This thesis is a part of a larger project that focuses on the development of a highspeed, high energy flywheel using both high-tech composites and levitating magneticbearings alongside a custom made, permanent magnetized generator built into theflywheel itself. The goal of the project is then to integrate this flywheel into anelectrical vehicle.The main focus of this thesis is the composite material. The composite is to be usedas a shell around the flywheel rotor. This composite shell fills two purposes. The firstis to act as the main energy carrying material, storing above 75% of the total energy inthe flywheel. The second purpose it to strengthen the machine, holding it together.This so that higher speeds than normally possible can be achieved, with the goal beingset to 30 000rpm.In order to be able to design the composite shell correctly a method of calculating theload stresses had to be developed. This was done by the creation of a Matlabprogram, named Spin2Win, capable of calculating the stresses inside a compositemetal hybrid flywheel. Using said Matlab code, combined with modelling andsimulations from SolidWorks, a fully-fledged flywheel was designed complete withdrawings and material specifications.The composite analysis surprisingly shows that the best combination of compositematerials is a mixture of both high strength carbon fibres alongside softer glass fibrescoupled with the weight of the central core. This allowed for control of the radialstresses which was shown to otherwise be the limiting factor when designing rotatingcomposite materials.One of the most interesting, and perhaps even unique, parts of the design is that theelectrical machine has been integrated into the flywheel’s composite shell. Having thetwo entities working together in order to control the radial stresses in thecomposite, by utilizing the weight of the permanent magnets.

This experimental work focusses on the magnetic thermal conductivity, κ_mag, of the one-dimensional two-leg spin ladder system Sr_14Cu_24O_41 and the spin chain system SrCuO_2. These two S = 1/2 antiferromagnetic Heisenberg compounds possess enormous magnetic contributions to the heat transport which in some cases exceed the phonon contributions by more than one order of magnitude. Despite of intense ongoing experimental and theoretical investigations, the underlying mechanism of the magnetic heat transport remains unclear. The study of κ_mag aims a better understanding of the basic physics which determine mobility, scattering and dissipation of the dispersing magnetic excitations. The most important tool used in this study is to selectively influence the structure and the electronic and magnetic properties of the compounds through doping. For this purpose single crystalline samples were produced using the Traveling Solvent Floating Zone technique, a crucible-free technology, which allows the growth of centimeter sized single crystals of high quality. In particular, the successful growth of large quantities of the hole-free ladders La_4Sr_10Cu_24O_41 allowed the realization of inelastic neutron scattering and, for the first time, the acquisition of the complete magnetic excitation spectrum of the spin ladder, composed not only by the triplon band, but also by the two-triplon continuum, permitting an accurate determination of the coupling constants in this system. The importance of the cyclic-exchange, previously unclear, was asserted. In order to study the scattering mechanisms of the magnetic excitations (triplons) off static defects in the two-leg ladder Sr_14Cu_24O_41, this compound was doped with tiny amounts of Zn. Occupying the Cu site in the ladders, the Zn plays the role of a non-magnetic defect, imposing an upper limit to the mean free path of the triplons. The thermal conductivity of Sr_14Cu_(14−z)Zn_zO_41, with z = 0, 0.125, 0.25, 0.5 and 0.75, shows a strong decrease of both the phononic and magnetic contributions with increasing z value. In particular, the decrease of the magnetic part