Rozprawy doktorskie na temat „Géométrie magnétique”
Utwórz poprawne odniesienie w stylach APA, MLA, Chicago, Harvard i wielu innych
Sprawdź 50 najlepszych rozpraw doktorskich naukowych na temat „Géométrie magnétique”.
Przycisk „Dodaj do bibliografii” jest dostępny obok każdej pracy w bibliografii. Użyj go – a my automatycznie utworzymy odniesienie bibliograficzne do wybranej pracy w stylu cytowania, którego potrzebujesz: APA, MLA, Harvard, Chicago, Vancouver itp.
Możesz również pobrać pełny tekst publikacji naukowej w formacie „.pdf” i przeczytać adnotację do pracy online, jeśli odpowiednie parametry są dostępne w metadanych.
Przeglądaj rozprawy doktorskie z różnych dziedzin i twórz odpowiednie bibliografie.
Martin, Witkowski Laurent. "Écoulements engendrés par un champ magnétique tournant en géométrie cylindrique". Grenoble INPG, 1997. http://www.theses.fr/1997INPG0218.
Pełny tekst źródłaPacitto, Grégory. "Instabilités hydrodynamiques à l'interface de ferrofluides en géométrie confinée". Paris 7, 2001. http://www.theses.fr/2001PA077107.
Pełny tekst źródłaBarthélemy, Quentin. "Etudes par RMN, µSR et chaleur spécifique de liquides de spins quantiques dans des matériaux à géométrie kagome". Thesis, université Paris-Saclay, 2020. http://www.theses.fr/2020UPASP065.
Pełny tekst źródłaThe kagome Heisenberg antiferromagnet decorated with quantum spins is a fascinating model where to look for exotic quantum states including the elusive spin liquids. Despite its apparent simplicity, there is still no consensus on the exact nature of the ground state and of the excitation spectrum. The current availability of quantum kagome materials, each with their own deviation to the pure Heisenberg model, allows to confront a fair amount of experimental results to theoretical predictions and study the effect of perturbations inherent to real materials. The experimental work presented in this manuscript examines two directions.First, we focus on the emblematic herbertsmithite ZnCu₃(OH)₆Cl₂, with perfect kagome planes. It is the closest materialization of the pure Heisenberg model so far (J~180 K) and exhibits attributes of a quantum spin liquid at low temperatures, with the absence of magnetic long-range order and the persistence of spin dynamics. As main perturbations, we can mention magnetic impurities between the kagome planes and a finite out-of-plane Dzyaloshinskii-Moriya component (Dz~0.06J). With a comprehensive ¹⁷O NMR study on a high-quality single crystal, we were able to measure the intrinsic (kagome) low-temperature static spin susceptibility and spin dynamics, and we show unambiguously that the excitation spectrum is gapless, at variance with valence bond crystal states and numerous spin liquid candidates that are characterized by gapped excitations. Moreover, the disclosed static spin susceptibility restores some convergence with a fermionic Dirac cone model now advocated for in most numerical works. To shed another light on the low energy density of states, we measured the specific heat on high-quality single crystals. The kagome intrinsic specific heat is obtained in high magnetic fields only: at low magnetic fields, another dominant contribution prevents any conclusive analysis. Remarkably, the kagome specific heat is found to be almost field-independent up to 34 T. This seems to be at odds with a fermionic nature for the excitations, which in such a case would form field-induced Fermi pockets. Yet, as expected for Dirac particles, the density of states appears to vanish in the zero-temperature limit. Although the exact nature of the excitations remains unclear, our results severely constrain future theoretical developments. Further, thanks to NMR and specific heat, we were also able to identify field-induced transitions or crossovers at very low temperatures, and we present a tentative phase diagram. We also used µSR to investigate the robustness of the spin liquid state, which extends at least up to 28 kbar. Second, we introduce two new closely related materials, that implement interesting variants of herbertsmithite: the insulating yttrium kapellasites YCu₃(OH)₆OxCl₃-x, with compositions x=0 and x=1/3. These two compounds are fortuitous outcomes of the ongoing – and still unsuccessful – effort for doping herbertsmithite, by replacing the divalent zinc cations by monovalent or trivalent cations. In yttrium kapellasites, copper ions also build a kagome lattice with antiferromagnetic in-plane couplings, and the structure forbids interlayer impurities. The ground state properties are determined using bulk thermodynamics and µSR on powdered samples. The x=0 compound has a perfect kagome geometry but displays an ordered ground state, which is attributed to a large Dzyaloshinskii-Moriya anisotropy. The x=1/3 compound has a slightly distorted kagome geometry displaying anisotropic interactions, but exhibits features of spin liquid physics. Yet, we show with a comparative ³⁵Cl NMR study, that single crystal samples of the latter material actually develop static magnetism at variance with the polycrystalline samples. We conclude that Dzyaloshinskii-Moriya anisotropy and/or the modified kagome magnetic lattice in the x=1/3 compound are likely at the origin of the low-temperature magnetic transition
Serret, Emmanuelle. "Etude de réseaux de nanojonctions Josephson : compétition entre le champ magnétique et la géométrie". Université Joseph Fourier (Grenoble), 2002. http://www.theses.fr/2002GRE10164.
Pełny tekst źródłaOrain, Jean-Christophe. "Frustration géométrique et nouveaux états quantiques de spins dans les composés vanadates fluorés à géométrie kagomé". Thesis, Université Paris-Saclay (ComUE), 2015. http://www.theses.fr/2015SACLS113/document.
Pełny tekst źródłaThe search for quantum liquid state is a very active field in condensed matter research. In two dimensions, the antiferromagnetic spin 1/2 kagome lattice seems to be the most able to stabilize such a ground state. Indeed, from recent theoretical investigations, we are now quite sure that this model has a quantum spin liquid ground state. However, we still do not know its nature, in particular the nature of its correlations. They could be short ranged with a gap in the excitation spectrum, or long ranged with a gapless excitation spectrum. On the experimental side, only few materials exist and only one possesses a geometrically perfect lattice, the Herbertsmithite. All the experiments that have been done on this compound reveal a gapless spin liquid state along with deviations to the spin 1/2 Heisenberg hamiltonian which could be responsible of the gap closure.This thesis deals with the experimental study, mainly by NMR and µSR, of new vanadium based kagomé compounds which are part of a newly synthesized family, the kagome fluoride vanadates. The material that we studied the most is a spin 1/2 kagomé compound based on V4+, (NH4)2[C7H14N][V7O6F18] (DQVOF). The magnetic model of this compound can be decomposed in two rather independent parts, trimerized kagome planes and quasi paramagnetic V3+ ions. The µSR studies, showing the absence of frozen moment down to 20 mK, reveal a spin liquid ground state in DQVOF. The heat capacity and the NMR experiments point out a gapless behavior despite trimerization and likely weak Dzyaloshinskii Moriya interactions. Our results demonstrate that the gapless ground state, whether intrinsic or due to deviation to the ideal hamiltonian, is a rather robust characteristic of kagome materials.Furthermore, we studied another compound of this family, (NH4)2[C2H8N][V3F12] (DDVF), which magnetic lattice is made of uncoupled kagomé planes based on V3+ (S = 1). The lattice shows large deviations to the ideal kagomé and the thermodynamic experiments and the µSR studies reveal a magnetic transition to a frozen state at 10 K with a long distance order which is effective only below 6 K
Renaud, Denis. "Caractérisation du propulseur PEGASES : diagnostics du filtre magnétique et du faisceau : optimisation de la géométrie". Thesis, Orléans, 2016. http://www.theses.fr/2016ORLE2018/document.
