Thesis advisor: Michael Graf / Senior thesis on muon spin relaxation study of LiHoXY1-XF4 and semi-adiabatic specific heat experiment on LiTb0.40Y0.60F4 / Thesis (BS) — Boston College, 2010. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Physics Honors Program. / Discipline: Physics.

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Thesis advisor: Michael J. Graf / This thesis is concerned with experimentally determining the magnetic and electronic states in a unique class of transition metal oxides known as the pyrochlore iridates, A₂Ir₂O₇ (A = Y or Rare earth). The extended nature of the 5d Ir orbitals in the iridates places these materials in a regime of intermediate electron correlation and large spin-orbit interaction such that this system may host several novel or topological states of matter which may be perturbed by incorporating different A-species. Additionally, the pyrochlore structure is geometrically frustrated and has been long been studied as a potential host of a number of exotic magnetic phenomenon. However, even after years of intense theoretical and experimental interest many fundamental questions still remain about the nature of the magnetic ground sates in this series which are of vital importance in understanding the roles of various interactions and potential of such novel phenomenon. The primary aim of this thesis is therefore to determine how magnetic order develops on the Ir sublattice in this series, particularly how it is perturbed through variation of the crystalline structure, magnetism of the A-site ions, and presence of mobile charges. This thesis is the first comprehensive experimental study of these effects which has utilized several complementary experimental probes of both bulk and local magnetism in a number of compounds. The techniques presented in this work include magnetotransport, bulk magnetization, elastic neutron scattering, and muon spin relaxation (µSR) measurements. All of the three compounds studied in this work (A = Y, Yb, and Nd) are shown to definitively exhibit long-range magnetic order on the Ir sublattice, which has previously only been inferred based on studies of other compounds. The compounds Y₂Ir₂O₇ and Yb₂Ir₂O₇ are correlated insulators at low temperature and are found to have identical configuration of the Ir moments, despite the presence of the large localized Yb³⁺. Numerical investigations presented here have provided the first conclusive evidence that this order is of the `all-in/all-out' type, consistent with recent resonant x-ray studies; additionally, we have shown that this order exists for all insulating compounds regardless of structural parameters or properties of the A-ion. On the other hand, Nd₂Ir₂O₇ is weakly metallic with Kondo-like behavior at low temperature, with long-range order only on the Ir site, in disagreement with previous results from neutron scattering. Measurements of the field dependent magnetization and Hall effect reveal a large anomalous Hall component, suggesting that the Nd<super>3+</super> may exhibit a spin-ice state with very short correlation length, while the Ir sublattice is likely in the `all-in/all-out' state. From this, it is determined that Nd₂Ir₂O₇ lies at an important cross-over point in the series in which correlation energy and conduction bandwidth yield chiral order with features akin to both the metallic unordered Pr₂Ir₂O₇ and those of the magnetically ordered insulators. These results are discussed with regard to recent theoretical models exploring the role of electron correlation, frustration and various exchange interactions in these materials. / Thesis (PhD) — Boston College, 2013. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Stoffe mit schnell zerfallendem Magnetresonanz (MR)-Signal sind mit herkömmlichen MR- Sequenzen nicht darstellbar. Solche Stoffe haben meist starke Bindungen, wie im menschlichen Körper beispielsweise Sehnen, Bänder, Knochen oder Zähne. In den letzten Dekaden wurden spezielle Sequenzen mit ultrakurzer Echozeit entwickelt, die Signale von diesen Stoffen messen können. Messungen mit ultrakurzen Echozeiten eröffnen der Kernspintomographie neue Anwendungsgebiete. In dieser Doktorarbeit werden die in der Literatur bekannten Methoden zur Messung mit ultrakurzen Echozeiten untersucht und evaluiert. Es werden zwei neue, in dieser Arbeit entwickelte Ansätze vorgestellt, die es zum Ziel haben, bestehende Probleme der vorhandenen Methoden bei robuster Bildqualität zu lösen, ohne auf Hardwareänderungen am Kernspintomographen angewiesen zu sein. Die ’Gradient Optimized Single Point imaging with Echo time Leveraging’ (GOSPEL) Sequenz ist eine Single-Point-Sequenz, die im Vergleich zu den bekannten Single-Point-Sequenzen eine stark reduzierte