High pressure, high magnetic field and low temperature techniques are required to investigate magnetic transitions and quantum critical behaviour in different ferromagnetic materials to elucidate how novel forms of superconductivity and other new states are brought about. In this project, several instruments for magneto-transport and neutron scattering measurements have been designed and built. They include inserts for a dilution refrigerator and pressure cells for resistivity, magnetic susceptibility and inelastic neutron scattering measurements. The technical drawings of the low temperature inserts and pressure cells were produced with Solid Edge computer-aided software and the performance and safety assessments were evaluated with the ANSYS finite element analysis package. The pressure cells developed include diamond anvil cells, piston cylinder cells and some auxiliary equipment. Pressure effects on the physical properties such as the electrical resistivity and magnetic ordering of some ferromagnetic materials were studied with the equipment developed. A two-axis rotating stage was developed and deployed with a dilution refrigerator combined within a superconducting magnet to measure various physical properties as a function of the orientation of the sample with respect to applied field at sub-Kelvin temperature. The rotating stage is made of Beryllium Copper (BeCu) alloy. In order to avoid the entanglement of the wires, custom-designed “flexi cables” - copper tracks printed on a Kapton foil with a yield of nearly 100% - to work with the rotating stage were manufactured. The performance of the rotating stage has been demonstrated by a quantum oscillation in the electrical resistivity study of a high field ferromagnetic superconductor URhGe. A miniature diamond anvil cell based on the turnbuckle principle has been designed. The cell, made of BeCu alloy, is 7mm in length and 7mm in diameter. It has been shown to reach a maximum pressure of 10 GPa with diamond anvils with 800 μm culets. The small dimensions of the cell allow it to fit into the existing sample environment such as Physical Properties Measurement System (PPMS) and Magnetic Properties Measurement System (MPMS) from Quantum Design, USA, and onto the customized two-axis rotating stage built for the dilution fridge. It also thermalizes rapidly allowing rapid cooling and heating during the experiments. The cell can be used to make both resistivity and magnetic susceptibility measurements. To ensure the hydrostaticity of the pressure around the sample in the turnbuckle cell, a gearbox was designed for cryogenic loading of liquid argon and room temperature gas loading of either helium or argon at a loading pressure of up to 0.3 GPa. Pressure effects on the Curie temperature of a PrNi ferromagnet were studied in a diamond anvil cell. Four-probe resistance measurements under pressures up to 9 GPa were carried out in a PPMS. The possibility of tuning the physical properties of the material by altering the pressures has been demonstrated. By analysing the results of the electrical resistivity measurements under pressures, it was concluded that the Curie temperature of PrNi increases with pressure at the rate of 0.85 K per GPa. The quantity ∆(δρ/δτ)which reflects some part of the entropy change also increases with pressure. The expected quantum critical point has not been observed in this material up to 9 GPa. A large volume high-pressure piston-cell for inelastic neutron scattering measurements has been designed and can reach a pressure of up to 1.8 GPa with a sample volume in excess of 400 mm3. The dimension of the part of the cell exposed to the neutron beam has been optimized to minimize the attenuation of the neutron beam. The novel design of the piston seal also eliminates the use of a sample container, which makes it possible to accommodate larger samples and reduces the absorption. The pressure in the cell is measured by a manganin pressure gauge placed next to the sample. The performance of the cell was illustrated by an inelastic neutron scattering study of UGe2.

No description available.

