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1
Borlenghi, Simone. "Electronic transport and magnetization dynamics in magnetic systems." Phd thesis, Université Pierre et Marie Curie - Paris VI, 2011. http://tel.archives-ouvertes.fr/tel-00590363.
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L'objectif de ce travail de thèse est de comprendre l'influence mutuelle entre le transport électronique et la dynamique de l'aimantation dans des nanostructures hybrides magnétiques métalliques. Dans une première partie on a développé un modèle théorique, basé sur la théorie des matrices aléatories, pour décrire au niveau microscopique le transport dépendent du spin dans une nanostructure hétérogène. Ce modèle, appélé CRMT (pour Continuous Random Matrix Theory) a ensuite été traduit dans un code de simulation qui permet de calculer les proprietés locales (couple de transfert de spin, accumulation de spin et courant de spin) et macroscopiques (résistance) du transport dans des conducteurs ohmiques. Le modèle a été validé en le comparant avec une théorie du transport quantique basée sur le calcul des fonctions de Green hors équilibre. Le couplage des ce deux modèles a permis d'effectuer une description multiéchelle du transport dans des nanostructures métalliques hybrides, où les parties ohmiques sont décrites par CRMT et les parties purement quantiques par le formalisme des fonctions de Green. CRMT a ensuite été incorporé dans un code de simulation micromagnétique, pour décrire de façon réaliste la texture spatiale de la dynamique de l'aimantation induite par le transfert de spin. L'originalité de cette approche réside dans la modélisation des mesures spectroscopiques utilisant une détection mécanique de la résonance ferromagnétique, conduites sur des oscillateurs à transport de spin. Ce travail a permis d'obtenir le diagramme de phase dynamique de l'aimantation, ainsi que les règles de sélection des ondes de spin et la compétition entre les modes propres du systeme lors du passage d'un courant continu à travers la multicouche, en accord partiel avec les données experimentales
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Borlenghi, Garoia Simone. "Electronic transport and magnetization dynamics in magnetic systems." Paris 6, 2011. http://www.theses.fr/2011PA066009.
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L'objectif de ce travail de thèse est de comprendre l'influence mutuelle entre transport électronique et dynamique de l'aimantation dans des nanostructures hybrides magnétiques métalliques. Dans une première partie on a développé un modèle théorique, basé sur la théorie des matrices aléatories, pour décrire à niveau microscopique le transport dépendent du spin dans une nanostructure hétérogène. Ce modèle, appélé CRMT (Continuous Random Matrix Theory) a ensuite été traduit dans un code de simulation qui permet de calculer les proprietés locales (couple de transfert de spin, accumulation de spin et courant de spin) et globales (résistance) de transport dans des conducteurs ohmiques. Le modèle a été validé en le comparant avec une théorie du transport quantique basée sur le calcul des fonctions de Green hors équilibre (NEGF-Non Equilibrium Green Function formalism). Le couplage des ce deux modèles a permis d'éffectuer une description multi-échelle du transport dans des nanostructures métalliques hybrides, où les parties ohmiques sont décrites par CRMT (plus pérformant du point de vue computationnel) et les parties purement quantiques par le formalisme des fonctions de Green. CRMT a ensuite été couplé à un code de simulation micromagnétique, pour prendre en compte la dynamique complexe de l'aimantation induite par le transfert de spin. L'originalité de cette approche réside dans la modélisation des expériences de résonance ferromagnétique conduites sur des oscillateurs a transport de spin. Ce travail a permis la mise en évidence des règles de sélection des ondes de spin induites par le transfert de spin, en accord avec les données experimentales
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Zhang, Shulei. "Spin Transport and Magnetization Dynamics in Various Magnetic Systems." Diss., The University of Arizona, 2014. http://hdl.handle.net/10150/333352.
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The general theme of the thesis is the interplay between magnetization dynamics and spin transport. The main presentation is divided into three parts. The first part is devoted to deepening our understanding on magnetic damping of ferromagnetic metals, which is one of the long-standing issues in conventional spintronics that has not been completely understood. For a nonuniformly-magnetized ferromagnetic metal, we find that the damping is nonlocal and is enhanced as compared to that in the uniform case. It is therefore necessary to generalize the conventional Landau-Lifshitz-Gilbert equation to include the additional damping. In a different vein, the decay mechanism of the uniform precession mode has been investigated. We point out the important role of spin-conserving electron-magnon interaction in the relaxation process by quantitatively examining its contribution to the ferromagnetic resonance linewidth. In the second part, a transport theory is developed for magnons which, in addition to conduction electrons, can also carry and propagate spin angular momentum via the magnon current. We demonstrate that the mutual conversion of magnon current and spin current may take place at magnetic interfaces. We also predict a novel magnon-mediated electric drag effect in a metal/magnetic-insulator/metal trilayer structure. This study may pave the way to the new area of insulator-based spintronics. In the third part of thesis, particular attention is paid to the influence the spin orbit coupling on both charge and spin transport. We theoretically investigate magnetotransport anisotropy and the conversion relations of spin and charge currents in various magnetic systems, and apply our results to interpret recent experiments.
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Pimental, Iveta Rombeiro do Rego. "Critical dynamics in spin glasses and dilute magnets." Thesis, University of Oxford, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.329986.
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Lago, Jorge. "Magnetic ordering and dynamics of two transition metal oxide systems." Thesis, University of Oxford, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.670216.
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Ellis, Kevin John. "Neutron and muon studies of spin dynamics in magnetic systems." Thesis, University of Huddersfield, 2013. http://eprints.hud.ac.uk/id/eprint/18079/.
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In this thesis I present an investigation on the spin dynamics observed during moment localisation, non-ergodic magnetic phase transitions, and weak itinerant electron magnetism. The pseudo-binary compound Y(Mn1-xAlx)2 has been investigated under the influence of equivalent opposing chemical and mechanical pressures using Muon Spin Relaxation. The results reveal the application of external mechanical pressure (4.5kbar) to destabilise the manganese moment, and produce a ground stte distinctly different to that seen under ambient pressure conditions. Short-range nuclear and spin correlations have been studies via diffuse neutron scattering, and through a combination of analysis techniques I have mapped the temperature dependence of these correlations and their evolution due to the substitution of manganese for aluminium. Applying new methods of hierarchical relaxation and non-extensive entopy I have studied the slow relaxation dynamics of the spin glass phase using Beutron Spin Echo spectroscopy. The results are dveloped further by applying the same analysis to a variety of glassy magnetic phenomena: spin glass freezing ((La1-xEr x)Al )Al2), and superparamagnetic blocking (Cr 1-xFe x). I have shown that within this framework the underlying freezing mechanisms result in distinctly different responses, and that in the case of spin glass relaxation an apparantly universal scaling relationship is present. Finally the results of a Muon Spin Relaxation study on the moment fluctuations in Au4V above the Curie temperature are reported. The temperature dependence of the muon spin relaxation rate is to be similar to that of the archetypal weak itinerant helimagnet, MnSi.
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Li, Dawei. "Relaxation dynamics in some reentrant disordered magnetic systems, FeNiCr, FeNiMn, CrFe." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp04/nq23627.pdf.
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Dobramysl, Ulrich. "On the Relaxation Dynamics of Disordered Systems." Diss., Virginia Tech, 2013. http://hdl.handle.net/10919/23757.
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We investigate the properties of two distinct disordered systems: the two-species predator-prey Lotka-Volterra model with rate variability, and an elastic line model to simulate vortex lines in type-II superconductors.
