This thesis presents an investigation of the variations in the magnetic field obtained in the Mackenzie Bay/Beaufort Sea area of the Canadian Arctic. It was found that the variations obtained at sea were strikingly correlated with those recorded at Point Atkinson, a fixed station on land, 150 miles from the survey area. In addition, it was found that the higher frequencies present in the marine records were severely attenuated with respect to the corresponding frequencies in the Point Atkinson recordings. It was concluded that the Mackenzie Bay/Beaufort Sea area is geomagnetically anomalous and that this situation is probably caused by higher electrical conductivity material underlying, the Mackenzie Bay/Beaufort Sea area, abutting lower conductivity material of the North American craton underlying Point Atkinson. This conclusion has important implications relating to the tectonic history of the Canadian Arctic. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

Par delà ses applications largement distribuées pour le stockage de l’information binaire, la spintronique vise le traitement Booléen de l’information. Des dispositifs de logique de spin (spin-FETs et portes logiques de spin) sont envisagés en se basant sur la propagation et la manipulation de porteurs polarisés en spin injectés dans un canal latéral depuis un contact magnétique. En dépit de deux décennies de recherche active, l’efficacité des dispositifs (notamment en termes de longueur de propagation du spin et d’amplitude du signal de spin) est toujours limitée quand le canal latéral est un métal ou un semi-conducteur conventionnel : la mise en évidence d’un medium adapté est nécessaire.Le transport dépendant du spin dans le graphène a été étudié dans le cadre de cette thèse. Dans un premier temps, l'impact et la structure de barrières tunnel de haute qualité déposées sur le graphène, nécessaires pour l'injection efficace de l'information de spin, ont été étudiés. A partir de ces résultats, des dispositifs complets d'injection, transport et détection de spin basés sur un feuillet de graphène épitaxié ainsi que sur une bicouche de graphène exfoliée ont alors été construits. Enfin, des mesures de transport du spin dans ces dispositifs ont été effectuées, puis analysées via les modèles classiques de drift/diffusion. Les forts signaux obtenus (gamme du MΩ), en validant l'existence d'un support capable de transporter le spin avec une très forte efficacité sur des distances macroscopiques (jusqu’à 200 µm), ouvrent la voie à une implémentation des concepts de traitement de l'information de spin. / Beyond its widely distributed applications for binary data storage, spintronics aims Boolean information processing. Spin logic devices (spin-FETs and spin logic gates) are envisioned through the propagation and the manipulation of a spin-polarized carriers injected into a lateral channel from a magnetic contact. In spite of two decades of active research, the devices efficiency (in particular in terms of spin propagation length and spin signal amplitude) is still limited when the lateral channel is made of conventional metals or semiconductors: a suitable host still lacks. Spin-dependant transport in graphene is investigated in this thesis. At first, the impact of the growth and on the structure of high quality tunnel barriers deposited on graphene, required to obtain an efficient injection of the spin information, has been studied. From these results, complete devices for spins injection, transport and detection based on an epitaxial graphene sheet and also on an exfoliated bilayer graphene flake were built. Finally, measurements of spin transport in these devices were carried, and then analyzed through classical drift/diffusion models. The strong signals obtained (MΩ range), by validating the existence of a substrate able to carry the spin information with a very large efficiency on macroscopic distances (up to 200 µm), open the way for the implementation of spin processing concepts.

