The work undertaken involves the use of functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG) as separate but complementary non-invasive functional brain imaging modalities. The aim in combining fMRI and MEG is centred around exploitation of the high temporal resolution available in MEG, and the high spatial resolution available in fMRI. However, whilst MEG represents a direct measure of neuronal activity, BOLD fMRI is an indirect measure and this makes the two modalities truly complementary. In both cases, the imaging signals measured are relatively poorly understood and so the fundamental question asked here is: How are the neuromagnetic effects detectable using MEG related to the metabolic effects reflected in the fMRI BOLD response? Initially, a novel technique is introduced for the detection and spatial localisation of neuromagnetic effects in MEG. This technique, based on a beamforming approach to the MEG inverse problem, is shown to yield accurate results both in simulation and using experimental data. The technique introduced is applied to MEG data from a simple experiment involving stimulation of the visual cortex. A number of heterogeneous neuromagnetic effects are shown to be detectable, and furthermore, these effects are shown to be spatially and temporally correlated with the fMRI BOLD response. The limitations to comparing only two measures of brain activity are discussed, and the use of arterial spin labelling (ASL) to make quantitative measurements of physiological parameters supplementing these two initial metrics is introduced. Finally, a novel technique for accurate quantification of arterial cerebral blood volume using ASL is described and shown to produce accurate results. A concluding chapter then speculates on how these aCBV measurements might be combined with those from MEG in order to better understand the fMRI BOLD response.

Experimental studies of magnetotunnelling in heterostructures have revealed series of resonances due to electrons tunnelling from a 2DEG in a lightly-doped emitter into magnetoquantised states in the collector contact of a single-barrier structure (Hickmott, 1987 and Snell et al. 1987) or in the quantum well of a double-barrier structure (Eaves et a1., 1988 and Leadbeater et a1., 1989). These experiments are very suitable for theoretical analysis since a transverse magnetic field (parallel to the barrier interfaces) has little effect on the electronic states of the 2DEG, provided the diamagnetic energy is much less than the binding energy of the bound state of the accumulation layer potential. The tunnelling electrons then have a small range of transverse momenta between +PF and -PF, where PF = l'lkF is the Fermi momentum in the 2DEG. This range determines the positions of the orbit centres of the magnetoquantised states into which the electrons are injected after emergence from the tunnel barrier. For the single-barrier heterostructures described in this thesis, these are interfacial Landau states corresponding to classical orbits in which the electron skips along the barrier interface. For double-barrier structures there are interfacial states at high magnetic fields and traversing states at low magnetic fields. Owing to the high electric field in the quantum well, the corresponding classical orbits are cycloidal trajectories which intersect both barrier interfaces (traversing states) or just one barrier interface (skipping states). The variation of the tunnel current I with magnetic field B and voltage V is calculated using the Bardeen transfer-Hamiltonian approach within a WKB approximation. The accumulation layer potential is modelled according to a simple variational solution. This enables a physical interpretation of the experimental results to be given in terms of the effect of the magnetic field on the effect ive barri er hei ght and the ampli tudes of the magnetoquantised wave functions at the barrier interfaces. Both of these effects are required to account for the observed dependence of current on magnetic field I(B) and the amplitudes of the oscillatory structure revealed in the derivative plots of dI/dB and d2I/dB2 accounts for: The model (a) the observation of two series of resonances corresponding to +PF and -PF electrons in experiments on (InGa)As/InP single-barrier structures. (b) the absence of the +PF series of resonances in GaAs/(A1Ga)As single-barrier structures. (c) the changeover from traversing to skipping states in GaAs/(A1Ga)As double-barrier structures and the characteristic decrease in oscillatory amplitudes in the changeover region.