We study the effects of intrinsic demographic variability with inheritance in the reaction rates of the Lotka-Volterra model via zero-dimensional Monte Carlo simulations as well as two-dimensional lattice simulations. Individuals of each species are assigned inheritable predation efficiencies during their creation, leading to evolutionary dynamics and thus population-level optimization. We derive an effective subspecies mean-field theory and compare its results to our numerical data. Furthermore, we introduce environmental variability via quenched spatial reaction-rate randomness. We investigate the competing effects and relative importance of the two types of variability, and find that both lead to a remarkable enhancement of the species densities, while the aforementioned optimization effects are essentially neutral in the densities. Additionally, we collected extinction time histograms for small systems and find a marked increase in the stability of the populations against extinction due to the presence of variability.
We employ an elastic line model to investigate the steady-state properties and non-equilibrium relaxation kinetics of magnetic vortex lines in disordered type-II superconductors. To this end, we developed a versatile and efficient Langevin molecular dynamics simulation code, allowing us to do a careful study of samples with or without vortex-vortex interactions or disorder allows us to disentangle the various complex relaxational features present in this system and investigate their origin. In particular, we compare disordered samples with randomly distributed point defects versus correlated columnar defects. We extract two-time quantities such as the mean-square displacement, the height and density correlations, to investigate the relaxation kinetics of the system of flux lines. Additionally, we compare the steady-state mean velocity and gyration radius as a function of an external driving current in the presence of point-like and columnar disorder. We validate our simulation algorithm by matching our results against a previously-used Monte Carlo algorithm, verifying that these microscopically quite distinct methods yield similar results even in out-of-equilibrium settings.<br>Ph. D.
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Schmiel, David R. "Effects of variations in controller gains on the dynamics of magnetic bearings." Thesis, This resource online, 1996. http://scholar.lib.vt.edu/theses/available/etd-11182008-063516/.
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Yadav, Nirbhay N. "Probing porous systems using nuclear magnetic resonance diffusometry." View thesis, 2009. http://handle.uws.edu.au:8081/1959.7/46601.
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Thesis (Ph.D.)--University of Western Sydney, 2009.<br>A thesis presented to the University of Western Sydney, College of Health and Science, School of Biomedical and Health Sciences, in fulfilment of the requirements for the degree of Doctor of Philosophy. Includes bibliographies.
11
Brown, Bart Lee II. "Pattern formations and relaxation dynamics in non-equilibrium systems." Diss., Virginia Tech, 2019. http://hdl.handle.net/10919/89346.
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We present an investigation of two non-equilibrium systems: spatial many-species predator-prey games and systems of interacting magnetic skyrmions.
We numerically study two predator-prey systems characterized by nested pattern formations. We first consider a six species game in which spiral patterns spontaneously form within coarsening domains. Through a systematic investigation of relevant correlation functions, the interface width, and other quantities, we show that the non-trivial in-domain dynamics affect the coarsening process and the interfacial properties. The exponents which govern domain growth, aging, and interface fluctuations differ from those expected from curvature driven coarsening. The response to perturbations of the reaction rates is also studied. Furthermore, we introduce a nine species model characterized by nested spiral pattern formations. Quantitative evidence of the existence of two length and time scales associated to the spiral levels is presented in the form of correlation lengths and a temporal Fourier analysis of the species densities. A generalized interaction scheme is proposed for dynamically generated hierarchies.
Magnetic skyrmions are particle-like spin configurations found in certain chiral magnets. We study the effect of the Magnus force on the relaxation dynamics through Langevin molecular dynamics simulations. The Magnus force enhances the disorder of the system at high noise strengths while we observe a dynamic regime with slow decaying correlations at low noise strengths. The different regimes are characterized by changes in the aging exponent. In general, the Magnus force accelerates the approach to the steady state. In the presence of quenched disorder, we find that the relaxation dynamics are more robust in systems with a strong Magnus force. We also examine periodically driven skyrmion systems and show that a transition from reversible to irreversible flow exists in the presence of attractive defects. The Magnus force enhances the irreversible regime in this case.
The work on predator-prey systems was supported by the U.S. National Science Foundation through Grant No. DMR-1606814 whereas the work on skyrmions was supported by the US Department of Energy, Office of Basic Energy Sciences (DOE-BES), under Grant No. DE-FG02-09ER46613.<br>Doctor of Philosophy<br>We present an investigation of two non-equilibrium systems: spatial many-species predator- prey games and systems of interacting magnetic skyrmions. We numerically study two predator-prey systems characterized by nested pattern formations. We first consider a six species game in which spiral patterns spontaneously form within coarsening domains. Through a systematic investigation of relevant correlation functions, the interface width, and other quantities, we show that the non-trivial in-domain dynamics affect the coarsening process and, to a greater extent, properties at the interface between competing groups of species. The exponents which govern domain growth, aging, and interface fluctuations are shown to differ from those expected in typical games of competition. We also study the change of the system due to a perturbation of the reaction rates, which could represent an abrupt change in the environment. Furthermore, we introduce a nine species model characterized by the emergence of nested spiral pattern formations. Quantitative evidence of the existence of two distinct spiral levels is presented. We also propose a generalized interaction scheme for dynamically generated spiral hierarchies. Magnetic skyrmions are particle-like spin configurations found in certain chiral magnets. We study the effect of the Magnus force on the dynamic properties of skyrmion systems through particle-based simulations. The Magnus force enhances the disorder of the system at high noise strengths while accelerating the formation of the triangular lattice at low noise strengths. We find that, in general, the Magnus force accelerates the approach to the steady state. In the presence of randomly placed attractive pinning sites, we find that a strong Magnus force can prevent caging effects and allow skyrmions to more easily move around pinning sites. We also examine periodically driven skyrmion systems and show that a transition from reversible to irreversible flow exists in the presence of attractive defects. The Magnus force is shown to enhance the irreversible regime in this case. The work on predator-prey systems was supported by the U.S. National Science Foundation through Grant No. DMR-1606814 whereas the work on skyrmions was supported by the US Department of Energy, Office of Basic Energy Sciences (DOE-BES), under Grant No. DE-FG02-09ER46613.
12
Margineda, Daniel. "Investigations of spin dynamics in magnetic systems and development of novel probes." Thesis, Cardiff University, 2017. http://orca.cf.ac.uk/111448/.
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This thesis presents the development of the first SQUID-based ac-magnetometer built in a dry dilution fridge. The possibility of expanding the frequency respond from tens of kHz of a commercial magnetometer to the bandwidth of muon-spin relaxation μSR (MHz), an indirect magnetic probe, which measures magnetic dynamics from the depolarisation of fundamental particles, is demonstrated. It opens a new scenario to investigate classical and quantum magnetic fluctuations by a direct probe in a range of frequencies that have an important role in exotic magnetic phases. Geometrical spin ices, frustrated magnets where magnetic excitations can be deconfined forming monopoles are good candidates to investigate. The study of magnetic fluctuations will help to understand the magnetic dynamics of these elementary excitations. Quantum spin liquids, frustrated systems without long-range order but with spins highly correlated that still fluctuate down to zero Kelvin are also interesting systems. The temperature dependence of their quantum fluctuations investigated by μSR have shown a dynamical plateau that might be corroborated by susceptibility measurements. The fabrication of the magnetometer has been combined with μSR investigations of the ground state of NbFe2 and CeRhIn5. The magnetic ground state of the ferromagnetic quantum critical point induced by growing around 1% Nbrich Nb1−yFe2+y is claimed to be reached in this kind of clean itinerant systems by a long–range spin density wave (SDW), although several attempts to identify the ground state by neutron scattering were unsuccessful. The μSR measurements prove that the ground state of the stoichiometric compound is governed by static and short-range correlations or by an incommensurate and helical SDW. The heavy fermion CeRhIn5 was investigated at ambient pressure due to the coexistence of antiferromagnetic and superconducting order in an intermediate region of pressures. The filamentary or bulk nature of the superconducting phase is still until debate and the onset of superconductivity at ambient pressure reported in some works may shed some light into the nature of the ground state. μSR and resistivity measurements were carried out to investigate the antiferromagnetic and helical phase without any signature of a superconducting transition.