An investigation of the electronic structure of some transition metal clusters comprising anti-ferromagnetically coupled, heterometallic arrays of eight metal ions that are wheel-shaped, is reported. The compounds were synthesized and provided by Dr. Grigore Timco of The University of Manchester and are formulated by their metal content as Cr7M, where M = a divalent 3d metal. Two families of wheels are the subject of this research, defined ‘green’ and ‘purple’ from their physical appearance. Within each family, the compounds are all isostructural. From simulation using a single Hamiltonian for Cr7M-purple compounds, where M = Zn, Mn, or Ni, it is shown that with only two exchange parameters, one JCr-Cr and one JCr-M, data from bulk magnetization, neutron scattering, Electron Paramagnetic Resonance (EPR) spectroscopy at multiple frequencies and specific heat measurements can be modelled and that there is transferability of parameters. Preliminary attempts to measure electron spin relaxation times for two of the purple wheels have shown values of T1 and T2 that are comparable with those of the more extensively studied green wheels and hence further studies in this area are warranted. Variable temperature Q- and W-band EPR spectra for a series of nine heterodimers comprising one green and one purple wheel, M=Zn, Mn or Ni in each case, are reported. For Cr7Zn-purple there is no magnetic exchange detected, whereas weak and quantifiable exchange is required to interpret the spectra from the other six dimers. EPR studies of three trimers of the form purple-green-purple are reported and the presence of magnetic exchange is identified by comparison with the spectra of the component single and double wheel compounds, although this is not quantified because of the numerical size of the simulations that are required. The process of comparing simulated to experimental spectra is a complex problem and one which is central to the work reported in this thesis. The problem of fitting has been investigated and two novel solutions, one based upon pixel mapping and the other based on wavelet transformation are proposed.

Submitted by Rafael Santana (rafael.silvasantana@ufpe.br) on 2017-07-10T18:32:41Z No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Dissertação_JM_Marin.pdf: 8178373 bytes, checksum: f54920c574e580233e6d0f22302111e3 (MD5) / Made available in DSpace on 2017-07-10T18:32:41Z (GMT). No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Dissertação_JM_Marin.pdf: 8178373 bytes, checksum: f54920c574e580233e6d0f22302111e3 (MD5) Previous issue date: 2016-03-10 / FACEPE / Rare earth chromites exhibit a diversity of interesting chemical and physical properties depending on the rare earth ion. EuCrO3 shows a weak spontaneous magnetic moment below a Néel temperature of 181 K; attributed to a slight canting of the Cr3+ magnetic moments that are otherwise antiferromagnetically aligned. We have synthesized chromites by using a combustion reaction method applying urea as fuel. X-ray diffraction (XRD), Fourier transformed Infrared Spectroscopy (FT-IR), Scanning and Transmission Electron Microscopy (SEM and TEM), Thermogravimetric analysis and differential scanning calorimetry (TGA-DSC) were used to study the samples. The magnetization was measured by using a vibrating sample magnetometer (VSM) and a Physical Properties Measurement System (PPMS) at a wide range of temperature (5 K to 300 K). Rietveld analysis of the XRD data yielded to an orthorhombic structure (Pnma) for EuCrO3, with an average crystallite size about 65 nm. Then, EuCrO3 was doped with iron forming a mixed chromite Eu1−xFexCrO3 (0 ≤ x ≤ 1.0). The presence of iron proved to be a source of microstrain in the system, due to the substitution of Eu3+ ions by Fe3+ in the perovskite structure. In fact, single phase EuCrO3 is paramagnetic at room temperature. However, above x>0.2, an enhance in the exchange interaction among the magnetic ions. The magnetic properties of Eu1−xFexCrO3 is accounted for by taking into consideration the lattice distortion introduced by the iron substitution in the EuCrO3 perovskite structure mainly due to the large difference in the ionic radius of Eu3+ and Fe3+ and in their magnetic moments. One of the most important contributions of our work has been focus on the relation between the bond angle of Cr3+-O2−-Cr3+ and the Néel temperature. This angle is responsible for the weak magnetic behavior of the rare earth chromites. We have found that as a product of the induced stress the Néel temperature is modified, so does the bonding angle. This behavior has been also found analyzing the shift towards higher values of the Hopkinson peak. To conclude, a termal treatment at 800°C for the Eu1−xFexCrO3 made at different times, shown a reduction in the microstrain of the system accompanied by an inhibition of the ferromagnetic-like character of the as-prepared samples. / As cromitas, à base de terras raras, exibem várias propriedades químicas e físicas interessantes dependendo do íon de terra rara. EuCrO3 mostra um momento magnético espontâneo fraco abaixo da temperatura de Néel (181 K); atribuída a uma ligeira inclinação dos momentos magnéticos do Cr3+ que são, de outra forma, antiferromagneticamente alinhados. Sintetizamos cromita de európio utilizando a técnica de reação por combustão. Difração de raiosX (XRD), espectroscopia de infravermelho por transformada de Fourier (FT-IR), microscopia electrónica de varredura e de transmissão (SEM e TEM), análises térmicas como termogravimetria e a calorimetria diferencial de varredura (TGA-DSC) foram utilizados para estudar nossas amostras. A magnetização foi medida usando um magnetómetro de amostra vibrante (VSM) e um PPMS (Physical Properties Measurement System) numa vasta região de temperatura (5 K até 300 K). O analises Rietveld dos difratogramas de XRD mostrou uma estrutura ortorrômbica (Pnma) para o EuCrO3, com um tamanho médio de cristalito de 65 nm. Depois, o sistema foi dopado com ferro formando cromitas mistas do tipo Eu1−xFex CrO3 (0≤ x ≤ 1,0). A presença de Fe provou ser uma fonte de “micro-strain” no sistema, devido à substituição dos íons de Eu3+ por aqueles de Fe3+. Na realidade, a fase pura de EuCrO3 é paramagnética à temperatura ambiente. No entanto, acima de x > 0,2, uma melhora na interação de troca entre os íons magnéticos é encontrada. As propriedades magnéticas de Eu1−xFexCrO3 são contabilizadas tendo em conta a distorção da estrutura introduzida pela substituição de ferro, principalmente devido à diferença no raio iónico de Eu3+ e Fe 3+ e dos seus momentos magnéticos. A relação entre o ângulo de ligação de Cr3+-O2−-Cr3+ (responsável pelo magnétismo fraco das cromitas de terras raras em baixa temperatura) e seu efeito na temperatura de Néel é estudada. Verificou-se que como produto do “strain” induzido, a temperatura de Néel é modificada e o mesmo acontece com o ângulo de ligação. Esse comportamento foi também encontrado analisando a mudança para valores mais elevados no pico de Hopkinson. Para concluir, um tratamento térmico feito a 800 °C para Eu1−xFexCrO3 durante tempos diferentes, mostraram que existe uma redução no “micro-strain” do sistema acompanhado por uma inibição do caráter ferromagnético das amostras (x > 0,2).