Pełny tekst źródłaThe PEGASES ion thruster differs from standard electric propulsion technologies through its use of electronegative gases, such as SF₆, as a propellant. Its operation relies on the trapping of electrons using a magnetic field and the creation of a plasma dominated by positive and negative ions. These ions are alternately accelerated to produce thrust, and later undergo a recombination to ensure beam neutrality. This thruster eliminates the need for an electron-producing neutralizer, which is a standard feature in other sources such as the Hall thruster. This thesis is divided into three parts. The first describes the development and implementation of a new EXB probe for the study of the ion beam properties, the identification of the beam chemical composition and the verification of the presence of negative and positive ion species. The second part concerns the design and application of a new laser photodetachment diagnostic for the measurement of the negative ion fraction. Lastly, a new ion-ion thruster with a circular geometry, known as AIPE, has been designed, constructed and successfully tested. This prototype eliminates the plasma asymmetry present in PEGASES and reveals the importance of the magnetic filter to source operation
Laribi, Elias. "Modélisation numérique de l'impact de la géométrie magnétique sur le plasma de bord des tokamaks". Electronic Thesis or Diss., Aix-Marseille, 2021. http://www.theses.fr/2021AIXM0298.
Pełny tekst źródłaThis phD work has shown that, in a limiter configuration, the elongation of magnetic surfaces has a stabilizing effect with respect to the edge plasma turbulence. This leads to a reduction of the edge plasma width when the elongation increases. Regarding triangularity, this phD work revealed that its effects on the edge plasma were weaker than those of elongation. A better understanding of these trends has been possible thanks to a theoretical work on a simplified interchange model. This work has also revealed the importance of the magnetic field spatial variability on the poloidal size of turbulent structures and therefore on the radial turbulent fluxes. In a second step, the study of a divertor configuration with one X point has shown that the the magnetic field spatial variability was so important that it could be one of the reasons that explain the enhancement of the low field side ballooned nature of edge plasma turbulence in a divertor geometry. Still concerning the divertor geometry, this phD also revealed that the internal material receives more heat compared to the external material when the toroidal magnetic field is oriented in the direct direction (= magnetic drift of the ions pointing towards the X point). A possible explanation of this observation is the existence of a significant plasma circulation from the low field side to the high field side at the X point that we observed in this configuration. Finally, we were able to confirm that one way to improve this heat flux distribution on the walls was to merge two X points perfectly (= ideal snowflake proposed in 2007 by Ryutov and Sukhanovsky)
Olariu, Areta. "Antiferromagnétisme frustré à deux dimensions : de la géométrie triangulaire dans NaCrO2 à la géométrie kagomé dans Cu3Zn(OH)6Cl2". Phd thesis, Université Paris Sud - Paris XI, 2007. http://tel.archives-ouvertes.fr/tel-00268177.
Pełny tekst źródłaNous présentons l'étude par RMN et µSR du composé triangulaire à côtés partagés NaCrO2, un système Heisenberg modèle de spin 3/2. Nous mettons ainsi en évidence un régime de fluctuations très étendu en dessous d'un maximum de chaleur spécifique, signature d'excitations originales spécifiques à cette classe de systèmes.
Nous présentons ensuite l'étude de l'Herbertsmithite ZnCu3(OH)6Cl2, première réalisation expérimentale d'une structure kagomé parfaite de spin 1/2. Nous montrons par µSR que dans la famille plus large des Paratacamites ZnxCu4-x(OH)6Cl2, lorsque x passe de 0 à 1, le système évolue à basse température d'un état ordonné à longue distance vers un état fluctuant, type liquide de spin, pour x>0.66. Les mesures de RMN de l'oxygène 17 dans l'Herbertsmithite nous ont permis d'isoler la contribution intrinsèque de celle des défauts issus des substitutions Cu/Zn. L'évolution de la susceptibilité, ainsi que les mesures de relaxation démontrent l'absence de gap dans le spectre des excitations magnétiques. Nos mesures suggèrent que ces excitations ont un poids constant sur toute la zone de Brillouin, comme prédit théoriquement. L'évolution de la raie des défauts est en accord qualitatif avec l'image théorique de la localisation de singulets non magnétiques autour de l'impureté.
Gardiennet-Doucet, Carole. "Exploitation de l'anisotropie de déplacement chimique pour l'étude de la géométrie moléculaire et des contacts intermoléculaires par résonance magnétique nucléaire à l'état solide". Nancy 1, 2005. http://www.theses.fr/2005NAN10010.
Pełny tekst źródłaThis work presents various methodologies to probe the molecular geometry and the nature of intermolecular contacts in organic and inorganic solids. The first part gives a brief description of basic principles of solid-state NMR that are used through these studies. In the second part, the variation of chemical shielding tensor principal values of carboxy carbon-13 and phosphate phosphorus-31 upon deprotonation is correlated with ionisation state, conformation and nature of the intermolecular interactions. The third part deals with the characterisation of the strong hydrogen bond involving the silanols of sodium hydrous polysilicates, using complementary methods such as 1H-29Si cross-polarisation dynamics and dipolar modulation of the 29Si chemical shift anisotropy at slow spinning frequency
Heitz, Millet Annie. "Étude conformationnelle par résonance magnétique nucléaire et le programme de calcul distance géométrie (DISGEO) d'un nouvel inhibiteur de trypsine extrait d'Ecballium elaterium". Montpellier 2, 1988. http://www.theses.fr/1988MON20197.
Pełny tekst źródłaRenault, Christophe. "Courbures et lignes de crête sur des images en niveaux de gris : Etude comparative et application aux sillons corticaux". Caen, 2001. http://www.theses.fr/2001CAEN2066.
Pełny tekst źródłaFillard, Pierre. "Riemannian processing of tensors for diffusion MRI and computational anatomy of the brain". Nice, 2008. http://www.theses.fr/2008NICE4002.