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Rantaharju, J. (Jyrki). "Magnetization dynamics in paramagnetic systems." Doctoral thesis, Oulun yliopisto, 2018. http://urn.fi/urn:isbn:9789526221205.
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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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Adams, Daniel J. "Magnetization Dynamics in Coupled Thin Film Systems." ScholarWorks@UNO, 2019. https://scholarworks.uno.edu/td/2578.
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A study is presented detailing experimental investigations of magnetization dynamics in nanostructured systems which are coupled magnetically. This work seeks to characterize the anisotropy of such systems through experimental techniques which probe microwave resonant absorption in the materials.
A custom-built experimental setup, designed and assembled in our labs, is explained in detail. This setup allows for angular-dependent ferromagnetic resonance (FMR) measurements in the sample plane through vector network analyzer spectroscopy and is adaptable to two different types of coplanar waveguides. This technique has proven effective for characterization of multiple types of magnetic systems, including multilayered structures as detailed here, with different types of anisotropies while allowing us to draw analogies with more common characterization techniques. The angular FMR setup has been used to study coupled systems, such as those coupled through the Ruderman–Kittel–Kasuya–Yosida interaction as well as exchange-biased structures. These types of coupled systems have technological impacts and are highly applied in the components of magnetoresistive random access memory. Using this new characterization technique, properties of synthetic antiferromagnets have been revealed which had not been observed before.
In addition to these experiments, magnetic susceptibility and FMR in exchange biased systems have been investigated at temperatures as low as 2 K. This investigation used a new FMR spectrometer and was one of the first studies to use this instrument.
For the first time a new method of identifying several types of coupling which can be present in layered nanostructures is presented and supported through comparison with known techniques, thus connecting a new characterization technique for layered structures with decades-old procedures. Many results within this work are also supported theoretically with computer simulations.
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Radu, Cosmin. "Study of Magnetization Switching in Coupled Magnetic Nanostructured Systems." ScholarWorks@UNO, 2008. http://scholarworks.uno.edu/td/894.
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A study of magnetization dynamics experiments in nanostructured materials using the rf susceptibility tunnel diode oscillator (TDO) method is presented along with a extensive theoretical analysis. An original, computer controlled experimental setup that measures the change in susceptibility with the variation in external magnetic field and sample temperature was constructed. The TDO-based experiment design and construction is explained in detail, showing all the elements of originality. This experimental technique has proven reliable for characterizing samples with uncoupled magnetic structure and various magnetic anisotropies like: CrO2 , FeCo/IrMn and Co/SiO2 thin films. The TDO was subsequently used to explore the magnetization switching in coupled magnetic systems, like synthetic antiferromagnet (SAF) structures. Magnetoresistive random access memory (MRAM) is an important example of devices where the use of SAF structure is essential. To support the understanding of the SAF magnetic behavior, its configuration and application are reviewed and more details are provided in an appendix. Current problems in increasing the scalability and decreasing the error rate of MRAM devices are closely connected to the switching properties of the SAF structures. Several theoretical studies that were devoted to the understanding of the concepts of SAF critical curve are reviewed. As one can notice, there was no experimental determination of SAF critical curve, due to the difficulties in characterizing a magnetic coupled structure. Depending of the coupling strength between the two ferromagnetic layers, on the SAF critical curve one distinguishes several new features, inexistent in the case of uncoupled systems. Knowing the configuration of the SAF critical curve is of great importance in order to control its switching characteristics. For the first time a method of experimentally recording the critical curve for SAF is proposed in this work. In order to overcome technological limitations, a new way of recording the critical curve by using an additional magnetic bias field was explored.
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Panicker, Anil T. "A systems dynamics economic evaluation methodology for high speed inter-city transportation." Thesis, This resource online, 1991. http://scholar.lib.vt.edu/theses/available/etd-10102009-020125/.
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Schavkan, Alexander [Verfasser], and Gerhard [Akademischer Betreuer] Grübel. "Dynamics of colloidal systems of magnetic nanoparticles under influence of magnetic fields investigated by XPCS / Alexander Schavkan ; Betreuer: Gerhard Grübel." Hamburg : Staats- und Universitätsbibliothek Hamburg, 2017. http://d-nb.info/1135725276/34.
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Schavkan, A. [Verfasser], and Gerhard [Akademischer Betreuer] Grübel. "Dynamics of colloidal systems of magnetic nanoparticles under influence of magnetic fields investigated by XPCS / Alexander Schavkan ; Betreuer: Gerhard Grübel." Hamburg : Staats- und Universitätsbibliothek Hamburg, 2017. http://nbn-resolving.de/urn:nbn:de:gbv:18-85618.
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Rodgers, Christopher T. "Magnetic field effects in chemical systems." Thesis, University of Oxford, 2007. http://ora.ox.ac.uk/objects/uuid:f5878b88-c5ba-4cbd-83af-857431aef66e.
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Magnetic fields influence the rate and/or yield of chemical reactions that proceed via spin correlated radical pair intermediates. The field of spin chemistry centres around the study of such magnetic field effects (MFEs). This thesis is particularly concerned with the effects of the weak magnetic fields B₀ ~ 1mT relevant in the ongoing debates on the mechanism by which animals sense the geomagnetic field and on the putative health effects of environmental electromagnetic fields. Relatively few previous studies have dealt with such weak magnetic fields. This thesis presents several new theoretical tools and applies them to interpret experimental measurements. Chapter 1 surveys the development and theory of spin chemistry. Chapter 2 introduces the use of Tikhonov and Maximum Entropy Regularisation methods as a new means of analysing MARY field effect data. These are applied to recover details of the diffusive motion of reacting pyrene and N,N-dimethylaniline radicals. Chapter 3 gives a fresh derivation and appraisal of an approximate, semiclassical approach to MFEs. Monte Carlo calculations allow the elucidation of several "rules of thumb" for interpreting MFE data. Chapter 4 discusses recent optically-detected zero-field EPR measurements, adapting the gamma-COMPUTE algorithm from solid state NMR for their interpretation. Chapter 5 explores the role of RF polarisation in producing MFEs. The breakdown in weak fields of the familiar rotating frame approximation is analysed. Chapter 6 reviews current knowledge and landmark experiments in the area of animal magnetoreception. The origins of the sensitivity of European robins Erithacus rubecula to the Earth’s magnetic field are given particular attention. In Chapter 7, Schulten and Ritz’s hypothesis that avian magnetoreception is founded on a radical pair mechanism (RPM) reaction is appraised through calculations in model systems. Chapter 8 introduces quantitative methods of analysing anisotropic magnetic field effects using spherical harmonics. Chapter 9 considers recent observations that European robins may sometimes be disoriented by minuscule RF fields. These are shown to be consistent with magnetoreception via a radical pair with no (effective) magnetic nuclei in one of the radicals.
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Yan, Hui. "Dynamics and real-time optimal control of satellite attitude and satellite formation systems." Texas A&M University, 2006. http://hdl.handle.net/1969.1/4283.
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In this dissertation the solutions of the dynamics and real-time optimal control of
magnetic attitude control and formation flying systems are presented. In magnetic
attitude control, magnetic actuators for the time-optimal rest-to-rest maneuver with a
pseudospectral algorithm are examined. The time-optimal magnetic control is bang-bang
and the optimal slew time is about 232.7 seconds. The start time occurs when the
maneuver is symmetric about the maximum field strength. For real-time computations,
all the tested samples converge to optimal solutions or feasible solutions. We find the
average computation time is about 0.45 seconds with the warm start and 19 seconds with
the cold start, which is a great potential for real-time computations. Three-axis magnetic
attitude stabilization is achieved by using a pseudospectral control law via the receding
horizon control for satellites in eccentric low Earth orbits. The solutions from the
pseudospectral control law are in excellent agreement with those obtained from the
Riccati equation, but the computation speed improves by one order of magnitude. Numerical solutions show state responses quickly tend to the region where the attitude
motion is in the steady state.