Cílem této práce je studium systému UCo1−xRuxGe nacházejícího se na rozhraní magnetické nestability. Podrobná studie magnetických a transportních vlastností po- lykrystal· s r·zným obsahem ruthenia odhalila komplexní změnu těchto parametr· v rámci celé série. UCoGe je slabý ferromagnet s TC ∼ 3 K a malým magnetickým momentem 0.035 µB/f.u. Pozorovali jsme strmý nár·st těchto hodnot až po hodnotu koncentrace ruthenia xmax ≈ 0.1(TC,max = 9 K a µ0 = 0.11 µB). Podstata tohoto ná- r·stu byla studována na mono krystalu o složení UCo0.88Ru0.12Ge získaném metodou plovoucí zóny. Difrakce polarizovaných neutron· provedená na tomto krystalu se slo- žením UCo0.88Ru0.12Ge vysvětlila posílení magnetismu změnou vzájemné orientace magnetických moment· na iontu uranu a kobaltu. Antiparalelní uspořádání, které bylo dříve publikováno pro UCoGe je změněno na paralelní uspořádání v dopova- ném UCo0.88Ru0.12Ge kdy dojde k reorientaci momentu na kobaltu. Silná anizotropie UCo0.88Ru0.12Ge se promítá do teplotní závislosti odporu a teplotní roztažnosti, které se dramaticky liší pro všechny tři osy. Tento prvotní nár·st TC a magnetického mo- mentu je následován poklesem směřujícím ke koncentraci xcr ≈ 0.31 kde magnetické uspořádání mizí. Podrobná studie kritických exponent· teplotních závislostí elek- trického odporu,...

