The prospect of a new generation of electronic devices based on the fundamental quantum property of angular momentum, known as spin, has led to the rapidly developing field of spintronics. It is envisioned that these advanced devices will have significant advantages over traditional charge based electronics in properties such as speed, power consumption and long coherence times. By combining the properties of magnetics with that of semiconductors, the novel class of materials known as dilute magnetic semiconductors (DMSs) are considered a promising system for exhibiting spintronic functionality. These materials are created by using molecular beam epitaxy (MBE) to incorporate into traditional semiconductors a quantity of transition metal atoms sufficient that ferromagnetism is exhibited. The most widely studied DMS is (Ga,Mn)As which has well characterised behaviour and can be processed using standard III-V fabrication techniques, thus providing an excellent basis for further study. In this research the properties of (Ga,Mn)As based systems are studied as the material dimensions are reduced to nanometre length scales. Three complementary approaches are used for this purpose. The first is to use ultra-high-resolution electron-beam lithography to construct devices. By being able to selectively remove material, laterally patterned structures can have sizes as small as 10 nm. The second approach is to exploit the atomic layer growth of MBE to allow the construction of epilayers and heterostructures with well defined vertical compositions. Thirdly, a theoretical k.p kinetic-exchange model allows the simulation of multilayer structures and an exploration of the parameter spaces available in such materials. Two systems are considered: lateral nanoconstricted magnetic tunnel junctions and vertically defined magnetic superlattices. The nanoconstrictions are analysed using low temperature magnetotransport techniques and novel anisotropic magnetoresistance (MR) effects are measured. Primarily, tunnelling anisotropic magnetoresistance (TAMR) is observed, demonstrating that it is a generic property of ferromagnetic tunnel devices and is therefore of wide interest for other spintronic systems. Secondarily, anisotropic switching behaviour is observed and is interpreted as Coulomb blockade anisotropic magnetoresistance (CBAMR). Additionally, the significance of the processing stages and material properties are highlighted. The magnetic superlattices are firstly considered on a theoretical basis in order to determine structural parameters in which a new MR effect might be observed. This effect derives from the interlayer exchange coupling (IEC) between the magnetic layers which can either be in parallel or opposed orientations. Based on the calculations, samples are measured using low temperature magnetotransport and magnetometry techniques in order to explore the possibility of some of the dramatic properties predicted in magnetic superlattice structures.

MRI has become an invaluable tool for diagnostic medicine. Its operation is based on the principles of electromagnetism that are dictated by Maxwell's equations. MRI relies on the existence of well defined, spatially and temporally controlled magnetic fields, which are usually generated by coils of wire. Human exposure to these fields has become a safety concern, especially with the increase in the strength of the magnetic fields used. In this thesis, problems in electromagnetism relevant to different areas in MRI and involving the calculation of solutions to both forward and inverse problems are investigated using techniques derived for computational mechanics. The first section of the work focuses on the development of an accurate technique for the solution of magnetostatic inverse problems using boundary element methods (BEM) with the aim of designing optimised gradient coils. This approach was found to be an extremely effective method which can be applied to a wide range of coil geometries and is particularly valuable for designs where the coil surface has low symmetry. BEM-based approaches to designing gradient coils that reduce the likelihood of peripheral nerve stimulation due to rapidly switched magnetic fields are also considered. In the second section of the work, a novel BEM tool to allow the calculation of solutions to quasi-static forward problems has been developed, and used for the evaluation of the electric fields induced in the human body by temporally varying magnetic fields, due to either gradient switching or body movements in strong static magnetic fields. This approach has been tested by comparison with analytic solutions for simply shaped objects, exposed to switched gradients or moving in large static fields, showing good agreement between the results of numerical and analytical approaches. The BEM approach has also been applied to the evaluation of the electric fields induced in human body models. This work involved the development of an appropriate theoretical framework for the study of conducting systems moving in magnetic fields. This involved correcting some misconceptions that had propagated in the literature and allowed the development of an effcient implementation of a BEM suited to this problem.

Looking inside the lungs without the danger of ionizing radiation side effects became available with magnetic resonance imaging using hyperpolarised noble gases. This technique has the potential to become a real tool for assessing in vivo ventilation, perfusion and even lung microstructure. The work covered in this research was aimed to improve the existing method for 3He polarisation and open the possibility to develop new modalities to probe the lung microstructure that could then be used in clinical trials. For this purpose, the polarisation facility was remodeled and new components were added. The rest of the work was focused on developing diffusion techniques that are more appropriate for the assessment of lung diseases. The improvement of the 3He polarisation facility consisted in the optimization of the gas flow path, implementation of a new dispensing method and new controlling protocol. The capacity of the polarisation system was increased by using a more powerful laser. The outcome of this was an increase in polarisation rate and a significant reduction of the dispensing time. Altogether this allow for clinical studies to be performed without too much delay. A clinical study aimed to distinguish differences between children born at term and premature was started on 70 volunteers. Three methods for measuring diffusion were used : spin echo diffusion weighted method, SPAMM tagging and MR diffusion spectroscopy. The first was previously used in the group and the last two were developed during this research. The results were correlated with basic pulmonary functional tests(spirometry and plethysmography) and also with the multiple breaths nitrogen wash-out results. No differences were found in the two groups. The results don’t agree with the current theories on lung growth and suggest that alveolarisation occurs even after the age of 8, possibly up to adult age. This is very important to be investigated further due to its clinical importance.

