We investigate whether quantum spin rings with nearest-neighbor Heisenberg or Ising exchange interactions can host solitary states. Using complete diagonalization techniques the system is described without classical or semiclassical approximation. In this case definitions used in connection with classical solitons are not applicable, one needs to redefine what solitary objects on a quantum spin system with translational symmetry ought to be. Thus, we start our contribution by defining which quantum states possess solitary character. In addition we discuss useful observables in order to visualize solitary quantum states. Then we demonstrate for various quantum spin rings that solitary quantum states indeed exist, and that they are moving around the spin ring without changing their shape in the course of time.

It’s well known that MnAl(C) material consists of a metastable phase (τ) with promising ferromagnetic properties, produced either by controlled cooling from the high-temperature hexagonal ε-phase or rapid cooling that freezes the ε-phase followed by low-temperature annealing. Due to the high cooling rates involved, additive manufacturing (AM) especially selective laser melting (SLM), has been identified as a possible method to retain the high-temperature ε-phase, hence containing a potential capacity to produce permanent magnets upon low-temperature annealing. Moreover, the competency of additive manufacturing to address manufacturing design complexity, material scarcity and tailored properties, yields a great opportunity to produce permanent magnets with suitable magnetic properties for complex applications. This work provides a systematic study on three main aspects; development of printing parameters for improved relative density of as-printed MnAl(C) samples; investigation of the influence of scanning strategies on the crystallographic texture of as-printed and annealed samples; investigation of the influence of annealing time and temperature on τ-phase purity and magnetic properties. It was found that laser remelting (multiple laser exposure) combined with specific scanning strategies is a promising path to enhance the relative density of as-printed samples. Some specific scanning strategies were found to be capable of retaining relatively strong crystallographic textured ε-phase in as-printed samples. Following the annealing process for ε→τ transformation, only a partial transformation of crystallographic texture was observed. Characterization of annealed samples through XRD (x-ray diffraction) and phase fractions calculations through Rietveld refinement reveals that relatively short annealing times and low temperatures result in incomplete ε→τ transformation. In addition, longer annealing times and higher temperatures surpass the complete ε→τ transformation and lead to the formation of equilibrium phases subsequently reducing the magnetic performance. Furthermore, the experimental findings demonstrated a pronounced influence of higher carbon content in the powder, resulting in improved magnetic properties.

This thesis investigates a number of transition-metal coordination polymers and iron-pnictide superconductors through the use of high magnetic fields, low temperatures, and on occasion, high pressures. The thesis will begin by describing my development of the proximity detector dynamic susceptometer, a novel technique that can be used for magnetometery and transport measurements in high magnetic fields. This technique is highly compact and has no moving parts, making it suitable for use in pressure cells, hence opening the way for a variety of new experiments. Through high-field magnetometery and other measurements, I will demonstrate that the pressure can be used to directly control the magnetic properties of the polymeric magnet CuF₂(H₂O)₂(pyrazine). In particular, I observe a transition from quasi-two-dimensional to quasi-one-dimensional antiferromagnetism at 9~kbar, driven by the rotation of the Jahn-Teller axis. I will then present a series of measurements on two coordination polymers, showing how a small chemical difference can lead to drastically different magnetic properties. I show that [Cu(pyrazine)H₂O(glycine)₂]ClO₄ is an excellent spin-chain, while the sister compound [Cu(pyrazine)(glycine)]ClO₄ is a dimerised material that shows a spin-gap and is disordered down to very low temperatures, but then undergoes a field-induced phase transition to an ordered phase. I will also describe a series of pulsed-field measurements of the upper critical field of the iron-based superconductors NaFe_1-xCo_xAs across the whole of the doping phase diagram. It is shown that paramagnetic pair-breaking effects dominate the critical field when the field is parallel to the crystal planes. In the parent compound the paramagnetic limit is equal to that expected from BCS theory, but becomes significantly enhanced above the BCS limit upon doping. It is shown that the multi-band nature of the superconductivity leads to a convex curvature in the evolution of the critical field as the temperature is reduced.