<p> This dissertation describes three related studies regarding the imbalance difference theory in modeling the conversion between differential mode and common mode/antenna mode signals. The topics covered are: rigorous derivation of imbalance difference theory for modeling radiated emission problems, modeling the conversion between differential mode and common mode propagation in transmission lines, and modeling the loading impedance on differential mode signals due to radiated emissions. </p><p> The imbalance difference theory describes a method for calculating the coupling between differential mode signals and common mode signals due to changes in electrical balance on a transmission line. It provides both physical insight and a simple technique for modeling the conversions between the two modes. </p><p> The first chapter presents a rigorous derivation of imbalance difference theory for modeling radiated emission problems. Although the theory has been successfully used to model a wide variety of important EMC problems over the past, it has not been rigorously derived. The derivation carefully defines the important quantities and demonstrates that imbalance difference calculations are exact provided that the differential-mode propagation is TEM and the current division factor, h, represents the actual ratio of currents on the two transmission line conductors excited by a common-mode source. This chapter also discusses the acquisition of the current division factor from 2D calculations of the cross-section of the transmission line. </p><p> The second chapter provides a rigorous development of the imbalance difference theory for three-conductor transmission lines where both the differential mode and common mode exhibit TEM propagation. It also derives expressions for the mode conversion impedances, which account for the energy converted from one mode to the other. They are essential for modeling the conversion between the two modes when they are strongly coupled. </p><p> The third chapter introduces conversion impedance to the existing imbalance difference theory model for modeling radiated emission problems, so that when the coupling between differential mode and antenna mode are strong, the imbalance difference theory can more accurately estimate the antenna mode current. </p><p> All three papers are about confirming, enriching and expanding the imbalance difference theory. The first chapter focuses on the rigorous derivation of theory for its most common application, radiated emission problems. The second chapter expands the theory to multi-conductor transmission line structure when the two modes are strongly coupled. The last chapter introduces conversion impedance to the theory in modeling radiated emission problems and improves the accuracy of the model at resonant frequencies. </p>

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.)

The magnetic spinel ferrite nanoparticle is exceptionally intriguing nanocrystal system due to the industrial importance of various technical applications and the scientific significance of studying the quantum origin of magnetism. Studies of quantum influences upon magnetic properties have revealed that the spin-orbit coupling and the net magnetization greatly affect the net magnetic properties of each spinel ferrite system differently. In case of cobalt ferrite where spin-orbit coupling is relatively large, increasing Cr3+ doping concentration, which has smaller magnetic moment and zero angular moment, decreases blocking temperature, saturation magnetization, remnant magnetization and coercivity. However, in case of manganese ferrite where spin-orbit coupling is relatively small, increasing Cr3+ doping concentration, reduces all the magnetic parameters except coercivity. The coercivity increases due to smaller magnetocrystalline anisotropy energy constant which forces the coercivity to increase as saturation magnetization decreases in accordance with Stoner-Wohlfarth theory. In order to improve product quality and quantity, synthesis routes in hot oleylamine and aminolytic reaction were developed. Both methods were proven to be extremely effective, environmental friendly, inexpensive, and simple routes in the synthesis of a variety of spinel ferrite systems including CoFe2O4, MnFe2O4, NiFe2O4, and ZnFe2O4 from a single source metal precursor.

Intensive development in the field of high-temperature superconductors (HTS) has resulted in long length HTS with high current carrying capacity. The performance of HTS in the form of multifilamentary silver sheathed Bi-2223 tapes is close to the requirements for power application. Several power apparatus prototypes based on HTS have been and are being demonstrated. Recently the first commercial order for a HTS power apparatus was placed. Tennessee Valley Authority ordered two 12 MVA synchronous condensers with HTS rotor windings for reactive power compensation. In most power devices the conductor is carrying an AC transport current while it is exposed to an AC magnetic field transverse to the current path. In certain applications such as multi-layer power transmission cables or the control winding in a controllable reactor, the conductors are exposed to a magnetic field component longitudinal to the tape axis that is parallel with the current path. In this thesis, the losses in HTS tapes in longitudinally applied magnetic field are investigated and a single phase small scale controllable reactor with a loss optimized HTS control winding is designed, constructed and tested. An experimental setup to measure the losses in tapes exposed to longitudinal magnetic fields, transport currents, frequencies, and temperatures is presented. The system is based on a calorimetric loss measuring method where the losses are determined by the temperature response of the conductor when exposed to magnetic field and/or carrying transport current. Semi-empirical loss models are developed from the results of the loss measurements of non-twisted multifilamentary Bi-2223 tapes. The losses when the sample is carrying transport currents and is exposed to longitudinal magnetic fields are found to consist of hysteresis, flux flow, and eddy current losses. The hysteresis losses can be modeled with the critical state model. The flux flow losses can be described with a power law dependent current voltage characteristic. The loss models are well suited for the design of power devices in which HTS is exposed to longitudinal magnetic fields. Based on the knowledge of the losses a single phase small scale controllable reactor with a loss optimized HTS control winding is designed, constructed and tested. A simple model of the magnetic circuit of the controllable reactor is presented. Calculations from the model are compared with experimental measurements. The feasibility to design controllable shunt reactors with large linear dynamic range, low losses and limited harmonic distortion is demonstrated. Compared to a copper control winding the HTS control winding increases the dynamic range and reduces the size due to the high current density and reduces the losses of the control winding. / <p>QC 20100927</p>

Thesis (Ph.D.)--Tufts University, 1998 . / Adviser: Yaacov Shapira. Submitted to the Dept. of Physics. Includes bibliographical references. Access restricted to members of the Tufts University community. Also available via the World Wide Web;

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.