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Stacked commercial second generation (2G) high temperature superconducting (HTS) tapes has been demonstrated to have great field trapping ability, with superior mechanical, thermal properties and crossed-field performance to HTS bulks. HTS stacks are considered as a very promising candidate for superconducting permanent magnets, which can be used for many applications like superconducting machines and levitation devices. However, the current magnetization methods for HTS stacks are the conventional magnetization methods developed for HTS bulks decades ago, which have various limitations. On the other hand, the recent progress on HTS flux pumping technology enables HTS coils to be successfully magnetized in an effective and efficient way. In this thesis, these two concepts were combined for the first time. Flux pumping for HTS stacks was proposed and developed as a new magnetization technique. I employed two types of flux pumps, the rotating magnet flux pump and the rectifier type flux pump, to magnetize the modified HTS stacks. The design of the flux pumps, the experimental results and the analysis of the observed behaviours are presented in the thesis. This research provides a new direction in the magnetization of HTS permanent magnets, which has the potential to make HTS permanent magnets more practical with lighter, more compact and efficient magnetization setups.

This thesis presents the results of a combined magnetic fabric and palaeomagnetic analysis of lower crustal rocks exposed in the Oman (Semail) ophiolite. This has long been an important natural laboratory for understanding the construction of oceanic crust at fast spreading axes and its subsequent tectonic evolution, but magnetic investigations in the ophiolite have been limited. Analyses presented here involve using: (i) magnetic anisotropies as a proxy for magmatic petrofabrics in lower crustal rocks in order to contribute to outstanding questions regarding the mode of accretion of fast-spread oceanic crust; and (ii) classical palaeomagnetic analyses to determine the nature of magnetization in these rocks and gain further insights into the regional-scale pattern of tectonic rotations that have affected the ophiolite. The extensive layered gabbro sequences exposed in the Semail ophiolite have been sampled at a number of key localities. These are shown to have AMS fabrics that are layer-parallel but also have a regional-scale consistency of the orientation of maximum anisotropy axes. This consistency across sites separated by up to 100 km indicates large-scale controls on fabric development and may be due to consistent magmatic flow associated with the spreading system or the influence of plate-scale motions on deformation of crystal mushes emplaced in the lower crust. Detailed analysis of fabrics in a single layer and across the sampled sections are consistent with either magmatic flow during emplacement of a melt layer into a lower crustal sill complex, or traction/drag of such layers in response to regional-scale stresses (e.g. mantle drag). Together, results support formation of the layered gabbros by injection of melt into sill complexes in the lower crust. New anisotropy data from the overlying foliated gabbros sampled at two key localities also provide insights into the style of melt migration at this crustal level. Fabrics are consistent with either focused or anastomosing magmatic upwards flow through this layer, reflecting melt migration beneath a fossil axial melt lens. Previous palaeomagnetic research in lavas of the northern ophiolitic blocks has demonstrated substantial clockwise intraoceanic tectonic rotations. Palaeomagnetic data from lower crustal sequences in the southern blocks, however, have been more equivocal due to complications arising from remagnetization. Systematic sampling resolves for the first time a pattern of remagnetized lowermost gabbros and retention of earlier magnetizations by uppermost gabbros and the overlying dyke-rooting zone. Results are supported by a positive fold test that shows that remagnetization of lower gabbros occurred prior to Campanian structural disruption of the Moho. NW-directed remagnetized remanences in the lower units are consistent with those used previously to infer lack of significant rotation of the southern blocks. In contrast, E/ENE-directed remanences in the uppermost gabbros imply a large, clockwise rotation of the southern blocks, of a sense and magnitude consistent with that inferred from extrusive sections in the northern blocks. Hence, without the control provided by systematic crustal sampling, the potential for different remanence directions being acquired at different times may lead to erroneous tectonic interpretation.