Thesis (PhD (Electrical and Electronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Certain types of electric machines are particularly susceptible to the proliferation of eddy currents flowing within the solid conducting regions in the rotor. Single-layer, non-overlapping windings within uneven open slots are some stator properties that can produce damaging, asynchronous magnetic field harmonics which manifest in the rotor as eddy currents. The ohmic losses caused by these eddy currents are a source of inefficiency and can cause a marked increase in the temperature of the rotor. This temperature rise can be dangerous for the magnets, which have to be kept within temperature limits to avoid partial or full demagnetization. The research work presented here is concerned with reducing the effect of eddy currents in the rotor magnets and solid rotor yoke of an electric machine. The work presents analytical methods to calculate the magnetic fields, eddy currents and solid loss in an electric machine due to current in the winding and due to the interaction of the permeance variation in the stator with the magnets in the rotor. A method is also suggested where the analytical theory can be used with a magnetostatic finite element solution to produce a transient solid loss result. The research work also investigates a method for optimal segmentation in both level and penetration, and provides some design suggestions. The work presents the method of partial magnet segmentation, which is a technique whereby thin incisions are made into the magnet material from one or both sides. Another method of partial rotor segmentation is also presented where the incisions are made into a portion of the magnet-facing solid yoke. These methods attempt to interrupt the flow of eddy currents and increase the resistance ‘seen’ by the eddy currents, while also keeping construction difficulty and cost to a minimum. The methods are verified using finite element calculations which are compared to measured results. The result is that partial magnet segmentation is a very useful, effective and practical method of segmenting magnets. The loss reduction profile can be similar to that of traditional full segmentation. The method of partial rotor segmentation also shows a large reduction in rotor power loss. With implementation of these methods on a test machine, one can expect an efficiency increase of more than 4 % / AFRIKAANSE OPSOMMING: Sekere tipes van elektriese masjiene is veral sensitief vir die vloei van werwelstrome in solied geleidende gebiede in die rotor. Enkellaag, nie-oorvleuelende wikkelings in oneweredige oop gleuwe is enkele stator eienskappe wat skadelike, asinchrone magneetveld harmonieke tot gevolg kan hê, wat as werwelstrome in die rotor manifesteer. Die ohmiese verliese wat deur hierdie werwelstrome teweeg gebring word is 'n bron van ondoeltreffendheid en kan lei tot 'n merkbare toename in die temperatuur van die rotor. Hierdie temperatuur styging hou gevaar in vir die magnete en moet binne temperatuur limiete gehou word om gedeeltlike of self volle demagnetisering te vermy. Die navorsing vervat in hierdie document is gemoeid met die vermindering van die effek van werwelstrome in die rotor magnete en in die soliede rotor juk van 'n elektriese masjien. Die werk bied analitiese metodes aan vir die berekening van die magneetvelde, werwelstrome en soliede verliese in ’n elektriese masjien as gevolg van strome in die wikkelings en die interaksie van die permeansie variasie van die stator met die magnete in die rotor. ’n Metode word ook voorgestel waar die analitiese teorie saam met ’n magnetostatiese eindige element oplossing gebruik word om ’n resultaat vir die oorgang soliede verliese te verkry. Die navorsingswerk ondersoek ook ’n metode vir die optimale segmentering in beide vlak sowel as penetrasie, en verskaf sekere ontwerp voorstelle. Die werk bied die metode aan van gedeeltelike magneet segmentering, wat 'n tegniek is waarvolgens dun insnydings gemaak word aan een of beide kante van die magneet materiaal. Nog ’n metode van gedeeltelike rotor segmentering word beskou waar die insnydings in in ’n gedeelte aan die magneetkant van die soliede rotor juk gemaak word. Hierdie metodes poog om die vloei van werwelstrome te onderbreek en die weerstand soos "gesien" deur die werwelstrome te verhoog, terwyl