Pełny tekst źródłaLes matrices symétriques et définies positives, ou tenseurs, sont aujourd'hui fréquemment utilisées en traitement et analyse des images. Leur importance a été mise à jour avec l'apparition récente de l'IRM du tenseur de diffusion (ITD) et de l'anatomie algorithmique (AA). Cependant, il est difficile de travailler avec : la contrainte de positivité doit être satisfaite à tout prix, ce qui n'est pas garanti avec les opérations matricielles standard. Dans ce travail, nous proposons deux alternatives au calcul euclidien sur les tenseurs. Au lieu de voir l'espace des tenseurs comme un espace vectoriel, nous le considérons comme une variété, i. E. , un espace courbe et lisse. Grâce à la géométrie riemannienne, il est alors possible de " déplier " cet espace et de généraliser aux tenseurs toute opération avec des implémentations étonnamment simples. Dans un deuxième temps, nous passons en revue les applications de tels cadres de calcul en ITD clinique et en AA du cerveau. En ITD, nous montrons qu'il est possible de traiter de manière optimale des données très bruitées typiques d'acquisitions cliniques, et de produire des reconstructions de fibres plausibles. En AA du cerveau, nous montrons qu'en considérant des repères anatomiques simples - les lignes sulcales - il est possible de mesurer précisément la variabilité interindividuelle du cortex. Finalement, nous développons un cadre nouveau pour étudier les corrélations anatomiques entre régions du cerveau, et présentons des résultats jusqu'à maintenant inconnus de dépendances entre sillons symétriques, et entre sillons à priori non reliés, soulevant ainsi de nouvelles questions sur l'origine de telles dépendances statistiques
St-Onge, Etienne. "Analyse et modélisation de la surface corticale et de l'architecture sous-jacente des axones". Mémoire, Université de Sherbrooke, 2016. http://hdl.handle.net/11143/8778.
Pełny tekst źródłaScotto, d'Abusco Manuel. "Modélisation numérique du transport turbulent cœur-bord dans un tokamak en géométrie réaliste par une méthode numérique avancée". Electronic Thesis or Diss., Aix-Marseille, 2022. http://www.theses.fr/2022AIXM0173.
Pełny tekst źródłaNowadays a challenge remains the design of optimized plasma scenarios for tokamak operation to control the heat flow from the core region to the wall. This calls for the development of efficient and reliable numerical codes with predictive capabilities for plasma simulations. The present work aims to develop a high-order finite elements code based on hybrid discontinuous Galerkin numerical scheme and an efficient implicit time integration method for solving non isothermal Braginskii reduced fluid equations in versatile tokamak and magnetic equilibrium geometries. The use of such numerical scheme allows to perform simulations with time evolving magnetic configurations, avoiding expensive re-meshing of the computational domain. The structure and the realization of such a numerical tool is presented. The feasibility of the latter is then investigated through a careful validation and benchmarking operation with SolEdge3X. Self-consistent sources of particle, due to plasma recycling, and energy due to Ohmic heating are introduced to perform 2D simulation of a full poloidal tokamak cross section. With such a model the main features of a detached plasma are investigated for the WEST tokamak machine. The first core-edge transport simulations of an entire WEST discharge (shot #54487) are shown from the start-up phase to the final plasma landing. Comparisons between experimental interferometry and synthetic simulation data show a remarkable agreement. The time evolution of the particles and heat fluxes at the wall, are analyzed and exploited to assess the tungsten sputtering, using both a simple cinematic model and the impurity tracker monte-carlo code ERO2.0
Matthieu, Branellec. "Impact du mode de propagation des fronts orogéniques sur la géométrie, la localisation et la chronologie de la déformation : Cas du Bassin de Neuquén, (Argentine)". Thesis, Pau, 2014. http://www.theses.fr/2014PAUU3042/document.
Pełny tekst źródłaThis PhD project deals with multiscale record of the tectonic signal in the Malargüe fold-and-thrust-belt (MFTB) located in the northern part of the Neuquén basin (Argentina). The first results presented rely on the study of the macroscopic finite strain in the fold-belt and the characterization of the active deformation of the San Rafael Block uplift. The cross-sections we produce show that structural inheritance related to the Jurassic extension is the main parameter controlling the belt structure. In addition we proposed that the building mechanisms that controlled the MFTB evolution by Miocene times are the same than those triggering the present day San Rafael block uplift. The second part of this work is dedicated to mesoscopic strain pattern analysis recorded by fracture networks. Throughout the MFTB, we are able to describe the occurrence of four main fractures sets emplaced in several stress regime that are linked (1) to the inheritance and (2) to the well-known compression phases from pre-folding to syn-folding settings. Finally the third part of this work describes the microscopic damage recorded by the anisotropy of magnetic susceptibility method. We mainly evidence that there is no clear gradient of magnetic fabrics from foreland to hinterland and that deformation is compartmentalized by structural inheritance. This atypical pattern of magnetic fabrics succession reveals that the matrix damage is governed by the same strain distribution as those observed at macroscopic scale thus providing a supplementary argument to consider the Andean system at these latitudes as singularly different from a classical Coulomb wedge
Borghesani, Valentina. "The neuro-cognitive representation of word meaning resolved in space and time". Thesis, Paris 6, 2017. http://www.theses.fr/2017PA066091/document.
Pełny tekst źródłaOne of the core human abilities is that of interpreting symbols. Notwithstanding decades of neuropsychological and neuroimaging work on the cognitive and neural substrate of semantic representations, many questions are left unanswered. The research in this dissertation attempts to unravel one of them: are the neural substrates of different components of concrete word meaning dissociated? In the first part, I review the different theoretical positions and empirical findings on the cognitive and neural correlates of semantic representations. Crucially, I propose an operational distinction between motor-perceptual dimensions (i.e., those attributes of the objects referred to by the words that are perceived through the senses) and conceptual ones (i.e., the information that is built via a complex integration of multiple perceptual features). In the second part, I present the results of the studies I conducted in order to investigate the automaticity of retrieval, topographical organization, and temporal dynamics of motor-perceptual and conceptual dimensions of word meaning. The results suggest that the neural substrates of different components of symbol meaning can be dissociated in terms of localization and of the feature of the signal encoding them, while sharing a similar temporal evolution
Lenglet, Christophe. "Geometric and variational methods for diffusion tensor MRI processing". Nice, 2006. http://www.theses.fr/2006NICE4083.