The formation of magnetic order in solids is a complex and subtle issue. There are a wide range of different types of magnetisation, all of which may be favoured under different circumstances. In this thesis we consider a novel combination of ideas where the formation of spatially modulated magnetisation is linked to a metamagnetic transition. In this we are inspired by a general principle of modulated phases intervening as intermediate states in phase transitions. In particular we draw analogies with the Fulde-Ferrell-Larkin-Ovchinnikov state of spatially modulated superconductivity. We study a mean-field theory for itinerant magnetism where the crystal lattice drives the formation of a rich phase diagram. A peak in the electronic density of states due to a van Hove singularity creates ferromagnetism and a metamagnetic transition. Furthermore we find that a modulated magnetic phase - a spin-spiral, becomes favoured along the metamagnetic transition line. The appearance of this phase causes the metamagnetic transition to bifurcate to enclose the modulated region. The topology of this reconstructed phase diagram shows remarkable similarity to that observed in experiments on Sr₃Ru₂O₇. This material shows a metamagnetic transition which can be tuned by field angle towards zero temperature. Before this point is reached a new phase with high and anisotropic resistivity appears. We believe that this anomalous phase can be explained by the formation of a phase of modulated magnetisation caused by a peak in the electronic density of states. This mechanism may also apply in a range of other materials as it is driven by rather generic features of the bandstructure.

Single-crystal Tm/Y and Tm/Lu superlattices have been grown using molecular beam epitaxy and their chemical structures have been determined using X-ray diffraction. Magnetisation measurements have revealed a more complicated phase diagram than that of pure Tm. Application of a field along the c-direction gave rise to an extra transition, and transitions were detected for the superlattices when the field was applied along the b-axis. In neutron diffraction studies, c-axis longitudinally modulated magnetic structures were found for both Tm/Y and Tm/Lu, which propagate coherently through the non-magnetic layers. In the case of Tm/Lu superlattices, there is evidence for ordering of the basal plane components.

The most important device for today's large scale information storage is the magnetic hard disk drive. This is because it can store vast amounts of data and also provides the fastest way of accessing this valuable information. A current state of the art commercially available hard disk has data rates in excess of 1 GHz which means the magnetic bits are required to reverse in less than one nanosecond. The areal density is greater than 10 Gbits/in2 which requires extremely small magnetic grains with sizes of approximately 10 nm. These are so small that their magnetisation can be affected by the ambient thermal energy and in extreme cases the information can be lost. It is imperative, for our rapidly growing system requirements, that we characterise and understand the mechanisms of thermally activated high frequency magnetisation dynamics, and that is the main aim of this thesis. Our approach is based upon the numerical solution of the Langevin equation of the problem. The solution of this stochastic differential equation is carried out by using Stratonovich stochastic calculus methods which generates the fluctuating trajectories of the individual magnetic moments. The model was first applied to the study of thermal relaxation and computed results showed non-exponential relaxation over small energy barriers. The coercivity was seen to rapidly increase for very high frequency field pulses which is important as regards to the ultimate switching speeds in recording. The short time scale breakdown of the Arrhenius-Neel law for a single magnetic moment is demonstrated and explained in terms of the dynamics of the precess ional motion. The variation in response as a function of the damping parameter is found to be an important factor determining the remanent magnetisation for a given pulse width. The effects of interactions between moments are described, including the apparent increase in effective damping. It is shown that interactions between grains can be described in terms of thermally excited spin waves.

