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This thesis is divided into two parts, experimental part presenting the synthesis of exchange spring magnets and theoretical part showing the magnetization dynamics of exchange spring magnets. For the synthesis, ferrite and metal alloy based magnets using mechanical milling and ultrasonic mixing are studied. This part discusses the difficulty in controlling the microstructure during synthesis. Several samples with varying volume fractions of the soft phase are synthesized. It is found that mechanical milling initiates a reaction and this leads to disintegration of the ferrite phase into its intermediate phases. Samples prepared using ultrasonic mixing however show presence of the ferrite phase up to very large volume fraction of the soft alloy phase which provides better perspective for the synthesis. Dynamics of the exchange spring system is studied theoretically using micromagnetic theory. Microwave assisted magnetization reversal are studied in the bulk bilayer exchange coupled system. We investigate the nonlinear magnetization reversal dynamics in a perpendicular exchange spring media using the Landau-Lifshitz equation. In the limit of the infinite thickness of the system, the propagation field leads the reversal of the system. The reduction of the switching field and the magnetization profile in the extended system are studied numerically. The possibility to study the dynamics analytically is discussed and an approximation where two P-modes are coupled by an interaction field is presented. The ansatz used for the interaction field is validated by comparison with the numerical results. This approach is shown to be equivalent to two exchange coupled macrospins.

Micromirror scanners are the most significant of the micro-optical actuator elements with applications in portable digital displays, automotive head-up displays, barcode scanners, optical switches and scanning optical devices in the health care arena for external scanning diagnostics and in vivo scanning diagnostics. Recent development in microscanning technology has seen a shift from conventional electrostatic actuation to electromagnetic actuation mechanisms with major advantages in the ability to produce large scan angles with low voltages, remote actuation, the absence of the pull-in failure mode and the acceptable electrical safety compared to their electrostatic counterparts. Although attempts have been made to employ silicon substrate based MEMS deposition techniques for magnetic materials, the quality and performance of the magnets are poor compared to commercial magnets. In this project, we have developed novel low-cost single and dual-axis polymer hard magnetic micromirror scanners with large scan angles and low power consumption by employing the hybrid fabrication technique of squeegee coating to combine the flexibility of polydimethylsiloxane (PDMS) and the superior magnetic performance of fine particle isotropic NdFeB micropowders. PCB coils produce the Lorentz force required to actuate the mirror for scanning applications. The problem of high surface roughness, low radius of curvature and the magnetic field interaction between the gimbal frame and the mirror have been solved by a part PDMS-part composite fabrication process. Optimum magnetic, electrical and time dependent parameters have been characterized for the high performance operating conditions of the micromirror scanner. The experimental results have been demonstrated to verify the large scan angle actuation of the micromirror scanners at low power consumption.

The field of low-dimensional quantum magnets has received lot of attention owing to the possibility of studying phenomena associated with the quantum nature of matter. Many materials that realize low-dimensional spin arrangements in their structure have been synthesized in the past twenty years due to the emergence and development of crystal growth techniques. These materials have been studied using various experiments in order to explore the wide range of interesting properties predicted theoretically for low-dimensional systems. In this pursuit, novel properties have been observed and many open questions have been raised. One such property that is typically observed in many low-dimensional quantum magnets is heat transport via magnetic excitations. Large magnitudes of magnetic heat conductivity has been found experimentally in materials belonging to this class in addition to the conventionally known phononic heat conduction, and interesting theoretical predictions like the divergence of heat conductivity in certain spin models exist, that have stimulated research in this field. This experimental work mainly deals with the crystal growth and heat transport properties of low-dimensional quantum magnets that include one-dimensional (1D) spin chain systems Sr$_2$CuO$_3$ and SrCuO$_2$, two-dimensional (2D) Heisenberg antiferromagnet La$_2$CuO$_4$, and a five-leg spin ladder La$_8$Cu$_7$O$_{19}$, with a view to understand propagating low-energy magnetic excitations and their interaction amongst themselves, other quasiparticles and impurities present in the systems. These interactions result in scattering processes that govern the magnitude and temperature dependence of heat conductivity. In spite of considerable theoretical and experimental work in the field of heat transport, a complete understanding