The Acoustic Paramagnetic Resonance (APR) of the chromous ion in two cubic host lattices, MgO and CaO, has been investigated for phonons of frequency around 9.5 GHz and temperatures in the range 1.51 K to 4.2 K. An iron-cored electromagnet produced magnetic fields continuously variable from 0 to 2 Tesla, the direction of magnetic field could be varied through 90 degree in the same plane as the phonon direction. Experiments were performed in which a variable uniaxial stress could be applied to a crystal of Cr2+ in MgO, giving information about the effects of local lattice strain on the APR spectrum. A change in the g-value of one resonance line with applied stress was observed, and, from this, the value of the strain coupling constant could be obtained. The experimental results enabled values for the parameters 3r (Jahn-Teller tunnelling splitting) and K (spin-orbit splitting) to be obtained, which were compared with those obtained by other methods. Further experiments were performed on Cr2+ in CaO in which the direction of the applied magnetic field could be changed by rotating the sample crystal about the phonon direction, enabling the field to be aligned with the crystal <111> direction. The theoretical prediction (Fletcher (1971)) that strain broadening of the resonance lines should be reduced under these conditions was demonstrated. Experiments were performed at two separate frequencies and two separate temperatures, and the results used as data for a computer programme which gave the values of the various parameters of the system, and thus allowed a low-lying energy level diagram to be predicted. There were similarities between this diagram and the accepted energy-level diagram of Cr2+ in MgO, as would be expected. The differences were attributed to die difference in the lattice constants of the host materials.

This thesis addresses the inhomogeneity and the high SAR values associated with the state-of-art 7 T high field MR system. The high signal to noise ratio associated with such high field systems ≥ 7 T is a continuous driver to use such systems where high resolution images are acquired at short acquisition time. However, these systems come with many challenges. The central brightening effect in MR images indicates a B1 degraded field uniformity. For example, at 7 T with an operational electromagnetic frequency of 298 MHz the wavelength is about 12 cm in tissues. At such short wavelengths, circuit and electromagnetic theory will be invalid an analytical solution and is no longer feasible to predict magnetic field distribution. In this thesis the full wave method, Transmission Line Modelling (TLM) technique is used in conjunction with Tikhonov regularisation inverse method in order to optimise phases and amplitude of elementary drive currents of four different coils for optimal uniformity and low SAR values. Two dimensional 8 and 16 rungs birdcage-like coils were first optimised. Then the optimisation was carried out for the three dimensional problem for 8 and 16 rungs birdcage-like coils and then compared to 32 and 64 multi-element coils. The travelling wave approach is a recent approach to overcome field inhomogeneity and high SAR values. An Antenna is used to couple head/body to a travelling wave RF signal. Using Finite Difference Time Domain (FDTD) method, a patch antenna has been designed, and the effect of using matching load for maximising the power flow in the magnet bore, field uniformity and reducing SAR values in the head have been explored. An end tapered waveguide has been designed for local imaging and tested on the 7 T Philips Acheiva system. Further developments have been suggested for the end tapered waveguide by suggesting the design of a dielectric transformer. It is envisaged that the waveguide approach is ideally suited for a multi-transmit system which would employ a number of waveguide ports.