Approximate models are often used for the study of relative motion of formation
flying satellites. A modeling error index is introduced for evaluating and comparing the
accuracy of various theories of the relative motion of satellites in order to determine the
effect of modeling errors on the various theories. The numerical results show the
sequence of the index from high to low should be Hill's equation, non- J2, small
eccentricity, Gim-Alfriend state transition matrix index, with the unit sphere approach
and the Yan-Alfriend nonlinear method having the lowest index and equivalent
performance. A higher order state transition matrix is developed using unit sphere
approach in the mean elements space. Based on the state transition matrix analytical
control laws for formation flying maintenance and reconfiguration are proposed using
low-thrust and impulsive scheme. The control laws are easily derived with high
accuracy. Numerical solutions show the control law works well in real-time
computations.
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Sánchez-Barriga, Jaime. "A photoemission study of quasiparticle excitations, electron-correlation effects and magnetization dynamics in thin magnetic systems." Phd thesis, Universität Potsdam, 2010. http://opus.kobv.de/ubp/volltexte/2010/4849/.
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This thesis is focused on the electronic, spin-dependent and dynamical properties of thin magnetic systems. Photoemission-related techniques are combined with synchrotron radiation to study the spin-dependent properties of these systems in the energy and time domains.
In the first part of this thesis, the strength of electron correlation effects in the spin-dependent electronic structure of ferromagnetic bcc Fe(110) and hcp Co(0001) is investigated by means of spin- and angle-resolved photoemission spectroscopy. The experimental results are compared to theoretical calculations within the three-body scattering approximation and within the dynamical mean-field theory, together with one-step model calculations of the photoemission process. From this comparison it is demonstrated that the present state of the art many-body calculations, although improving the description of correlation effects in Fe and Co, give too small mass renormalizations and scattering rates thus demanding more refined many-body theories including nonlocal fluctuations.
In the second part, it is shown in detail monitoring by photoelectron spectroscopy how graphene can be grown by chemical vapour deposition on the transition-metal surfaces Ni(111) and Co(0001) and intercalated by a monoatomic layer of Au. For both systems, a linear E(k) dispersion of massless Dirac fermions is observed in the graphene pi-band in the vicinity of the Fermi energy. Spin-resolved photoemission from the graphene pi-band shows that the ferromagnetic polarization of graphene/Ni(111) and graphene/Co(0001) is negligible and that graphene on Ni(111) is after intercalation of Au spin-orbit split by the Rashba effect.
In the last part, a time-resolved x-ray magnetic circular dichroic-photoelectron emission microscopy study of a permalloy platelet comprising three cross-tie domain walls is presented. It is shown how a fast picosecond magnetic response in the precessional motion of the magnetization can be induced by means of a laser-excited photoswitch. From a comparision to micromagnetic calculations it is demonstrated that the relatively high precessional frequency observed in the experiments is directly linked to the nature of the vortex/antivortex dynamics and its response to the magnetic perturbation. This includes the time-dependent reversal of the vortex core polarization, a process which is beyond the limit of detection in the present experiments.<br>Diese Dissertation beschäftigt sich mit den elektronischen, spinabhängigen und dynamischen Eigenschaften dünner magnetischer Systeme. Auf dem Photoeffekt basierende Untersuchungsmethoden werden zusammen mit Synchrotronstrahlung eingesetzt, um die spinabhängigen Eigenschaften dieser Systeme im Energie- und Zeitbereich zu untersuchen.
Im ersten Teil dieser Arbeit wird mit spin- und winkelaufgelöster Photoemission die Stärke von Elektronenkorrelationseffekten in der spinabhängigen elektonischen Struktur von ferromagnetischerm bcc Fe(110) und hcp Co(0001) untersucht. Die experimentellen Ergebnisse werden verglichen mit theoreteischen Berechnungen im Rahmen der Näherung der Drei-Körper-Streuung und der dynamischen Molekularfeldtheorie, zusammen mit Berechnungen des Photoemissionsprozesses im Rahmen des Ein-Stufen-Modells. Ausgehend von diesem Vergleich wird gezeigt, dass die gegenwärtig fortgeschrittensten Rechnung, obgleich sie die Beschreibung von Korrelationseffekten in Fe und Co verbessern, zu kleine Massenrenormalisierungen und Streuraten ergeben, was zu der Forderung nach verfeinerten Vielteilchentheorien unter Einbeziehung von nichtlokalen Fluktuationen führt.
Im zweiten Teil wird unter Kontrolle durch die Photoelektronenspektroskopie im Detail gezeigt, wie Graphen durch chemische Gasphasenabscheidung auf den Übergangsmetall-Oberflächen Ni(111) und Co(0001) aufgebracht und mit einer Monolage Au interkaliert werden kann. Für beide Systeme wird eine lineare E(k)-Dispersion masseloser Dirac-Fermionen im Graphen-pi-Band in der Nähe der Fermi-Energie beobachtet. Spinaufgelöste Photoemission des Graphen-pi-Bandes zeigt, dass die ferromagnetische Polarisation von Graphen/Ni(111) und Graphen/Co(0001) vernachlässigbar ist und dass Graphen/Ni(111) nach Interkalation mit Au eine Spin-Bahn-Aufspaltung aufgrund des Rashba-Effekts zeigt.
Im letzten Teil wird eine zeitaufgelöste Studie des Röntgenzirkulardichroismus mit Photoelektronenmikroskopie präsentiert, die an einer Permalloy-Probe durchgeführt wurde, die drei als Stachelwände ausgebildete Domänenwände enthält. Es wird gezeigt, wie eine schnelle magnetische Antwort auf der Pikosekundenskala in der Präzessionsbewegung der Magnetisierung durch einen laserangesteuerten Photoschalter erzeugt werden kann. Durch Vergleich mit einer mikromagnetischen Rechnung wird gezeigt, dass die relativ hohe Präzessionsfrequenz, die im Experiment beobachtet wird, in unmittelbarer Beziehung steht zu den Eigenschaften der Vortex/Antivortex-Dynamik und ihrer Antwort auf die magnetische Störung. Das schließt die zeitabhängige Umkehr der Vortexkernpolarisation ein, einem Vorgang der jenseits der Nachweisgrenze der gegenwärtigen Experimente liegt.
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Landers, Joachim [Verfasser], and Heiko [Akademischer Betreuer] Wende. "Study of magnetic relaxation dynamics in soft matter nanoparticle composite systems / Joachim Landers ; Betreuer: Heiko Wende." Duisburg, 2017. http://d-nb.info/1127527770/34.
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Stagraczyński, Stefan Piotr [Verfasser]. "Magnetic dynamics and spin currents in quantum spin systems strongly coupled to environment / Stefan Piotr Stagraczyński." Halle, 2017. http://d-nb.info/114951289X/34.
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Liu, Peiwen. "Ultrafast Optical Control And Characterization Of Carrier And Spin Dynamics In Novel Magnetic Topological Insulator Systems." W&M ScholarWorks, 2021. https://scholarworks.wm.edu/etd/1627047889.
