In this thesis the structural and magnetic properties of two epitaxial heterostructures, Co₇₅Fe₂₅/Cu(110) and Co/Cu(311), are presented. The initial stages of growth are investigated, along with their interaction with gaseous adsorbates and overlayers. Furthermore, MgO is grown and studied for use as a tunnel barrier. For the growth of Co₇₅Fe₂₅/Cu(110) a 2D percolation magnetic phase transition is observed with a critical exponent γ = 2.385 ± 0.069, in excellent agreement with the theoretical value γ = 2.389. The changing ratio of the in-plane cubic and uniaxial anisotropy with film thickness leads to a rotation of the in-plane magnetic anisotropy. This rotation can be reversed by the addition of a submonolayer Cu overlayer. Dosing the system with oxygen revealed three distinct regions in the evolution of the coercive field: a decrease in the coercive field (I), the formation of a distinct peak (II), and a steady increase in the coercive field (III). The effect of dosing with hydrogen is qualitatively similar, although the observed features are generally less pronounced. In contrast to the CoFe/Cu(110) system for Co/Cu(311), a purely uniaxial magnetic anisotropy was observed for films of up to 15 ML; hence no rotation of the magnetic easy axis was observed. What is more, the results suggest a 3D not 2D phase transition.

The injection of spin polarised electrons is a fundamental challenge in the emergent field of spin electronics and is a prerequisite for the readout and initialisation for a spin qubit. This work is an investigation into spin injection from magnetic thin film contacts into (100) GaAs/InGaAs quantum wells as a function of energy, magnetic field, applied bias and temperature. The spin injection efficiency is derived from optical polarisation measurements in the oblique Hanle geometry. The Heusler alloy Co₂MnGa is investigated as a potential spin injector and is compared directly with Fe on the same III-V device substrate. <i>Ab-initio</i> methods are used to investigate the spin polarised electronic band structure of some of these half-metallic alloys – in particular Co₂ TiSn.

No description available.

This dissertation aims at a deeper understanding of the microstructural factors that origin and influence non-ohmic behaviour in ZnO ceramics. The third chapter presents results and discussion of the classical simulation results. We have carried out calculations on a large number of twist and tilt grain boundary systems and we have investigated the energetics and segregation properties of cobalt and barium to the Σ7 twist grain boundary core. We have determined the energies of a large number of twist and tilt boundaries and seen how they fit into known models. The structural units of the Σ13 tilt boundaries were identified and systems for the <i>ab initio </i>calculations selected. The electronic structure calculations on twist grain boundaries are presented in chapter four, along with the results for segregation of antimony and bismuth to the Σ7 and Σ13 boundaries. We have found a limit doping that is approximately the same for very different twist and tilt boundaries and suggests a maximum for the density of interfacial impurity states. The impurity states found were of a donor type and, although they were approximately in the right position in the band gap, they did not correspond to the acceptor states associated with varistor behaviour. The fifth chapter describes the results on tilt grain boundaries and doping with antimony, bismuth and defect complexes. We have investigated the shallow interface states that result from the tilt grain boundaries, the states introduced by oxygen interstitial impurities, the effect of Zn vacancies and the electronic structure of defect complexes. The sixth chapter presents a model intergranular film boundary. The charges, interface states and energetics are analysed as well as the possible origin of the compensation mechanism in intergranular thin films. We suggest that these films can be electrically active. The seventh chapter contains unrelated work on solid phases of Met-Car analogue clusters. We have calculated a number of clusters assembled phases and related their properties to the superconductive characteristics of <i>C</i>₂₀ solids.

This dissertation describes the development of atomic force microscope cantilevers as surface stress sensors for monitoring surface processes at the solid-liquid interface. Micromechanical bending-beams are highly sensitive stress sensors; stress changes as small as 10<SUP>-4</SUP>Nm<SUP>-1</SUP> can be detected using this type of bending-beam, whereas typical stress changes associated with monolayer processes are ˜0.1-1Nm<SUP>-1</SUP>. As well as offering high sensitivity, micromechanical cantilevers have a fast response time (˜0.5ms in liquids) which means they are ideally suited for fast stress measurements. Textured (111) surfaces were prepared by thermal evaporation of Au onto one face of the Si<SUB>3</SUB>N<SUB>4</SUB> cantilevers. These cantilevers were used as electrodes to monitor the stress changes associated with electrochemical processes, in conjunction with electrochemical measurements such as cyclic voltammetry and chronoamperometry. The potential-dependent adsorption of C1<SUP>-</SUP> and I<SUP>-</SUP> anions gave rise to stress changes of ˜0.5-1Nm<SUP>-1</SUP>, and in both cases, adsorption of the anion was associated with an increase in compressive surface stress. Underpotential deposition (UPD) of Pb and Ag on Au(111) has been studied in detail. For Pb UPD on Au(111) a compressive stress change of ˜1Nm<SUP>-1</SUP> was measured on deposition of one monolayer. More detailed features in the surface stress-potential curve have been directly related to features in the cyclic voltammogram and known structural processes such as the potential-dependent compression of the Pb monolayer. The measured stress changes for Ag UPD on Au(111) were also ˜1Nm<SUP>-1</SUP> and did not display a strong dependence on the nature of the anion present in the electrolyte.

