these vortices around the antidots.

Phase formation and transformation in mechanically alloyed iron-rich Sm-Fe-C is the principal subject of this thesis. Ternary Sm-Fe-C is a complicated system. The strategy was therefore to start with a binary system. A series of mechanically alloyed R$ sb2$Fe$ sb{17}$ powders were investigated for a better understanding of both the Sm-Fe alloy system in general and the Sm$ sb2$Fe$ sb{17}$ compound in particular. The objective was to learn (1) what is the steady-state in the mechanically alloyed R$ sb2$Fe$ sb{17},$ and (2) how the 2-17 structure is formed from the mechanically alloyed precursors. Phase formation and transformation in the mechanically alloyed Sm$ sb2$Fe$ sb{17}$C$ sb{x}$ with various carbon contents was then studied. The objective in this case was to learn (1) how the 2-17 structure with interstitial carbon is formed, (2) what is the maximum C content in the 2-17 structure, the critical content $x sb{c},$ and (3) what phase(s) is (are) formed with $x>x sb{c}.$ / Phase transformation from Sm$ sb2$Fe$ sb{17}$C$ sb{x}$ to Sm$ sb2$Fe$ sb{14}$C was the second subject for study. As required by this study, the grain refinement process was investigated first. The objective was to prepare the nanocrystalline Sm$ sb2$Fe$ sb{17}$C$ sb{x}$ with various grain sizes. Emphasis was on the ball milling of Sm$ sb2$Fe$ sb{17}$/graphite mixture in the hope of forming a nano-scale mixing of Sm$ sb2$Fe$ sb{17}$ and graphite by ball milling. Solid-solid reaction between the Sm$ sb2$Fe$ sb{17}$ and graphite leading to the formation of nanocrystalline Sm$ sb2$Fe$ sb{17}$C$ sb{x}$ was then studied. The phase transformation from Sm$ sb2$Fe$ sb{17}$ was carried out with nanocrystalline Sm$ sb2$Fe$ sb{17}$C$ sb{x}$ samples. Samples prepared by other methods were also studied. The objective was to learn (1) what the transformation product is and (2) what the kinetics of the phase transformation and its grain size dependence are. (Abstract shortened by UMI.)

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007. / Source: Dissertation Abstracts International, Volume: 69-02, Section: B, page: 1214. Adviser: Jian-Ming Jun. Includes bibliographical references (leaves 122-128) Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-05, Section: B, page: 3221. Adviser: Phillip Geil. Includes bibliographical references. Available on microfilm from Pro Quest Information and Learning.

Orientador : Carlos Roberto Grandini / Banca: Jorge Roberto Pimentel / Banca: Jesuína Lopes de Almeida Pacca / Resumo: Nesta pesquisa, foi feita uma análise em dez livros didáticos de física para o ensino médio, enfocando as atividades experimentais referentes aos conteúdos da eletricidade e magnetismo. A metodologia "análise de conteúdo" foi a técnica utilizada para nortear o trabablho. Foram elaborados instrumentos ou fichas de análise, que permitiram a coleta dos dados sob três níveis: estrutural, operacional e conceitual. Primeiramente, na análise estrutural, verificou-se a presença de atividades práticas e como elas se subdividem em relação aos conteúdos do eletromagnetismo. Analisou-se o tipo da atividade experimental; a presença de títulos, esquemas, figuras e/ou fotos; e a presença de uma relação de material. Os aspectos qualitativo/quantitativo e individual/coletivo também foram analisados. Posteriormente, no nível operacional, foi investigada a ação do aluno nas etapas do processo científico. Finalmente, na análise conceitual, verificou-se a presença de falhas conceituais e investigou-se o funcionamento das montagens. A segurança dos experimentos também foi avaliada. Concluiu-se discutindo as concepções de ensino e ciência implícitas nas atividades. Os dados foram organizados através de gráficos e tabelas, e a sua interpretação permitiu conhecer melhor o tipo de ensino experimental que está chegando ao aluno do ensino médio, através do livro didático / Abstract: In this research, it was made an analysis in ten physical textbooks of the high school level, focusing the experimental activities of the electricity and magnetism contents. The methodology "analysis of contents" was the technique used to the work. We made instruments to the analysis that allowed the collect of data about three levels: structural, operational and conceptual. First, in the structural analysis, it was verified the presence of experimental activities and how they are divided about contents of electromagnetism. It was analyzed the kind of the experimental activity, the presence of titles; schemes, figures or photos and material lists were verified. The aspects quantitative/qualitative and individual/collective were analyzed, too. After that, in the operational level, was researched the student's action in the stages scientific process. Finally, in the conceptual analysis was verified some conceptual problems and the manner as the experimental works. The experiments were constructed and analyzed. The safety of the experiments was verified, too. It was made a discussion about teaching and science conceptions. The data were organized in graphics and tables, that allowed to know better the kind of experimental teaching made to the students / Mestre