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Magnetic topological insulators (MTIs) are of considerable interest in developing novel spintronics and quantum computing applications. Under the topological protection by time-reversal Z2 invariant number, magnetic topological insulators are provided with robust electronic and magnetic properties against local perturbations. The quantum anomalous Hall effect (QAHE), which harbors dissipationless chiral edge states in MTIs, provides a competitive platform for future low-power consumption and high-speed spintronic devices. Although the present studies on both bulk and surface magnetic properties in MTIs have made significant progress, the in-depth understanding of the exchange couplings and the interaction between the two magnetization sources is far from completion. In addition, the optical control of the non-trivial properties in MTIs is important in achieving novel applications for ultrafast optoelectronics and optical spintronics. The goal of this dissertation is to understand and manipulate the dynamical spin coupling as well as the carrier relaxation dynamics in MTIs, using the static magneto-optical Kerr effect (MOKE) and the time-resolved magneto optical Kerr effect (TRMOKE) techniques. First, a pronounced spin-valve-like structure of dynamical magnetization is observed in Cr-(Bi,Sb)2Te3/CrSb bilayer heterostructure through the pump-modulated MOKE characterization. The characters of the soft and resilient ferromagnetic phases are distinguished in terms of the spin coupling between the dynamical surface and bulk ferromagnetism. The dynamical bulk ferromagnetic ordering is softened by a laser-induced heat effect on the lattice, while the dynamical surface magnetization is enhanced via the strengthening Dirac-hole-mediated exchange coupling. In addition, the pump-fluence-dependent measurement of the exchange-bias effect provides further evidence for the enhancement of MTI surface magnetization at the MTI/AFM interface. Lastly, we propose a theoretical model that includes the long-range p-d exchange coupling and a Dirac-hole-mediated exchange interaction and estimate the exchange coupling energies in the MTI/AFM bilayer structure. Second, ultralong carrier lifetimes (3~20 ns) of the optically pumped surface states are observed in Cr-(Bi,Sb)2Te3 MTI which corresponds to the slow radiative recombination within the gapped Dirac cone. The photoinjection dependency of radiative lifetimes suggests a strong Coulomb screening effect of electron-hole plasma on surface excitons. The experimental results are consistent with the theoretical simulation. On the other hand, the nonradiative nature of bulk electron relaxation is identified with a lifetime of ~1000 ps by photoinjection- and temperature- dependent reflectivity measurements. The finding of long-lived excited carriers in MTI improves the understanding of the general carrier dynamics in topological insulators-based materials.
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Assi, Hiba. "Non-Equilibrium Relaxation Dynamics in Disordered Superconductors and Semiconductors." Diss., Virginia Tech, 2016. http://hdl.handle.net/10919/70858.
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We investigate the relaxation properties of two distinct systems: magnetic vortex lines in disordered type-II superconductors and charge carriers in the Coulomb glass in disordered semiconductors.
We utilize an elastic line model to simulate magnetic flux lines in disordered type-II superconductors by performing Langevin molecular dynamics simulations. We study the non-equilibrium relaxation properties of flux lines in the presence of uncorrelated point-like disorder or extended linear defects analyzing the effects of rapid changes in the system's temperature or magnetic field on these properties. In a previously-equilibrated system, either the temperature is suddenly changed or the magnetic field is abruptly altered by adding or removing random flux lines to or from the system. One-time observables such as the radius of gyration are measured to characterize steady-state properties, and two-time correlation functions such as the vortex line height autocorrelations are computed to investigate the relaxation dynamics in the aging regime and therefore distinguish the complex relaxation features that result from the different types of disorder in the system. This study allows us to test the sensitivity of the system's non-equilibrium aging kinetics to the selection of initial states and to make closer contact to experimental setups.
Furthermore, we employ Monte Carlo simulations to study the relaxation properties of the two-dimensional Coulomb glass in disordered semiconductors and the two-dimensional Bose glass in type-II superconductors in the presence of extended linear defects. We investigate the effects of adding non-zero random on-site energies from different distributions on the properties of the correlation-induced Coulomb gap in the density of states and on the non-equilibrium aging kinetics highlighted by the autocorrelation functions. We also probe the sensitivity of the system's equilibrium and non-equilibrium relaxation properties to instantaneous changes in the density of charge carriers in the Coulomb glass or flux lines in the Bose glass.<br>Ph. D.
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Elgabarty, Hossam [Verfasser]. "Probing the Structure and Dynamics of Disordered Systems by MD-averaged ab initio Magnetic Resonance / Hossam Elgabarty." Berlin : Freie Universität Berlin, 2013. http://d-nb.info/1042186073/34.
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Chaturvedi, Harshwardhan Nandlal. "Dynamics of Driven Vortices in Disordered Type-II Superconductors." Diss., Virginia Tech, 2019. http://hdl.handle.net/10919/86844.
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We numerically investigate the dynamical properties of driven magnetic flux vortices in disordered type-II superconductors for a variety of temperatures, types of disorder and sample thicknesses. We do so with the aid of Langevin molecular dynamics simulations of a coarsegrained elastic line model of flux vortices in the extreme London limit. Some original findings of this doctoral work include the discovery that flux vortices driven through random point disorder show simple aging following drive quenches from the moving lattice state to both the pinned glassy state (non-universal aging) and near the critical depinning region (universal aging); estimations of experimentally consistent critical scaling exponents for the continuous depinning phase transition of vortices in three dimensions; and an estimation of the boundary curve separating regions of linear and non-linear electrical transport for flux lines driven through planar defects via novel direct measurements of vortex excitations.<br>Ph. D.<br>The works contained in this dissertation were undertaken with the goal of better understanding the dynamics of driven magnetic flux lines in type-II superconductors under different conditions of temperature, material defects and sample thickness. The investigations were conducted with the aid of computer simulations of the flux lines which preserve physical aspects of the system relevant to long-time dynamics while discarding irrelevant microscopic details. As a result of this work, we found (among other things) that when driven by electric currents, flux lines display very different dynamics depending on the strength of the current. When the current is weak, the material defects strongly pin the flux lines leaving them in a disordered glassy state. Sufficiently high current overpowers the defect pinning and results in the flux lines forming into a highly ordered crystal-like structure. In the intermediate critical current regime, the competing forces become comparable resulting in very large fluctuations of the flux lines and a critical slowing down of the flux line dynamics.
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Naugle, Cameron R. "Rotordynamic Analysis of Theoretical Models and Experimental Systems." DigitalCommons@CalPoly, 2018. https://digitalcommons.calpoly.edu/theses/1867.
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This thesis is intended to provide fundamental information for the construction and
analysis of rotordynamic theoretical models, and their comparison the experimental
systems. Finite Element Method (FEM) is used to construct models using Timoshenko
beam elements with viscous and hysteretic internal damping. Eigenvalues
and eigenvectors of state space equations are used to perform stability analysis, produce
critical speed maps, and visualize mode shapes. Frequency domain analysis
of theoretical models is used to provide Bode diagrams and in experimental data
full spectrum cascade plots. Experimental and theoretical model analyses are used
to optimize the control algorithm for an Active Magnetic Bearing on an overhung
rotor.
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Klingler, Stefan Horst [Verfasser], Sebastian T. B. [Akademischer Betreuer] Goennenwein, Sebastian T. B. [Gutachter] Goennenwein, and Christian [Gutachter] Back. "Magnetization dynamics in coupled magnetic systems / Stefan Horst Klingler ; Gutachter: Sebastian T.B. Goennenwein, Christian Back ; Betreuer: Sebastian T.B. Goennenwein." München : Universitätsbibliothek der TU München, 2019. http://d-nb.info/118325962X/34.
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Walker, Philipp [Verfasser], and Jan [Akademischer Betreuer] Lipfert. "Millisecond dynamics of biological systems investigated with magnetic tweezers and small-angle X-ray scattering / Philipp Walker ; Betreuer: Jan Lipfert." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2018. http://d-nb.info/1189584611/34.
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Simmons, Stephanie. "Creation and control of entanglement in condensed matter spin systems." Thesis, University of Oxford, 2011. http://ora.ox.ac.uk/objects/uuid:b9c5ad90-30e2-4e44-8c51-37d46eabc92f.