Radical aircraft and propulsion system architecture changes may be required to continue historic performance improvement rates as current civil aircraft and engine technologies mature. Significant fuel-burn savings are predicted to be achieved through the Distributed Propulsion concept, where an array of propulsors is distributed along the span of an aircraft to ingest boundary layer air and increase propulsive efficiency. Studies such as those by NASA predict large performance benefits when integrating Distributed Propulsion with the Blended Wing Body aircraft configuration, as this planform geometry is particularly suited to the ingestion of boundary layer air and the fans can be redesigned to reduce the detrimental distortion effects on performance. Additionally, a conventional aircraft with Distributed Propulsion has not been assessed in public domain literature and may also provide substantial benefits. A conceptual aircraft design code has been developed to enable the modelling of conventional and novel aircraft. A distributed fan tool has been developed to model fan performance, and a mathematical derivation was created and integrated with the fan tool to enable the boundary layer ingestion modelling. A tube & wing Distributed Propulsion aircraft with boundary layer ingestion has been compared with a current technology reference aircraft and an advanced turbofan aircraft of 2035 technology. The advanced tube & wing aircraft achieved a 27.5% fuel-burn reduction relative to the baseline aircraft and the Distributed Propulsion variant showed fuel efficiency gains of 4.1% relative to the advanced turbofan variant due to a reduced specific fuel consumption, produced through a reduction in distributed fan power requirement. The Blended Wing Body with Distributed Propulsion was compared with a turbofan variant reference aircraft and a 5.3% fuel-burn reduction was shown to be achievable through reduced core engine size and weight. The Distributed Propulsion system was shown to be particularly sensitive to inlet duct losses. Further investigation into the parametric sensitivity of the system revealed that duct loss could be mitigated by altering the mass flow and the percentage thrust produced by the distributed fans. Fuel-burn could be further reduced bydecreasing component weight and drag, through decreasing the fan and electrical system size to below that necessary for optimum power or specific fuel consumption.

Les développements récents en physique, et technologiques, ont permis l'élaboration de nouveaux matériaux magnéto-électro-élastique, comme les composites multicouches piézoélectriques / piézomagnétiques. Leur coefficient magnétoélectrique, très grand en comparaison de celui des matériaux constitués d'une seule phase, a suscité récemment un grand nombre de travaux, menant au développement de capteurs, d'actionneurs, de systèmes de conversion d'énergie magnétique-électrique, et dans les mémoires à état solide.Cette thèse porte sur l'étude théorique et expérimentale des ondes acoustiques de surface dans des structures multicouches piézomagnétiques. Une description théorique des matériaux magnéto-élastiques, reposant sur la dérivation d'un modèle de matériau piézomagnétique équivalent, est utilisée conjointement à une technique numérique afin de calculer les courbes de dispersion et les formes des modes des ondes acoustiques se propageant dans des composites piézo-électro-magnétiques déposés sur un substrat. Ce modèle, très général, peut être utilisé pour différents types de structures et pour une intensité et une direction quelconques du champ magnétique externe appliqué. Les structures réalisées en salle blanche sont constituées d'un film mince de 20 couches de TbCo2 (5nm)/FeCo(5nm) déposé sur un substrat de LiNbO3 entre deux peignes interdigités. Une comparaison, entre les variations de la vitesse de phase d'ondes acoustiques de surface induites par l'application d'un champ magnétique externe modélisées et mesurées, est réalisée. Un bon accord quantitatif entre les mesures et les calculs théoriques, et cela pour toutes les orientations du champ magnétique (suivant l'axe facile ou l'axe difficile) et pour différents modes acoustiques, est obtenu. Le mode transverse horizontal présente les plus grandes variations de vitesse, proche de 20% pour un film dont l'épaisseur serait celle de la longueur d'onde acoustique

The morphology, composition, and magnetic properties of NiFe thin films were characterized. Films with thicknesses up to 137 nm were deposited in an RF induction evaporator at high vacuum (10^-8 mbar). Time resolved magneto-optic Kerr effect microscopy (TR-MOKE) was used to measure the Gilbert damping constant, an important dynamic magnetic property with applications to magnetic data storage. The composition of each film was measured with energy-dispersive X-ray (EDX) microscopy and used to determine the weight percent of Ni and Fe in each film. A trend of increased damping with increased thickness was found, in agreement with published results. Magnetic properties and roughness were found to differ significantly from previous films grown in the same vacuum chamber by Rudge, and are attributed to different growth modes produced by differing deposition conditions. However, the weight percent of Ni in each film was found to be inconsistent, deviating by up to 7% from the Ni80Fe20 evaporation source. Inconsistent composition, caused by the inability to control deposition parameters, prevents insight into Gilbert damping from being drawn from the analysis. / Graduate

