Astrophysical observations of the solar photosphere uncover a wealth of detailed structures that arise from the interaction of vigorously convecting plasma and the internally generated magnetic elds. The most prominent features are sunspots, which exhibit sub-structures on a range of scales. Speci cally within the umbra is an intensity pattern consisting of individual small bright points, referred to as umbral dots. These states are thought to indicate the presence of localised magnetoconvective motions. This thesis discusses the applications of magnetoconvection to the umbra, with the aim of investigating the occurrence of steady and oscillatory localised states known as convectons. Convectons are isolated convective plumes from which magnetic ux is at least partially expelled. In two-dimensional Boussinesq magnetoconvection we examine both a simpli ed model, in which the vertical structure has been reduced, and a fully-resolved model. In performing parametric surveys of the steady modes we attempt to understand how localised states di er between the two models. Examining the oscillatory localised cells we locate, for the rst time, these states in the fully-resolved system. Both of these models are horizontally periodic. We nd that by altering these horizontal boundaries so that they are impermeable to uid motions does not impede the existence of these states but leads to the additional existence of a new set of solutions that are localised at the boundaries. To examine the bifurcation structure of these states we develop a numerical continuation model. However, due to the limitations of the continuation program, AUTO-07p, this model has restricted symmetries and impermeable horizontal boundaries. Despite these simpli cations the symmetries of the model ensure that convectons can still be found and in addition allows the examination of the wall states. The remainder of this thesis focuses on compressible magnetoconvection. In studying oscillatory convectons in two-dimensions we nd a new type of oscillation not found in the Boussinesq models. This state no longer retains Boussinesq point symmetry but has more gentle extended up ows characteristic of a three-dimensional cylindrical plume. In three dimensions a new type of steady convecton is found with a broken symmetry such that the cross-section corresponds to a single overturning roll.

This thesis describes a new technique for determining the magnitude of the palaeomagnetic field, and the results of applying it to a transition from reversed to normal polarity. Several successful experiments were carried out on modern lavas and on archaeomagnetic material to test the accuracy of the technique. The results of these experiments demonstrate that this technique gives accurate, consistent results. The technique was applied to samples from lavas that were extruded during a palaeomagnetic field reversal. The variation of the magnitude of the palaeofield with time was successfully determined with a mean error (standard deviation) of only O.03T (3000 gamma). The results indicate that the palaeofield was large and st~ble during a period when the virtual magnetic north pole seems to have lingered at the geographic equator. This and other published results suggest that an intermediate state .of the geomagnetic field can exist which is sometimes as strong as the more usual normal and reversed states, and which endures metastably for short periods of time. This means', among other things, that any single transition may appear to be quite complex, although the average transition is known to involve fairly simple geometry. It also constrains future theories of the generation of the geomagnetic field to include the phenomenon of "intermediate metastable states".

No description available.

This thesis work addresses the characterisation of structural and magnetic properties of Ni2MnGa based Heusler alloys. The alloy series Mn1+xNi2-xGa has been investigated experimentally by magnetization measurements, X-ray and neutron scattering. The systematics is established as a function of Mn content of the magnetic and structural properties. Of particular interest is the development of the martensitic phase as a function of Mn content, its hysteresis behaviour and the influence of chemical order as established by a suitable heat treatment. A comprehensive experimental characterisation is given of the alloys series Mn1+xNi2-xGa for 0≤x≤1. The experimental investigation is complemented by modelling of domains in pure Ni2MnGa. Based on a detailed experimental determination of transformation matrices for the austenite martensite transformation a characterisation is given of domain walls and their crystallographic orientation. The descriptions and models which exist in the literature are extended to enable the modelling of individual domain walls within a single crystal of Ni2MnGa and its extension to several domain walls. The framework for this modelling is extended to enable the description of domain wall networks. Particular domain wall network configurations are identified and described. Within the topic of shape memory materials in general, and of the ferromagnetic shape memory Ni2MnGa based alloys in particular, this thesis work offers insight into some important aspects of their physical properties. A range of approaches is used to address some of the characterisation issues for these compounds.

