The increased signal-to-noise ratio available at high magnetic field makes possible the acquisition of clinically useful MR images either at higher resolution or for quantitative methods. The work in this thesis is focused on the development of quantitative imaging methods used to overcome difficulties due to high field MRI systems (> 3T). The protocols developed and presented here have been tested on various studies aiming at discriminating tissues based on their NMR properties. The quantities of interest in this thesis are the longitudinal relaxation time T1, as well as the magnetization transfer process, particularly the chemical exchange phenomenon involving amide protons which is highlighted particularly well at 7T under specific conditions. Both quantities (T1 and amide proton transfer) are related to the underlying structure of the tissues in-vivo, especially inside the white matter of the brain. While a standard weighted image at high resolution can provide indices of the extent of the pathology, a robust measure of the NMR properties of brain tissues can detect earlier abnormalities. A method based on a 3D Turbo FLASH readout and measuring reliably the T1 in-vivo for clinical studies at 7T is first presented. The other major part of this thesis presents magnetization transfer and chemical exchange phenomena. First a quantitative method is investigated at 7T, leading to a new model for exchange as well as contrast optimization possibility for imaging. Results using those methods are presented and applied in clinical setting, the main focus being to image reliably the brain of both healthy subjects and Multiple Sclerosis patients to look at myelin structures.

pintronics is a rapidly developing field in solid state physics based on the quantum property of spin angular momentum. It has the potential to offer a new generation of electronic devices exploiting spin properties instead of, or in addition to, charge. Such quantum-based devices are expected to demonstrate significant advantages over traditional charge based electronics with a promise of faster data processing speeds and lower power consumption. One of the most widely studied spintronic materials is the dilute magnetic semiconductor gallium manganese arsenide ((Ga,Mn)As). This continues to be a valuable test ground for spintronics applications due to its close relation to the traditional, and well-characterised, semiconductor GaAs, and its relatively high Curie temperature despite values remaining some way off the much sought-after room temperature. The two primary focuses of this thesis are phase-coherent transport and critical phenomena, both of which whilst well understood in metals have seen limited work in (Ga,Mn)As. Critical behaviour in particular has not been extensively studied despite continued disputes over theoretical models and resistance peak positions relative to Curie temperature. Studies of both these areas are presented within this thesis split over four main chapters. The first of these chapters acts as a general introduction to spintronics, and includes both a brief history of the subject, and a theoretical overview focused on the structure and properties of (Ga,Mn)As. This introductory chapter also includes an in-depth review of nanofabrication including typical processing techniques and their applications to the study of spintronics in Nottingham. The second chapter presents a comprehensive study of critical phenomena within (Ga,Mn)As, showing how the behaviour of magnetic properties close to Tc are strongly correlated between samples. Both magnetisation and susceptibility are found to demonstrate behaviour very close to that predicted by the Heisenberg model; a result in strong agreement with theoretical work. The study of critical behaviour is carried over into the third chapter with transport measurements showing that resistance data can be directly used to accurately measure sample Curie temperature by finding the peak in the derivative deltaR/deltaT. This potentially offers an alternate approach to calculating Tc that is faster and cgeaper than the more conventional magnetometry or Arrott plot methods. Analysis is also carried out on the resistance peak which is expected to follow the critical behaviour of the specific heat. The final experimental chapter focuses on the development of nanoring fabrication processes in (Ga,Mn)As including the difficulties associated with fabricating nanoscale structures, the testing performed to achieve high quality, reproducible structures, and the final adopted recipe. This chapter then details early test measurements on these devices including an initial study on the first structures within a dilution refrigerator, and prelimenary work on a second improved batch at 4He temperatures. This work will act as a foundation for the future aim of conducting a full phase-coherence phenomena study in highly optimised (Ga,Mn)As samples grown in Nottingham.