The interest in the growth of III-V compound semiconductors such as GaAs and AlGaAs on high index planes has increased tremendously over the last few years. The structural, optical and electrical properties III-V based structures are found to improve by, growing on (nil) planes. For example the amphoteric nature of silicon (Si) facilitates the Molecular Beam Epitaxy (MBE) growth of p-type GaAs/AlGaAs heterostructures on (311)A that have higher hole mobilities than those based on the conventional Be-doped p-type on (100) GaAs plane. The incorporation of intentional impurities, such as Si or Be in III-V semiconductors, have desirable effects in terms of controlling the electrical conductivity of the materials. However, other unintentionally incorporated impurities and defects have deleterious effects on the electrical and optical properties of III-V based devices. In this thesis, current-voltage-temperature (I-V-T), capacitance-voltage (C-V) Deep Level Transient Spectroscopy (DLTS) and Laplace DLTS techniques have been used to investigate defects in several MBE III-V epilayers and modulated structures grown both on the conventional (100) and non-(100) GaAs substrates. These include: (i) n-type silicon-doped (n11)B (n = 2-5) GaAs epitaxial layers; (ii) n-type silicon-doped (100) and (311)B GaAs/AlGaAs multi-quantum well (MQW); (iii) n-type silicon-doped (100) MQWs grown at different substrate temperatures, arsenic overpressures and arsenic species (As2 and As4); (iv) p-type Be-doped (100) and (311)A AlGaAs epitaxial layers; (v) GaAs/AlGaAs two dimensional electron gas (2DEG); (vi) commercially grown high electron mobility transistors (HEMT). The main findings of the experimental results are given in the following: 1. n-type silicon-doped (n11)B (n = 2-5) GaAs: the overall density of defects is highest in (211)B and lowest in (511)B. The number of detected defects is minimum in (511)B. The common carbon background impurity in MBE is observed only in (100) substrates. 2. n-type silicon-doped (100) and (311)B GaAs/AlGaAs MQWs: the concentration of the only trap is higher in (100) than in (311)B orientation. Furthermore, in (100) the observed trap electrically charged, while it has neutral nature in (311)B. 3. n-type silicon-doped (100) MQWs grown at different substrate temperatures, arsenic overpressures and arsenic species (As2 and As4): the average trap concentration for As2 samples is lower than Asa samples. In addition, the concentration of the common VAs-related point defect decreases with increasing growth temperature and arsenic overpressure. 4. p-type Be-doped (100) and (311)A AlGaAs: the number of hole traps in (311)A decreases from five to one when the Be-doping level varies from 1 x 1016 cm-3 to 1 x 1017 cm3. For (100) the detected hole levels are three, four and two for Be-concentrations of 1ix 016 cm -3 , 3x1016 cm-3 and 1x1017 cm 3, respectively. In addition, an electron emitting level is observed only in (100) samples doped to 1 x1017 cm 3. 5. GaAs/AlGaAs 2DEG and HEMT devices: one major defect, assigned to the DX center, is common in both in-house grown 2DEG and commercially HEMT devices. It behaves as a generation-recombination center, and its concentration is directly related to the silicon doping level in the AlGaAs layer. The HEMT devices which showed poor frequency response are found to have the highest concentration of the DX center.

The III-V ferromagnetic semiconductor Gallium Manganese Arsenide ((Ga,Mn)As) is one of the most interesting and well studied materials in spintronics research area. The first chapter is a brief introduction to spintronics, the properties of (Ga,Mn)As and the growth technique molecular beam epitaxy (MBE). Then the thesis presents a detailed study of the effect on the Curie temperature (TC) of varying the growth conditions and post-growth annealing procedures for epitaxially grown (Ga,Mn)As materials. The results indicate that it is necessary to optimize the growth parameters and post-growth annealing procedure to obtain the highest TC. From detailed magnetotransport studies, the carrier densities of high TC (Ga,Mn)As and H-doped (Ga,Mn)As have been achieved. It is found that the anomalous Hall resistance is the dominant contribution even at room temperature for these samples, which means it is incorrect to obtain carrier densities directly from Hall slope at high temperature. The results also show that the as-grown and lightly annealed H-doped (Ga,Mn)As samples have relatively high Curie temperatures down to low carrier density which make them good candidates for showing strong gate control of ferromagnetism. Besides (Ga,Mn)As, this thesis also discusses the studies of III-V ferromagnetic semiconductors (Ga,Mn)(As,P), (Al,Ga,Mn)As and some heterostructures based on these materials. The experimental investigation shows that a (Ga,Mn)(As,P) single layer grown on GaAs substrate has perpendicular anisotropy easy axis after annealing. It also demonstrates a method to suppress the diffusion of interstitial Mn ions during low temperature annealing from specific layers in (Al,Ga,Mn)As based heterostructures. The magnetometry study shows that the individual layers in the heterostructure have tailored magnetic properties, which makes this material useful for the further investigation of tunnelling magnetoresistance and spin transfer torque phenomena.