of the scattering mechanisms is lacking. The work tries to add to the experimental knowledge about magnetic heat transport in such systems and presents cases which motivate the need for theoretical understanding of aspects of heat transport. The focus of this work was twofold. One part focusses on the single crystal growth using the travelling-solvent floating zone (TFSZ) method of materials which realize low-dimensional spin systems in their structure. The TFSZ method is indispensable for growing large single crystals of extraordinary purity, which can be used for investigations using neutrons and other techniques like heat conductivity measurements that probe anisotropic properties. The other part deals with the experimental results on heat transport and other thermodynamic properties of these materials. In order to study the behaviour of the magnetic heat conductivity at high temperatures, and the effect of small amount of magnetic and non-magnetic impurities on the heat transport of 2D Heisenberg antiferromagnet La$_2$CuO$_4$, single crystals of pure La$_2$CuO$_4$, and Ni- and Zn-doped versions, La$_2$Cu$_x$Ni$_{1-x}$O$_4$ and La$_2$Cu$_x$Zn$_{1-x}$O$_4$ for $x$ = 0.001 and 0.003, were grown using the TFSZ method. Heat transport in the pure compound was experimentally investigated for the first time up to very high temperatures of 813 K using two methods, namely the steady state method for low temperatures and the dynamic flash method for measuring high temperature conductivity. Analysis of the magnon mean-free path using empirical models based on semi-classical theories, and qualitative comparison to theoretical calculations seems to suggest that scattering between magnons might play an important role in addition to scattering of magnons with phonons and defects, and that the spin-spin correlation length could be crucial in limiting the mean free path of magnons at high temperatures. These experimental results and indications of probable scattering mechanisms based on non-rigorous analyses and comparisons, strongly motivate the need for theoretical studies. Heat conductivity measurements on the Ni- and Zn- doped versions of La$_2$CuO$_4$ are still incomplete and inconclusive, and hence have not been reported in this work. Heat transport experiments on Ni- and Ca-doped Sr$_2$CuO$_3$ were performed, with a motivation to investigate the role of disorder induced by impurities lying within the spin chains (Ni) and those lying outside the spin chains (Ca), on the heat transport in this system. In both the cases, the magnetic heat transport is observed to be strongly suppressed upon doping. Empirical analysis of the data seems to suggest that in the temperature regime of 100-300 K, the temperature dependence of the mean-free path of magnetic excitations for the Ni- and Ca-doped samples can be described by scattering with defects (Ni and Ca impurities) and phonons alone. However, surprisingly, a strong increase of phononic conductivity is observed perpendicular and parallel to the spin chains of the Ni-doped compounds compared to the pure compounds, whose explanation seems to lie in the existence of an additional dissipative scattering mechanism present in the pure compounds that is lifted upon doping, possibly due to the presence of a spin gap in the doped compounds. The effect of Ni on the Sr$_2$CuO$_3$ and SrCuO$_2$ was also investigated by studying the low energy regime of the spin excitation spectrum using other microscopic probes like nuclear magnetic resonance (NMR) and inelastic neutron scattering (INS). Large single crystals of SrCu$_x$Ni$_{1-x}$O$_2$, with $x$ = 0.01 were grown and used in these experiments that observed the presence of a spin gap in the Ni-doped sample. Further theoretical investigations are however required to understand the possible role of the spin gap in influencing the spin-phonon scattering mechanism, and its relevance to the observed enhancement in phononic conduction. Although we observe that in the case of both 1D and 2D systems, a semi-classical kinetic model for heat transport along with empirical models of scattering processes describe the temperature dependence of the measured heat conductivity surprisingly well in the temperature regime up to 300 K and 800 K respectively, interpretations based on these analyses must be treated as only preliminary, and as a step towards understanding microscopically the scattering mechanisms involved in low-dimensional systems such as the ones discussed in this work. In the direction of exploratory research towards synthesis of novel low-dimensional materials, two cuprate compounds were synthesized in the form of single crystals using the floating zone method for the first time, namely, a five leg $S=tfrac{1}{2}$ antiferromagnetic spin ladder compound La$_8$Cu$_7$O$_{19}$ and an insulating delafossite LaCuO$_{2}$. A bulk 3D antiferromagnetic ordering is observed in La$_8$Cu$_7$O$_{19}$. Heat conductivity of La$_8$Cu$_7$O$_{19}$ is observed to be purely phononic and no contribution from magnetic excitations seem to exist, although the measurements indicates that there is a large anisotropy in heat transport. However, detailed diffraction experiments using x-rays and neutrons indicate that both the crystal and magnetic structures are complicated, and that the details of the structure prevent La$_8$Cu$_7$O$_{19}$ from being a perfect realization of a five-leg spin ladder.

