This thesis describes the characterisation of GaMnAs, related compounds and heterostructures. GaMnAs and other (III,Mn)V compounds have provided many interesting insights into fundamental physics, and are of considerable potential interest commercially in the field of spintronics. This study examines a set of samples grown by molecular beam epitaxy and characterised using several techniques: primarily this study makes use of the x-ray absorption techniques, x-ray magnetic circular dichroism(XMCD) and x-ray absorption spectroscopy (XAS). In addition, x-ray diffraction (XRD), transport measurements and super conducting quantum interference device (SQUID) magnetometry were used as complimentary techniques. GaMnAs layers with epitaxial Fe grown on top, are shown to have a sub-nanometre interfacial layer which remains polarised above room temperature. A detailed understanding of these systems is obtained by applying the element specific nature of XMCD in combination with two different probing depths to explore separately the nature of the coupling of the bulk and interfacial region. The coupling between the interfacial layer and the Fe is shown to be strongly antiferromagnetic (AF). A weaker coupling is also shown to exist between the Fe and the bulk of the \gamnas layer below the Curie temperature (Tc). This coupling is also AF at low fields, leading to an exchange bias for the entire layer. Doping of GaMnAs with P is shown to have several effects on the magnetic properties of the GaMnAs layer. Changes in the layer strain are observed using high resolution XRD. This strain also manifests itself in the Mn L_2,3 XMCD spectra and the relationship between the two is shown to be linear. A pronounced effect on the magnetic anisotropy is observed using SQUID measurements, with the easy axis switching from in-plane, in the compressively strained GaMnAs, to out-of-plane in the higher doped GaMnAsP layers. A decrease in total magnetic moment per Mn atom and Tc are observed with increased doping. This is inferred not to be due to a direct effect of the P on the local surrounding of the Mn ions, owing to the striking similarity of the XMCD spectra. This is instead attributed to reduced participation of Mn ions in the magnetic ordering. Finally, K edge XMCD is used to reveal the element specific nature of unoccupied states near the Fermi level in a set of GaMnAs and (In,Ga,Mn)As samples with differing Mn doping levels . The character of the holes in low-doped samples is shown to be markedly different than for those in the highly doped metallic samples. A transfer of orbital magnetic moment from the Mn to the As sites is observed on crossing the metal-insulator transition, with the large XMCD on Mn sites in low doped samples interpreted as a sign of hole localisation around the Mn ion.

Numerical simulation techniques play an important role due to their flexibility in dealing with a broad range of complex geometries and material responses. This flexibility requires substantial computational time and memory. Most numerical methods use structured grid for graphical discretization, although this approach is straightforward it is not ideal for smoothly curved boundaries. In this thesis the two-dimensional Transmission Line Modelling (TLM) method based on unstructured meshes is adopted. TLM is an established numerical simulation technique that has been employed in a variety of applications area. Using unstructured meshes to discretize the problem domain permits smooth boundary presentation which provides significant enhancement in the flexibility and accuracy of the TLM simulations. An algorithm is developed to implement Unstructured Transmission Line Modelling (UTLM) which is carefully designed for simplicity and scalability of model size. Several examples are employed to test the accuracy and efficiency of the UTLM simulations. Delaunay meshes, as a type of unstructured meshes, provide good quality triangles but have the disadvantage of providing close to zero transmission line length which has impact on the maximum permissible time step for stable operation. In this thesis, a simple perturbation method for relaxing the minimum link length and clustering triangles in pairs is presented, which permits substantial increase in time step and hence computational runtime to be made without compromising the simulation stability or accuracy. Also, a new model for relaxing the short link lines that fall on the boundaries is presented. UTLM method is based on temporal and spatial sampling of electromagnetic fields which results in dispersion. In this thesis, dispersion characteristics of the unstructured TLM mesh are investigated and compared against structured TLM results for different mesh sizes and shapes. Unlike the structured TLM mesh, the unstructured mesh gives rise to spatial mode coupling. Intermodal coupling behaviour is investigated in a statistical manner upon the change of the mesh local characteristics.