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The highly parallel nature of the fundamental principles of quantum mechanics means that certain key resource-intensive tasks --- including searching, code decryption and medical, chemical and material simulations --- can be computed polynomially or even exponentially faster with a quantum computer. In spite of its remarkably fast development, the field of quantum computing is still young, and a large-scale prototype using any one of the candidate quantum bits (or 'qubits') under investigation has yet to be developed. Spin-based qubits in condensed matter systems are excellent candidates. Spins controlled using magnetic resonance have provided the first, most advanced, and highest fidelity experimental demonstrations of quantum algorithms to date. Despite having highly promising control characteristics, most physical ensembles investigated using magnetic resonance are unable to produce entanglement, a critical missing ingredient for a pure-state quantum computer. Quantum objects are said to be entangled if they cannot be described individually: they remain fundamentally linked regardless of their physical separation. Such highly non-classical states can be exploited for a host of quantum technologies including teleportation, metrology, and quantum computation. Here I describe how to experimentally create, control and characterise entangled quantum ensembles using magnetic resonance. I first explore the relationship between entanglement and quantum metrology and demonstrate a sensitivity enhancement over classical resources using molecular sensors controlled with liquid-state nuclear magnetic resonance. I then examine the computational potential of irreversible relaxation processes in combination with traditional reversible magnetic resonance control techniques. I show how irreversible processes can polarise both nuclear and electronic spins, which improves the quality of qubit initialisation. I discuss the process of quantum state tomography, where an arbitrary quantum state can be accurately measured and characterised, including components which go undetected using traditional magnetic resonance techniques. Lastly, I combine the above findings to initialise, create and characterise entanglement between an ensemble of electron and nuclear spin defects in silicon. I further this by generating pseudo-entanglement between an ensemble of nuclear spins mediated by a transient electron spin in a molecular system. These findings help pave the way towards a particular architecture for a scalable, spin-based quantum computer.
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Regan, David Gabriel. "NMR DIFFUSION MEASUREMENTS OF COMPARTMENTALIZED AND MULTICOMPONENT BIOLOGICAL SYSTEMS: Studies of Tropoelastin, the Self Association of N Methylacetamide, and q-Space Analysis of Real and Model Cell Suspensions." University of Sydney. School of Molecular and Microbial Biosciences, 2002. http://hdl.handle.net/2123/514.
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Molecular diffusion is an inherent feature of all fluid systems. The processes and interactions that characterize these systems are in some way dependent upon the mobility of the component molecules. Pulsed field-gradient spin-echo nuclear magnetic resonance (PGSE NMR) is a powerful tool for the study of molecular diffusion; for heterogeneous systems, such as those typically found in biology, this technique is unsurpassed in the diversity of systems that yield to its probing. The aim of the work presented in this thesis was to use an integrated NMR-based approach, in conjunction with computer modeling, for the study of molecular diffusion in compartmentalized and multicomponent biological systems. Erythrocyte suspensions provided an ideal experimental system for the study of compartmentalized diffusion in cells. Water exchanges rapidly between the intra- and extracellular regions and, as the major constituent of the cell, provides a strong and predominant proton NMR signal. In addition, the cells are known to align in the strong static magnetic field of the spectrometer. As a consequence of these two properties, the signal intensity from a suitably designed series of PGSE NMR experiments exhibits a series of maxima and minima when graphed as a function of the magnitude of the spatial wave number vector q. The apparently periodic phenomenon is mathematically analogous to optical diffraction and interference and is referred to here as diffusion-coherence. It is the characterization of this phenomenon, with the aid of computer-based models, which was the focus of a major section of the work described herein. Two quite distinct molecular systems formed the basis of the work in which I investigated diffusion in multicomponent systems. Both systems involved molecules that undergo self-association such that at equilibrium a population distribution of different oligomeric species is present. The first of these was tropoelastin, the monomeric subunit of elastin, which under certain conditions aggregates to form a coacervate. The second system was N-methylacetamide (NMA) which also undergoes extensive self-association. NMA oligomers have previously been studied as peptide analogues due to the presence in the monomer of a peptide linkage. In this work the aim was to use PGSE NMR diffusion measurements, in a manner that is in many ways analogous to analytical ultracentrifugation, to obtain estimates of hydrodynamic and thermodynamic parameters. Computer modeling was also used extensively in this section of work for the interpretation of the experimental data.
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Johnson, Kevin Robert. "In Vivo Coronary Wall Shear Stress Determination Using CT, MRI, and Computational Fluid Dynamics." Diss., Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/14482.
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Wall shear stress (WSS) has long been identified as a factor in the development of atherosclerotic lesions. Autopsy studies have revealed a strong tendency for lesion development at arterial branch sites and along the inner walls of curvature areas that, in theory, should experience low WSS. Calculations of coronary artery WSS have typically been based upon average models of coronary artery geometry with average flow conditions and then compared to average lesion distributions. With all the averaging involved, a more detailed knowledge of the correlation between WSS and atherosclerotic lesion development might be obscured. Recent advancements in hemodynamic modeling now enable the calculation of WSS in individual subjects. An image-based approach for patient-specific calculation of in vivo WSS using computational fluid dynamics (CFD) would allow a more direct study of this correlation. New state-of-the-art technologies in multi-detector computed tomography (CT) and 3.0 Tesla magnetic resonance imaging (MRI) offer potential improvements for the measurement of coronary artery geometry and blood flow.
The overall objective of this research was to evaluate the quantitative accuracy of multi-detector CT and 3.0 Tesla MRI and incorporate those imaging modalities into a patient-specific CFD model of coronary artery WSS. Using a series of vessel motion phantoms, it has been shown that 64-detector CT can provide accurate measurements of coronary artery geometry for heart rates below 70 beats per minute. A flow phantom was used to validate the use of navigator-echo gated, phase contrast MRI at 3.0 Tesla to measure velocity of coronary blood flow. Patient-specific, time-resolved CFD models of coronary WSS were created for two subjects. Furthermore, it was determined that population-average velocity curves or steady state velocities can predict locations of high or low WSS with high degrees of accuracy compared to the use of patient-specific blood flow velocity measurements as CFD boundary conditions. This work is significant because it constitutes the first technique to non-invasively calculate in vivo coronary artery WSS using image-based, patient-specific modeling.
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Kurcyus, Katarzyna [Verfasser], Valentin [Akademischer Betreuer] Riedl, Helmuth [Gutachter] Adelsberger, and Markus [Gutachter] Ploner. "Using magnetic resonance spectroscopy to study neural activity: The role of inhibitory and excitatory neurotransmitters for systems-level brain dynamics / Katarzyna Kurcyus ; Gutachter: Helmuth Adelsberger, Markus Ploner ; Betreuer: Valentin Riedl." München : Universitätsbibliothek der TU München, 2017. http://d-nb.info/1172880034/34.
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Le, Toan T. "A Single-Stage Passive Vibration Isolation System for Scanning Tunneling Microscopy." DigitalCommons@CalPoly, 2021. https://digitalcommons.calpoly.edu/theses/2272.
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Scanning Tunneling Microscopy (STM) uses quantum tunneling effect to study the surfaces of materials on an atomic scale. Since the probe of the microscope is on the order of nanometers away from the surface, the device is prone to noises due to vibrations from the surroundings. To minimize the random noises and floor vibrations, passive vibration isolation is a commonly used technique due to its low cost and simpler design compared to active vibration isolation, especially when the entire vibration isolation system (VIS) stays inside an Ultra High Vacuum (UHV) environment. This research aims to analyze and build a single-stage passive VIS for an STM. The VIS consists of a mass-spring system staying inside an aluminum hollow tube. The mass-spring system is comprised of a circular copper stage suspended by a combination of six extension springs, and the STM stays on top of the copper stage. Magnetic damping with neodymium magnets, which induces eddy currents in the copper conductor, is the primary damping method to reduce the vibrations transferred to the mass-spring system. FEMM and MATLAB® are used to model magnetic flux density and damping coefficients from eddy current effect, which will help determine the necessary damping ratios for the VIS. Viton, which demonstrates a high compatibility with vacuum environments, will also serve as a great damping material between joints and contacts for the housing tube. Viton will be modeled as a Mooney-Rivlin hyperelastic material whose material parameters are previous studied, and Abaqus will be used as a Finite Element Analysis software to study the Viton gaskets’ natural frequencies. The natural frequencies of the aluminum hollow tube will also be investigated through Abaqus.
