Fault pseudotachylytes commonly form by frictional melting due to seismic slip. These fine-grained clastic rocks result from melt quenching and may show a high concentration of fine ferromagnetic grains. These grains are potentially excellent recorders of the rock natural remanent magnetization (NRM). The magnetization processes of fault pseudotachylytes are complex and may include the following: i) near coseismic thermal remanent magnetization (TRM) acquired upon cooling of the melt; ii) coseismic lightning induced remanent magnetization (LIRM) caused by earthquake lightnings (EQL); iii) post seismic chemical remanent magnetization (CRM) related to both devitrification and alteration. Deciphering these magnetization components is crucial to the interpretation of paleointensities to see if coseismic phenomena such as EQL’s were recorded within these rocks. Hence the paleomagnetic record of fault pseudotachylytes provides an independent set of new constraints on coseismic events. Fault pseudotachylytes from the Santa Rosa Mountains, California host a magnetic assemblage dominated by stoichiometric magnetite, formed from the breakdown of ferromagnesian silicates and melt oxidation at high temperature. Magnetite grain size in these pseudotachylytes compares to that of magnetite formed in friction experiments. Paleomagnetic data on these 59 Ma-old fault rocks reveal not only anomalous magnetization directions, inconsistent with the coseismic geomagnetic field, but also anomalously high magnetization intensities. Here we discuss results of rock magnetism and paleointensity experiments designed to quantify the intensity of coseismic magnetizing fields. The REM’ paleointensity method, previously tested on meteorites, is particularly well suited to investigate NRMs resulting from non-conventional and multiple magnetization processes. Overall findings indicate an isothermal remanent magnetization (IRM) in some, but not all, specimens taken from four different Santa Rosa pseudotachylyte samples. The cause of this IRM may be attributed to an LIRM produced by ground lightning (less likely), LIRM produced by an EQL (more likely), or a VRM imparted during laboratory preparation (not likely). The anomalously high NRM recorded in a few specimens points to LIRM as the most likely explanation for the dominant origin of magnetization.

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.

abstract: This dissertation research has involved microscopic characterization of magnetic nanostructures using off-axis electron holography and Lorentz microscopy. The nanostructures investigated have included Co nanoparticles (NPs), Au/Fe/GaAs shell/core nanowires (NWs), carbon spirals with magnetic cores, magnetic nanopillars, Ni-Zn-Co spinel ferrite and CoFe/Pd multilayers. The studies have confirmed the capability of holography to describe the behavior of magnetic structures at the nanoscale. The phase changes caused by the fringing fields of chains consisting of Co NPs were measured and calculated. The difference between chains with different numbers of Co NPs followed the trend indicated by calculations. Holography studies of Au/Fe/GaAs NWs grown on (110) GaAs substrates with rotationally non-uniform coating confirmed that Fe was present in the shell and that the shell behaved as a bar magnet. No fringing field was observed from NWs with cylindrical coating grown on (111)B GaAs substrates. The most likely explanation is that magnetic fields are confined within the shells and form closed loops. The multiple-magnetic-domain structure of iron carbide cores in carbon spirals was imaged using phase maps of the fringing fields. The strength and range of this fringing field was insufficient for manipulating the carbon spirals with an external applied magnetic field. No magnetism was revealed for CoPd/Fe/CoPd magnetic nanopillars. Degaussing and MFM scans ruled out the possibility that saturated magnetization and sample preparation had degraded the anisotropy, and the magnetism, respectively. The results suggested that these nanopillars were not suitable as candidates for prototypical bit information storage devices. Observations of Ni-Zn-Co spinel ferrite thin films in plan-view geometry indicated a multigrain magnetic domain structure and the magnetic fields were oriented in-plane only with no preferred magnetization distribution. This domain structure helps explain this ferrite's high permeability at high resonance frequency, which is an unusual character. Perpendicular magnetic anisotropy (PMA) of CoFe/Pd multilayers was revealed using holography. Detailed microscopic characterization showed structural factors such as layer waviness and interdiffusion that could contribute to degradation of the PMA. However, these factors are overwhelmed by the dominant effect of the CoFe layer thickness, and can be ignored when considering magnetic domain structure. / Dissertation/Thesis / Doctoral Dissertation Engineering 2015