Thesis (MScEng)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: Slip-synchronous permanent magnet generators (SS-PMG) is a recently proposed direct-grid connected direct-drive generator topology for wind power applications. It combines a permanent magnet synchronous generator and a slip permanent magnet generator through a common permanent magnet rotor. In this study the possibility of using an eddy current coupling as the slip permanent magnet generator is investigated. The eddy current coupling has the attractive advantage of completely removing cogging and load torque ripple, which are known problems in the SS-PMG. However, the analytical modelling of the eddy current coupling is complex. Three different topologies are considered for the eddy current coupling. A finite element model is presented for the eddy current coupling. It is shown that 2D finite element methods are inaccurate compared to 3D finite element methods when solving eddy currents in eddy current couplings. In order to test the accuracy of the finite element modelling of a large eddy current coupling a prototype slip rotor is designed to operate with an existing permanent magnet rotor. Two topologies are optimally designed and compared for the slip rotor, using 3D finite element transient simulations. One of the designed topologies is used for the construction of the prototype slip rotor. The manufactured eddy current coupling allows for comparison between the 3D finite element simulations and measured results, which shows an excellent correlation. Based on observations of the 3D finite element simulations an analytical approximation of the eddy current coupling is proposed for low slip frequencies. It is shown that the analytical model is very dependent on the accurate modelling of the eddy current paths in the slip rotor, something that is difficult to determine accurately. An approximation is made, again based on 3D finite element simulations, which allows the accurate modelling of the current paths for different axial lengths. The analytical model is used for rapid design optimisation of both the slip rotor and permanent magnet rotor of the eddy current coupling, for two different eddy current coupling topologies. The optimised eddy current coupling design with the best results is compared to existing slip permanent magnet generator technologies. The eddy current coupling is shown to have the potential to be a feasible alternative to existing slip permanent magnet generator topologies for application in slip-synchronous permanent magnet generators. It has excellent torque versus slip behaviour, and no cogging or load torque ripple. However, the manufacturing and assembly process of the proposed slip rotor has to be improved for the eddy current coupling to be a realistic competitor to the existing slip permanent magnet generator technologies. / AFRIKAANSE OPSOMMING: Die glip-sinkroon permanente magneet generator is ʼn direk-aangedrewe wind generator wat direk aan die krag netwerk gekoppel kan word. Dit kombineer ʼn permanente magneet sinkroon generator en ʼn permanente magneet glip generator deur middel van ʼn gemeenskaplike permanente magneet rotor. In hierdie studie word die moontlike gebruik van ʼn werwelstroom skakel as plaasvervanger vir die permanente magneet glip generator ondersoek. Die werwelstroom skakel het geen vertandings of las draaimoment rimpeling nie, wat bekende probleme vir die glip-sinkroon permanente magneet generator is. Die analitiese modellering van die werwelstroom skakel is egter nie eenvoudig nie. Daar word drie verskillende topologieë oorweeg vir die werwelstroom skakel. ʼn Eindige element model word ontwikkel vir die werwelstroom skakel. Dit word bevind dat 2D eindige element analise onvoldoende is vir die berekening van werwelstrome in die werwelstroom skakel, en gevolglik word 3D eindige element modelle gebruik in hierdie studie. Om die akkuraatheid van die eindige element model te beproef word ʼn prototipe glip rotor optimaal ontwerp deur middel van eindige element analise. Die glip rotor vorm saam met ʼn bestaande permanente magneet rotor ʼn werwelstroom skakel. Vir hierdie ontwerp word twee werwelstroom skakel topologieë gebruik, en met mekaar vergelyk. Die topologie wat beter presteer word gebruik vir die vervaardiging van die glip rotor. ʼn Vergelyking van die gemete waardes van die vervaardigde werwelstroom skakel en die resultate van die 3D eindige element simulasies dui daarop dat die 3D eindige element modellering ʼn baie goeie voorspelling van die werklikheid is. ʼn Analitiese model vir die werwelstroom skakel onder lae glip toestande is ontwikkel deur gebruik te maak van observasies uit die 3D eindige element