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Vannette, Matthew Dano. "Dynamic magnetic susceptibility of systems with long-range magnetic order." [Ames, Iowa : Iowa State University], 2009.
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Sunny, Ajin. "SINGLE-DEGREE-OF-FREEDOM EXPERIMENTS DEMONSTRATING ELECTROMAGNETIC FORMATION FLYING FOR SMALL SATELLITE SWARMS USING PIECEWISE-SINUSOIDAL CONTROLS." UKnowledge, 2019. https://uknowledge.uky.edu/me_etds/146.
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This thesis presents a decentralized electromagnetic formation flying (EMFF) control method using frequency-multiplexed sinusoidal control signals. We demonstrate the EMFF control approach in open-loop and closed-loop control experiments using a single-degree-of-freedom testbed with an electromagnetic actuation system (EAS). The EAS sense the relative position and velocity between satellites and implement a frequency-multiplexed sinusoidal control signal. We use a laser-rangefinder device to capture the relative position and an ARM-based microcontroller to implement the closed-loop control algorithm. We custom-design and build the EAS that implements the formation control in one dimension. The experimental results in this thesis demonstrate the feasibility of the decentralized formation control algorithm between two satellites.
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Yu, Howard. "Spin Dynamics in Novel Materials Systems." The Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1435582765.
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Hall, Dennis A. (Dennis Alan) 1970. "Dynamic nuclear polarization of biological systems at high magnetic fields." Thesis, Massachusetts Institute of Technology, 1998. http://hdl.handle.net/1721.1/9635.
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Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 1998.<br>Includes bibliographical references.<br>Dynamic nuclear polarization methods were studied at high magnetic field strength and were applied to improve the sensitivity of the nuclear magnetic resonance spectroscopy of biological solids. Studies of the dynamics of electron-nuclear polarization transfer via the solid effect and thermal mixing at 5 Tesla are described for two systems: the free radical BDPA doped into polystyrene and the nitroxide TEMPO in a water:glycerol matrix. A model for thermal mixing at high magnetic fields in paramagnetic systems such as TEMPO which exhibit partially inhomogeneous EPR lines is developed in which electron-electron cross relaxation across the EPR line is explicitly included. The TEMPO/water/glycerol matrix is exploited for polarization transfer to biological solutes. As a demonstration, enhancements of up to two orders of magnitude were exhibited in the high-resolution "1N magic-angle spinning spectra of the protein T4- lysozyme. The potential of this method as a general signal enhancement tool for biological systems is assessed. These dynamic nuclear polarization experiments at 5 Tesla require high-power microwave irradiation at or near the EPR frequency. To that end, a cyclotron resonance maser, or gyrotron, is described. This 140 GHz gyrotron, which under conventional operation produces millisecond pulses, has been adapted to operate at -100 W in a quasi-CW mode for tens of seconds, the time required for electron-nuclear polarization transfer.<br>by Dennis A. Hall.<br>Ph.D.
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Wigström, Lars. "Multidimensional magnetic resonance imaging : new methods for analysis of cardiovascular dynamics /." Linköping : Univ, 2003. http://www.bibl.liu.se/liupubl/disp/disp2003/tek807s.pdf.
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MEHRPARVAR, MAHSHID. "Control Systems for Experimental Magnetic Materials Characterization Using Dynamic Preisach Models." Thesis, KTH, Elektrisk energiomvandling, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-160704.
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The eciency of electrical machines is of major concern due to their widespread usage and the globally increasing awareness of energy consumption issues. Iron losses have a signicant impact on the total and thus researchers and manufacturers of electrical machines are developing dierent strategies in order to reduce them. The iron losses are highly dependent on the magnetic material that is used and thus it is necessary to identify its relevant characteristics. In this work, the development of a control system for inducing a pure sinusoidal magnetic ux density in the magnetic material is described. This is necessary in order to perform characterisation of the magnetic material. The main diculty is the highly non-linear and hysteretic relationship between the magnetic eld strength and the magnetic ux density. In order to mitigate the eect of the hysteresis, a mathematical inverse model was used in the control system. To nd a suitable model, an extensive study of literature was performed and discussed in this work. The Preisach model and its dynamic extension was chosen as the most suitable approach. A detailed description of both theory and implementation details is provided in this work. Furthermore, the model is validated by comparing simulation against measurement data for two dierent materials. In the last part of this work, the inverse model is combined with a controller to form a feedback control system. Two dierent control schemes are investigated: a simpler PI controller and a more elaborate disturbance observer (DOB) based control scheme. The DOB is used to observe the hysteresis inversion error and the observation is used to correct for the error. The controller's ability to produce a pure sinusoidal magnetic ux density was assessed by simulations with dierent magnetic materials at varying frequencies.<br>Verkningsgraden for elmotorer ar av okande intresse pa grund av deras omfattande anvandning och vaxande oro for globala energiforbrukningsfragor. Jarnforluster har ett stort inytande i de totala forlusterna och ar darfor ett viktigt omrade for forskare och tillverkare av elektriska motorer. Jarnforlusterna beror till stor del av det magnetiska materialet som anvands i konstruktion av elmotorer och det ar darfor nodvandigt att identiera materialets egenskaper. I det har arbetet beskrivs utvecklingen av ett reglersystem for att inducera en ren sinusformat magnetisk odestathet i ett magnetiskt material. Detta ar nodvandigt for att kunna bestamma det magnetiska materialets egenskaper under kontrollerade forhallanden. Huvudsvarigheten ar det icke-linjara sambandet mellan magnetiska faltstyrkan och odest atheten. Sambandet formar en hysteres och for att eliminera dess inytande anvandes en matematisk invers model. For att hitta en lamplig model genomfordes en literaturstudie och Preisach modellen och dess dynamiska utokning valdes. I detta arbete nns en detaljerad beskrivning av bade teorin bakom modellen och dess implementering. Modellen utvarderades genom att jamfora matvarden med simulationsresultat for olika magnetiska material. I sista delen av detta arbete kombineras inversmodellen med ett reglerssystem for att kunna uppna en sinusformat odestathet i det magnetiska materialet. Tva olika regleralgoritmer utvarderas, en enklare PI-regulator och en regulator som inkluderar en sa kallat "Disturbance Observer" (DOB). DOB:n anvandes for att observera felet som uppstar vid invertering av hysteresen och for att korrigera felet. De bada regulatorernas formaga attaterskapa en ren sinusformat magnetisk odestathet testas genom att genomfora simulationer for olika magnetiska material vid varierande frekvenser.
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Buttini, Thiago Malta. "Algoritmo de autoidentificação para o controle autônomo de vibrações em sistemas rotativos." Universidade de São Paulo, 2011. http://www.teses.usp.br/teses/disponiveis/18/18149/tde-03102011-210959/.
