Graduation date: 1998

Studying dynamics of magnetization relaxation in excited magnetic materials is important both for understanding the rates and pathways of magnetization relaxation and for the potential use in spin-based electronics and data storage devices in the future. Previous studies have demonstrated that the size of nanocrystals is an important factor for energy relaxation in quantum dots and metal nanoparticles. Since magnetization relaxation is one of energy relaxation pathways, the size of nanocrystals may be also an important factor for magnetization relaxation in nanoscale magnetic materials. The goal of this study is to have a better understanding of magnetization relaxation in nanoscale magnetic materials. In particular, we focused on the correlation between the nanocrystal size and the rates of spin-lattice relaxation (SLR), a magnetization relaxation pathway, in magnetic nanocrystals. The size-dependent magnetization relaxation rate after optically induced demagnetization in colloidal Fe3O4 nanocrystals was measured by using time-resolved Faraday rotation (FR). Fe3O4 nanocrystals were chosen as the model system to study the correlation between the size of nanocrystals and the rates of SLR due to the wellestablished synthetic procedure of making nanocrystals with various sizes and narrow size dispersion. Faster SLR rates were observed in smaller Fe3O4 nanocrystals. The results suggested the surface of nanocrystals have higher efficiency of SLR than the interior region by using a simple model to analyze the SLR rates of Fe3O4 nanocrystals with various sizes. Higher efficiency of SLR at the surface may be due to the stronger spin-orbit coupling at the surface relative to the interior region. In addition to magnetization dynamics studies, the effect of oxidation on static FR in iron oxide nanocrystals (between Fe3O4 and y-Fe2O3) was studied. The results indicated FR signal is linearly correlated to the strength of optical transition between Fe2 and Fe3 in Fe3O4 for a given size of nanocrystals.

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On-line nuclear orientation at low temperature has become an important technique for the study of nuclei far from stability, through measurements of nuclear moments and other quantities of spectroscopic interest. The theory of low temperature nuclear orientation and its application to the study of nuclear structure are reviewed. Of particular importance to the on-line measurement, in which a wide range of short-lived nuclei are available for study, is the question of how fast these nuclei can be cooled to the lattice temperature, and thus oriented. To address this, the theory of nuclear spin-lattice relaxation, relevant to the online technique, is outlined. In particular, quantitative methods to deal with cases in which the spin-lattice relaxation time is comparable with the isotope half-life have been developed and applied. One of the major current interests in nuclear structure physics is to investigate how the neutron-proton interaction influences the structure of nuclei that are transitional, between well established regions of spherical and deformed nuclei. In such nuclei, intruder excitations, which signal the onset of deformation, are observed low in energy. Using the Daresbury on-line isotope separator, an extensive study of the decay of ¹¹⁸I to ¹¹⁸Te has been performed using nuclear orientation techniques, combined with γ - γ and conversion electron spectroscopy measurements. Interpretation of the results obtained for ¹¹⁸Te within the framework of IBM-2, gives strong evidence for the existence of such an ,em>intruder configuration in this nucleus. On-line experiments have also been performed in which a range of neutrondeficient Cs nuclei has been oriented for the first time. In these measurements the hyperfine field of CsFe has been determined as (+)40.8(7) T, and also the Korringa constant for the system ¹²¹Cs^mFe has been measured (using a new technique) to be C_k = 0.059(l6)sK. These results have been applied to the case of ¹¹⁸Cs^m, for which the magnetic moment has been measured to be 5.4(1.1)nm. This large value clearly indicates the presence of the [404]9/2 orbital, which provides further evidence for the existence of intruder states in this region.