Thesis (PhDEng)--University of Stellenbosch, 2003. / ENGLISH ABSTRACT: The advent of new high energy product permanent magnet materials has opened great opportunities for novel electrical machine topologies with advantageous features such as high efficiency and high power/weight ratio. Amongst others, axial field permanent magnet (AFPM) machines with ironless stators are increasingly being used in power generation applications. Because of the absence of the core losses, a generator with this type of design can operate at a substantially high efficiency. Besides, the high compactness and disc-shaped profile make this type of machine particularly suitable for compact integrated power generation systems. Due to construction problems, the generator application of this type of machine has been limited to quite a low power range. There is a need to investigate the performance capability of this type of AFPM machine in the upper medium power level. The focus of this thesis is on the design optimisation of the air-cooled AFPM generator with an ironless stator. A design approach that directly incorporates the finite element field solution in a multi-dimensional optimisation procedure is developed and applied to the design optimisation of a 300 kW (at unity power factor) AFPM generator. To enable an overall design optimisation of the machine, different design aspects, such as the cooling capacity, the mechanical strength and eddy loss, are also studied in this research. To enable the free movement of the rotor mesh with respect to the stator mesh, the air-gap element originally proposed by Razek et. al. is derived for Cartesian coordinate systems. For minimising the large computation overhead associated with this macro element, a number of existing time-saving schemes have been utilised together with the derived Cartesian air-gap element. The developed finite element time-step model is applied to calculating the steadystate performance of the AFPM machine. Since the flux distribution in an AFPM machine is three dimensional by nature, calculating the eddy current loss by merely using a simple analytical method may be subject to a significant error. To overcome this problem, the two dimensional finite element field modelling is introduced to perform accurate field analysis. To exploit the full advantages of the twodimensional finite element modelling, a multi-layer approach is proposed, which takes into account the variation of the air-gap flux density in the conductors with regard to their relative positions in the air-gap. To account for the radial variation of the field, a multi-slice finite element modelling scheme is devised. The thermal analysis is an important aspect of the design optimisation of AFPM machines. From a design point of view, it is preferable to have a simple but effective method for cooling analysis and design, which can easily be adapted to a wide range of AFPM machines. In this thesis a thermofluid model of the AFPM machine is developed. The fluid flow model is needed for calculating the air flow rate, which is then used to find the convective heat transfer coefficients. These are important parameters in the subsequent thermal calculations. Experimental investigations have been carried out to verify each of the above-mentioned models/methods. With these models implemented, the design optimisation of an air-cooled ironless stator 300 kW (at unity power factor) AFPM generator is carried out. The performance measurements done on the fabricated prototype are compared in this thesis with predicted results. The study shows that the proposed design approach can be applied with success to optimise the design of the AFPM machine. The advantages of high power density, high efficiency, no cogging torque and good voltage regulation make this type of AFPM machine very suitable for power generator applications. The optimum steady-state performance of the AFPM machine shows that this machine with an ironless stator is an excellent candidate for high speed power generator applications, even in the upper medium power level. The good cooling capacity of this type of machine holds the promise of its being a self-cooled generator at high power ratings. / AFRIKAANSE OPSOMMING: Die uitvinding van nuwe hoë energiedigtheid permanent magneet materiale het groot geleenthede vir nuwe elektriese masjien topologië laat ontstaan met voordelige eienskappe soos hoë benuttingsgraad en hoë drywing/gewig verhouding. Onder andere word