A probe for measuring the specific heat of superconductors at low temperatures and in high magnetic fields has been built and commissioned. The probe has been tested using the relaxation method on samples of copper and the accuracy of the data is 1.3 % between 5 K and 30 K, data taken using the long range pulse method has a resolution of 10 mK. Specific heat measurements have been performed on members of the series (Pb(_1)-(_x))Cu(_1.8x)Mo(_6)S(_8), (Sn(_1-x))Eu(_x)Mo(_6)S(_8) and (Pb(_1-x)M(_x))Mo(_6)S(_8) where M = Gd and Eu, from 3 K up to 30 K and in magnetic fields up to 15 T. Additional results from resistivity, susceptibility, magnetisation. X-ray diffraction, transmission electron microscopy and electron dispersive-ray measurements are also presented. These data have been compared to results from other authors and are analysed in terms of the BCS and GLAG theories of superconductivity and the magnetic properties of these materials. The mean field model has been used to calculate numerically the magnetic contribution to the specific heat (cm) of both ferromagnetic and antiferromagnetic systems as a function of temperature and applied field both above and below the ordering temperature. In addition an approximate analytic form for the magnetisation has been used to calculate Cm above the ordering temperature. Expressions have been derived for the saturation value of the peak in C(_m): C(^sat)(_m) = 1.1245n(_cell)RJI(J+1) and the temperature dependence of the peak with applied field ȡ(μ(_o)H(_ext))/ȡT(_peak)=6.540/g(_J)(J+1). They allow the simple calculation of the values of J and g(_J)(J + 1) from specific heat data. The magnetic contribution to the specific heat of the samples (Sn(_0.65)Eu(0.35)Mo(_6)S(_8)) and (Sn(0.50)Eu(_0.50)Mo(_6)S(_8)) have been modelled using these calculations and excellent agreement is found by considering the magnetic ions as free ions. The sample is accurately modelled by including an additional minority phase (Gd(_2)S(_3)). The approximate expressions have also been used to analyse data on high temperature superconductors producing values of J and g(_J)}{J + 1) consistent with a doublet ground state. The properties of Chevrel phase materials have been determined as a function of doping level. The critical temperature is degraded by doping but an increase in the critical current density is observed in the series (Pb(_1-x)Cu(_1-8x)Mo(_6)S(_8) for very low levels of doping. Increases of up to 28 % in the upper critical field, that are probably due to the compensation effect and an increase in the normal state resistivity, are also observed in the series (Sn(_1-x)Eu(_x)Mo(_6)S(_8)) at high levels of doping and in the series (Pb(_1-x)Gd(_x)Mo(_6)s(_8) for low levels of doping.

A model for studying magnetisation during FC-PEDRI experiments has been developed and verified experimentally. The optimum EPR irradiation frequency, which maximises the Overhauser enhancement while minimising the RF power deposition in sample, was investigated using the FC-DNP technique. Experiments to verify the model were carried out in the EPR frequency range between 20 and 180 MHz and with different applied ERP RF power for free radicals with different EPR linewidths. The optimum EPR irradiation frequency was found to be dependent on the applied power level and on the EPR linewidths. Methods to obtain NMR images with T1-weighted were also investigated. The most successful method was found to be a combination of Inversion Recovery and FC-PEDRI. This technique yields both high T1 contrast between samples and good contrast between images with different T1 values. The free radical distribution is recorded with the same SNR as those collected with the conventional FC-PEDRI techniques. Rapid imaging of FC-PEDRI has also been studied. The number of EPR irradiation periods can be optimised to give a good free radical distribution image while reducing the power deposition in the sample. With 4 EPR irradiation periods, the difference image displays the free radical distribution comparable with the conventional technique (64 EPR irradiation periods). The power deposition in the sample is reduced by the factor of 16 and acquisition time is reduced by a factor of 4. Finally, inversion recovery spin preparation has been added to the hybrid fast sample. It was found that an IR hybrid FISP FC-PEDRI pulse sequence could be used to generate T1 contrast in images which show similar detail and intensity of the free radical distribution as those obtained using hybrid FISP-PEDRI.