indicates an increased scattering of the triplons off Zn defects. The analysis of κ_mag, using a kinetic model, allows the extraction of the triplon mean free path l_mag. This quantity was successfully correlated to the mean Zn-Zn distance along the ladders, confirming the validity of the employed kinetic model and corroborating results of previous works. In SrCuO_2, the magnetic contribution to the thermal conductivity appears as a hump-like anomaly on the high-T back of the low-T phonon peak. In order to better separate the magnetic contribution from the phononic background, small amounts of Sr were substituted by the smaller and lighter Ca, leading, on the one hand, to an increased scattering of the phonons and consequently to a suppression of the phononic thermal conductivity. On the other hand, since Ca is isovalent to Sr, no significant changes of the magnetic properties of the system are expected: a magnetic peak belonging to κ_mag should appear. Measurements of the thermal conductivity of Sr_(1−x)Ca_xCuO_2 for x = 0, 0.0125, 0.025, 0.05 and 0.1 show indeed a systematic decrease of the phonon thermal conductivity with increasing x. However, against initial expectations, no magnetic peak appears. Instead, the magnetic thermal conductivity decreases at intermediate and low temperatures with increasing doping level, indicating a strong influence of the Ca dopant on the magnetic system. Surprisingly, no changes of κ_mag occur at higher temperatures, where κ_mag remains constant for all doping levels. To explain this intriguing temperature and doping dependence of κ_mag, three scenarios are proposed. One of the scenarios is based on the phenomenon of mutual spinon and phonon heat transport, the so called spin-phonon-drag mechanism. Another scenario assumes an effective scattering of spinons off Ca defects. In a third scenario, the appearance of a gap in the doped compounds is considered. The obvious effect of the Ca dopant on the magnetic thermal conductivity motivated a more detailed investigation of the doping dependence of electronic and magnetic properties in Sr_(1−x)Ca_xCuO_2. NMR data reveal the presence of a magnetic gap for the x = 0.1 compound. The doping dependent evolution of the specific heat at low-T is consistent with this result. Furthermore, susceptibility data may be explained within a segmentation of the spin chains, which in turn can be also related to the opening of a gap. These results strongly support that the reduction of κ_mag in the Ca doped compounds is related to a smaller number of magnetic excitations participating in the heat transport due to the presence of the gap. A possible reduction of the chain length, as suggested by the susceptibility data, is also consistent with the scenario of a reduced κ_mag due to an increased scattering of magnetic excitations. In spite of these partially consistent results, there are still no clear-cut explanations for the evolution of κ_mag upon doping. In particular, it cannot be completely ruled out that a fraction of the Ca dopant goes into the chains, a point which has to be urgently clarified in order to allow a correct interpretation of the data.

Nanokristalline Permanentmagnete zeigen ungewöhnliche magnetische Eigenschaften aufgrund von Oberflächen- und Grenzflächeneffekten, die verschieden von denen massiver oder mikrokristalliner Materialien sind. Diese Arbeit zeigt Ergebnisse einer systematischen Untersuchung der Beziehung zwischen Mikrostruktur und magnetischen Eigenschaften von isotropen nanokristallinen (Nd,Pr)(Fe,Co)B-Permanentmagneten. Hochkoerzitive Magnete vom Typ (Nd,Pr)FeB wurden durch hochenergetisches Mahlen in der Kugelmühle oder Rascherstarrung hergestellt. Der Einfluss geringer Mengen von Zusätzen wie Dy und Zr und die Substitution von Nd durch Pr auf die magnetischen Eigenschaften wird dargestellt. Weiterhin wurde eine Einschätzung des Warmumformverhaltens dieser Materialien durchgeführt. Hochenergetisches Kugelmahlen einer Legierung mit der Anfangszusammensetzung Pr9Nd3Dy1Fe72Co8B6.9Zr0.1 führte, nach Glühbehandlung, zu fast einphasigem Magnetpulver mit einem maximalen Energieprodukt von (BH)max~140 kJm-3. Das hochenergetische Kugelmahlen wurde zu einer sehr vielseitigen Technik zur Herstellung hochleistungsfähiger Nanokompositmagnete weiterentwickelt. Das Zulegieren unterschiedlicher Anteile von weichmagnetischem alpha-Fe ist damit sehr effektiv möglich. Der Zusatz von 25 Gew.