An Electromagnetic Pulse (EMP) resulting from, for example, the detonation of a nuclear weapon is characterised by a wave of electromagnetic activity able to couple with power lines and electro-sensitive equipment with the potential of rendering an establishment or on a greater scale, a whole city impotent. Protection against such occurrences is of paramount importance. It is now accepted that an important consideration when devising protective schemes against such phenomena is an accurate understanding of the effects on propagating waveforms such as those coupled to wires, when electrical breakdown of the dielectric material surrounding such wires occurs, otherwise known as electrical discharge. Such issues can occur around the affected wires if the electric fields generated exceed the dielectric strength of the surrounding medium, typically air or soil. Under these circumstances, the signature of the coupled waveform is known to change in characteristic ways. The form and degree of distortion needs to be understood if the harmful effects are to be prevented by protection systems put in place. The purpose of this thesis is to first describe the mechanisms that lead to the development of the Nuclear–Electromagnetic Pulse (NEMP) and the mechanisms of the discharge that can result once such pulses have coupled to a wire. Next, some of the previous corona-modelling approaches are discussed. Many of the modelling approaches have been applied to 1-D transmission-line simulations. When 3-D simulations have been performed, the Finite-Difference (Time Domain) or FD-TD approach seems to be the preferred method. At the time of writing, no 3-D Transmission Line simulations of discharge phenomena around wires were available. Hence, here, the 3-D Transmission Line Modelling Method (TLM) is described with a view to modelling such behaviour. In particular, the Embedded-Wire-Node (EWN) is used to model the discharge development around the wire. This is a fine-wire technique used to reduce computational fatigue. The node can be adapted to accept changes related to electrical discharge allowing for a real-time, self-consistent recreation of such effects. The 3-D TLM approach proves to be a decent candidate to the modelling of such behaviour. Both advantages and disadvantages of this method are discussed.

Combining stainless steel with polyester fibres adds an attractive conductive behaviour to the yarn. Once knitted in such a manner, fabrics develop sensing properties that make them suitable for wearable applications as a consequence of the variation of their conductivity when subject to structural deformations. The use of such textiles as accurate strain/motion sensors can be considered the new frontier in wearable sensor applications, in opposition to their use as simple strain/motion detectors as mainly done until now. This thesis aims to characterise the electrical properties of knitted conductive textiles made of 20% stainless steel and 80% polyester fibres in the context of their application as fabric strain sensors. However, the use of conductive textiles as sensors is challenging and requires the combined study of their electrical and mechanical behaviours. In the first part of this thesis, the fabric resistance variation is analysed with a purposely built measurement system during a considerable number of stretch-recovery cycles performed by an Instron tensile machine. From the resulting electrical characterisation, it is possible to select the best knit pattern candidates for the second part of the study, which regards their integration into a supporting garment and further investigation of their electrical and mechanical properties in view of their application as wearable motion sensors. Two acquisition systems capture both free and constrained motions of a participant's knee and the electrical resistance variation due to combined deformation of samples and supporting leggings. In both parts of this thesis, a detailed analysis in terms of linearity, stability, sensitivity and hysteresis confirms that the overall sensing performance progressively decreases during extended use as a consequence of the short-term irreversible fabric deformation and does not significantly recover after short rest periods. For the development and use of high performance fabric-based strain sensors, it is therefore evident the importance of the long-term maintaining of a reversible stretch-recovery behaviour not only of the sensing fabrics but also of the supporting garments they are integrated in.