The processing power of the computer has increased at extraordinary rates over the last few decades. However, even today's fastest computer can take several hours to find solutions to some complex mathematical problems, and there are instances when a high powered supercomputer may be impractical, with the need for near instant solutions just as important. One approach to this kind of problem is through machine learning. This research investigates the application of various neural network techniques, and aims to solve known complex inverse problems in the field of magnetic and optical recording. Investigations were conducted to determine anisotropy distributions from transverse susceptibility data ( an area of magnetism which is proving to be very important, as it gives information about the overwrite characteristics of the media). The neural networks produced results which were very impressive, and compared well with other more conventional methods (solutions were obtained in a fraction of the time). More importantly, it suggested that this technique was indeed feasible, and could be used to solve a series of similar inverse problems. The neural network was then modified to investigate diffraction patterns from a compact disc. This was an interesting application which had practical uses in industry, as the idea of reducing the testing time for each disc was very attractive. Simulations on theoretical data were successful, and suggested that this method could be carried out experimentally on an online testing system. The final part of the research involved the extraction of important features from magnetisation maps and magnetic force microscopy images. The use of neural networks to study general image analysis is well established, but its use in magnetism was quite novel. The results obtained were surprisingly good as the images investigated hardly contained enough data for the human eye to observe, and features such as percolation and vortices were observed on a variety of samples. This success has prompted suggestions as to ways in which this approach can be expanded to solve similar problems.

Over recent years the investigation into the magnetic behaviour of nanostructured p ermalloy has become more advanced due to improvements in numerical micromag-netic methods on the theoretical side and high accuracy electron-beam lithography methods exp erimentally. The interest in such structures of magnetic material is increasing mainly due to the possible potential use in future high-density magnetic storage media a pplications. When the material is discretised into a nanoelement structure at the sub micron level t heoretical micromagnetic techniques may be employed in order to investi-gate the magnetization behaviour. This thesis describes a theoretical study of the hysteresis and domain behav iour in thin film permalloy nanoelements. To carry out our investigations we have developed a dynamical micromagnetic model based on t he use of the finite element method. The results presented in this t hesis begin with a test of the performance of our model. We then proceed with an investigation into the effect of size, elongation and geometry on the transition states for single nanoelements. The investigation is then extended to look at t he magnetization behaviour of arrays of interacting nanoelements in relation to their separation and material propert ies . The reversal mechanism of the arrays is very sensitive to the degree of disorder. In the case of an aligned uniaxial anisotropy a highly symmetric cooperative switch-ing mechanism is observed. A large anisotropy has t he effect of stabilizing states during the reversal process leading to distinctive switching. A random anisotropy breaks this high symmetry sufficiently to reduce the cooperative switching leading to a relatively random reversal of individual elements. The theoretical predictions are compared with experimental observations.

This work has involved the study of the magnetic behaviour of small magnetic nanoparticle systems. Due to the reduced size of magnetic nanoparticles they present distinctive properties, such as size and surface effects, that have been analysed in this work, as well as the effect of interactions in such systems. The samples chosen for the study were magnetite particles in the form of a ferrofluid and Co nanoclusters in a nonmagnetic matrix of Cu. Both systems present very narrow particle size distributions, which facilitates the interpretation of the data. The samples have been subjected to basic characterisation, which includes the determination of the distribution of magnetic particle sizes using the magnetisation curves at room temperatures, TEM microscopy and X-ray diffraction, in the case of the ferrofluid samples. For the nanoclusters, a time of flight spectrometer has been used to obtain the number of atoms per cluster. Many of the measurements have been performed at low temperatures, where thermal effects are minimised. For such measurements the samples have been frozen in a zero applied field, so that they have a random distribution of magnetic moments prior to the measurement. The energy barrier distributions have been calculated via the temperature decay of remanence (TDR). From this study, an effective anisotropy constant has been calculated. For the study of the interactions, surface and size effects, magnetisation, susceptibility (ZFC), remanence and delta-M curves, as well as the time dependence of magnetisation have been studied. The attempt frequency of the different particle size systems has been calculated using different techniques. The basic magnetic behaviour can be explained on the basis of the Neel blocking model. It has been found that the systems with the smaller particles have significant surface effects, which are enhanced at lower temperatures. Interactions, which are weak due to the low concentration of magnetic material in the samples (<10%), have been found to be overall demagnetising and the evolution of the magnetic properties with dilution has been explained. As is the case for the surface effects, interaction effects are stronger at low temperatures due the reduction of thermal effects. The experimental results have been compared with calculations from a Montecarlo model for fine particles, which includes the effects of concentration, anisotropy, particle size and temperature.