die aksiaalveld permanente magneet (AVPM) masjiene met kernlose stators toenemend gebruik vir elektriese generator toepassings. As gevolg van die afwesigheid van kernverliese kan 'n generator met hierdie tipe ontwerp teen 'n aansienlik hoë benuttingsgraad werk. Daarbenewens maak die hoë kompaktheid en skyfvorm-profiel die masjien in besonder geskik vir die ontwikkeling van kompak geïntegreerde drywing generator stelsels. As gevolg van konstruksie probleme is die toepassing van hierdie tipe masjien as generator beperk tot redelik lae drywingsgebiede. Dit is nodig om die werkverrigtingsvermoë van hierdie tipe AVPM masjien in die boonste medium drywingsgebied te ondersoek. Die fokus van hierdie tesis is op die ontwerp-optimering van 'n lugverkoelde AVPM generator met 'n kernlose stator. 'n Ontwerpsbenadering wat die eindige element veldoplossing in 'n multi-dimensionele optimeringsprosedure insluit, is ontwikkel en toegepas op die ontwerpsoptimering van 'n 300 kW (by eenheidsarbeidsfaktor) AVPM generator. Om 'n globale ontwerpsoptimering van die masjien te kan doen is verskillende ontwerpsaspekte soos die verkoelingskapasiteit, meganiese sterkte en werwelverliese ook in hierdie navorsing bestudeer. Om die vrye beweging van die rotormaas ten opsigte van die statormaas te verseker is die lugspleet-element, soos oorspronklik deur Razek et al voorgestel, afgelei vir Cartesiaanse koórdinaat stelsels. Om die lang berekeningstyd geassosieer met hierdie makro-element te minimaliseer is 'n aantal bestaande tydbesparende metodes saam met die ontwikkelde Cartesiaanse lugspleet-element gebruik. Die ontwikkelde eindige element tydstapmodel is toegepas om die bestendige werkverrigting van die AVPM masjien te bereken. Aangesien die vloedverspreiding in 'n AVPM masjien van nature drie-dimensioneel is, kan die berekening van die werwelstroomverliese tot aansienlike foute lei as eenvoudige analitiese metodes gebruik word. Om hierdie probleem te oorkom is twee-dimensionele eindige element modellering gebruik om akkurate veld-analise te doen. Om die volle voordele van die twee- dimensionele eindige element modellering te eksploiteer is 'n multi-laag benadering voorgestel wat die variasie van die lugspleetvloeddigtheid in die geleiers met betrekking tot hulle relatiewe lugspleetposisies in ag neem. Om voorsiening te maak vir die radiale variasie van die veld, is 'n multi-skyf eindige element modelleringstegniek ontwikkel. Die termiese analise is 'n belangrike aspek van die ontwerpsoptimering van AVPM masjiene. Vanuit 'n ontwerpsoogpunt is dit verkieslik om 'n eenvoudige maar tog effektiewe metode van verkoelingsanalise en -ontwerp te hê wat maklik toegepas kan word op 'n wye reeks van AVPM masjiene. In hierdie tesis word 'n termovloeimodel van die AVPM masjien ontwikkel. Hierdie vloeimodel is nodig vir die berekening van die lugvloeitempo, wat op sy beurt weer nodig is om die konveksie hitte-oordrag koëffisiënte te bepaal. Hierdie is belangrike parameters in die opvolgende termiese berekeninge. Eksperimentele ondersoek is uitgevoer om elkeen van die bogenoemde modelle en metodes te verifieer. Nadat hierdie modelle geïmplimenteer is, is die ontwerpsoptimering van 'n 300 kW (by eenheidsarbeidsfaktor) lugverkoelde kernlose stator AVPM generator uitgevoer. Die werkverrigtingmetings gedoen op 'n vervaardigde prototipe masjien, word in hierdie tesis vergelyk met voorspelde resultate. Daar word getoon dat die voorgestelde ontwerpsbenadering met sukses toegepas kan word om die ontwerp van die AVPM masjien te optimeer. Die voordele van hoë drywingsdigtheid, hoë benuttingsgraad, geen vertandingsdraaimomente en goeie spanningsregulasie maak hierdie masjien baie aantreklik vir generator toepassings. Die optimum bestendige werkverrigting van die AVPM masjien toon dat hierdie masjien met 'n kernlose stator 'n goeie kandidaat is vir hoë spoed generator toepassings, selfs in die boonste medium drywingsgebied. Die goeie verkoelingskapasiteit van hierdie tipe masjien hou die belofte in van'n selfverkoelde generator by hoë drywing aanslae.

No description available.

A high resolution photoconductivity investigation of two 13 -3 photon magneto-absorption (TPMA) in n-InSb (n - 9 x 10 cm ) has been performed. This is the first time that two-photon absorption in a semiconductor has been studied with cw lasers only. With a stable cw CC>2 laser and a highly sensitive sampling and magnetic field modulation technique, a minimum of 4 2 transitions in the TPMA photoconductivity spectra can be observed. Most of these transitions are a result of the usual spherical approximation TPMA selections rules (An =0, ±2; As = 0 for e ⊥ B and Δn = 0; Δs = 0 for e || B) . However, some transitions, in particular several near the TPMA band edge, are not explained by these rules. The TPMA spectra have been found to depend upon crystallographic orientation. This has not been previously observed. The temperature variation of the fundamental energy gap Eg between 2 and 100° K is also obtained from TPMA experiments.