Pełny tekst źródłaThis thesis deals with the development of new processing tools for Diffusion Tensor Magnetic Resonance Imaging (DT-MRI). This recent MRI technique is of utmost importance to acquire a better understanding of the brain mechanisms and to improve the diagnosis of neurological disorders. We introduce new algorithms relying on Riemannian geometry, partial differential equations and front propagation techniques. The first part of this work is theoretical. After a few reminders about the human nervous system, MRI and differential geometry, we study the space of multivariate normal distributions. The introduction of a Riemannian structure on that space allows us to define statistics and intrinsic numerical schemes that will constitute the core of the algorithms proposed in the second part. The properties of that space are important for DT-MRI since diffusion tensors are the covariance matrices of normal laws modeling the diffusion of water molecules at each voxel of the acquired volume. The second part of this thesis is methodological. We start with the introduction of original approaches for the estimation and regularization of DT-MRI. We then show how to evaluate the degree of connectivity between cortical areas. Next, we introduce a statistical surface evolution framework for the segmentation of those images. Finally, we propose a non-rigid registration method. The last part of this thesis is dedicated to the application of our tools to two important neuroscience problems: the analysis of the connections between the cerebral cortex and the basal ganglia, implicated in motor tasks, and the study of the anatomo-functional network of the human visual cortex
Amalric, Marie. "Etude des mécanismes cérébraux d'apprentissage et de traitement des concepts mathématiques de haut niveau". Thesis, Paris 6, 2017. http://www.theses.fr/2017PA066143/document.
Pełny tekst źródłaHow does the human brain conceptualize abstract ideas? In particular, what is the origin of mathematical activity, especially when it is associated with high-level of abstraction? Is mathematical thought independent of language? Cognitive science has now started to investigate this question that has been of great interest to philosophers, mathematicians and educators for a long time. While studies have so far focused on basic arithmetic processing, my PhD thesis aims at further investigating the cerebral processes involved in the manipulation and learning of more advanced mathematical ideas. I have shown that (1) advanced mathematical reflection on concepts mastered for many years does not recruit the brain circuits for language; (2) mathematical activity systematically involves number- and space-related brain regions, regardless of mathematical domain, problem difficulty, and participants' visual experience; (3) non-verbal acquisition of geometrical rules relies on a language of thought that is independent of natural spoken language. Finally, altogether these results raise new questions and pave the way to further investigations in neuroscience: - is the human ability for language also irrelevant to advanced mathematical acquisition in schools where knowledge is taught verbally? - What is the operational definition of the fields of “mathematics” and “language” at the brain level?
Machat, Mohamed. "Computational geometry for the determination of biomolecular structures". Thesis, Paris 6, 2017. http://www.theses.fr/2017PA066359/document.
Pełny tekst źródłaStructural biology has allowed us expand our knowledge of living organisms. It is defined as the investigation of the structure and function of biological systems at the molecular level. Studying a biomolecule's structure offers insight into its geometry, as angles and distances between the biomolecule's atoms are measured in order to determine the biomolecular structure. The values of these geometrical parameters may be obtained from biophysical techniques, such as X-ray crystallography or nuclear magnetic resonance (NMR) spectroscopy. One of the most used methods to calculate protein structures from geometric restraints is simulated annealing. This method does not guarantee an exhaustive sampling of protein conformational space, which is a shortcoming as one protein may adopt multiple functional conformations, and it is important to determine them exhaustively. In this PhD project, the efficiency of a new method - derived from operations research and computational geometry - is studied in order to answer this question: How does this method explore the conformational spaces of small proteins? This method - implemented within the iBPprot software framework - treats protein structure determination as a distance geometry problem, which the interval branch-and-prune algorithm tries to solve by the full exploration of its solutions space. The results obtained by iBPprot on a set of test proteins, with sizes ranging from 24 to 120 residues and with known structures, are analyzed here. Using short-range exact distance restraints, it was possible to rebuild the structure of all protein targets, and for many of them it was possible to exhaustively explore their conformational spaces. In practice, it is not always possible to obtain exact distance restraints from experiments. Therefore, this method was then tested with interval data restraints. In these cases, iBPprot permitted the sampling of the positions of more than 70% of the atoms constituting the protein backbone for most of the targets. Furthermore, conformations whose r.m.s. deviations closer than 6 Angstrom to the target ones were obtained during the conformational space exploration. The quality of the generated structures was satisfactory with respect to Ramachandran plots, but needs improvement because of the presence of steric clashes in some conformers. The runtime for most performed calculations was competitive with existing structure determination method
Robert, Julien. "Systèmes magnétiques à frustration géométrique : approches expérimentale et théorique". Phd thesis, Grenoble 1, 2007. http://www.theses.fr/2007GRE10192.
Pełny tekst źródłaLn this thesis, the magnetic properties of geometrically frustrated systems have been studied, using experimental and theoretical approaches. Ln the manuscript, the study of magnetic lattices based on corner-sharing triangles is reported in three different parts. The first part concerns the La3Cuz VOg compound, formed by weakly coupled frustrated planar clusters, each one being constituted of 9 coupled spins 1/2. Ln this system, different regimes are successively stabilized when the temperature is decreased: the high temperature paramagnetic regime of individual spins is followed by a paramagnetic regime of collective pseudo-spins 1/2 associated to each cluster below 20 K. Finally, short range inter-cluster correlations emerge below 2 K, indicating a hierarchical rise of the correlations. The following parts are dedicated to the study of the dynamical properties of the kagomé lattice. We first show that the langasite compound Nd3GasSi014, in which the anisotropic magnetic moments carried by the Nd3+ ions form a kagomé lattice, does not present any magnetic ordering down to 2 K, despite a Curie- Weiss temperature of a few tens of Kelvin. Moreover, we could observe a slowing down of the spin fluctuations below 300 K. Finally, we present a numerical study of the spin dynamics of the Heisenberg antiferromagnet on the kagomé lattice. We show that unexpected coherent excitations develop below T/J=O. OI, in spite of the very short spin-spin correlation length in the system. Ln addition, sorne of the excitations are characterized by a non-uniform distribution of spectral weight. This is understood as an effect of the particular geometry of this network
Albuquerque, Gonçalo Marcos Baia︣o de. "Précession de l'aimantation en géométrie confinée : aspects physiques et numériques". Paris 11, 2002. https://tel.archives-ouvertes.fr/tel-00002175.
