Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.

Thesis (Ph.D.)--University of Delaware, 2008. / Principal faculty advisor: John Q. Xiao, Dept. of Physics and Astronomy. Includes bibliographical references.

In this thesis, the potential of MRI phase to reveal the magnetic properties of tissue is thoroughly investigated. Extending beyond the well established field of quantitative susceptibility mapping (QSM), consideration is given to the influence of susceptibility anisotropy, chemical exchange, non-spherical susceptibility inclusions and multi-compartmental signal evolution from both a theoretical and experimental viewpoint. The first experimental chapter discusses a collaborative project with Cardiff University Brain Research Imaging Centre (CUBRIC) and the Department of Neurobiology in Tel-Aviv University to investigate neuroplasticity changes in rats. In this chapter the contribution from the SPMIC is described, detailing an analysis pipeline to generate and optimise isotropic susceptibility and R2∗ maps from multi-echo gradient echo (GE) in-vivo rat datasets obtained on a 7T Bruker Biospec 70/30 MR system. The second experimental chapter investigates the potential of frequency difference mapping (FDM), a recently developed phase processing technique which characterises multi-compartmental signal evolution to probe microstructure content. In this chapter, a novel FDM processing algorithm is introduced which does not require the use of sophisticated phase unwrapping and image filtering. To test the effectiveness of the FDM algorithm on experimental data, ten healthy volunteers underwent a single-slice, sagittal, multi-echo GE scan on a Philips Achieva 7T MR system. Results revealed consistent contrast over the corpus callosum relative to the surrounding tissue and a high sensitivity to small changes in microstructure content. Though fitting experimental magnitude and frequency difference evolution to a three-pool white matter model, the signal evolution over the corpus callosum is described in terms of physical properties of the nerve fibres. The final experimental chapter investigates the magnetic properties of skeletal muscle. A skeletal muscle sample (in the form of a section of pork tenderloin) was placed within an agar phantom and scanned via a multi-orientation, dual-echo GE protocol on a Philips Achieva 7T MR system. Measured phase data were post-processed and novel least-squares minimisation algorithms were developed to investigate the isotropic susceptibility, anisotropic susceptibility, chemical exchange and cylindrical microstructure inclusion properties of the sample. Analysis revealed that the skeletal muscle sample was significantly more diamagnetic than agar, χI = (−121 ± 22)ppb, and a large chemical exchange contribution was also observed within the tissue, E = (31 ± 11)ppb, relative to the surrounding agar. The experimental protocol was not sensitive enough to reveal quantitative information about the anisotropic susceptibility properties of the sample, yielding an an upper bound of 10 ppb. No evidence of cylindrical susceptibility inclusions was observed within the experimental field perturbation maps.

Ultra-High field (7T) MRI provides high sensitivity which allows for new qualitative and quantitative methodologies to be developed, that provide clinically useful information. The work presented in this thesis is focussed on developing a quick and reliable quantitative MT and CEST methodology, taking account of the difficulties encountered at high field. The method developed here has been tested on various studies, in both healthy and diseased brain, in an effort to aid the understanding of myelination in the human brain. The work in this thesis uses the quantitative measure of MT as a marker for myelination, and it shows strong correlations between MT-based myelination and functional connectivity, as well as very strong correlation between MT and NOE. These findings showcase the potential of NOE as a myelin marker as well, as long as the MT vs. NOE relationship remains the same in pathology. Myelination is investigated (via MT and NOE) in Multiple Sclerosis (MS) and Glioma, showing a strong coupling between the two exists even in pathology. Amide Proton Transfer (APT) is also investigated in Glioma, showing similar trends to MT and NOE. High resolution anatomical images can provide valuable information on the extend of the pathology, but quantitative information of the NMR properties of tissue (like MT, NOE and APT) has the potential to detect earlier abnormalities, and give a quantitative measure of healing or degeneration caused by pathology.

Image quality is degraded by involuntary movement of the subject in an MRI scanner. It is fairly challenging in MRI of the brain to monitor the involuntary head movement accurately. Though there are a few techniques to monitor head movement of the subject for prospective motion correction, it is still an unsolved problem in MRI. In this study, head movement inside an MR scanner is monitored via measurement of changes in the voltage induced in head mounted coils by switched magnetic field gradients. The motion of a rigid body such as the human head is decomposed into two components: namely translation and rotation. There are three degrees of freedom (DOFs) for translational motion i.e. translation along the x, y and z axes and three rotational degrees of freedom for rotational motion i.e. rotation about the x, y and z axes. Head movement is monitored in a gradient field by measuring the change in induced voltage in head mounted coils. To calculate the change in induced voltage I follow two approaches: circular loops simulation, analytical as well as numerical calculations. I show that by using a standard method one can form a linear model to identify the position and orientation of the coils. An experimental arrangement is set up to check the validity of the analytical and numerical calculations. Experiments carried out with a rig of five coils verified that the changes in induced voltage in the coils is linear with respect to the changes in position of the coils. The linear model is also verified by comparing estimated positions obtained by using the coils to those found by image realignment of fast field echo (FFE) images using Statistical Parametric Mapping (SPM). We experimentally evaluate the new approach for monitoring head movement inside an MR scanner, which exploits the linear variation of the voltages induced in a set of coils by time-varying magnetic field gradients with respect to small changes in position/orientation. This approach was tested by attaching five coils to a structured agar phantom and a healthy volunteer's head. The results suggest that it is possible to estimate the position and orientation with 0.22 mm and 0.24˚ root-mean-square error using this set-up. The new approach could be used for prospective or retrospective motion correction. An experiment is also carried out by using free running EPI (Echo Planar Imaging) to track the head movement inside an MR scanner. There is a strong relation between head movement and EPI waveforms, the central point of the experiment is to track the head displacements via measuring induced voltage in the coils by using EPI waveforms during execution of free running EPI. The results obtained from the experiment reveal that the method is promising.