-% alpha-Fe führt zu einem hohen (BH)max=178kJm-3. Dies wird auf eine sehr effektive Austauschkopplung zwischen den hart- und weichmagnetischen Phasen zurückgeführt. Die Natur der intergranularen Wechselwirkungen kann durch die Wohlfarth´sche Remanenzanalyse (?deltaJ-plot¡§) beschrieben werden. Im speziellen wurden deltaJ-Diagramme für verschiedene (i) alpha-Fe Gehalte, (ii) Korngrößen und (iii) Austauschlängen erstellt. Es konnte gezeigt werden, dass in den Nanokompositmagneten auf Pr-Basis keine Spinumorientierung auftritt. Abschließend zeigt die Arbeit die Möglichkeit der Nutzung einer mechanisch aktivierten Gas-Festkörper-Reaktion auf, mit der eine sehr feinkörnige Mikrostruktur erhalten wird. Die Untersuchungen wurden mit stöchiometrischen Nd2(Fe1-xCox)14B-Legierungen begonnen (x=0-1). Die Verbindungen wurden unter höheren Wasserstoffdrücken und Temperaturen gemahlen, wodurch sie zu NdH2+delta und krz-(Fe,Co) (x=0-0.75) oder kfz-Co (x=1) entmischt wurden. Die Korngrößen des rekombinierten Nd2(Co,Fe)14B-Materials liegen im Bereich von 40-50 nm. / Nanocrystalline permanent magnets present unusual magnetic properties because of surface/interface effects different from those of bulk or microcrystalline materials. This work presents results of a systematic investigation of the relationship between microstructure and magnetic properties in isotropic nanocrystalline (Nd,Pr)(Fe,Co)B permanent magnets. Highly coercive (Nd,Pr)FeB-type magnets have been produced using high energy ball milling and melt-spinning. The influence of small amounts of additives, Dy and Zr, and the substitution of Nd by Pr on the microstructural and magnetic properties are shown. An assessment of the hot deformation behaviour has been carried out. Intensive milling of an alloy with starting composition Pr9Nd3Dy1Fe72Co8B6.9Zr0.1 yields, after annealing treatment, nearly single-phase magnet powders with a maximum energy product (BH)max?î140kJm-3. Co has a beneficial effect on the intrinsic magnetic properties but also on the microstructure, with a mean grain size of 20nm. Intensive milling is used to produce high-performance nanocomposite magnets by blending this latter alloy with different fractions of soft magnetic alfa-Fe. Addition of 25wt.% alfa-Fe leads to a high (BH)max=178 kJm-3 due to an effective exchange-coupling between the hard and the soft magnetic phases. The intergrain interactions between the crystallites of the nanocomposite structure are analysed. Demagnetisation recoil loops of the nanocomposite magnets show relatively open minor loops due to the exchange-spring mechanism. Information about the intergrain interactions during demagnetisation are obtained by plotting the deviation of the demagnetising remanence from the Wohlfarth-model (¡§deltaJ-plot¡¨). Exchange-coupling phenomena are studied by analysing the evolution of the corresponding deltaJ values when varying (i) the alfa-Fe content, (ii) the annealing temperature, i.e. the grain size and (iii) the measurement temperature. Low temperature measurements do not reveal any sign of spin reorientation for these Pr-based nanocomposite magnets. The work concludes showing the possibility of using a mechanically activated gas-solid reaction to obtain an effective grain refined microstructure starting from stoichiometric Nd2(Fe1-xCox)14B alloys (x=0-1). These compounds were milled under enhanced hydrogen pressure and temperature leading to their disproportionation into NdH2+delta and bcc-(Fe,Co) (x=0-0.75) or fcc-Co (x=1). Grain sizes of recombined Nd2(Fe,Co)14B materials were found to be 40-50nm.