We present a controlled microscopic study of hole dynamics in both a gapped and a gapless quantum spin liquid. Our approach is complementary to previous phenomenological works on lightly doped quantum spin liquids as we introduce mobile holes into the ground state of the exactly solvable Kitaev honeycomb model. In the spatially anisotropic (Abelian) gapped phase of the model, we address the properties of a single hole [its internal degrees of freedom as well as its hopping properties], a pair of holes [their absolute and relative particle statistics as well as their interactions], and the collective state for a finite density of holes. Our main result is that the holes in the doped model possess internal degrees of freedom as they can bind the fractional excitations of the undoped model and that the resulting composite holes with different excitations bound are distinct fractional particles with fundamentally different single-particle properties and different experimental signatures in the multi-particle ground state at finite doping. For example, some hole types are free to hop in two dimensions, while others are confined to hop in one dimension only. Also, distinct hole types have different particle statistics and, in particular, some of them exhibit non-trivial (anyonic) relative statistics. At finite doping, the respective hopping dimensionalities manifest themselves in an electrical conductivity that is either approximately isotropic or extremely anisotropic. In the gapless phase of the model, we consider a single hole and address the possibility of a coherent quasiparticle description. Our main result is that a mobile hole has a finite quasiparticle weight which vanishes in the stationary limit. Although this result is obtained in terms of an approximate variational state, we argue that it is also applicable for the exact ground state of the doped model.

<p> The temperature dependences of the deuteron spin-lattice relaxation times, T1, in ferroelectric ammonium sulfate, (ND4)2SO4 , and ammonium fluoroberyllate, (ND4)2BeF4, have been studied by transient methods over the range 80 to 47S°K. The ability to resolve the individual correlations of the two independent ND4 groups in the deuteron experiment has thrown new light on the reorientation of the ammonium groups and their behaviour in the phase transition. It has been proposed that the mechanism of the phase transition in ammonium sulfate is a disordering, with respect to the a b plane in the paraelectric phase, of the ND4 dipoles which also make the dominant contribution to the spontaneous polarization. The present N.M.R. results show that in (ND4)2so4 ,T1 at both non-equivalent ND4 groups is affected at the phase transition, whereas in (ND4)2BeF4 , it is affected at only one ND4 group. This suggests that the phase transitions in these two materials are rather different and that in ammonium sulfate both types of ammonium ions are involved whereas in ammonium fluoroberyllate only one type is involved in the transition. Such a conclusion is supported by evidence from measurements of thermal and dielectric properties. An earlier study of the proton spectrum in (NH4)2so4 reported a value of 33 gauss^2 for the second moment at 20°K. This is considerably smaller than the expected rigid lattice value of 5O gauss^2 and it was concluded that while some of the NH4+ groups are effectively rigid at 20°K others are still reorienting. Our new results for the proton second moment confirm the low value down to 4.2°K but our results for T1 indicate that the proton line is not motionally narrowed. </p> / Thesis / Doctor of Philosophy (PhD)

Since the high $T_c$ superconductivity was discovered in iron pnictides in 2008, the interplay between the reduced dimensionality, magnetism and unconventional superconductivity has been attracting renewed interest. Recently, Bao et al. and Tang et al. discovered a series of quasi-one-dimensional (quasi 1D) superconductors: \K($T_c=6.1 K$), \Rb($T_c=4.8 K$), and \Cs($T_c=2.2 K$). In this thesis, we will discuss microscopic investigation of \Cs based on nuclear magnetic resonance techniques. The first chapter is a brief introduction to this series of superconductors. The second chapter is a summary of NMR techniques and theory. In the third part, I summarize $^{133}$Cs NMR and $^{75}$As Nuclear Quadrupole Resonance (NQR) measurements on a powder sample of \Cs ($T_c < 1.6$~K). From the $^{133}$Cs NMR Knight shift $^{133}K$ measured at the Cs1 site, we show that the uniform spin susceptibility $\chi_{spin}$ increases from 295~K to $\sim$ 60~K, followed by a mild suppression; $\chi_{spin}$ then levels off below $\sim$10~K. Low frequency Cr spin dynamics, reflected on $^{75}$As $1/T_1T$ (the nuclear spin-lattice relaxation rate $1/T_1$ divided by temperature $T$), shows an analogous trend as $\chi_{spin}$. Comparison with the results of $1/T_1T$ near $T_c$ with \K($T_c=6.1$~K) and \Rb($T_c=4.8$~K) establishes a systematic trend that substitution of K$^{+}$ ions with larger alkali ions progressively suppresses Cr spin fluctuations together with $T_c$. / Thesis / Master of Science (MSc)