Echozeit ermöglicht. Es wird gezeigt, dass dadurch ein deutlich besseres Signalzu-Rausch-Verhältnis (SNR) von Stoffen mit schnell zerfallendem Signal erreicht wird. Das Problem der sehr langen Messzeit bei Single-Point-Verfahren wird mit der ’Pointwise Encoding Time reduction with Radial Acquisition’ (PETRA) Sequenz gelöst. Bei diesem Ansatz wird der k-Raum-Außenbereich radial und das k-Raum-Zentrum single-point-artig abgetastet. Durch die Kombination beider Akquisitionsstrategien ist eine schnelle und robuste Bildgebung mit ultrakurzer Echozeit und ohne Hardwareänderungen möglich. Wie bei anderen Ansätzen sind bei der PETRA-Sequenz die Bildgebungsgradienten zum Anregungszeitpunkt bereites angeschaltet. Es wird untersucht, welchen Einfluss ungewollte Schichtselektionen auf die Bildgebung haben können und ein Korrekturalgorithmus entwickelt, mit dem sich dadurch entstehende Artefakte im Bild beheben lassen. Die Limitationen des Korrekturalgorithmus sowie mögliche Artefakte der PETRA-Sequenz werden untersucht und diskutiert. Erste Anwendungsbeispiele der PETRA-Sequenz bei verschiedenen Feldstärken und Applikationen werden demonstriert. Wie bei anderen Sequenzen mit ultrakurzen Echozeiten sind die Gradientenaktivitäten bei der PETRA- und GOSPEL-Sequenz gering, wodurch die Messung sehr leise sein kann. Lautstärkemessungen zeigen, dass bei Messungen mit der PETRA-Sequenz der Geräuschpegel um nur ein bis fünf dB(A) im Vergleich zum Hintergrundgeräuschpegel steigt. Es wird demonstriert, dass sich dadurch neue Anwendungsgebiete eröffnen könnten. Vergleichsmessungen zwischen einer T1-gewichteten PETRA- und einer MPRAGE-Messung weisen Bilder auf, die in Kontrast, Auflösung, SNR und Messzeit vergleichbar sind. Mit den in dieser Arbeit entwickelten Methoden konnten Probleme bestehender Ansätze gelöst und offene Fragen beantwortet werden. Die Ergebnisse können helfen, Applikationen von Sequenzen mit ultrakurzen Echozeiten in der klinischen Routine weiter zu etablieren. / Tissues with fast decaying magnetic resonance (MR) signal are not measureable with conventional MR sequences. These tissues mostly have strong covalent bondings, like in the human body tendons, ligaments, bones and teeth. In the last decade, special MR sequences with ultrashort echo times have been developed that are able to depict signal from those tissues. Ultrashort echo time imaging opens new application fields for magnetic resonance imaging. In this thesis, the known methods for imaging with ultrashort echo times are investigated and evaluated. Two new approaches that were developed in this work are presented. They aim to solve the problems of the previous methods and to allow for robust image quality. No hardware changes should be required for the MR scanner. The ’Gradient Optimized Single Point imaging with Echo time Leveraging’ (GOSPEL) sequence is a single-point sequence. Compared to the known single-point sequences, GOSPEL enables a reduced echo time. It is demonstrated that this allows for an enhanced SNR for tissues with fast decaying signal. The problem of very long measurement times with single point sequences is solved with the ’Pointwise Encoding Time reduction with Radial Acquisition’ (PETRA) sequence. In this approach, outer k-space is acquired with radial half-projections while the k-space center is acquired single-pointwise. The combination of these two acquisition strategies allows for fast and robust ultrashort echo time imaging without the need for hardware changes. Comparable to other approaches, the imaging gradients at the PETRA sequence are already switched on during the excitation pulse. The influence of unwanted slice-selectivity of the pulse is investigated. A newly developed correction algorithm is presented that eliminates artefacts due to unwanted slice-selectivity. The limitations of the correction approach are presented and discussed. A number of application examples of the PETRA sequence at different field strengths is demonstrated. The PETRA and GOSPEL sequence, and other ultrashort echo time sequences, have very limited gradient activities. Due to this, the measurements can be kept very silent. Acoustic noise measurements show that the acoustic noise level during PETRA examinations is only raised by one to five dB(A). It is demonstrated, that this might enable new applications. Comparing measurements between T1-weighted PETRA images and MPRAGE images lead to images with comparable contrast, resolution, SNR and measurement times. With the methods developed in this thesis, issues of existing ultrashort echo time approaches can be solved and answers to open questions are given. The outcomes could help to further establish the use of ultrashort echo time sequences in clinical routine applications.