In this dissertation we reported our experimental investigation of the magnetic properties of nine low-dimensional vanadium compounds. Two of these materials are completely new (Pb2V5O12 and Pb2VO(PO4)2) and were found during our search for new low-dimensional vanadium oxides. Among the other seven vanadium compounds studied, three were physically investigated for the first time (Sr2VO(PO4)2, BaZnVO(PO4)2 and SrZnVO(PO4)2). Two had hitherto only preliminary, and wrongly interpreted, susceptibility measurements reported in the literature (Sr2V3O9 and Ba2V3O9) while the remaining two (Li2VOSiO4 and Li2VOGeO4) were previously investigated in some detail but the interpretation of the data was controversial. We investigated the magnetic properties of these materials by means of magnetic susceptibility and specific heat (Cp(T)) measurements (as well as single crystal ESR measurements in the case of Sr2V3O9). We synthesized the samples necessary for our physical studies. That required a search of the optimal synthesis conditions for obtaining pure, high quality, polycrystalline samples. Single crystals of Sr2V3O9 and Pb2VO(PO4)2 were also successfully grown. Pb2VO(PO4)2, BaZnVO(PO4)2, SrZnVO(PO4)2, Li2VOSiO4 and Li2VOGeO4 were found to be experimental examples of frustrated square-lattice systems which are described by the J1-J2 model. We found that Li2VOSiO4 and Li2VOGeO4 posses a weakly frustrated antiferromagnetic square lattice while Pb2VO(PO4)2, BaZnVO(PO4)2 and SrZnVO(PO4)2 form a more strongly frustrated ferromagnetic square lattice. Pb2V5O12 is structurally and compositionally related to the two dimensional A2+V4+nO2n+1 vanadates. Its structure consists of layers formed by edge- and corner-shared square VO5 pyramids. The basic structural units are plaquettes consisting of six corner-shared pyramids pointing in the same direction, which form a spin lattice of novel geometry. / In dieser Dissertation berichteten wir über unsere experimentelle Untersuchung der magnetischen Eigenschaften von neun Niedrigdimensionalen vanadiumverbindungen. Zwei dieser Materialien sind vollständig neu (Pb2VO12 und Pb2VO(PO4)2) und wurden während unserer Suche nach neuen Niedrigdimensionalen Vanadiumoxiden gefunden. Unter den anderen sieben studierten Vanadiumverbindungen, wurden drei physikalisch zum ersten Mal nachgeforscht (Sr2VO(PO4)2, BaZnVO(PO4)2 und SrZnVO(PO4)2). Zwei hatten bisher nur einleitendes, und falsch gedeutet, magnetische Susceptibilitaet Messungen, die in der Literatur berichtet wurden (Sr2V3O9 und Ba2V3O9) während die restlichen zwei (Li2VOSiO4 und Li2VOGeO4) vorher in irgendeinem Detail aber in der Deutung der Daten waren umstritten nachgeforscht wurden. Wir forschten die magnetischen Eigenschaften dieser Materialien mittels der magnetischen Susceptibilitaet und der spezifischen Waerme (Cp(T)) nach (sowie ESR-Messungen des einzelnen Kristalles im Fall von Sr2V3O9). Wir synthetisierten die Proben, die für unsere körperlichen Studien notwendig sind. Das erforderte eine Suche der optimalen Synthesezustände für das Erreichen der reinen, hohen Qualität, polykristalline Proben. Einzelne Kristalle von Sr2V3O9 und von Pb2VO(PO4)2 wurden auch erfolgreich gewachsen. Pb2VO(PO4)2, BaZnVO(PO4)2, SrZnVO(PO4)2, Li2VOSiO4 und Li2VOGeO4 werden gefunden, um experimentelle Beispiele der frustrierten Quadrat-Gittersysteme zu sein, die durch das J1-j2 model. Wir fanden daß posses Li2VOSiO4 und Li2VOGeO4 ein schwach frustriertes antiferromagnetische quadratisches Gitter, während Pb2VO(PO4)2, BaZnVO(PO4)2 und SrZnVO(PO4)2 ein stärker frustriertes ferromagnetisches quadratisches Gitter bilden. Pb2V5O12 strukturell und zusammenhängt kreativ mit den zweidimensionalen vanadates A2+V4+nO2n+1 beschrieben werden. Seine Struktur besteht aus den Schichten, die durch Rand und Ecke-geteilte quadratische Pyramiden VO5 gebildet werden. Die grundlegenden strukturellen Maßeinheiten sind die plaquettes, die aus sechs Ecke-geteilten Pyramiden bestehen, die in die gleiche Richtung zeigen, die ein Drehbeschleunigunggitter von Romangeometrie bilden.