The field of paleomagnetism attempts to understand in detail the the processes of the Earth by studying naturally occurring magnetic samples. These samples are quite unlike those fabricated in the laboratory. They have irregular shapes; they have been squeezed and stretched, heated and cooled and subjected to oxidation. However micromagnetic modelling allows us to simulate such samples and gain some understanding of how a paleomagnetic signal is acquired and how it is retained. Micromagnetics provides a theory for understanding how the domain structure of a magnetic sample alters subject to what it is made from and the environment that it is in. It furnishes the mathematics that describe the energy of a given domain structure and how that domain structure evolves in time. Combining micromagnetics and ever increasing computer power, it has been possible to produce simulations of small to medium size grains within the so-called single to pseudo single domain state range. However processors are no longer built with increasing speed but with increasing parallelism and it is this that must be exploited to model larger and larger paleomagnetic samples. The purpose of the work presented here is twofold. Firstly a micromagnetics code that is parallel and scalable is presented. This code is based on FEniCS, an existing finite element framework, and is shown to run on ARCHER the UK’s national supercomputing service. The strategy of using existing libraries and frameworks allow future extension and inclusion of new science in the code base. In order to achieve scalability, a spatial mapping technique is used to calculate the demagnetising field - the most computationally intensive part of micromagnetic calculations. This allows grain geometries to be partitioned in such a way that no global communication is required between parallel processes - the source of favourable scaling behaviour. The second part of the theses presents an exploration of domain state evolution in increasing sizes of magnetite grains. This simulation, whilst a first approximation that excludes magneto-elastic effects, is the first attempt to map out the transition from pseudo-single domain states to multi domain states using a full micromagnetic simulation.

This thesis deals with solving stationary magnetic field in material with non-constant magnetic reluctivity in realistic geometry. To this end, we extended adgfem software. Software adgfem implements discontinuous Galerkin method and so far has been used mainly to solve convection- diffusion problems and lacked streamlined approach to computational mesh generation. This thesis contains step-by-step guideline to creation of complex geometry using software SALOME. This mesh is then converted to format suitable for adgfem using newly written convertor datToAdgfem. Mesh created in this way is then used for calculation of non-linear static magnetic field.

La thèse a porté sur l'étude par spectroscopie magnéto-optique de boites quantiques magnétiques et de nanofils semiconducteurs. Ces nanostructures à base de semiconducteurs magnétiques dilués sont élaborées et étudiées dans l'équipe depuis une quinzaine d'année. L'intérêt de ces structures repose sur la possibilité de contrôler et d'étudier très finement le couplage d'un petit nombre de spins localisés avec des porteurs libres. La première partie de la présente thèse a porté sur l'étude des conditions de formation de polarons magnétiques (orientation d'un petit ensemble de spins par l'intermédiaire d'un porteur ou d'une paire électron-trou) dans des boites quantiques CdMnTe. La formation de polarons magnétiques encore assez mal comprise avec les systèmes 0D tels que les boites quantiques auto-assemblées. Une série d'échantillons originaux de boites quantiques magnétiques auto-assemblées (concentration en manganèse intermédiaires) a été élaborée par épitaxie par jets moléculaires dans l'équipe. L'étude de ces échantillons pendant la thèse a permis d'étudier pour la première fois la formation de polarons magnétiques avec des boites quantiques magnétiques chargées négativement par spectroscopie magnéto-optique. La deuxième partie de la présente thèse a porté sur l'étude de nanofils semiconducteurs de boites quantiques insérées en nanofils. Ce projet vise à insérer et étudier des boites quantiques magnétiques dans des nanofils semiconducteurs (ANR Magwires). / This thesis is a part of a project which aims at fabricating a nanometer-sized magnetic object in and at optimizing its properties. A unique feature of Diluted Magnetic Semiconductors (DMS) is a magnetic polaron: a local ferromagnetic order of the spins of the magnetic atoms induced by the strong exchange interaction with the spin of an exciton optically injected in the quantum dot. The first part of the present work is devoted to a study of the optical properties of self-assembled DMS quantum dots. Using time-resolved and CW magneto-optical spectroscopy, we demonstrate the formation of the magnetic polaron associated with a negatively charged exciton or a neutral exciton. We show that the magnetic polaron associated with a negatively charged exciton is characterized by a stronger exchange field and a higher polaron energy than the one of associated with a neutral exciton. A theoretical description is provided in terms of a newly developed model which, in contrast to the widely used "exchange box" model, takes into account the Coulomb interaction between an electron and a hole. The numerical calculation based on this theory allows us to demonstrate its pertinence and to obtain parameters of the magnetic polaron which are in good agreement with the experiment. The second part of the present work is devoted to a study of semiconductor nanowire heterostructures. Such a geometry provides better access to "single object" studies and a better control of the size and the position of a quantum dot which is inserted in a nanowire. The unique anisotropy properties and the possibility of doping with magnetic impurities make nanowires promising candidates for future studies of DMS heterostructures. As a first step, the study of the optical properties of non-magnetic single nanowires with and without an inserted quantum dot, have been performed in the present work. Polarization-resolved photoluminescence experiments combined with cathodoluminescence allowed us to demonstrate the spatial localization of the emitted light. The influence of the strain on the photoluminescence energy is discussed for the nanowire with the core-shell structure. The single-photon emitter properties are characterized by photon correlation measurements. The exciton confinement in the quantum dot inserted in the nanowire is investigated by means of the temperature dependence of the photoluminescence. From the first part, we have in hand the spectroscopic and modeling tools to study the properties of a magnetic polaron with several carriers. In the second part, the fabrication and spectroscopy study of more promising nanostructure to host the magnetic polaron have been performed.