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Vibrações são intrínsecas às máquinas rotativas e, embora não possam ser completamente eliminadas, devem ser controladas de modo a se evitar fadiga e até mesmo falha da máquina. Neste contexto, devido à sua capacidade de alterar as características dinâmicas destas máquinas, os mancais ativos são uma solução efetiva a fim de se reduzir vibrações em rotores, permitindo não só maior ciclo de vida, mas também aumento de confiabilidade e desempenho. Frequentemente, o projeto do sistema de controle destes mancais baseia-se em um modelo matemático da planta, o qual pode ser de difícil obtenção e, devido à adoção de hipóteses simplificadoras (inerentes ao processo de modelagem), pode ser impreciso. Com base nestes conceitos, propõe-se a utilização de uma técnica de controle do tipo proporcional-derivativa baseada em medições de resposta em frequência (livre de modelos matemáticos) aplicada ao controle de vibrações em sistemas rotativos, contornando dificuldades de modelagem. Esta técnica é testada experimentalmente em uma bancada de testes cujos elementos de atuação são os eletromagnetos de um mancal ativo, e um algoritmo para a identificação automática das FRFs do sistema (algoritmo de autoidentificação) é desenvolvido e implementado, permitindo, de forma autônoma, o cálculo dos ganhos ótimos do controlador PD visando atenuação de vibrações. Com base nos resultados obtidos, tem-se que este trabalho é um estudo preliminar que pode viabilizar o desenvolvimento de um mancal ativo inteligente, o qual, a partir de medições do deslocamento do eixo, seria capaz de obter a resposta em frequência do sistema e determinar, de forma automática, os ganhos ótimos do controlador, possibilitando o controle autônomo de vibrações em sistemas rotativos, a partir de um algoritmo de autoidentificação e de uma metodologia de controle livre de modelos.<br>Vibrations are intrinsic to rotating machinery and, although they cannot be completely eliminated, it is important to control this kind of motion with the objective of avoiding fatigue and even failure of the machine. In this context, due to their capacity of changing the dynamic characteristics of these machines, active bearings are an effective solution to reduce vibration in rotors, allowing not only longer lifecycle, but also higher performance. Frequently, the design of the control system of these bearings is based on a mathematical model of the plant, whose obtainment can be hard and, due to the adoption of simplifying hypotheses (inherent to the modeling process), it may be imprecise. Keeping in mind these concepts, this dissertation proposes the use of a proportional-derivative control technique based on frequency response measurements (free of mathematical models) applied to the vibration control of rotating systems, overcoming modeling difficulties. This technique is experimentally tested in a test rig whose actuation elements are the electromagnets of an active bearing, and an algorithm for automatic identification of the system\'s FRFs (self-identification algorithm) is developed and implemented, allowing, in an autonomous way, the calculation of the optimum gains of the PD controller aiming at controlling vibrations. Based on the obtained results, this work consists in a preliminary study that may enable the development of a smart active bearing, which, from measurements of the shaft\'s displacement, would be capable of obtaining the frequency response of the system and determine, automatically, the optimum gains of the controller, making it possible the autonomous vibration control in rotating systems, from a self-identification algorithm and a model-free control methodology.
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Stockem, Irina [Verfasser]. "Modeling and simulation of the static and dynamic magnetic properties of nanostructured magnetic multilayer systems / Irina Stockem." Bielefeld : Universitätsbibliothek Bielefeld, 2016. http://d-nb.info/1105645479/34.
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McCallum, Duncan C. (Duncan Craig). "Dynamic modelling and control of a magnetic bearing-suspended rotor system." Thesis, Massachusetts Institute of Technology, 1988. http://hdl.handle.net/1721.1/14554.
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Dai, Huiguang. "Dynamic behavior of maglev vehicle/guideway system with control." Case Western Reserve University School of Graduate Studies / OhioLINK, 2005. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=case1117563035.
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Zhang, Bin [Verfasser]. "Static and Dynamic Magnetic Properties of Exchange-coupled Thin Film Systems / Bin Zhang." Berlin : Freie Universität Berlin, 2014. http://d-nb.info/1060717913/34.
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Yudin, Dmitry. "Trends in Magnetism : From Strong Correlations to “-onics” Technology." Doctoral thesis, Uppsala universitet, Materialteori, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-238177.
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Despite of enormous progress in experimental nanophysics theoretical studies of low-dimensional electron systems still remains a challenging task. Indeed, most of the structures are strongly correlated, so that an effective perturbative treatment is impossible due to the lack of a small parameter. The problem can be partly solved within the dynamical mean-field theory (DMFT) paradigm, nevertheless the correlations in physically relevant high-temperature superconductors are of purely non-local nature. The recently developed dual fermion approximation, combining field-theoretical diagram technique and numerical methods, allows for explicit account of spatial correlations. The approximation was shown to be of fastest convergence compared with standard DMFT extensions, and along with renormalization group is used here to study Fermi condensation on a triangular lattice near van Hove singularities. The still debated phenomenon of Fermi condensation is believed to be a precursor to strongly correlated low-temperature instability and is found in this thesis to be robust even at high temperature, making its experimental verification feasible. Unlike homogeneous ferromagnetic ordering a variety of non-collinear ground state configurations emerge as a result of competition among exchange, anisotropy, and dipole-dipole interaction. These particle-like states, e.g. magnetic soliton, skyrmion, domain wall, form a spatially localized clot of magnetic energy. Consistent study of spin, which essentially is a quantum mechanical entity, led to the emergence of spintronics (spin-based electronics) and magnonics (photonics with spin waves), in the meanwhile topologically protected magnetic solitons and skyrmions might potentially be applied for data processing and information storage in next generation of electronic technology (rapidly advancing solitonics and skyrmionics). An ability to easily create, address, and manipulate such structures is among the prerequisite forming a basis of "-onics" technology. It is shown here that spins on a kagome lattice, interacting via Heisenberg exchange and Dzyaloshinskii-Moriya coupling, allow the formation of topologically protected edge states through which a skyrmion can propagate. Not only can chemical methods be used to design novel functionality, but also geometric structuring. It is demonstrated that for graphene sandwiched between two insulating media external circularly-polarized light serves as an effective magnetic field. The direct practical implication permits to control light polarization and induce spin-waves propagating on the surface of e.g. a topological insulator. The newly discovered Dirac materials, graphene and three-dimensional topological insulators, are not easy to handle. In fact, the quasiparticle band function is gapless preventing them from being used in integrated circuits, nevertheless the problem is shown here to be partially relaxed by placing a vacancy on top of it.
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Rößler, Stefan [Verfasser]. "Magnetic imaging of static and dynamic states in systems of reduced dimensions / Stefan Rößler." Aachen : Shaker, 2015. http://d-nb.info/1069048666/34.
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Yao, Li. "Magnetic field modelling of machine and multiple machine systems using dynamic reluctance mesh modelling." Thesis, University of Nottingham, 2006. http://eprints.nottingham.ac.uk/10224/.
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This thesis concerns the modified and improved, time-stepping, dynamic reluctance mesh (DRM) modelling technique for machines and its application to multiple machine systems with their control algorithms. Improvements are suggested which enable the stable solution of the resulting complex non-linear equations. The concept of finite element (FE) derived, overlap-curves has been introduced to facilitate the evaluation of the air-gap reluctances linking the teeth on the rotor to those on the stator providing good model accuracy and efficient computation. Motivated industrially, the aim of the work is to develop a fast and effective simulation tool principally for evaluating salient pole generator system designs including the generator, exciter and the automatic voltage regulator (AVR). The objective is to provide a modelling system capable of examining the detail of machine operation including saturation of main and leakage flux paths, slotting and space harmonics of the windings. Solutions are obtained in a sufficiently short computational time to facilitate efficient iterative design procedures in an industrial design office. The DRM modelling technique for electrical machines has been shown in this thesis to be a fast and efficient tool for electrical machine simulation. Predicted results for specific machine and system designs have been compared with FE solutions and with experimental results showing, that for engineering purposes, the technique yields excellent accuracy. The DRM method has a great advantage in multiple machine simulations. This is because magnetic field calculations are limited to evaluating only the most important information so saving computation time. A brushless generating system including the excitation system and control scheme has been modelled. Additionally a cascaded, doubly fed induction generator for wind generator applications has also been modelled. These different applications for the dynamic reluctance mesh method have proved that this approach yields an excellent machine and machine-system evaluation and design tool.
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Bakker, Lennard Frank. "Brake orbits and magnetic twistings in two degrees of freedom Hamiltonian dynamical systems." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp04/nq20547.pdf.
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