This thesis investigates the electron-phonon interactions in fabricated Semiconductor Devices i.e. Schottky Diodes, Quantum Well (QW) embedded p-i-n photodiodes, two dimensional electron gas (2DEG) and two-dimensional hole gas (2DHG) structures all on (311) GaAs substrates. Fast electrical measurements were performed on Schottky diodes fabricated on the (311B) GaAs surface. Optical excitation generates a compressive strain pulse in an Al film which is at the opposite side of the GaAs substrate. Two coherent acoustic phonon modes were generated in the (311B) GaAs substrate: quasi-longitudinal phonons (QLA) and quasi-transverse phonons (QTA). These two modes were detected successfully using the Schottky diode, the dependence of QLA and QTA signals on different applied bias, pump powers and temperatures were investigated. The dependence of signals on bias reveals that the temporal position of the signals for QLA and QTA moves slightly earlier times with increasing reverse bias because the width of the depletion layer increases with increasing negative bias. The first peak arrival time for QLA phonons in the measured signal moves to earlier times with increasing pump power. This was caused by the nonlinear acoustic properties of the GaAs substrate. Conversely, QTA phonons arrival time was not dependent on pump power which indicates the elastic nonlinear effects are not crucial for QTA signals. Investigation of the temperature dependence reveals that the arrival time changes to later times with increasing temperature because of the decreasing sound velocity with increasing temperature. The polarity of the experimental signals was compared to the simulated signals of electron-phonon coupling interactions concludes the predominant role of piezoelectric phonon-electric transformation mechanism over deformation potential. Detailed theoretical calculations show the dominance of the piezoelectric mechanism up to frequencies of 70GHz and 250GHz for longitudinal and transverse phonons respectively. Low dimensional structures i.e. Quantum well embedded p-i-n photodiode detector (QW-p-i-n), two-dimensional electron gas (2DEG) and two-dimensional hole gas (2DHG), fabricated on the (311) GaAs surface, were also investigated as detectors for longitudinal and transverse acoustic signals. The first experimental evidence of high frequency tail in transverse signal is observed using GaAs based QW-p-i-n detector. The longitudinal signal confirms the existence of the GHz oscillations, with the most significant occurring 170 GHz, while in transverse acoustic signal we observed a high frequency tail from 300 ps to 800 ps. Fast Fourier transforms (FFT) analysis of this tail shows the existence of high frequency phonons up to 200 GHz. Photocurrent pump-probe measurements was also performed with three different Al film thicknesses (20nm, 30nm and 100nm). Experiments performed with 20nm film thickness clearly shows a weak signal at 250 GHz in the amplitude spectrum which is not observed for 30 nm and 100 nm aluminium films. This observation indicates that the thinner aluminium films can generate higher frequency phonons. The longitudinal and transverse signals were also detected by the 2DHG; for QLA predominantly the peak is negative while for QTA it is positive in contrast to the 2DEG experiments due to the reason that in 2DEG experiments the origin of the signal is due to the decrease in carrier density but for 2DHG the signal is due to the increase in carrier density. This thesis studies the interaction of subterahertz acoustic phonons with charge carriers with the aim of developing high speed acoustoelectric detectors for subterahertz compressional and shear acoustic waves, and it has been shown that the Schottky diodes are successful in this application.