Pełny tekst źródłaThis work starts with a derivation of brown's and the landau-lifshitz-gilbert equations from a general variational approach and their rewriting in a dimensionless invariant form. We proceed with the description of the precessional dynamics of a thin film in the single spin, or macrospin, limit. We present a thorough analysis of its switching phase diagram, laying emphasis on the determination of ballistic trajectories corresponding to ringing-free switching events in a minimal time. These are intimately connected with a precessional motion around the demagnetising field arising from the initial out-of-plane magnetisation motion. On the other hand, transverse susceptibility numerical experiments in s state 500 nm x 250 nm x 5 nm platelets allow to quantitatively establish the degree of non-macrospin like behaviour in inhomogeneous magnetisation distributions through the recalculation of the phenomenological damping parameter. We consider next the detailed characteristics of optimised switching events mimicking the macrospin ballistic trajectories. These are based on an efficient two-pulse time-shifted procedure leading to a controlled and coherent fast precessional switching in submicron-sized platelets. We tackle next the impact of real world parameters (such as edge roughness, artificial antiferromagnetic magnetostatic coupling, and structural disorder) on the switching properties of the platelets under study. In the appendices we propose a self-contained description of the numerical procedures employed in this work, focusing on their innovative contributions: the damping parameter recalculation as a means of quantifying time domain errors, the development of an optimised semi-implicit crank-nicolson integration scheme for the landau-lifshitz-gilbert equation, and an analysis of the effects of spatial discretisation on the dynamics of small-scale magnetisation distributions
Toussaint, Grégoire. "Apport des écoulements secondaires et pulsés et de la vélocimétrie IRM à la filtration de fluides biologiques". Compiègne, 2000. http://www.theses.fr/2000COMP1310.
Pełny tekst źródłaBenoit, Jérôme. "Symétrie, géométrie, topologie et spins : spins de Heisenberg à la limite continue, membranes magnétiques". Cergy-Pontoise, 1999. http://www.theses.fr/1999CERG0076.
Pełny tekst źródłaMachat, Mohamed. "Computational geometry for the determination of biomolecular structures". Electronic Thesis or Diss., Paris 6, 2017. http://www.theses.fr/2017PA066359.
Pełny tekst źródłaStructural biology has allowed us expand our knowledge of living organisms. It is defined as the investigation of the structure and function of biological systems at the molecular level. Studying a biomolecule's structure offers insight into its geometry, as angles and distances between the biomolecule's atoms are measured in order to determine the biomolecular structure. The values of these geometrical parameters may be obtained from biophysical techniques, such as X-ray crystallography or nuclear magnetic resonance (NMR) spectroscopy. One of the most used methods to calculate protein structures from geometric restraints is simulated annealing. This method does not guarantee an exhaustive sampling of protein conformational space, which is a shortcoming as one protein may adopt multiple functional conformations, and it is important to determine them exhaustively. In this PhD project, the efficiency of a new method - derived from operations research and computational geometry - is studied in order to answer this question: How does this method explore the conformational spaces of small proteins? This method - implemented within the iBPprot software framework - treats protein structure determination as a distance geometry problem, which the interval branch-and-prune algorithm tries to solve by the full exploration of its solutions space. The results obtained by iBPprot on a set of test proteins, with sizes ranging from 24 to 120 residues and with known structures, are analyzed here. Using short-range exact distance restraints, it was possible to rebuild the structure of all protein targets, and for many of them it was possible to exhaustively explore their conformational spaces. In practice, it is not always possible to obtain exact distance restraints from experiments. Therefore, this method was then tested with interval data restraints. In these cases, iBPprot permitted the sampling of the positions of more than 70% of the atoms constituting the protein backbone for most of the targets. Furthermore, conformations whose r.m.s. deviations closer than 6 Angstrom to the target ones were obtained during the conformational space exploration. The quality of the generated structures was satisfactory with respect to Ramachandran plots, but needs improvement because of the presence of steric clashes in some conformers. The runtime for most performed calculations was competitive with existing structure determination method
Darwaz, Khamsa. "Etude expérimentale et en simulation comportementale des"pertes fer"dans des circuits magnétiques de géométrie simple". Lyon, INSA, 1995. http://www.theses.fr/1995ISAL0016.
Pełny tekst źródła[Energy losses are constituting one of the principal limitations met with energy transformers and electrical machines construction. In the mean time, electronic converters are imposing new working conditions to magnetic circuits, that classical calculation methods can't easily take into account. The work exposed in this paper is about the development of adapted tools, designed to respond to these new constraints. We have chosen a technical representation that globally describes the dynamic behavior and evaluates iron losses on a circuit scale. The behavioral model used allows us to estimate with a good accuracy the losses in ferromagnetic materials, under various excitation conditions. The simulations are running on persona! computer and calculation time takes about a minute. The parameters used in the mode! are automatically and quickly obtain from UNIX station using a limited number of experimental data. These parameters are calculate once and remain constant throughout the simulations. The conditions and the validity's limits of the described method are evaluate using the mode! Accuracy, obtained by comparing results with a direct measure realised in a laboratory under the most widely encountered electrical conditions. ]
Marthouret, Frank. "Développement de modèles dynamiques comportementaux de circuits magnétiques : prise en compte de l’hystérésis et de la géométrie". Lyon, INSA, 1995. http://www.theses.fr/1995ISAL0098.
Pełny tekst źródłaModelling of the properties of magnetic circuits needs to take into account hysteresis, dynamical phenomena, and the geometry of the magnetic circuit. A behavioural model has been developed in the CEGELY. This model takes into account hysteresis and dynamic phenomena but is limited to simple geometries such as toroid circuits. In despite of good performances, this model presents important shortcomings in the context of electro technology. We propose new improvements of the model in order to increase its performances and which allow to take into account more complex geometries. In a first time, we have developed a step by step programmation of the Preisach model, model used to generate static hysteresis loops. The Preisach model has been improved in order to predict accurately magnetic characteristics at low peak flux density. In a second time, we have introduced a more realistic representation of the magnetic field inside the magnetic circuit. Hence, electrical and magnetic characteristics are predicted with a good accuracy, even in the case of fast transient operations. In a third time, we have studied the case of complex geometries. The magnetic circuit is divided into elementary circuits and the resulting model predicts the magnetic and electrical behaviour of geometries involving several limbs. The simulated results obtained agree satisfactorily with the experimental data. This study offers new prospect for the optimisation of industrial applications, by the use of an accurate and fast behavioural model of magnetic circuits
Raoux, Arnaud. "Magnétisme orbital et aspects géométriques de la théorie des bandes". Thesis, Université Paris-Saclay (ComUE), 2017. http://www.theses.fr/2017SACLS041/document.
Pełny tekst źródłaMy research project has been to studythe orbital magnetic response of a electron gas inthe periodic potential of a crystal. Its purpose isto generalize Landau’s diamagnetism and Peierls’formula for one-band crystals. The main goal wasto generalize Peierls’ work to any number of bands.Then, I applied the obtained formula to 2-bandsystems in order to highlight the role of interbandeffects in the orbital susceptibility. The susceptibilitycan be written using Berry curvature, quantityassociated to interband effects, as well as the metrictensor. In particular, I show that a band isulatorcan have a non-vanishing magnetic responseeven if the chemical potential lies in the gap. Moreover,I study a model where the geometric propertiescan be tuned without changing the dispersionrelation. This tuning can drastically modifythe orbital magnetic response
Ottenwaelder, Xavier. "Coupleurs ferromagnétiques : aspects géométrique, topologique et électrochimique". Paris 11, 2001. http://www.theses.fr/2001PA112326.