simulasies. Die analitiese model is baie afhangklik van die modellering van die werwelstrome se stroompaaie, iets wat moeilik is om akkuraat te bepaal. ʼn Benadering word gemaak wat die akkurate modulering van die stroompaaie moontlik maak vir verskillende aksiale lengtes. Die analitiese model word dan gebruik vir vinnige optimering van die werwelstroom skakel se ontwerp vir twee verskillende werwelstroom skakel topologieë. Die geoptimeerde ontwerp wat die beste resultate toon word vergelyk met bestaande permanente magneet glip generators. Dit word gewys dat die werwelstroom skakel die potensiaal het om ʼn uitvoerbare alternatief tot die permanente magneet glip generator te wees, vir gebruik in glip-sinkroon permanente magneet generators. Die werwelstroom skakel toon baie goeie draaimoment teenoor glip gedrag, en het geen vertandings of las draaimoment rimpeling nie. Voordat die werwelstroom generator ʼn realistiese kompeteerder teenoor die bestaande glip-sinkroon tegnologie is, moet daar verbeterde vervaardigings maniere gevind word vir die voorgestelde glip rotor.

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Les récents progrès des supraconducteurs à haute température critique (SHTC) rendent ces matériaux très prometteurs pour les applications de grande envergure. Ils montrent des propriétés stupéfiantes, particulièrement à très basse température où ils sont capables de transporter des courants extrêmement élevés. Ces conducteurs sont construits sur un substrat en alliage de nickel qui leurs permettent de supporter des efforts mécaniques considérables. Pour ces raisons, les SHTC sont à présent au cœur de nombreux projets pour bâtir une nouvelle génération d’aimants produisant des champs intenses. Ce travail est mené dans le cadre du projet Nougat dont l’objectif est la conception d’un insert générant 10 T à l’intérieur d’un aimant plus grand de 20 T. Malgré leurs remarquables propriétés, les conducteurs SHTC montrent d’importantes inhomogénéités de performance ce qui, ajouté à leur forte capacité calorifique, peut provoquer des échauffements très localisés. La transition locale du matériau vers un état dissipatif est difficile à détecter notamment à cause de la lente propagation de l’évènement. Le signal de transition risque donc d’être noyé dans le bruit de mesure et être à l’origine de dommages sévères voir irréversibles du conducteur. Pour répondre à ce problème, des électro-aimants sont fabriqués en remplaçant l’isolation électrique par un ruban métallique. Ces aimants sont appelées Bobines Métal-Isolée (BMI). Dans cette configuration et dans le cas spécifique d’une inhomogénéité sur le conducteur, le courant court-circuite le défaut en passant sur la spire suivante ce qui permet d’empêcher la destruction du bobinage. Cependant, dans ce type de bobinage, la stabilité et la linéarité du champ magnétique généré peut être un problème et les dynamiques lentes de ce type de bobine les empêchent de prétendre à être utilisé pour certaines applications. Pour des raisons pratiques, le prototype final du projet Nougat sera construit avec un bobinage métalliquement isolé mais le travail mené ici se concentre sur les bobines électriquement isolées, plus particulièrement l’étude de la transition dans le bobinage afin d’identifier des possibilités de protection fiables. Quelques résultats d’échantillons BMI sont cependant présentés pour comparaison. Dans un premier temps, les contraintes générales de conception de l’insert NOUGAT sont définies, principalement les exigences pour le conducteur et les calculs préliminaires de performance. Cette étude souligne le besoin de caractériser le conducteur SHTC sous haut champ et à basse température, dans des conditions similaires à celles du fonctionnement final du prototype d’insert. Dans un second temps, cette caractérisation expérimentale d’échantillons courts est implémentée dans un modèle pour simuler le comportement de bobines pour la construction d’aimants. L’objectif de ce modèle est d’étudier le départ de transition et sa propagation dans le bobinage. Une des particularités du modèle est la simulation de l’entièreté du bobinage dans le but de prendre en compte les inhomogénéités de performance du conducteur, dont les mesures en continu sont fournies à 77 K en champ propre par les constructeurs. Le dernier axe de ce travail est l’étude de plusieurs échantillons bobinés pour permettre notamment la comparaison entre bobinage isolé électriquement et métalliquement. Leurs comportements électrique, mécanique et thermique sont examinés ainsi que les problèmes de couplage dus à la présence d’un aimant extérieur. Les résultats expérimentaux sont comparés aux calculs préliminaires ainsi