Since its first successful growth in 1996, the ferromagnetic semiconductor (Ga,Mn)- As has had a great inuence on the research field of semiconductor spintronics. Among the outstanding characteristics of this material the large spin-orbit interaction for the holes in the valence band plays a major role, since it is responsible for some of the most interesting properties of (Ga,Mn)As, like the magnetocrystalline anisotropy, the magnetoelastic coupling and the extraordinary contributions to the magnetotransport. Furthermore, the combination of large magnetic anisotropies, large spin stiffness and relatively small magnetic moments renders (Ga,Mn)As a hard ferromagnetic system with excellent micromagnetic properties, including mean-field like magnetization and macroscopic single-domain characteristics, that can be described both phenomenologically and microscopically, thanks to the relatively simple band structure. Finally, the interplay between ferromagnetism and semiconductivity, arising from the hole-mediated nature of the ferromagnetic interaction in (Ga,Mn)As, allows for the remarkable possibility of manipulating its magnetic properties by varying the state of the holes using non-magnetic parameters like electric fields, electric currents, light or strain. This circumstance could in principle be very useful to improve the process of writing information in magnetic memories, which is currently performed, not very efficiently, with magnetic fields. However, it does seem unlikely that (Ga,Mn)As will become a relevant material for technological applications since the highest Curie temperature so far obtained for (Ga,Mn)As is still well below room temperature. Nonetheless the study of (Ga,Mn)As remains a fervent research area since it allows to explore a variety of novel functionalities and spintronics concepts that could in future be implemented in other systems. For this reason (Ga,Mn)As is often referred to as a test bench material for semiconductor spintronics. This Thesis presents the results of a series of experimental investigations showing how dfferent approaches can be used to manipulate the magnetic anisotropy in (Ga,Mn)As thin films. In Chapter 4 the properties of the ferromagnetic semiconductor (Ga,Mn)(As,P) are investigated through structural, magnetometry, transport and magnetotransport measurements. By varying the amount of phosphorus incorporated it is possible to vary the sign of the in-built growth strain, to which the magnetic anisotropy in (Ga,Mn)As is extremely sensitive. It is in fact shown that samples with large enough phosphorus concentrations are characterized by a perpendicular-to-plane magnetic easy axis, which is an extremely useful property since it allows to detect the orientation of the magnetization via anomalous Hall effect and polar magneto-optical Kerr effect. Furthermore, it is demonstrated that by varying the temperature or the post-growth annealing time it is possible to obtain a reorientation of the magnetic easy axis from an in-plane direction to the perpendicular-to-plane direction in some samples, which is another interesting aspect of this material. Chapter 5 consists of a study exploring the effects of piezoelectric-induced strain on the magnetic anisotropy of a highly-doped annealed (Ga,Mn)As sample bonded to a piezoelectric actuator. It is shown that large and reversible rotations of the magnetic easy axis can be achieved in this sample by varying the voltage applied to the piezoelectric actuator, thus demonstrating that strain-mediated electric control of ferromagnetism is effective even in the limit of high doping levels and high Curie temperatures, where direct electric control of ferromagnetism via carrier manipulation is not possible. Furthermore, the results obtained from magnetotransport and SQUID magnetometry measurements are compared, extracting the dependence of the piezo-induced uniaxial magnetic anisotropy constant upon strain in both cases and discussing why the magnetotransoport measurements are believed to be more accurate than SQUID magnetometry measurements in evaluating the inverse magnetostriction effects in (Ga,Mn)As-piezoelectric actuator hybrid systems. Finally, Chapter 6 contains the results of an investigation attempting to use ultrashort strain pulses to switch the magnetization direction in a (Ga,Mn)(As,P) sample on fast time scales. These pulses are generated by femtosecond optical excitation of a metal transducer film deposited on the back of the substrate and travel ballistically through it until they reach the sample under investigation. Despite demonstrating that this method can indeed be used to induce a fast irreversible switching of the magnetization orientation in the (Ga,Mn)(As,P) sample, time-resolved magnetotransport measurements show that the switching is not triggered by the strain pulse, but rather by the transverse heat pulse, the latter being generated with the strain pulse during the optical excitation of the metal film. It is shown that the switching occurs through domain-related processes and the possible mechanisms behind its cause are speculated.

A current-switched superconducting field-cycling NMR spectrometer has been designed and built for studying the role of quantum tunnelling in molecular dynamics. The instrument is designed for work in the solid state with sample temperatures extending from 4K up to 300K. The maximum field-switching rate is 10Ts-1. Among the samples studied in this thesis is the nuclear spin-relaxation and proton tunnelling. Concerted double proton transfer in the hydrogen bonds of carboxylic acid dimers is well established as the model system for translational quantum tunnelling. The model system has been chosen to illustrate the smooth quantum-to-classical transition and at all temperatures the proton transfer is characterised by a single correlation time. Quadrupolar interactions introduce an additional relaxation to the proton spin polarisation. The enhanced relaxation of the proton spin appears as a dip in the proton magnetisation curve. This technique is employed to measure the quadrupolar transition frequency of 14N and 35Cl and determine the structure of heroin hydrochloride. The introduction of a second spin species has a significant effect on the spin-lattice relaxation. Compared with homonuclear systems, the spectral density acquires additional components characterised by the sum and difference Larmor frequencies of the two nuclei. Further, instead of a single relaxation time, there are four elements of a relaxation matrix. Therefore, the magnetisation recovery becomes bi-exponential and the initial polarisation state of the second nucleus strongly affects the magnetisation recovery of the nucleus which is being observed. We shall report on the results of spin-lattice relaxation investigations on 1H-13C, 1H-19F systems. The role of heteronuclear interactions in spin-lattice relaxation and the newly developed methodology of field-cycling relaxometry will be discussed. This represents the first 13C field-cycling NMR experiment and the first to measure the field dependence of the off-diagonal element of the relaxation matrix.