konstruksie kompleksiteit en koste tot ’n minimum beperk word. Die metodes word bevestig deur eindige element berekeninge wat met gemete resultate vergelyk word. Die gevolg is dat gedeeltelike magneet segmentering 'n baie nuttige, doeltreffende en praktiese metode van die segmentering van magnete is. Die verliesverminderingsprofiel van gedeeltelike segmentering kan soortgelyk wees aan dit van tradisionele volle segmentering. Die metode van gedeeltelike rotor segmentering toon ook 'n groot afname in rotor drywingsverlies. Met die implementering van hierdie metodes op ’n toetsmasjien, kan ’n mens ’n verhoging in benuttingsgraad verwag van meer as 4 %

Thesis (PhD)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: In hierdie tesis word ’n nuwe lug kern permanent magnet (PM) lineêre generator (LG) vir toepassing tot direk aangedrewe (DA) oseaan golf energie omsetters (GEO) ontwikkel. Die nuwe LG word ontwikkel vanaf die lineêre dubbel-kant topologie in ’n poging om probleme met die huidige longitudinale vloed (LV) yster kern LGs, wat tot dusvêr oorheersend voorkom in eksperimentele DA-GEOs, te oorkom. Die grootste probleem met hierdie LGs is die masiewe aantrekkings kragte tussen hul yster stators en die PM transleerders. ’n Groot hoeveelheid strukturuele staal word benodig om die luggaping te handhaaf, terwyl die las op die laars ook ’n groot probleem is. Die nuwe LG gebruik ’n lug kern stator wat alle aantrekkings kragte tussen die stator en transleerder elimineer en dus die nodige strukturuele material verminder. Die topologie van die transleerder is ook van so ’n aard dat die netto aantrekkings kragte op enige spesifieke PM ideaal nul is; dit verminder die strukturuele materiaal selfs verder. Die transleerder het ook ’n nuwe transversale vloed pad wat die sogenaamde paarwyse vloed koppeling wat in LV-LGs voorkom, en die negatiewe effekte daarvan, verhoed. ’n Aantal nuwe bydraes tot die veld van LGs vir DA-GEO word in hierdie tesis gemaak. ’n Nuwe topologie lug kern PMLG is ontwikkel soos bespreek. Dit sluit in die ontwikkeling van analitiese en eindige element modelle en ’n optimerings prosedure wat vinnig optimale dimensies vir minimum aktiewe massa van die nuwe LG vind. In die ontwerp word dit ook gevind dat die drywingsdigtheid van LGs verbeter kan word deur zero oorvleuling tussen die die stator en transleerder by die slag endte toe te laat. ’n 1 kW prototipe van die nuwe LG word ontwerp en gebou; die uitvoerbaarheid van die konstruksie vir die nuwe topologie op ’n klein skaal word dus gedemonstreer. ’n Unieke toets opstelling word ook ontwerp en is gebasseer op bestaande toerusting in die vorm van ’n wind turbine generator en rug-aan-rug spannings bron omsetters. Met die toets opstelling word ’n enkel frekwensie golf ge-emuleer om die teorie en simulasies te verifieer en word ook ’n voorspellende beheer strategie geimplementeer, wat vir die eerste keer gedemonstreer word vir LGs vir DA-GEOs. Goeie ooreenstemming tussen die gemete en gesimuleerde data bevestig die voorgestelde modellerings en ontwerps metodes. / AFRIKAANSE OPSOMMING: In this thesis, a novel air-cored permanent magnet (PM) linear generator (LG) is developed with application to direct drive (DD) wave energy converters (WECs). The novel LG is developed from the linear double-sided topology in an attempt to overcome the problems with current longitudinal flux (LF) iron-cored LGs, which have so far been dominant in experimental DD-WECs. The biggest problem with these LGs is the massive attraction forces between their iron stators and PM translators. A large amount of structural steel is required to maintain the air gap, while the load on the bearings is also a large concern. The novel LG uses an air-cored stator which eliminates any attraction forces between the stator and translator and hence reduces the required structural material. Furthermore, the topology of the translator is such that the net attraction force on any particular PM is ideally zero, which even further reduces the structural material required for the translator. A new transverse circulating flux path is also introduced in the translator which prevents pair-wise flux coupling and its negative effects as observed in LF-LGs. A number of new contributions are made to the field of LGs for DD-WECs in this thesis. A novel topology aircored PMLG is developed