L'utilisation de l'équation de Dirac/Weyl conduit à une simplification conceptuelle dans une description de la structure de la bande dans les solides à faible échelle d'énergie. En particulier, les excitations d'électrons-trous peuvent être considérées comme analogues au cas relativiste tel que conductivité optique linéaire, le suppression de backscattering ou la manifestation des arcs de Fermi et la chiralité des particules. En outre, la phase semi-métallique est également un élément crucial pour la classification des matériaux. La taille de le gap est affectée qualitativement par le type de dispersion d'énergie par un croisement continu des bandes linéaires à paraboliques. Cela peut être compris comme une limite classique ou ultra-relativiste du mouvement d'une particule massive.La spectroscopie infrarouge de la transformation de Fourier est une technique unique pour étudier les excitations optiques dans une large gamme d'énergies et représente en combinaison avec le champ magnétique élevé un outil puissant pour sondage de la structure électronique et surmonte le principal obstacle des systèmes sans gap qui est un dopage fort en raison de désordre structurel.La première partie du travail est consacrée à l'arséniure de cadmium, où nous élaborons une approche de distinction qualitative entre les systèmes Dirac et Kane qui ont été utilisés pour prouver sur la base de la réponse magnéto-optique observée la réalisation du modèle Kane presque sans gap avec une similitude frappante avec HgCdTe, en contradiction avec l'existence de cônes purement Dirac. La magnéto-réflectivité dans un champ magnétique à champ élevé la résonance cyclotron caractéristiques par un radical-B dépendance avec un comportement particulier dans la limite quantique. En revanche, la magnéto-transmission montrait des transitions de niveau Landau qui doit être interprétées que comme un type plat-à-cône afin de préserver une cohérence totale du modèle. Les cônes de Dirac prédits par la théorie sont susceptibles de coexister dans le modèle de Kane sous la forme d'une sous-structure décrite par le modèle de Bodnar qui se rapproche de la structure cristalline complexe par une simple cellule antifluorite qui permet d'utiliser la théorie du k.p classique.Dans la deuxième partie, nous nous concentrons sur le bismuth comme isolant topologique 3D archétype. Nous étudions une condition particulière obéie pour le BHZ-hamiltonien qui apporte des propriétés intriguantes comme une relation inhabituelle de spin gap et la résonance du cyclotron, l'épinglage spécifique entre les fancharts des sous-groupes Landau ou les g-facteurs compensés dans les bandes de conduction et de valence. Les mesures de photoluminescence ont montré une émission directgap, ce qui donne un nouvel aperçu de la structure largement acceptée à partir des données ARPES, où la “chameau structure” de la bande de valence doit être expliquée dans le confinement de surface et le point de Dirac de l'état de surface doit être repositionné par rapport aux bandes en bulk. La réponse magnéto-optique peut être pleinement expliquée dans une image classique du paramagnétisme de Pauli comme un simple effet d'occupation. Un tel comportement se manifeste dans la transmission en tant que fractionnement progressif du bord d'absorption interbande avec une saturation successive due à la polarisation spin partielle ou totale des électrons. Le dichroïsme relatif entraîne également une forte rotation de Faraday linéaire décrite par un modèle simple de la constante Verdet qui ne dépend pas sur le niveau de Fermi. / The use of the Dirac/Weyl equation leads to a conceptual simplification in a description of the band structure in solids at low energy scales. In particular, electron-hole excitations can be regarded as an analogue to the relativistic case with several expected phenomena to be observed in the condensed systems such as a suppressed back-scattering, linear optical conductivity or the manifestation of the Fermi arcs and particle's chirality. Moreover, the semimetallic phase also symbolizes a boundary between the trivial and topological insulators and thus play a crucial role for the material classification. The size of the gap qualitatively affects the type of the energy dispersion by a continuous crossover from the linear to parabolic bands. This fact can be easily understood as a classical or ultra-relativistic limit of the motion of a free massive particle.Infrared Fourier transform spectroscopy is a unique technique for studying optical excitations in a wide range of energies and it represents in combination with the high magnetic field a powerful tool for probing electronic structure and overcomes the main obstacle of the gapless systems that is a strong doping due