Spin torque oscillator devices have presented themselves as an energy efficient method of generating microwave frequencies in recent years. These are devices which rely on giant magnetoresistance to create a device resistance which oscillates at microwave frequencies due to the microwave oscillation of a magnetically free layer in the device. A spin torque oscillator can be improved by using a vortex core oscillator as the magnetically free layer offering a narrower linewidth and greater synchronisation possibilities, at the expense of a lower power output. The desire to tune the frequency of oscillation of these devices has been the focus of a great deal of research in recent years and one promising avenue of investigation is to alter the frequency of oscillation by inducing a strain anisotropy in such a device by the use of a piezoelectric transducer. In order for the induced anisotropy to be large a material must be used which exhibits a strong magnetostrictive effect such as Fe1−xGax, a material which exhibits strong magnetostrictive properties without the need for rare earth elements. This thesis describes investigations into the magnetisation dynamics of nanostructures fabricated from magnetostrictive thin films of Fe1−xGax under conditions of in-plane uniaxial anisotropy induced by an applied stress. Chapter 4 describes investigations into the effects of altering the thickness of sputter grown Fe1-xGax films on the crystalline anisotropy of the films. It was found that the intrinsic magnetocrystalline uniaxial anisotropy within the films increased with film thickness. The cubic anisotropy was shown to be roughly constant with respect to film thickness except when the film was 20nm thick when the cubic anisotropy of the sample was anomalously high. Investigations of the magnetostrictive properties of these materials revealed sputter grown thin films to exhibit similar magnetostrictive properties as bulk material and thin films grown by molecular beam epitaxy. X-ray analysis performed by Dr. P. Wadley and Prof. V. Holy failed to explain the relationships between film thickness and magnetocrystaline anisotropy observed in the samples, but suggested that the average grain size increases as the thickness of the film increases. Chapter 5 describes the results of time resolved XMCD PEEM measurements performed at the Diamond Light Source synchrotron facility performed in order to investigate the magnetisation dynamics within a series of Fe1−xGax squares. It was found that these squares demonstrated no significant response to an applied stress, probably due to strong shape anisotropy. Preliminary work to investigate Ni squares revealed that they do exhibit a strong response to stress. The dynamic response of the Ni squares was notsuccessfully measured however. Chapter 6 presents results of micromagnetic simulations performed to predict the effects of strain-induced anisotropy on magnetic square nanostructures fabricated from Fe1−xGax. Time resolved simulations demonstrated the ability of a strain induced anisotropy to modify the frequency of oscillation of the vortex core oscillations and the confined spin wave modes as well as the amplitude of the magnetic field pulse required to induce switching of the polarisation of the vortex core. The effects of size and uniaxial anisotropy on the spin wave modes within square devices was studied and an s shaped spin wave mode was shown to form in the presence of a uniaxial anisotropy.