Research into antiferromagnetic materials for application in spintronics has rapidly expanded in recent years. The prediction and observation of spin based phenomena with antiferromagnets as the active components, has expanded the field and there is a need for high quality materials that are compatible with existing III-V semiconductor systems to expand this research. Copper manganese arsenide is one such material and will be the subject of this thesis. Early studies had shown that this material grows epitaxially on both gallium arsenide and gallium phosphide substrates by molecular beam epitaxy. This thesis builds on this early work by further characterising CuMnAs, improving the techniques used to grow it, and enhancing our understanding of the material. A key result of this thesis is that the Néel temperature of CuMnAs can be studied using temperature dependent transport measurements. This method allows for a range of layer thickness (from between 5 and 140 nm) to be studied. We find that the Néel temperature of CuMnAs is suppressed by around 100K when the layer thickness is less than 10nm. At the thicknesses studied there is agreement (around (480±5)K) with the more established neutron diffraction technique for measuring Néel temperature, which was also used to determine the magnetic structure of the CuMnAs studied. In addition to measurement of the Néel temperature of CuMnAs, a detailed study is made in this thesis of the ideal growth conditions for ultrathin (sub 10nm) films of CuMnAs. Post-growth examination of ultrathin layers of CuMnAs showed that significant portions of material were missing due to poor adhesion. This thesis shows the results of the development of several different nucleation and growth methods, which were used to improve the adhesion of the CuMnAs layer to the substrate. These methods are evaluated using atomic force microscopy, x-ray diffraction, magnetometry and transport measurements. CuMnAs has previously shown to strongly prefer growth under stoichiometric conditions, as non-stoichiometric conditions have tended to favour the formation of clusters of the excess material. In excess Mn conditions these clusters are ferromagnetic MnAs inclusions that are conducting and contribute to the magnetic behaviour. This thesis presents the results of a simulation study of the conductivity of ferromagnetic elements in a non-ferromagnetic medium. This approach could be extended to allow the number of inclusions in a CuMnAsl layer to be approximated from transport measurements. Finally, this thesis will also look at the effects of alloying CuMnAs with phosphorous. This reduces the lattice constants of the material while retaining the same crystal and magnetic structure. In thick films of the alloy the Néel temperature increases from that of CuMnAs.

This thesis reports on recent results which pave the way for future experiments in the emerging field of quantum magnonics. Chapter 1 presents a brief outline of the field of magnonics, which provides the context in which quantum magnonics has begun to develop. Chapter 2 provides an introduction to the theory of spin waves, which is necessary to understand the experiments reported in the thesis. In Chapter 3, the experimental methods and materials used to carry out the investigations in the thesis are described. Chapter 4 describes the coupling of resonant magnon modes in a sphere of yttrium-iron garnet to photon modes in a coplanar-waveguide resonator. Strong coupling is achieved to multiple magnon modes, and a theoretical model is used to identify the magnon modes which couple most strongly to the photon mode. In Chapter 5, the behaviour of propagating magnon modes is investigated in a waveguide formed from a thin film of yttrium-iron garnet. Two different configurations are investigated supporting different types of propagating mode, namely backward-volume and surface spin waves. Simulations are performed which reproduce the main features of the data. Chapter 6 characterises the effect of the gadolinium-gallium garnet substrate on propagating spin waves. The magnitude of this effect is dependent on both the orientation and temperature of the sample. Finally, Chapter 7 provides a short summary of the results of the thesis, and speculates on how they may inform future work in the field.

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Double perovskites with a single s = ½ magnetic ion and rocksalt order can show geometric frustration, due to arrangement of electron spins such that they cannot satisfy all nearest-neighbour antiferromagnetic interactions simultaneously. This can give rise to exotic magnetic states at low temperature. Compounds with unpaired 4d and 5d electrons have in this respect been studied far less than 3d compounds. Here, results of studies on the compounds Sr2CuWO6, Ba2YWO6 and LaSrMgWO6 are presented. The synthesis of the previously reported compound Ba2Y1WO6 (W5 +, 5d1) was attempted by many methods, but yttrium deficiencies were present in all samples. This led to the series Ba2YxWO6 with ⅔ ≤ x ≤ 0.8, dependent on the synthesis conditions. The x = ⅔ compound is known to exist as a metastable cubic phase and an 18H rhombohedral thermodynamic phase. The one-third vacant B sites on the x = ⅔ cubic material are doped with lithium, resulting in the Ba2Y2/3LixWO6 compound. This thesis focuses on the new x = 0.75 phase, and characterises its structural and magnetic properties. The Jahn-Teller distorted Sr2CuWO6, with the Cu2+ (3d9) magnetic ion, has an elongated c axis leading to separation of ab planes. This compound had been proposed as a pseudo-2D spin liquid model candidate, following a lack of evidence of transition to long-range order in SQUID magnetometry and heat capacity. However, recent μSR measurements did show a transition to long-range ordered state at 24K. This thesis details further bulk and local probe measurements which indicate that the low-temperature state is type-2 antiferromagnetic, and which point to a thermally activated spin-liquid-like state which occurs between 24K and ∼100K. La0.5Sr1.5MgWO6, has also been synthesised. The 1:1 compound LaSrMgWO6 had previously been reported as W5+ and described as “pseudo-cubic”: X-ray and neutron diffraction studies here characterise the low-temperature structure within the P2₁/n monoclinic space group, and suggest that no ordering of the atoms on the A site is observed.