to the structural disorder.The first part of the work is devoted to cadmium arsenide, where we elaborate an approach to qualitatively distinguish between the Dirac and Kane systems that was used to prove on the basis of the observed magneto-optical response the realization of the nearly gapless Kane model with a striking similarity to HgCdTe, contradicting the existence of purely Dirac cones. The magneto-reflectivity revealed a strong splitting of the plasma edge that turns into the cyclotron resonance characteristic by a squareroot-of-B dependence in the high magnetic field with a particular behaviour in the quantum limit independent on the initial Fermi level. In contrast, the magneto-transmission revealed interband Landau level transitions that could be only interpreted as a flat-to-cone type in order to preserve a full consistency of the model. The Dirac cones predicted by theory are feasible to coexist within the Kane model in the form of a substructure described by the Bodnar model that approximates the complex crystal structure by a simple antifluorite cell, which allows to use the conventional k.p-theory.In the second part, we focus on bismuth selenide entitled as an archetypal 3D topological insulator. We study a peculiar condition fulfilled for the BHZ-hamiltonian that brings intriguing properties such as an unusual relation of the spin gap and cyclotron resonance, the specific pinning between fancharts of Landau subsets or the compensated g-factors of the conduction and valence bands. The photoluminescence measurements showed a direct-gap emission, that gives a new insight to the widely accepted structure from ARPES data, where the declared camel-back structure of the valence band needs to be explained within the surface confinement and the Dirac point of the surface state should be repositioned with respect to the bulk bands. The magneto-optical response can be fully explained in a classical picture of the Pauli paramagnetism as a purely occupational effect. Such behaviour is evinced in the transmission as a gradual splitting of the interband absorption edge with a successive saturation due to the partial or total spin polarization of electrons. The related dichroism drives also a strong linear Faraday rotation described by a simple model of the Verdet constant that depends only on the Fermi level.

O tema deste trabalho é a modelização computacional das propriedades magnéticas de sistemas nanoparticulados a temperatura finita. Estes materiais, que são de grande interesse acadêmico e aplicado, possuem uma sensibilidade atípica às flutuações térmicas, um fenômeno conhecido como superparamagnetismo. Por essa e outras peculiaridades, eles apresentam um comportamento extremamente rico e complexo que se estende por uma gama ampla de situações experimentais, indo desde eras geológicas em aplicações na área de geomagnetismo, a fenômenos ultra-rápidos em dispositivos eletrônicos e tratamentos clínicos. O modelo empregado, conhecido como teoria de Néel-Brown, introduz na equação dinâmica magnética um termo estocástico para lidar com as flutuações térmicas. Sua validade é bastante geral, podendo ser aplicado para simular uma quantidade enorme de experimentos. Implementamos uma biblioteca numérica extremamente eficiente, que permite tratar sobre um mesmo escopo estas diferentes situações. Neste trabalho, focamos no problema de histerese dinâmica que vêm recebendo considerável atenção nos últimos anos motivado, principalmente, pela aplicação de nanopartículas magnéticas em tratamentos de tumores por uma técnica conhecida como magneto-hipertermia. / This thesis concerns the use of computer models to study the magnetic properties of nanoparticles at a finite temperature. These materials, which are of great academic and applied interest, are known to have an enhanced sensitivity to thermal fluctuations -- a phenomenon known as superparamagnetism. Such a peculiar nature is responsible for a large number of interesting physical phenomena, which are known to extend over a wide range of experimental situations. These include, among others, geomagnetism, ultra-fast devices and oncological treatments. The model employed, known as the Néel-Brown theory, introduces in the dynamical equation an stochastic term representing the thermal fluctuations. It\'s range of validity is quite broad, thus being applicable to all of the aforementioned situations. We implemented a highly efficient numerical library, whose scope extends over a large range of experiments. In this thesis we focused on the problem of dynamic hysteresis, which has receive considerable attention in recent years. This was motivated, among other things, by the potential use of nanoparticles in magneto-hyperthermia treatments.