Scanning probe microscopy techniques were employed to investigate C60 molecules adsorbed on Si(111)-(7x7) and Ag-Si(111)-(√3x√3)R30o using imaging, spectroscopy, and manipulation methods. First, dynamic scanning tunnelling microscopy revealed the lowest unoccupied molecular orbital features of C60 molecules adsorbed on Si(111)-(7x7) with extremely high resolution at 77 K. Experimental data were compared with Hückel molecular orbital theory simulations to determine the orientation of the molecules on these surfaces. Second, C60 molecules were imaged with a qPlus atomic force microscope, in the attractive force regime and appeared as bright spherical protrusions. The potential energy of interaction between the AFM tip and C60 molecules adsorbed on Si(111)-(7x7) was quantified by force spectroscopy. Furthermore, a C60 molecule was transferred to the scanning probe microscope tip and used as molecular probe to image the Si(111)-(7x7) surface and other C60 molecules. The on-tip C60 molecule was imaged with high precision. Hückel molecular orbital theory calculations accurately predicted the shape and characteristics of molecular orbitals observed with dynamic scanning tunnelling microscopy, which were strongly dependent on molecular symmetry, orientation, and adsorption angle. Using qPlus atomic force microscopy, chemical reactivity was probed close to or at the carbon atom positions in the C60 cage. Density functional theory simulations showed that an (iono)covalent bond formed between a carbon atom and the underlying Si adatom was responsible for contrast formation. The pair potential for two C60 molecules was also determined experimentally and found to be in very good agreement with the Girifalco potential (Girifalco, L.A., J. Phys. Chem., 1992. 96(2): p. 858). Using Hückel molecular orbital theory, the mutual orientation of a C60 molecule adsorbed on the STM/AFM tip and a C60 molecule adsorbed on the Si(111)-(7x7) surface was determined via comparison of simulated images to the experimental data. Individual C60 molecules were also manipulated with qPlus atomic force microscopy. Manipulation of single C60 molecules was performed on the Ag-Si(111)-(√3x√3)R30o surface using scanning tunnelling microscopy at room temperature and at 100 K. The interaction was predominantly attractive. Due to weak molecule-substrate interaction, a short-range chemical force between the C60 molecule and the tip was considered to be responsible for the manipulation process.

The relationship between meal structure and composition can modulate gastrointestinal processing and the resulting sense of satiety. This applies also to the fat component of meals and particularly to the surface area available for digestion. The main hypothesis underpinning this thesis work was that fat emulsion droplet size has a profound effect on fat digestion and, in turn, on the gastrointestinal and satiety responses. To test this hypothesis two fat emulsion meal systems were used. They had exactly the same composition but a small (termed the Fine emulsion, with a droplet size of 400 nm) or a large (termed the Coarse emulsion, with a droplet size of 8 μm) emulsified fat droplet size. The two fat emulsion systems were manufactured and characterised using a range of bench techniques, in vitro digestion models and MRI techniques in vitro. The difference in microstructure caused different temporal creaming characteristics for the emulsions and different percentage hydrolysis profiles in a gastric digestion model in vitro. The Fine emulsion showed initial rapid hydrolysis whilst the Coarse emulsion showed an initial slow hydrolysis phase with the hydrolysis rate increasing at later stages. This indicated that there was indeed a droplet size effect on fat hydrolysis whereby the smaller droplet size with a larger surface area hydrolysed faster than a larger droplet size. The emulsions’ performance was finally tested in vivo in healthy volunteers using MRI in a series of pilot studies leading to a main physiological study. Creaming differences in the gastric lumen were addressed by redesigning the meals using a locust bean gum (LBG) thickener that made them stable throughout the gastric emptying process. A main three-way physiological and satiety study in healthy volunteers showed that a highly emulsified, intragastrically stable emulsion delayed gastric emptying, increased small bowel water content and reduced consumption of food at the end of the study day. Finally, magnetic resonance imaging, relaxometry and spectroscopy were further evaluated to assess fat emulsion parameters in vitro and in vivo in the gastric lumen. Main static magnetic field and droplet size effects on T2 relaxation times of the Fine and the Coarse emulsions were observed. There was reasonable correlation between m-DIXON and spectroscopy methods to quantify fat fraction both in vitro and in vivo. Differences in T2 relaxation times for different droplet sizes of 20% fat emulsions were detected in vitro. These changes were however difficult to separate from creaming effects in vivo with a view of drawing meaningful inferences on droplet sizes. The main conclusion from this work was that manipulating food microstructure especially intragastric stability and fat emulsion droplet size can influence human gastrointestinal physiology and satiety responses and that MRI and MRS provide unique non invasive insights into these processes. This improved knowledge could help designing foods with desired health-promoting characteristics which could help to fight the rising tide of obesity.

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.