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Dissolution dynamic nuclear polarization (DNP) provides several orders of magnitude of NMR signal enhancement by converting the much larger electron spin polarization to nuclear spin polarization. Polarization occurs at low temperature (1.4K) and is followed by quickly dissolving the sample for room temperature NMR detection. DNP is generally applicable to almost any small molecules and can polarize various nuclei including 1H, 19F and 13C. The large signal from DNP enhancement reduces the limit of detection to micromolar or sub-micromolar concentration in a single scan. Since DNP enhancement often provides the only source for the observable signal, it enables tracking of the polarization flow. Therefore, DNP is ideal for studying chemical processes. Here, quantitative tools are developed to separate kinetics and spin relaxation, as well as to obtain structural information from these measurements. Techniques needed for analyzing DNP polarized sample are different from those used in conventional NMR because a large, yet non-renewable hyperpolarization is available. Using small flip angle pulse excitation, the hyperpolarization can still be divided into multiple scans. Based on this principle, a scheme is presented that allows reconstruction of indirect spectral dimensions similarly to conventional 2D NMR. Additionally, small flip angle pulses can be used to obtain a succession of scans separated in time. A model describing the combined effects of the evolution of a chemical process and of spin-lattice relaxation is shown. Applied to a Diels-Alder reaction, it permitted measuring kinetics along with the effects of auto- and cross-relaxation. DNP polarization of small molecules also shows significant promise for studying protein-ligand interaction. The binding of fluorinated ligands to the protease trypsin was studied through the observation of various NMR parameter changes, such as line width, signal intensity and chemical shift of the ligands. Intermolecular polarization transfer from hyperpolarized ligand to protein can further provide information about the binding pocket of the protein. As an alternative to direct observation of protein signal, a model is presented to describe a two-step intermolecular polarization transfer between competitively binding ligands mediated through the common binding pocket of the protein. The solutions of this model relate the evolution of signal intensities to the intermolecular cross relaxation rates, which depend on individual distances in the binding epitope. In summary, DNP provides incomparable sensitivity, speed and selectivity to NMR. Quantitative models such as those discussed here enable taking full advantage of these benefits for the study of chemical processes.

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Abstract This thesis reports simulations of direct observables in electron and nuclear spin relaxation experiments in an example paramagnetic system, as well as polarization transfer occurring in a spin-exchange optical pumping (SEOP) experiment. Studies of paramagnetic relaxation are important, e.g., in the development of agents used for enhanced contrast in magnetic resonance imaging. SEOP is used to produce hyperpolarized noble gases, which are then used to, e.g., enhance sensitivity in structural studies of matter with nuclear magnetic resonance. Presently the theory, available software and hardware for such computational modeling have reached a state in which quantitative reproduction of the experimentally observed magnetization decay is possible from first principles. The present multiscale computations are carried out from first principles combining molecular dynamics simulations of atomistic motion and quantum-chemical electronic structure calculations of the spin interaction parameters that enter the effective spin Hamiltonian. A time series of the spin Hamiltonian is then explicitly used to propagate spin dynamics in the system, and dynamical time constants of the magnetization are obtained through ensemble averaging. The complete decay of electron spin magnetization could be followed directly within the duration of the simulation, whereas the nuclear spin relaxation rates were extracted using Kubo’s theory regarding generalized cumulant expansion and stochastic processes. The extracted electron and nuclear spin relaxation rates for the chosen prototypic system, the aqueous solution of Ni²⁺, are in quantitative and semi-quantitative agreement, respectively, with the available experimental results. The simulations of polarization transfer corroborate the empirical observations on the importance of van der Waals complexes and binary collisions in the spin-exchange process. Long van der Waals complexes represent the overwhelmingly most significant kind of individual events, but the short binary collisions can also give a relatively important contribution due to their vast abundance. This thesis represents a first study in which first principles-calculated trajectories of individual events could be followed. The simulations reported in this thesis were run without any empirical parametrization and thus represent a significant step in first-principles computational modeling of magnetization dynamics.

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