as described. This includes the development of analytical and finite element models and an exhaustive optimisation procedure for quickly finding optimal dimensions for minimum active mass of the novel LG. In the design it is also found that the power density of LGs can be improved by allowing zero overlap between the stator and translator at the stroke ends. A 1 kW prototype of the novel LG is designed and built; the feasibility of constructing the novel LG on a small scale is as such demonstrated. A unique test rig is designed based on existing equipment in the form of a wind turbine generator and back-to-back voltage source converters. The test rig allows emulation of a monochromatic wave for verifying the theory and simulations and also allows for implementation of a predictive control strategy, which is for the first time demonstrated for LGs for DD-WECs. Good agreement between measured and simulated data confirms the presented modelling and design methods.

Thesis (MScEng (Electrical and Electronic Engineering))--Stellenbosch University, 2007. / Particle accelerators today have numerous magnets that require controlling. These include magnets for analysing, beam-path selection, focusing, etc. Also, design specifications are becoming tighter. A typical modern magnet power supply is expected to have a resolution of 16-bit and a stability of 10 ppm. This thesis addresses two research areas. First, various aspects of high-performance accelerator magnet power supplies are investigated. An isolated dual-stage 3.5 kW converter is designed. The concept is verified through practical measurements. The control system and high-resolution pulse-width modulation are implemented within a field-programmable gate array. Second, a nuclear-magnetic resonance probe is designed and simulated. It is intended to provide a measurement of field-strength for feed-back purposes. Some adjustments are made with existing technology in order to decrease the time between successive measurements to the order of 10 μs. Also, the support systems (central processing unit, hardware drivers, etc.) are designed, implemented in the field-programmable gate array and tested successfully.

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This research work focuses on the geometry and shape effects on submicron magnetic material. A web based micromagnetics program is written to model the hysteresis loop of nano magnetic samples with arbitrary geometry shapes and multiple magnetic materials. Three material samples have been modeled with this program along with nano magnets with a variety of geometric shapes. Shape anisotropy has been introduced to a permalloy ring by adding a cross-tie structure with various widths. The in-plane hysteresis loop and reversal behavior have no notable difference in direction parallel to the cross-tie, but greatly changed in perpendicular and diagonal directions. The switching field distribution is significantly reduced. The two distinct "onion" bit states of the modified ring elements are stabilized in the hysteresis in the diagonal direction. The changes in the modified rings make them better candidates for Magnetic Random Access Memory elements. Two Pac-Man elements, PM I and PM II, geometrically modified from disc and half disc respectively, are modeled. The PM I element undergoes a magnetic reversal through a two-stage mechanism that involves nucleation in the left and right middle areas followed by vortex core formation and vortex core motion in the lower middle area. The reversal process of the PM II element lacks the vortex core formation and motion stage. The switching field of the PM I and PM II elements are the same but the switching field distribution of the PM II elements is much narrower than that of the PM I element. Only the PM II element meets MRAM application requirements. The thickness dependence of the magnetic properties of a core-shell structure has been studied. The nano particles have a cobalt core and a permalloy shell. The nano spheres are the same size but with various shell thickness. Simulations reveal a multi-stage reversal process without the formation of a Bloch wall for thin-shell structure and smooth reversal process with the formation and motion of a Bloch wall for thick-shell structure. Gradual transition of the hysteresis loop patterns has been observed.