The main motivation behind this work was to explore the potential of nuclear magnetic resonance (NMR) to study small molecule diffusion in porous materials and the relationship between structure and transport processes, with particular emphasis on liquid diffusion in polymers. The diffusion of small hydrocarbons into a range of semicrystalline polyethylene (PE) samples is studied. Under the penetration conditions employed the polymer crystallinity is retained, supporting the idea that the penetrant resides only in the amorphous region. Magnetic resonance imaging (MRI) is used to monitor the bulk transport of penetrating liquids into the polymers. Pulsed Gradient Spin Echo (PGSE) NMR is used to measure the self diffusivity of the penetrant. The transport diffusivity estimated from MRI shows good agreement with the self diffusivity when the degree of crystallinity is taken into account. The data presented show the importance of the phase and chain structure of the polymer on the diffusion and mobility of the penetrating species. Deuterium NMR relaxation time and line shape analysis is used to measure the correlation time of various motional process. By comparing the correlation time for translational motion with the PGSE self diffusivity it has been possible to estimate the penetrant jump distance to be in the order of a few nanometers. Molecular Dynamics (MD) simulations of hexane diffusion in amorphous PE are shown to give reasonable agreement with PGSE results. A mesoscopic lattice model is used to include the effect of the impenetrable crystallites on the diffusion process. Preliminary results using <SUP>31</SUP>P NMR spectroscopy to study the mobility and molecular scale distribution of additives show that some additives intimately mixed with the polymer while some reside in a discrete domain of around 100 nm. Diffusion weighted imaging is used to study liquid diffusion into polyethylene oxide (PEO) hydrogel. Despite the diffusion kinetics showing Fickian behaviour the concentration dependence of the diffusivity calculated from the PGSE measurements and from the concentration profiles are significantly different, highlighting the importance of the polymer chains in controlling the diffusion process. Three dimensional imaging shows that structural heterogeneities in catalyst support pellets can be characterised by a fractal parameter. The importance of slice thickness in determining the distribution of T<SUB>1</SUB> times from a 2D image is highlighted. NMR cryoporometry is used to measure the pore size distribution and the results compare well in nitrogen adsorption, and T<SUB>1</SUB> measurements. NMR cryoporometry is combined with MRI and the spatial variations in the pore size distribution are shown to be similar to those previously observed through NMR spin density and relaxation imaging.

No description available.

Thin films of the high-T<SUB>c</SUB> superconductor YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-x</SUB> (YBCO) are the key to devices that function at the temperature of liquid nitrogen. The main priority in depositing YBCO films is the ability to deposit smooth epitaxial films with the desired crystal structure and the correct level of oxygenation. This thesis describes the crystal structure, composition and initial growth stages of YBCO films prepared by planar dc high pressure sputtering. X-ray diffraction (XRD), atomic force microscopy (AFM), and electrical measurements have been used to study the films. Prior to a description of these experiments, two preliminary chapters are included in this thesis. The first chapter is an introduction to some basic concepts of superconductivity, and the crystal structure as well as the physical properties of YBCO. The second chapter reviews important issues and compares the different deposition techniques for preparing YBCO thin films. In a preliminary study of YBCO films deposited under different PxD conditions (the product of pressure and target-to-substrate distance), it was found that a PxD of 2000 Pa-mm is necessary to prevent preferential resputtering. A study of the thermalisation of the high energy particles emanating from a sputtering target was carried out on both Nb films and YBCO films. The sputtering atmospheres and the time for which the ceramic YBCO target has been operated under such atmosphere was found to influence the superconducting critical temperatures of sputtered YBCO films. The effect of oxygenation of the target surface on the superconducting properties of films deposited was studied with various amounts of hydrogen gas added to the sputtering gas or residual water vapour in the vacuum system. The initial stages of YBCO films grown on SrTiO<SUB>3</SUB> substrates were investigated with different film thickness. For the 1-nm-thick film flat island nuclei with one-unit-cell height were found using AFM which is consistent with the observation by XRD. Secondary phases were found to appear in the 2-nm-thick film. The effect of secondary phase precipitates on the superconducting properties and surface morphology of the films is discussed. Spiral growths were found to appear in films with thicknesses larger than 24nm, and their origin is discussed. To study the effect of substrate lattice misfit on the initial stages of film growth, NdGaO<SUB>3</SUB> substrates were chosen.

No description available.

The work contained in this thesis addresses the single and multicomponent adsorption of hydrocarbons in silicalite. Both experimental and numerical work is presented. Experimental measurements are made using a volumetric technique and many equilibrium adsorption studies are reported for the first time. Multicomponent adsorption studies have revealed several novel adsorption phenomena. Intermolecular interaction within silicalite has been probed using deuterium nuclear magnetic resonance (<SUP>2</SUP>H NMR) experiments. For the first time data are presented for both single and multicomponent adsorbates within the structure and the effect of co-adsorption upon molecular microdynamics is investigated. Incorporating <SUP>2</SUP>H NMR data obtained with the network model for the adsorbent in a numerical simulation known as Monte Carlo Lattice Dynamics, has enabled equilibrium predictions to be made for single and multicomponent adsorption. On comparison with the experimental data obtained the results achieved by Monte Carlo Lattice Dynamics are very good and demonstrate the strength of the simulation orthodoxy. Molecular mechanic simulations, together with data obtained from <SUP>2</SUP>H NMR and volumetric studies have also been used to probe the behaviour of the external surface of silicalite in the study of adsorbate uptake kinetics. Results indicate that a surface-moderated insertion can affect the uptake of a component from the gas phase and hence affect the gross adsorption uptake properties of the material. Binary component studies have also been performed in which the interplay between adsorbate surface mobility and molecular desorption rates can affect a separation between components.