Pełny tekst źródłaThis work concerns the synthesis and the study of dinuclear complexes capable of anchoring other complexes at their periphery to form polynuclear compounds. Two pathways towards the target to input of a ferromagnetic coupling in the initial complexes have been developed. The first part is dedicated to the static approach where the properties of the complexes cannot be tuned after synthesis. 1) A dinucleating ligand whereby the two magnetic orbitals are orientated in a quasiperpendicular fashion leads to a feeble ferromagnetic coupling. With this synthon hexanuclear complexes were obtained. 2) The topological aspect of the interaction is inspected through a mecanism based on spin polarisation. Biscopper(II) complexes in meta or para positions of benzenic rings lead to differently coupled systemes. The ions are highly coupled despite the large separation between them. Polynuclear compounds containing 6, 9 and 16 Cu(II) ions have been isolated from the meta substituted derivatives and they all retain the ferromagnetic coupling. .
Arsigny, Vincent. "Traitement de données dans les groupes de Lie : une approche algébrique. Application au recalage non-linéaire et à l'imagerie du tenseur de diffusion". Phd thesis, Ecole Polytechnique X, 2006. http://tel.archives-ouvertes.fr/tel-00121162.
Pełny tekst źródłaMelchy, Pierre-Eric. "Frustration géométrique: le cas des antiferromagnétiques triangulaires". Grenoble, 2010. http://www.theses.fr/2010GRENY041.
Pełny tekst źródłaThis doctoral dissertation presents a thorough determination of the phase diagrams of classical Heisenberg triangular antiferromagnet (HTAF) and its anisotropic variants based on theoretical and numerical analysis (Monte Carlo). At finite-field HTAF exhibits a non-trivial interplay of discrete Z3 symmetry and continuous S1 symmetry. They are successively broken (discrete then continuous) with distinct features at low and high fields: in the latter case the ordering is along transverse direction; in the former case an intermediate collinear phase is stabilised before 120-degree structure is. Due to zero-field behaviour, transition lines close at (T,h) = (0,0). Single-ion anisotropy is here considered. Easy-axis HTAF for moderate anisotropy strength 0 < d
Bono, David. "Etude par RMN et MuSR des composés antiferromagnétiques fortement frustrés à géométrie de bicouches kagomé". Phd thesis, Université Paris Sud - Paris XI, 2004. http://tel.archives-ouvertes.fr/tel-00008532.
Pełny tekst źródłaOuedraogo, Edgard. "Caractérisation géométrique et acoustique de la diaphyse osseuse par tomographie ultrasonore". Cergy-Pontoise, 2002. http://www.theses.fr/2002CERG0163.
Pełny tekst źródłaOur objective is to derive quantitative sound speed images of cortical bone using ultrasonic transmission tomography. The classical acquisition procedure of projection data is modified: the transducers are orientated according to Snell's law of refraction in the aim of optimizing sound propagation as parallel longitudinal rays inside bone. This strategy allows the subsequent application of straight-ray reconstruction by the backprojection technique, which is a classical procedure in X ray tomography. The method has been validated with Plexiglas® solid cylinders and tubes immersed in water. Improved sound velocity images are then derived using conventional Radon transform of experimental time-of-flight data. The method is then extended to in vitro human femur immersed in water. The procedure leads to significant restoration enhancement over the non corrected image. These preliminary quantitative images using combined reflected and transmission ultrasound are promising results for bone imaging
Amice, Marc. "Le complexe granitique de Cabeza de Araya (Estrémadure, Espagne) : zonation, structures magmatiques et magnétiques, géométrie, discussion du mode de mise en place". Toulouse 3, 1990. http://www.theses.fr/1990TOU30115.
Pełny tekst źródłaChapuis, Yann. "Frustration géométrique, transitions de phase et ordre dynamique". Phd thesis, Grenoble 1, 2009. http://www.theses.fr/2009GRE10130.
Pełny tekst źródłaThis PhD thesis is about geometrically frustrated magnetic materials which crystallise in the pyrochlore structure, made of corner sharing regular tetrahedra. Firstly, we show that the growth of the spin liquid Tb2Ti2O7 presents many difficulties : some physical properties are sample dependent. Anyhow, two characteristic temperatures (2 and 50 K) are deduced by different experimental techniques, irrespective of the sample. Then, about the ordered spin ice Tb2Sn2O7, the study of spin dynamics suggests the existence of a complex relaxation mechanism, characterized by several fluctuation times. One of them is tau ≈ 10^{-10} s. Microscopic and macroscopic measurements carried out on these two compounds show that the lowest two crystal field level are singlets, and they are separated by an energy of about 2 K. About the XY antiferromagnet Er2Ti2O7, some results suggest the existence of exitations, responsible for low temperature spin dynamics. Finally, there inconsistencies between the interpretation of some experimental results about the existence of a low temperature spin dynamics in the Heisenberg antiferromagnet Gd2Sn2O7
Chapuis, Yann. "Frustration géométrique, transitions de phase et ordre dynamique". Phd thesis, Grenoble 1, 2009. http://tel.archives-ouvertes.fr/tel-00463643.
Pełny tekst źródłaPerrin, Yann. "Réseaux artificiels à frustration géométrique". Thesis, Université Grenoble Alpes (ComUE), 2016. http://www.theses.fr/2016GREAY054/document.