Compact, autonomous computing systems with integrated transducers are imperative to deliver advances in healthcare, navigation, livestock monitoring, point of care diagnostics, remote sensing, internet-of-things applications, smart cities etc. Reflecting this need, there has been sustained growth in the market for transducers. Polymer based transducers, which meld highly desirable properties such as low cost, light weight, high manufacturability, biocompatibility and flexibility, are quite attractive. Doping polymers with magnetic materials results in the formation of magnetic composite polymers, enhancing the attractive traits of polymer transducers with magnetic properties. This dissertation is dedicated to the development of magnetic polymer transducers, which are suitable for energy harvesting and saline fluid transduction. The first-ever magnetic composite energy harvester capable of converting vibrations from the practically relevant low-frequency range into electrical energy was fabricated and tested. The harvester was realized by fabricating an array of PDMS-iron nanowire nanocomposite cilia on a planar coil array and exhibits a linear frequency response. This energy harvester design was further improved by increasing the doping concentration of the composite, adding a composite proof mass and improving the microfabricated coil. These changes manifest in an energy harvester that not only increases the power density by 4 orders of magnitude over the previous design but also possesses large operational bandwidth. The composite structure, comprising of the cilia and the proof mass has a frequency response comprised of two closely spaced resonant peaks facilitating the desirable broadband behavior at low frequency.A polymer-based magneto hydrodynamic pump prototype capable of actuating saline fluids was developed. The benefit of this pumping concept to operate without any moving parts is combined with simple and cheap fabrication methods and a magnetic composite material, enabling a high level of integration together with the advantages of mechanical flexibility. The pump electrodes are created by laser printing of graphene on polyimide, while the permanent magnet is molded from an NdFeB powder - polydimethylsiloxane (PDMS) composite. These materials were leveraged to fabricate an integrated, low profile magneto hydrodynamic pump, suitable for deployment in lab on chip systems.

No description available.

This dissertation consists of four chapters, each of which is either published in a peer-reviewed journal, or in submission. These chapters developed from the testing of the hypothesis that the lithospheric mantle contains significant magnetic regions that contribute to the magnetism observed/measured, either at or close to the Earth’s surface, or from satellite data. Chapter 1 “Eight good reasons why the mantle could be magnetic” (2014) published in Tectonophysics by Ferré, Friedman, Martín-Hernández, Feinberg, Till, Ionov and Conder, addresses the motivation for this project and establishes the probability of upper mantle contribution to magnetic anomalies. My role with this manuscript was to produce figures using my previously collected data (Figures 2, 4, and 6), compile known data on the magnetic properties of minerals in mantle peridotites (Table 1), provide discussion for and against each argument made, and edited the manuscript. Chapter 2 “Remanent magnetization in fresh xenoliths derived from combined demagnetization experiments: Magnetic mineralogy, origin and implications for mantle sources of magnetic anomalies” (2014) published in Tectonophysics by Martín-Hernández, Ferré, and Friedman, investigates the natural remanent magnetization of mantle xenoliths. Notably, it establishes that the natural remanent magnetization of these xenoliths is derived from a thermoremanent magnetization (primary) and not from chemical remanent magnetization (secondary) origin. My primary role in this study was to provide preliminary magnetic and petrologic data and analysis of the samples. Secondary responsibilities were to prepare the samples, edit the manuscript and provide discussion on the results. Chapter 3 “Craton vs. rift uppermost mantle contributions to magnetic anomalies in the United States interior” (2014) published in Tectonophysics by Friedman, Feinberg, Ferré, Demory, Martín-Hernández, Conder, and Rochette begins to compare magnetic properties across different tectonic settings. The metasomatized cratonic upper mantle of the United States interior contains ferromagnetic phases that exist at temperatures lower than the Curie temperature. This upper mantle would likely contribute to magnetic anomalies. Alternatively, the high geotherm and sulfide-rich mantle near the Rio Grande Rift precludes this area from mantle contribution to magnetic anomalies. As first author I prepared samples, ran experiments, processed data, produced figures, wrote the manuscript and applied for funding. Chapter 4 “What is magnetic in the mantle wedge?” (2015) submitted to Geology, examines the mantle wedge beneath multiple island arcs. Magnetic anomalies in island arc settings have been attributed to a serpentinized mantle wedge. While this material is not available to test, metasomatized mantle, common to the mantle wedge, is available. Metasomatized mantle is mostly paramagnetic, and thus supports that stepwise dehydration of a subducting slab may produce positive and negative anomalies in the mantle wedge. As first author I prepared samples, ran experiments, processed data, produced figures, wrote the manuscript and applied for funding.

No description available.

No description available.