Quantum dots offer a number of advantages over standard fluorescent dyes for monitoring biological systems including high luminescence, stability against photobleaching, and a wide range of fluorescence wavelengths from blue to infrared depending on the particle size. In this work, we investigated in using the protein cage apoferritin as a template for the synthesis of colloidal quantum dots. We obtained apoferritin after reductive dissolution of the ferritin iron core and showed that the protein structure was left intact during this process. We further studied the solubility of ferritin, apoferritin and cationized ferritin in organic and fluorinated solvents by hydrophobic ion pairing methodology in order to expand the possibility of using an apoferritin template for the synthesis of quantum dots in organic media. We then focused on the synthesis and fluorescence properties of PbS quantum dots in aqueous solution. PbS dots are thermally stable and emit in the range 1,100 to 1,300 nm depending on their size. We demonstrated the encapsulation of these PbS quantum dots within the cavity of the iron storage protein apoferritin using two routes: 1) the disassembly/reassembly of apoferritin subunits trapping previously synthesised PbS quantum dots; and 2) use of the channels present in the protein shell to allow the entrance of Pb2+ and S2- ions leading to formation of quantum dots in the apoferritin cavity. We show that PbS-apoferritin composites emit in the near infrared region which make them promising labels for biological applications. Furthermore, we demonstrated that PbS QDs can be excited via a bioluminescence resonance energy transfer (BRET) using luciferin from Luciola mingrelica which could be developed into a self-illuminating labelling system. Finally, in order to make PbS-apoferritin composites selectively attachable to biomolecules during labelling experiments, the apoferritin was modified by the incorporation of analogues of methionine introducing azido groups absent in the proteins. The azido groups can then be selectively modified in complex mixtures e.g. cell lysates using `bio-orthogonal' reactions such as the Cu(I) catalysed Staudinger ligation and Huisgen cycloaddition. This would allow highly selective addition of receptor targeting or cellular permeation of peptides to the outer surface of the apoferritin shell.

This thesis describes the growth and characterisation of four different III-V semiconductor materials. Growth was primarily performed by molecular beam epitaxy, while characterisation, which was largely structural, was carried out mainly using X-ray difraction and atomic force microscopy. Growth of low temperature(LT)GaAs was undertaken to investigate whether a phase transition accompanies the structural transition which occurs when GaAs is grown at temperatures below ~ 150C. It was found however, that LT GaAs remains zinc-blende, albeit with a signicant degree of disorder. Migration enhanced epitaxy was subsequently used to grow LT GaAs, resulting in single crystalline GaAs at growth temperatures down to 115C. The possibility of using AlN as a source for nitrogen, in the growth of GaAs based dilute nitrides was explored. No conclusive evidence has been presented to suggest that small amounts of nitrogen were incorporated into the GaAs lattice. The potential for ScN to be used as a buffer layer/interlayer to reduce the defect density in cubic GaN (c-GaN) was investigated. It was found that ScN grows on c-GaN(0 0 1)/GaAs(0 0 1) in a (1 1 1) orientation, leading to overgrowth of GaN occurring in the hexagonal phase. If the ScN interlayer was sufficiently thin (<3nm), then overgrowth of GaN was cubic, but no evidence of a reduction in stacking fault density was observed. Growth of ScN on GaAs(0 0 1) was also found to result mainly in a (1 1 1) orientation, but films were of poor quality. Growth of ScN on ScAs(0 0 1) was subsequently explored. ScN was found to grow in a (0 0 1) orientation, with both smoother surfaces and improved material quality than ScN(1 1 1). Growth of GaN atop ScN(0 0 1) was found to be c-GaN(0 0 1), but insuffcient studies have been carried out to determine the effect on material quality. During the growth of InGaN, it was found that unmounted substrates lead to large temperatures rises (>100C) for In rich compositions. Modelling heat absorption due to bandgap, phonon and plasmon absorption showed that this is due primarily to the large number of free carriers and not to the narrow bandgap (wrt substrate). The preliminary doping of In0.8Ga0.2N with Mn was investigated. The amount of Mn that can be incorporated without causing a signicant reduction in film quality was found to increase with decreasing growth temperature.

Phase images of the human brain acquired using gradient echo based Magnetic Resonance Imaging techniques show excellent contrast at 7T. This contrast is attributed to small variations in magnetic susceptibility that perturb the main magnetic field and thus yield a spatial variation of the NMR frequency. The work described in this thesis is primarily concerned with mapping the distribution of magnetic susceptibility within the human brain using these phase images. The main technical challenges of the project were first to extract accurate field maps based on phase data, and then to solve the ill-posed problem of inverting these field maps to reconstruct susceptibility (ϰ) maps. In initial work, simulations of field shifts based on known ϰ -distributions are compared to field maps acquired in vivo to highlight the non-local relationship between measured field offsets and the underlying susceptibility. These simulations were carried out using a recently derived Fourier method. The bulk of the thesis is then devoted to a detailed study of the process of inverting field maps generated from phase data using the Fourier relationship to yield quantitative 3D ϰ -maps. Unfortunately, the inversion problem is ill-posed and requires careful conditioning, either through rotation of the sample being imaged or through regularisation. A simple k-space threshold is introduced to condition the inversion and the preliminary results of applying this method to brain data from healthy subjects and patients with Parkinson's disease and multiple sclerosis are presented. The results suggest that susceptibility mapping is sensitive to iron deposition and could be a useful tool in investigating the progression of neurodegeneratived diseases. Iterative inversion algorithms, which deal with noise more robustly and allow more sophisticated filtering techniques to be employed, are then presented. These powerful regularisation methods are compared to previously described techniques, and are shown to yield high quality whole-brain ϰ -maps.