Pełny tekst źródłaSince a decade, a big interest has grown about geometrically frustrated nanomagnets arrays. They allow experimental realisation of theoretical spin models, that sometimes have no natural counterpart. In addition to their high flexibility, these networks provide a direct access to local spin configurations. The work presented in this manuscript aims to realise experimentally the so-called "square ice" model, using nanomagnets array. This highly frustrated model has already been theoretically studied. It is found to have a massively degenerated ground state, associated to a residual entropy at low temperature. In this thesis, we present two approaches that should achieve the square ice model.The first one consists to introduce additional magnets in the conventional square nanomagnets array. They can act through an effective coupling between the magnets of the initial network. The condition required is that additional magnets should behave passively against main magnets. We will show in a theoretical study that a reciprocal space analysis of the Hamiltonian fails to grasp the essential properties of the new model. By computing the energy of a number of random spins configurations, we will show that the square ice model can be achieved. A special attention will be paid to the influence of the range and the nature of magnets interactions. Using finite-difference micromagnetic simulations, we will determine the geometries adapted to an experimental realisation. We were able to make such networks using the microfabrication tools available in the laboratory. Our magnets are made of thin permalloy films. This allows the magnets to reach a superparamagnetic regime during a thermal annealing. We observed that additional magnets have the expected influence on square nanomagnets arrays. However, an experimental bias caused a high residual magnetisation in some networks. This effect has unfortunately hided the correlations expected in the square ice model. But this work shown an unexpected effect. Additional magnets appear to enhance thermal fluctuations in the networks.The second approach is to raise the magnets oriented in one of the two directions of the square lattice. For the study of this new system, a methodology similar to that described above will be used. Theoretically, the control of the elevation can explore three different spins models, including the square ice model. We made arrays for different elevations, estimated by micromagnetic calculations. For technical reasons, we worked with thick permalloy nanomagnets. Their thickness makes them insensitive to thermal fluctuations. Fluctuations are then introduced using a decreasing rotating magnetic field. Our simulations show that this particular dynamic stimulates emergence of ferromagnetic correlations. The field demagnetisation then reduce the elevations required for achieving the square ice model. We have experimentally observed that the elevation has an effect perfectly consistent with our previsions. Experimental structure factors show that we succeed to achieve the square ice model with nanomagnets. This approach allowed us to observe, for the first time, a Coulomb phase in the real space. This work opens interesting perspectives for studying the excitations of this phase. It has previously been shown that they are similar to classical magnetic monopoles
Borghesani, Valentina. "The neuro-cognitive representation of word meaning resolved in space and time". Doctoral thesis, Università degli studi di Trento, 2017. https://hdl.handle.net/11572/367587.
Pełny tekst źródłaBolteau, Blandine. "La magnétisante histoire de la goutte fakir ou étude des propriétés de mouillage de surfaces superhydrophobes à géométrie magnétiquement modulable". Thesis, Sorbonne université, 2018. http://www.theses.fr/2018SORUS062/document.
Pełny tekst źródłaDuring this thesis, we have developped superhydrophobic surfaces whose wettability can be controlled by an external magnetic stimulus. Formulating a network of elastomeric and magnetic micro-pillars with high aspect ratio allows the orientation of the pillars through magnetic forces, hence an adaptable surface roughness. Moreover, modulating the geometry, elasticity and magnetization of pillars allowed us to highlight the following conclusions.We have seen first that in agreement with the literature, without magnetic field, the wetting hysteresis increased with the surface fraction. However, it remains constant varying the elasticity of pillars. This conclusion is confusing, because at the pillar scale, there is indeed a difference of mobility between rigid and flexible pillars due to the force exerted by the triple line.We then demonstrated that the deflexion of the pillars can change significantly the sliding angle due to the applied magnetic field. Moreover, sliding of the droplet on such a surface is promoted when pillars are deflected against the slope.Finally, we managed to control the displacement of a droplet on a surface which is tilted with an angle below the sliding angle : it moves forward from the surface only if magnetic actuation is applied. This surfaces will be an attractive source of study in order to understand how to modulate wetting and liquid flow in fakir state
Chabert-Pelline, Corinne. "Géométrie et cinématique de la branche occidentale de l'Arc de Castellane (Alpes de Haute-Provence-France)". Phd thesis, Université Pierre et Marie Curie - Paris VI, 1996. http://tel.archives-ouvertes.fr/tel-00800341.
Pełny tekst źródłaJiang, Zhifan. "Évaluation des mobilités et modélisation géométrique du système pelvien féminin par analyse d’images médicales". Thesis, Lille 1, 2017. http://www.theses.fr/2017LIL10003/document.
Pełny tekst źródłaThe better treatment of female pelvic mobility disorders has a social impact affecting particularly aged women. It is in this context that this thesis focuses on the development of methods in medical image analysis, for the evaluation of pelvic mobility and the geometric modeling of the pelvic organs. For this purpose, we provide solutions based on the registration of deformable models on Magnetic Resonance Images (MRI). All the results are able to detect the shape and quantify the movement of a part of the organs and to reconstruct their surfaces from patient-specific MRI. This work facilitates the simulation of the behavior of the pelvic organs using finite element method. The objective of these developed tools is to help to better understand the mechanism of the pathologies. They will finally allow to better predict the presence of certain diseases, as well as make surgical procedures more accurate and personalized
Qiu, Ruihao. "Étude théorique des instabilités de type ferroïques dans des géométries confinées et des réseaux distordus". Thesis, Bordeaux, 2017. http://www.theses.fr/2017BORD0660/document.
Pełny tekst źródłaIn this thesis, we present a theoretical study of two types of ferroic instabilities: the ferroelectric instability in novel confined geometries and magnetic instabilities controlled by the distortion of the underlying crystal lattice. On the one hand, we consider in detail the ferroelectric instability, specifically, in the nanotubes and the spherical nanoshells and develop a phenomenological theory for describing such an instability. We determine how the emergence of polarization is affected bythe thickness of the nanoparticle, the dielectric properties of the surrounding media and the interfacial boundary conditions. We finnd an intriguing topological finite-size effect that can promote an unexpected competition between two different types of distribution of polarization - irrotational and vortex-like - in the ultra-thin limit. One the other hand, we employ a different formalism to investigate the structural, electronic and magnetic properties of the rare-earth manganites. Specifically,we conduct a theoretical investigation from first-principles calculations. First, we predict a pressure-induced A-AFM insulator to FM metal transition on EuMnO3 under hydrostatic pressure, that is unprecedented in the multiferroic rare-earth manganites RMnO3. This investigation is extended to the study to the epitaxial strain effects on both EuMnO3 and TbMnO3 thin films. We show that epitaxial strain generates a much richer phase diagram compared to hydrostatic pressure. We predict novel magnetically-induced insulator { metal and polar { non-polar transitions. More specifically, we find that both the multiferroic E-AFM order and the polar metallic E*-AFM state are stabilized in TbMnO3 by means of epitaxial strain. In the contrast, we find a novel epitaxial-strain-induced multiferroic E-AFM state in EuMnO3 that cannot be obtained by means of just hydrostatic pressure
Benzid, Khalif. "Etude de l'effet de l'anisotropie magnétique sur la phase dynamique et sur la phase géométrique des bits quantiques de spins électroniques d'ions de métaux de transition Mn2+, Co2+, Fe3+ isolés et des complexes d'ions Fe3+ dans l'oxyde de zinc monocristallin". Thesis, Strasbourg, 2016. http://www.theses.fr/2016STRAE009/document.