<p>In the growing field of spintronics there is a strong need for development of flexible lightweight semi-conducting magnets. Molecular organic-based magnets are attractive candidates since it is possible to tune their properties by organic chemistry, making them so-called “designer magnets”. Vanadium tetracyanoethylene, V(TCNE)_x, is particularly interesting since it is a semiconductor with Curie temperature above room temperature (T_C~400 K). The main problem with these organic-based magnets is that they are extremely air sensitive. This thesis reports on the frontier electronic structure of the V(TCNE)_x by characterization with photoelectron spectroscopy (PES) and near edge x-ray absorption fine structure (NEXAFS) spectroscopy. It also presents a new and more flexible preparation method of this class of organic-based thin film magnets. The result shows improved air stability of the V(TCNE)_x prepared with this method as compared to V(TCNE)_x prepared by hitherto used methods.</p>

Fundamental conservation laws predict a dissipationless transport behavior in one-dimensional S=1/2 spin chains. This truly ballistic heat transport suggests anomalously large life times and mean free paths of the elementary excitations of the spin chain, spinons. Despite this rigorous prediction, in any real system, the transport is dissipative, due to the interaction of spinons with defects and phonons. Nevertheless, a promising large magnetic thermal conductivity \\kappa_{mag} has been observed in a few copper-oxide systems. Characteristic for these cuprate systems is a large exchange interaction J along the spin chain. However, due to the limited number and knowledge of the systems showing a large \\kappa_{mag}, it has been difficult, to identify overarching trends. The goal of this thesis therefore is twofold. First, to test new compounds for the appearance of magnetic heat transport and second, to broaden the understanding of the known compounds by studying the influence of various kinds of impurities. In particular, three families of materials are studied. First, the thermal conductivity \\kappa(T) of the compounds TiOBr and TiOCl is investigated. Below room temperature the compounds undergo two phase transitions T_{c2} and T_{c1}. Above T_{c2} the compounds contain S=1/2 spin chains with J_{Cl}=676 K and J_{Br}=375 K respectively, formed by direct orbital overlap of the Ti-atoms. Below T_{c1} the chains dimerize to form a non-magnetic ground state. The thermal conductivity exhibits pronounced anomalies at T_{c2} and T_{c1} confirming the transitions being of second and first order respectively. Surprisingly, \\kappa(T) appears to be dominated by phonon heat conduction, since no indications of a significant magnetic contribution is found. This is in contrast to the expectation of a spin chain system. In this context possible scenarios to understand the unusual behavior of the thermal conductivity are discussed. Second, two related materials, the single chain Sr_{2}CuO_{3} and the double chain SrCuO_{2} are investigated. In high purity samples huge magnetic heat conductivities and concomitantly, extremely large spinon mean free paths of >0.5 µm for Sr_{2}CuO_{3} and >1 µm for SrCuO_{2} are observed. This demonstrates that \\kappa_{mag} is only limited by extrinsic scattering processes, which is a clear signature of ballistic transport in the underlying spin model. Additionally, various subtle modifications of the spin chain are studied. Due to the large mean free path a pristine picture of the intrinsic incidents is expected. In particular, a chemical pressure is applied to the spin chain by doping SrCuO_{2} with Ca. This has a surprisingly strong effect on \\kappa_{mag}. Furthermore, the influence of magnetic Ni and non-magnetic Mg doping is studied for SrCuO_{2}. While Ni-doping has a large impact on the magnetic thermal conductivity, Mg-doping shows no influence. In order to clarify this surprising behavior, \\kappa_{mag} is compared to measurements of the single chain compound Sr_{2}CuO_{3}. Third, the magnetic thermal conductivity of the spin chain material CaCu_{2}O_{3} doped with non-magnetic Zn impurities is studied. \\kappa_{mag} of the pure compound is linear up to room temperature, which is indicative of a T-independent scattering rate of the magnetic excitations. Both, magnitude and T-dependence of \\kappa_{mag} exhibit a very unusual doping dependence. At moderate Zn-doping the linear temperature dependence of \\kappa_{mag} is preserved and the absolute value of \\kappa_{mag} increases. A slight suppression of \\kappa_{mag} occurs only at high Zn doping, where, surprisingly, the T-dependence of \\kappa_{mag} changes from linearity to one with a higher power of T . In order to clarify this surprising behavior, the results are compared to a detailed study of the g-tensor of the impurities in the material by means of ESR experiments, which reveal a change of the impurity type with increasing Zn-content.