qu’aux simulations de modèle. Un protocole expérimental est également proposé pour évaluer les performances d’une bobine sans risque et est testé avec succès. / Recent improvements in High Temperature Superconductors (HTS) make them promising for large scale applications. They show astonishing properties, especially at very low temperature where they are able to carry high amount of current. These conductors are also built on a nickel alloy substrate allowing them to face severe mechanical stresses. For these reasons, HTS are now placed at the heart of numerous projects for building a next generation of high field magnets. This work is conducted in the NOUGAT project, which intends to design and build a 10 T HTS insert working in a 20 T background field. Despite their outstanding properties, HTS conductors show strong inhomogeneities in their performance along their length. This added to their high heat capacity can be at the origin of local hot spots. The transition to a dissipative state on this local area is then difficult to detect; because of the low speed of its propagation. The transition signal is likely to be lost in the high noise level environment, which can lead the winding to severe or irreversible damages.One way of dealing with this problem is to create coils where the electric insulation is removed and replaced by a metallic layer, the so-called Metal-as-Insulation winding technique. In this configuration and in a case of an inhomogeneity, the current bypasses through the turn-to-turn contact resistance and prevents the winding from burning. However, in such kind of winding field stability and linearity can be an issue. The slower dynamics obtained with this method prevent its use in some applications.The main focus of this work is therefore insulated coils especially the study of the transition behaviour to explore the possibility of reliable protections. In the same extend, the present work also considers safe ways to evaluate the performance of a wound sample in order for it to work with appropriate margins when at nominal operating conditions. This would decrease the need of a strong detection/protection system: the high heat capacity of the conductor makes windings unlikely to be damaged by an outside event.For practical reasons, the NOUGAT project will be built using MI technique, and therefore some MI coils results are presented in this work for comparison purpose. In a first step, the general design constrains for the NOUGAT project insert are defined, especially tape requirements and performance calculations. This underlines the necessity of characterizing the HTS conductor under high field and at low temperature, under conditions similar to the expected operation of the final insert prototype.In a second step, this experimental short sample characterization is implemented in a model to simulate the behaviour of pancake coils designed to build magnet. The aim of the model is to investigate on the transition start and propagation inside the winding. One of the main specificities is the simulation of the whole winding taking as an input the continuous critical current density measurement given by the providers at 77 K, self-field.The last axis of this work is the study of several wound samples allowing comparison between both insulated and metallic insulated windings. Their electric, magnetic, mechanic and thermal behaviours are examined as well as the coupling issues while working inside an outer magnet. Experimental results are compared to preliminary calculations and modelling results. An experimental protocol to safely evaluate coil performance margins is also proposed and tested successfully.Conclusions are then drawn about the possibility to operate safely full scale HTS magnets with present day conductor performances.

High Tc superconductors are enabling in the generation of extremely high magnetic fields. Flux pumping is a promising technology which can be used to operate HTS magnets without significant loss. In this decade, several HTS flux pumps based on travelling magnetic waves have been developed, yet their physics is still unclear. This thesis established a framework in the area of flux pumping for HTS coils. It revealed the underlying physics of existing travelling wave flux pumps, which is an important theoretical contribution. Based on the thorough understanding of flux pumping mechanism, the author proposed two novel types of flux pumps. The new inventions make flux pumping much easier, more controllable, and much less energy consuming. These flux pumps may promote the future applications of HTS magnets. This thesis can be a guidebook for researchers and engineers in developing flux pumps.