The Onsager-Kaufman method is used to evaluate the partition functions of "face-centred" square Ising lattices (with zero magnetic field), i.e. square lattices which have an additional spin in the centre of each square, these spins interact with the four spins at the apices of the surrounding squares. For a certain range of the ratio of the interaction energy between pairs of apical spins, and, that between a central spin and an apical spin, there are three infinities in the specific heat - temperature curve. The same result, may hold for square Ising lattices in which both first and second nearest neighbours interact, but, Kaufman's method cannot be used for this problem.

No description available.

This thesis presents investigations of liquid ingress and vapour uptake in different porous media including textile fabrics and activated carbons, monitored by means of a unilateral NMR instrument. The aim of this work is to assess protective materials which prevent toxic liquid ingress and toxic vapour uptake from contaminating materials and personnel. A high performance fabric made of a combination of coated and not coated fibres can provide extremely high protection against toxic liquids. By incorporating an adsorbent layer between two highly repellent layers, an “intelligent” fabric that can prevent complete penetration through the composite system by toxic vapours can be constructed. This project was undertaken with a low-field unilateral profile NMR Mouse® (MObile Universal Surface Explorer) which can collect signal from a thin and flat sensitive volume (ca. 1.5 cm x 1.5 cm x 0.6 mm) up to 10 mm above it, and in a non-invasive manner. The instrument uses a strong inherent magnetic field gradient (11.38 T.m-1) in conjunction with pulsed radio frequency waves. The method makes use of Fourier Transformed NMR in order to spatially resolve 1D vertical profiles for each measurement over a field of view exceeding 500 µm and with a spatial resolution of 15 µm. One system investigated was a laminate heterogeneous layered fabric, made of a horizontal stack of three individual layers, each 70 μm thick, constructed from entangled fibres of 10 µm in diameter. The top and bottom layers are strongly repellent to the oil used as a model that represents a simulant for a toxic liquid, whilst the middle layer is non-repellent and allows oil to absorb inside. The ability of the liquid challenge (source of hydrogen-containing molecules in the form of a droplet) to penetrate the textile structure depends on the external pressure applied to the droplet; the effect of this pressure is extensively investigated in this work. In further experiments, the middle layer of the fabric was replaced by an activated carbon cloth textile layer (200 μm thick) made of woven fibres. Activated carbon is an extremely powerful adsorbent into which one or more challenge(s) can be adsorbed at a time and has therefore a great aptitude to contribute to the effectiveness of protective textiles. In this work it is demonstrated that the ingress of liquids as a function of pressure vertically applied to the top of the sample can be dynamically monitored via a vertical stack of 1D NMR profiles. It was possible to non-invasively and non-destructively quantify the amount of liquid penetrating into the middle non-repellent layer (or activated carbon cloth layer) as a function of pressure. The ability of unilateral NMR to monitor the adsorption process of vapours onto activated carbons cloths was also explored. The monitoring technique was extended to other types of activated carbons including monoliths and composites. Spatially resolved one dimensional profiles of both the transverse NMR relaxation rate (R2) and the absolute quantity of adsorbate present at a given position across the selected slice were obtained for three different activated carbons (monolith, composite and cloth) which are challenging samples from the point of view of their electrical conductivity. The NMR amplitude is shown to strongly correlate with T2 and, when calibrated, the NMR amplitude is shown to reveal the saturation level. The NMR relaxation rate reveals subtle information about pore filling, which allows quantitative assessment of the system’s saturation level, without the need for calibration, provided that the molecule under investigation is known. Differing dynamic variations in R2 are seen for one adsorbate to another. For the case of ingress into textile cloth layer, the data suggests that the bigger pores are filled before the smaller pores.

No description available.

No description available.