In this thesis the Drift Diffusion enhanced Hodgkin Huxley model is developed. This model uses the Drift Diffusion equations to model the bulk solutions both within a neuron and in the surrounding extracellular media. The Hodgkin Huxley ion channel behaviour is incorporated into the membrane regions through the use of an altered diffusion coefficient. Firstly the model is applied to the case of intracellular and extracellular media separated by a single membrane. Secondly the model is applied to a cell within a restricted extracellular space. This takes a slice through a cell and is therefore termed a double membrane model, since there are two membrane layers. Finally the model is used to determine whether there is any charge and field buildup on a gold surface located 100 nm from the cell. The results from this could then be used in future to model Surface Plasmon Resonance experiments which may form the basis of novel neuronal activity detectors.

This thesis describes the effects of a post-growth hydrogenation on as-grown samples and device structures based on III-N-V and III-V semiconductor compounds. The spectral response of quantum wells (QWs) or superlattices (SLs) are tuned by the control dissociation of N-H complexes using a focused laser beam (photon assisted dissociation) or by thermal annealing. These approaches could be implemented in other materials and heterostructure devices, and offer the advantage of enabling an accurate control of the spectral response of a device using a layer compound with a single N- concentration. A focused laser beam is also used to diffuse hydrogen from the p-type contact layer towards the III-N-V superlattice in the intrinsic region of a p-i-n diode, thus creating preferential injection paths for the carriers and creating nanoscale light emitting diodes. Opportunities for realizing a movable micron size-light emitting diode (-LED) are also demonstrated. Moreover, room temperature electroluminescence from semiconductor junctions formed from combinations of n-InSe, p-InSe, p-GaSe and n-In2O3 is demonstrated. These p-n junctions are fabricated using mechanical exfoliation of Bridgman-grown crystals and a simple mechanical contact method or thermal annealing. These results demonstrate the technological potential of mechanically formed heterojunctions and homojunctions of direct band gap layered GaSe and InSe compounds with an optical response over an extended wavelength range, from the near-infrared to the visible spectrum. These layered crystals could be combined in different sequences of layer stacking, thus offering exciting opportunities for new structures and devices.

Physical inactivity has been linked to poor health and disease progression, particularly in older people. This has led to an increasing interest in the effects of physical activity, physiological function and ageing. Importantly, negative health traits generally attributed to ageing, such as frailty, cognitive decline and brain atrophy, may in part result from decreased habitual physical activity levels, and be preventable with increased exercise. Here, we use magnetic resonance imaging (MRI) techniques to quantify the cortical haemodynamic and metabolic responses to acute low/moderate intensity exercise in healthy young and older volunteers, to investigate how this response is influenced by ageing and cardiorespiratory fitness. In addition, structural MRI is used to investigate global and regional grey matter volume, and cortical thickness in young and older adults, and to assess its association with age and cardiorespiratory fitness. In the final Chapter, methods are developed to study the impact of a 16-day limb immobilisation on brain and muscle function using ultra-high field, 7 T MRI. Functional magnetic resonance imaging (fMRI) is used to assess changes in brain motor cortex function over the course of 16 days of upper limb immobilisation, and assess whether any changes are observed associated with the loss of voluntary handgrip strength over the same time period. Muscle MRI is performed to assess changes in muscle cross sectional area, and muscle magnetic resonance spectroscopy (MRS) developed with the aim of quantifying changes in forearm muscle IMCL and EMCL content over the immobilisation period.

Magnetic Resonance Spectroscopy is a powerful non-invasive tool for investigating liver metabolism in vivo. PRESS and STEAM localized 1H MRS can be used to provide liver lipid measurements calculated from the fat to water peak ratios. Surface coils can also be used to measure other metabolites using multinuclear MRS, 13C MRS directly measures glycogen levels and 31PMRS measures ATP and other phosphate metabolites. This thesis reports on studies undertaken to develop these techniques and describes a number of in vivo investigations that explored the effects of dietary interventions on hepatic metabolism.