A determinação das principais propriedades magnéticas de um ímã permanente geralmente requer sistemas de medição sofisticados e custosos. Este trabalho analisa e propõe um sistema de caracterização de ímãs permanentes de Terras Raras à temperatura ambiente de baixo custo e de implementação mais simples e rápida. Ele consiste de um circuito magnético formado pelo ímã permanente sob caracterização, uma culatra de aço, um entreferro variável e uma sonda de efeito Hall com seu respectivo gaussímetro para a medição da intensidade do campo magnético no entreferro. Como introdução se apresenta o estado da arte da caracterização dos ímãs permanentes, conceitos básicos de magnetismo e em relação aos ímãs permanentes, sua história, os principais tipos, sua importância e suas aplicações principais. Os resultados teóricos e experimentais são comparados a resultados certificados, validando o método como proposto. / Determining the main magnetic properties of a permanent magnet usually requires sophisticated and expensive measurement setups. This work analyses and proposes a low-cost system to characterize Rare Earth permanent magnets at ambient temperature with easy and fast implementation. It consists of a magnetic circuit with the permanent magnet under test, appropriate pole pieces, a variable air gap and a Hall Effect probe with its respective Gaussmeter. As an introduction, it is presented the state of the art of permanent magnets characterization, basic concepts of magnetism and its history, the main types of permanent magnets, their importance and applications. The theoretical and experimental results are compared to certified ones, validating so the method as proposed.

Les tokamaks visent à produire de l'énergie par fusion thermonucléaire en chauffant un plasma d'hydrogène jusqu'à 150 millions K et en le confinant à l’aide d’un champ magnétique intense créé par des aimants transportant d’importants courants. La supraconductivité est un atout précieux ici car permettant de réduire la taille des aimants et leur consommation énergétique en contrepartie d’un refroidissement cryogénique. Cependant, dans les tokamaks, des variations de champ magnétique apparaissent (ex : décharge du solénoïde central) et génèrent des pertes par induction dans les aimants. Si leur température augmente trop, ils peuvent perdre leur état supraconducteur lors d’une transition brutale appelée "quench": afin de les protéger, ils sont déchargés de leur courant entraînant ainsi la perte du plasma. Nous avons concentré notre travail sur la modélisation de ces pertes car leur connaissance est cruciale pour le bon dimensionnement du refroidissement des aimants et la prédiction des limites opérationnelles du tokamak. Afin d'améliorer la compréhension physique de ce phénomène complexe et de proposer des solutions simples mais réalistes, facilement intégrables dans des plateformes multiphysiques déjà fortement sollicitées par la modélisation d'autres effets, nous avons choisi d'adopter une approche analytique. Les câbles présents dans les tokamaks ayant une architecture assez complexe (centaines de brins torsadés ensemble), nous avons mené des études analytiques et expérimentales aux différentes échelles du câble; nous comparons ensuite les résultats de notre approche à ceux d'autres modèles existants (ex : numériques) et, lorsque cela est possible, à l'expérience. / Tokamaks aim at producing energy by thermonuclear fusion heating a hydrogen plasma up to 150 million K and confining it with an intense magnetic field created by magnets carrying important currents. Superconductivity is a very valuable asset in this field since it allows to reduce the size of the magnets and their energy consumption in exchange for cooling them down to cryogenic temperatures. However, in tokamaks, magnetic field variations occur (e.g. due to the central solenoid discharge) and generate induction losses in the magnets. If their temperature increases too much, they lose their superconducting properties in a brutal transition called "quench": to protect their integrity, they are then discharged and the magnetic confinement of the plasma is lost. We have therefore focused on the modeling of these losses - more precisely on the “coupling losses” - since their knowledge is crucial to safely adapt the cryogenic cooling of the magnets and predict the operating limits of the tokamak. In order to both enhance the physical understanding of this complex phenomenon and provide simple but realistic solutions that can easily be integrated in multiphysics platforms already heavily solicited by the modeling of other effects, we have chosen to adopt an analytical approach on this problem. The cables commonly considered for tokamaks presenting a rather complex architecture (several hundreds of strands twisted together in specific patterns), we have carried out analytical and experimental studies at the different scales of the cable; we then compare the results of our approach to other existing ones (e.g. numerical models) and, when possible, to the experiment.