A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 2016 / The theoretical and experimental investigation of electron spin-resonance relaxation to deposit thermal energy into liquid gadolinium-based contrast agents for cancer hyperthermia treatment is presented. Previous works suggest that using protons in water are inadequate, with a thermal deposition rate of approximately 1 ◦C per two years. A novel component of this research relies on the use of gadolinium-chelated molecules, which are currently used as contrast agents in clinical MRI scans. The chelating agents, or ligands, investigated are Gadobenate (MultiHance R ), Gadopentetate (Magnevist R ), Gadoterate (DotaremR ) and Gadoteridol (ProHance R ). The gadolinium atom has seven unpaired electrons in its inner f shell orbital and as a result has a 660 times stronger paramagnetic response when placed in an external magnetic field. The research tests the hypothesis that by using an appropriate external homogeneous DC magnetic field, together with a radiofrequency excited resonator, that a measurable amount of thermal energy is deposited into a liquid gadolinium-based contrast agent. The aim of this research is to ultimately discover a new cancer hyperthermia treatment. The research theory suggests that a temperature rate of 13.4 ◦C · s−1 can be achieved using the gadolinium-based contrast agents under certain experimental conditions, and a maximum of 29.4 ◦C · s−1 under more optimal conditions. The temperature rates are calculated using parameter values commonly found in literature and practice. The simulation and design of the DC magnetic field coil system is discussed, together with the simulation results and design parameters of the radiofrequency loop-gap resonator. The experimental results and analysis indicate that the selected contrast agents have varied responses based on their chemical nature and that only two out of the four contrast agents, Dotarem and ProHance, show a measurable effect albeit sufficiently small that statistical techniques were necessary to distinguish the effect from background. A model fit to the data is performed in order to determine the spin-lattice relaxation time of the contrast agents under the specified experimental conditions. The model estimate is significantly smaller than the values found in literature under similar conditions, with a spin-lattice relaxation time τ1e of approximately 0.2 ps compared to the literature value of 0.1 ns. Although the observed electron spin resonance heating rate is in the milli-Watt range it is still notably larger (167 000 times) compared to the heating rate obtained using protons. The low temperature rates suggest that a more suitable agent or molecule with a larger spin-relaxation time be used, in order to achieve clinical useful temperature rates in the range of 14 ◦C · s−1. / MT2017

Knowledge of energy dissipation and relaxation in electron, spin, and lattice degrees of freedom is of fundamental importance from both a technological and scientific point of view. In this dissertation, the electronic and magnetization dynamics of photoexcited colloidal cobalt substituted iron oxide nanocrystals, CoxFe3-xO4, were investigated through transient absorption and pump-probe Faraday rotation measurements. In this dissertation, linearly polarized femtosecond optical pulses at 780 nm were used to excite the weak absorption originating from the intervalence charge transfer transition (IVCT) between Fe2+ and Fe3+ ions of Fe3O4 nanocrystals. The timescale and corresponding relaxation processes of electronic relaxation dynamics of the excited IVCT state were first discussed. Size effect on electronic relaxation dynamics in Fe3O4 nanocrystals is not distinct on the basis of result from this study. One interesting feature of electronic dynamics data of photoexcited Fe3O4 nanocrystals is the creation of coherent acoustic phonons. Information on lattice temperature was obtained by measuring the period of coherent acoustic phonon as a function of excitation fluence and fit into a simple model based on Lamb’s theory. Since optical control of the magnetization can be either through optical or heating mechanisms, quantitative estimation of degree of demagnetization caused by lattice temperature is made by using Langevin function. The result from such estimation indicates the effect of lattice temperature rise on magnetization is too small to significantly affect the magnetization of Fe3O4 nanocrystals. Magnetization dynamics were studied via pump-probe Faraday rotation measurements. Optical excitation with near-infrared pulse resulted in an ultrafast demagnetization in 100fs. The energy of the excited state then relaxed through spin-lattice relaxation (SLR). Effects of surface spin and chemical tuning on the SLR were investigated by comparing the magnetization recovery timescales of nanocrystal with different particle sizes and cobalt concentration respectively. The experimental result is explained by a simple model where interior and surface spins contributed to the spin-lattice relaxation process differently. The observations suggest that spin-orbit coupling of the surface is stronger and less sensitive to stoichiometric variation than the interior spins of the nanocrystals.

Thesis (Ph.D.)--McMaster University (Canada), 1994. / Source: Dissertation Abstracts International, Volume: 56-08, Section: B, page: 4313. Adviser: A. D. Bain.