The idea of miniaturizing devices down to the nanoscale where quantum ffeffects become relevant demands a detailed understanding of the interplay between classical and quantum properties. Therefore, characterization of newly produced nanoscale materials is a very important part of the research in this fifield. Studying structural and magnetic properties of nano- and molecular magnets and the interplay between these properties reveals new interesting effects and suggests ways to control and optimize the respective material. The main task of this thesis is investigating the magnetic properties of molecular magnetic clusters and magnetic nanoparticles recently synthesized by several collaborating groups. This thesis contains two main parts focusing on each of these two topics. In the first part the fundamental studies on novel metal-organic molecular complexes is presented. Several newly synthesized magnetic complexes were investigated by means of different experimental techniques, in particular, by electron spin resonance spectroscopy. Chapter 1 in this part provides the theoretical background which is necessary for the interpretation of the effects observed in single molecular magnetic clusters. Chapter 2 introduces the experimental techniques applied in the studies. Chapter 3 contains the experimental results and their discussion. Firstly, the magnetic properties of two Ni-based complexes are presented. The complexes possess different ligand structures and arrangements of the Ni-ions in the metal cores. This difffference dramatically affffects the magnetic properties of the molecules such as the ground state and the magnetic anisotropy. Secondly, a detailed study of the Mn2Ni3 single molecular magnet is described. The complex has a bistable magnetic ground state with a high spin value of S = 7 and shows slow relaxation and quantum tunnelling of the magnetization. The third section concentrates on a Mn(III)-based single chain magnet showing ferromagnetic ordering of the Mn-spins and a strong magnetic anisotropy which leads to a hysteretic behavior of the magnetization. The last section describes a detailed study of the static and dynamic magnetic properties of three Mn-dimer molecular complexes by means of static magnetization, continuous wave and pulse electron spin resonance measurements. The results indicate a systematic dependence of the magnetic properties on the nearest ligands surrounding of the Mn ions. The second part of the thesis addresses magnetic properties of nano-scaled magnets such as carbon nanotubes fifilled with magnetic materials and carbon-coated magnetic nanoparticles. These studies are eventually aiming at the possible application of these particles as agents for magnetic hyperthermia. In this respect, their behavior in static and alternating magnetic fifields is investigated and discussed. Moreover, two possible hyperthermia applications of the studied magnetic nanoparticles are presented, which are the combination of a hyperthermia agents with an anticancer drug and the possibility to spatially localize the hyperthermia effffect by applying specially designed static magnetic fifields.

Thesis (M.S.)--Mississippi State University. Department of Physics and Astronomy. / Title from title screen. Includes bibliographical references.