The technique of functional magnetic resonance imaging is rapidly moving from one of technical interest to wide clinical application. However, there are a number of questions regarding the method that need resolution. Some of these are investigated in this thesis. High resolutionf MRI is demonstrated at 3.0 T, using an interleaved echo planar imaging technique to keep image distortion low. The optimum echo time to use in fMRI experiments is investigated using a multiple gradient echo sequence to obtain six images, each with a different echo time, from a single free induction decay. The same data are used to construct T2* maps during functional stimulation. Various techniques for correcting the N/2 ghost are tested for use in fMRI experiments, and a method for removing the image artefact caused by external r. f. interference in a non-linearly sampled matrix is presented. The steps in the analysis of fMRI data are detailed, and two new non-directed analysis techniques, particularly for data from single events, as opposed to epoch based paradigms, are proposed. The theory behind software that has been written for fMRI data analysis is also given. Finally, some of the results from an fMRI study into the initiation of movement are presented, illustrating the power of single event experiments in the separation of cognitive processes.

The aim of this work was to implement a thorough method for quantifying the errors introduced to frame-based neurosurgical stereotactic procedures by the use of MRI. Chang & Fitzpatrick's reversed gradient distortion correction method was used, in combination with a phantom, to measure these errors. Spatial distortion in MR images of between 1 mm and 2 mm was measured. Further analysis showed that this typically introduced an additional error in the coordinate of the actual treatment point of 0.7 mm. The implications of this are discussed. The main source of distortion in the MR images used for stereotaxis was found to be the head ring. A comparison between imaging sequences and MR scanners revealed that the spatial distortion depends mainly on the bandwidth per pixel of the sequence rather than other differences in the imaging sequences. By comparison with a phase map distortion correction technique, the imaging parameters required to allow successful distortion correction with the reversed gradient method were identified. The most important was the use of full Fourier spin echo acquisitions. The reversed gradient correction method was applied to two contemporary EPI techniques. Considerable improvement was seen in the production of ADC maps after the images had been corrected for distortion. The method also was shown to be valid in application to BOLD fMRI data.

Interest in coherent remote sensing systems has stimulated investigations in the properties laser propagation through extended atmospheric turbulence. This thesis investigates the statistics of phase, and phase related, observables using analytical and computational techniques, together with experimental results. The phase screen technique is used to simulate perturbations to the refractive index of a medium through which the radiation propagates. Several different turbulence models (Gaussian correlated noise, Kolmogorov turbulence, Tatarski and Von Karman spectral models) are investigated, and their relative merits for describing experimental conditions and descriptive statistical measures are compared and contrasted. The phase power spectrum is crucial to an understanding of the practical operation of a coherent imaging system, and later part of the thesis is devoted to the investigation of a LIDAR system in particular. Several turbulence regimes are investigated, from an analytical treatment of a weakly turbulent, extended atmosphere, to large 3D computations designed to simulate experimental arrangements. The 3D simulation technique presented herein has been developed to allow for the investigation of temporal statistics. New power law behaviours are found to appear in temporal frequency spectra which differ from the -8/3 power law form that has been accepted in much of the literature. Strongly turbulent regimes result in a -2 power law while the use of a Gaussian beam profile in an extended medium gives a -11/3 power law under weak turbulence conditions. Please note: Pagination in electronic reproduction differs from print original. The print version is the version of record.