The work described in this thesis consists primarily of the development of an Electron Spin Resonance (E.S.R.) spectrometer, having a very rapid response time and therefore capable of observing E.S.R. signals from free radicals within microseconds of their formation. The development has proceeded in three phases corresponding to response times of 1 millisecond, 50 microseconds and 1 microsecond. The special apparatus required for each phase is described and the results obtained with each on a trial system, benzophenone, are discussed. The most important requirement of a rapid response spectrometer is a wide bandwidth in all the signal handling circuitry. It is shown how this bandwidth has been achieved in practice and how such an extension of bandwidth leads to a loss in spectrometer sensitivity. The use of signal averaging to recover the lost sensitivity is described and it is shown how by this means it has proved possible to obtain usable E.S.R. signals within 1 microsecond of the radicals formation. It is seen that broadening of the magnetic energy levels in accordance with the uncertainty principle makes any attempt to pursue E.S.R. detection to times appreciably shorter than 1 microsecond pointless. The final spectrometer is described in detail and. consists of a conventional spectrometer with the normal 100 kHz magnetic field modulation replaced by a 2 MHz modulation and detection system specially designed to achieve a wide bandwidth. A highly stable magnetic field is required if efficient signal averaging is to be achieved over long periods. This is ensured by the use of a Hall-effect probe to monitor and stabilise the field. Radicals are produced by a pulsed ultra violet laser, and the information contained in the resulting E.S.R. signal is stored and averaged digitally. Three ways of operating the apparatus are described, the first providing the maximum possible information from the system but involving a considerable amount of manual data processing if a spectrum is required. The second and third methods both provide a spectrum directly and more rapidly but at the expense of all the kinetic information and a possible loss in sensitivity. The three methods largely complement each other and are all in use. For all the development work benzophenone has been chosen as the molecule for investigation. Spectra are obtained for the ketyl radical anion produced from the photolysis of this molecule in alkaline aqueous isopropanol and the ketyl radical produced from its photolysis in liquid paraffin. The latter is seen to display a previously unsuspected population inversion in the magnetic spin states when first formed, the result of which is that during the first few tens of microseconds of the radical's life it displays a totally emitting E.S.R. spectrum. Previously the only emitting spectra to be seen in the liquid phase displayed emission in only some of their hyperfine lines, an effect explicable in terms of a polarisation of the nuclear spin energy levels. Totally emitting spectra require a polarisation of the electron spin energy levels when the radical is first observed. The initially emitting signal decays to a conventional absorbing signal with. a characteristic relaxation time. This in turn decays to zero in a normally much longer tine as the radicals themselves react and are lost. Using the apparatus in the first mode of operation provides a complex decay curve which yields information on the initial spin polarisation, the relaxation time and the kinetics of the radical loss. Such curves are illustrated and a computer program is described which simulates the complex decay curves and thereby aids their analysis. Spectra are obtained for both emitting and absorbing radicals and a second program is described for simulating such spectra and thereby confirming their analysis and hence yielding hyperfine splitting constants for the radical. A possible qualitative explanation to explain the emission phenomenon is proposed and it is shown how this corresponds to the recently observed phenomenon Chemically Induced Dynamic Nuclear Polarisation (CIDNP), in which partially emitting N.M.R. spectra are observed from the products of free radical reactions. By analogy the name CIDEP, Chemically Induced Dynamic Electron Polarisation, is given to the emission effect observed here. The mechanism supposes that the radicals are initially formed in pairs of which some recombine and others escape. The relative ease of escape and recombination is dependent upon the spin state of the radical pair and thus both the recombination products and the escaped radical exhibit a polarisation of the energy level populations. In the latter this drives rise directly to the phenomenon of CIDEP whilst in the former the non-equilibrium electron population 'pumps' the nuclear spin states via an Overhauser effect thereby producing the relatively long-lived nuclear polarisation observed as CIDNP. Finally the future development and use of the apparatus is considered and various possible ways of overcoming the data handling problems of the first mode of operation are discussed.