La manipulation de la matière au niveau nanométrique a ouvert depuis une quinzaine d'années de nouveaux champs fondamentaux et applicatifs pour les scientifiques et les industriels. Dans ce nouveau paradigme, la nanoélectronique quantique se propose de fonder une nouvelle électronique basée sur les phénomènes quantiques de la matière et plus particulièrement sur la nature quantique des électrons. Ce projet de thèse s'articule autour d'un système électronique quantique hybride supraconducteur/nanotube de carbone (CNT) dénommé nano-SQUID. Ce dispositif présente une boucle supraconductrice contenant deux jonctions CNT en parallèle. Il couple de façon unique les propriétés d'un interféromètre supraconducteur SQUID avec celles de jonctions Josephson à boîte quantique moléculaire. A travers des expériences de transport réalisées, à des températures de quelques dizaines de milli-Kelvins, dans un cryostat à dilution inversé, nous avons étudié les interactions électroniques entre une boîte quantique nanotube et des électrodes supraconductrices. Nous nous sommes particulièrement focalisés sur l'influence de l'état de spin du nanotube sur le courant supraconducteur, qui peut, dans certaines conditions, conduire à la réalisation d'un jonction-. Par un contrôle électrostatique des paramètres microscopiques du dispositif nous avons ainsi pu définir un diagramme de phase expérimental des transitions 0- d'une jonction Josephson à boîte quantique. La dernière partie de cette thèse a porté sur l'utilisation du nano-SQUID comme magnétomètre. En effet, en couplant un aimant moléculaire au CNT composant le SQUID, il a été montré théoriquement qu'il est possible de détecter le retournement d'aimantation d'un spin unique. Nous avons ainsi couplé au nano-SQUID l'aimant moléculaire Double Decker Holmium et réalisé les premières mesures de détections magnétiques aux résultats prometteurs. / The manipulation of matter at the nano-scale has opened, since fifteen years, new fundamental and application avenues for science and industry. In this new paradigm, quantum nano-electronics propose to start a new electronics based on quantum effects of matter and more particularly on the quantum nature of electrons. This thesis project deals with an electronic hybrid superconductor/carbon nanotube (CNT) system called nano-SQUID. This device has a superconducting loop containing two CNT junctions in parallel. This unique system couples the properties of a superconducting interferometer (SQUID) with those of molecular quantum dot (QD) Josephson junctions (CNT junction). Through transport experiments performed in a reversed dilution cryostat at temperatures of several tens of milli-Kelvin, we studied the electronic interactions between a nanotube quantum dot and superconducting electrodes. We specifically focused on the influence of the magnetic state of the nanotube on the superconducting current flowing through. Depending on the QD spin state, the CNT Josephson junction can behave as a -junction. Finally a complete electrostatic control allowed us to define an experimental 0- phase diagram of a QD Josephson junction.

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 e Cerâmicos à temperatura ambiente de baixo custo e de implementação mais simples. Para isso, o Histerisímetro de Entreferro Variável, HEV, é concebido e estudado. Ele consiste de um circuito magnético formado pelo ímã permanente sob caracterização, uma culatra de aço, um entreferro variável, bobinas de magnetização e desmagnetização de baixa potência, uma sonda de efeito Hall com seu respectivo gaussímetro, um fluxímetro com sua bobina exploradora, o circuito eletrônico de comando das bobinas de excitação e a interface a um sistema de instrumentação virtual. Para demonstrar os desenvolvimentos do trabalho são apresentados o estado da arte da caracterização dos ímãs permanentes, conceitos básicos de magnetismo em relação aos ímãs permanentes, instrumentação necessária para sua caracterização, a concepção e modelagem do HEV, os resultados dos testes realizados e uma abrangente análise de suas incertezas. Com isso é demonstrado que o Histerisímetro de Entreferro Variável é uma alternativa de caracterização de ímãs permanentes com desmagnetização linear com incertezas dentro do que é definido pela norma. / The determination of the main magnetic properties of permanent magnets usually requires sophisticated and expensive measuring systems. This work analyses and proposes a low-cost system for the characterization of Rare Earth and Ceramics permanent magnets at room temperature with simpler implementation. It comprehends a magnetic circuit with the permanent magnet under test, a steel yoke with appropriate pole pieces, a variable air gap, low power magnetization and demagnetization coils, a Hall Effect probe with its respective gaussmeter, a fluxmeter with its own search coil, a coil driver electronic circuit and a virtual instrumentation system (computer plus graphical interface software). To demonstrate the developments of the work, it is presented the state of the art of permanent magnets characterization, basic concepts of magnetism related to permanent magnets, the instrumentation required for its characterization, the HEV design and modeling, the results of the tests and a comprehensive analysis of related uncertainties. Thus, it is shown that the Variable Gap Hysteresimeter is an alternative characterization system for linear demagnetization permanent magnets with uncertainties within those defined by the standards.