Pełny tekst źródłaWe studied by pulsed EPR (p-EPR), the quantum coherence of electronic spins qubits of isolated transition metal ions of Mn2+, Co2+, Fe3+ and Fe3+/Cs+ as well as Fe3+/Na+ complexes, all found as traces in mono-crystalline ZnO. Indeed, we experimentally demonstrated that the magnetic anisotropy can alter the coherence of the dynamic phase of electronic spins qubits. We found a small decoherence for Mn2+ and Fe3+, spins having a small uniaxial magnetic anisotropy, and on the contrary, we found a very strong decoherence for Co2+ spins having a very strong uniaxial magnetic anisotropy. We found that the electronic spins of the Fe3+/Cs+ complex, having a more complex tensor magnetic anisotropy compared to the simplest uniaxial one of isolated Fe3+ spins in ZnO, have almost the same coherence time. By the perturbation method, we have found theoretically an additional term to the usual geometric Berry phase, due to the magnetic anisotropy which exists in any system having a spin S>1/2
Laporte, Sébastien. "Reconstruction 3D du squelette humain pour la biomécanique par radiographie biplane à dose minimale d'irradiation". Paris, ENSAM, 2002. http://www.theses.fr/2002ENAM0024.
Pełny tekst źródłaSakly, Nahed. "Investigations structurale et physique du système d'oxydes à chaînes de spins Ising (Sr, Ca)1+xCoxMn1-xO3". Thesis, Normandie, 2020. http://www.theses.fr/2020NORMC232.
Pełny tekst źródłaThis manuscript presents the experimental study of structural and physical properties of a spin chain compounds Sr4-xCaxCoMn2O9, belonging to the large oxides family A1+XA'XB1-XO3. In this series, the 1D chains are made up of the octahedra MnO6 (Mn4+) and trigonal prisms CoO6 (Co2+) connected by the faces, and distributed over a triangular lattice with an antiferromagnetic coupling between them. These chains exhibit a strong Ising-type magnetic anisotropy, originating from the cation Co2+ (HS, 3d7, S = 3/2). First of all, the structural study of these chains reveals that they can be immcommensurate, due to a change in the degree of oxidation of cobalt depending on the synthesis conditions. Then, we were interested in the study of the two compounds Sr4CoMn2O9 (x=0) and Sr2Ca2CoMn2O9 (x=2). The x=0 compound showed the absence of long-range magnetic ordering (LRO) and dynamic spin relaxation responses, typical of Single-Ion Magnet (SIM) and Single-Chain Magnet (SCM), of which the amplitude of their characteristic peaks depends on the (in)commensurability. On the other hand, in x = 2, only the SIM response was observed at low temperature, and which coexists with the LRO at TN ~ 28 K. The neutron diffraction data show that this LRO is compatible with a partially disordered antiferromagnetic state (PDA). A particular pre-transitional regime was also observed between TN and T* (~ 32.5 K), which was considered to be a precursor effect of LRO. Furthermore, a magneto-electric (ME) coupling has also been demonstrated within this compound. The mechanism of this ME coupling has been discussed as a result of exchange-striction phenomenon. Finally, we studied the magnetic anisotropy in oriented samples, whose grain morphology was optimized by different heat treatments
Nussle, Thomas. "Theoretical and computational studies of the thermomechanics of magnetic materials". Thesis, Tours, 2019. http://www.theses.fr/2019TOUR4007.
Pełny tekst źródłaOne of the utmost interesting properties of matter is magnetism. This property, which is a macroscopic consequence of quantum physics, is subjected and couples to several reservoirs. Among them, two are most relevant, namely the thermal and mechanical reservoirs. We build a Hamiltonian model for the coupling between -- classical -- magnetism and elasticity, which relies on the -- underlying -- anticommuting nature of spin, so as to describe the coupled dynamics of these degrees of freedom.The first step is to study the behavior of the classical spin -- or magnetic moment -- when coupled to different -- stochastic -- baths. First a spin bath, so as to investigate if and how such an effective model can mimic the couplings, to different magnetic moments but also to the elastic structure of the compound. A different approach is then followed where, through a Nambu dynamics model for spin precession, the ways in which this spin can be coupled to a bath, additively or multiplicatively, are studied in order to make out which is better suited to describe coupling phenomena in magnetism. Those are then studied numerically, initially stochastically, with the appropriate averaging procedure over different realizations and then deterministically, by building an effective model for the moments of the statistical distributions. This model is obtained by truncating the thus derived hierarchy of moments, so as to construct a quicker and deterministic method to deduce magnetic properties of a system.The second step is to construct models for magnetoelastic coupling, which we do via ``virtual'' particles carrying both localized magnetic moment and mechanical strain tensor. We begin by a Lagrangian formulation for the precession of spin, which is coupled to a dynamical elastic solid by a magnetoelastic coupling term. This enables us to study their coupled dynamics in a way that is fully consistent with all the symmetries, which ensures a consistent description.We then shift to a Hamiltonian description where spin is interpreted as a composite -- commuting -- variable, which is a product of underlying and not observable -- anticommuting -- variables. Such a spin interacts with a couple of canonically conjugate variables representing the elastic medium, in an extended Poisson structure. Finally, for each of these two models, we numerically study the influence of an external stress on the switching behavior of the Néel order parameter and spin accumulation for a NiO toy model antiferromagnet, induced by an external spin-transfer-torque
Bâty, Xavier. "Recalage de séquences cardiaques spation-temporelles IRM et TEP/SCAN". Phd thesis, Université d'Angers, 2007. http://tel.archives-ouvertes.fr/tel-00346306.
Pełny tekst źródłaHouwaart, Torsten. "Cobalt porphyrins on coinage metal surfaces - adsorption and template properties". Thesis, Lyon, École normale supérieure, 2014. http://www.theses.fr/2014ENSL0927.
Pełny tekst źródłaThis thesis is a theoretical study on the cobalt porphyrin - coinage metal surface interface with the DFT code VASP. The necessary DFT framework has been introduced in chapter 1. The structure of the Java program jBardeen for STM simulation is explained in chapter 2 and the source code is attached as Appendix. A study of the adsorption of CoTPP on coinage metal surfaces has been undertaken in chapter 3. Different parameters of the calculation have been evaluated: the adsorption site and the geometry of both the molecule and surface have been investigated with respect to the xc-functional and dispersion correction used. A most stable adsorption site -bridge down- is identified. Consequently, this most stable site was investigated for its electronic structure. Calculated STM images with the jBardeen code were compared with an experiment of CoTPP on a Cu(111) surface with sub monolayer coverage. In chapter 4 an Fe adatom was introduced to the CoTPP on Ag(111) system. Three symmetrically different binding sites for the Fe atom were identified on the macrocycle, labelled the bi-, brd- and bru-positions for bisector, bridge down and bridge up respectively. A magnetic moment could be evidenced which was mainly located on the Fe atom. Possible pathways between the four symmetrically equivalent bisector sites were investigated with different methods. Single point calculations in vacuum and Nudged Elastic Band (NEB) of the whole system revealed a barrier height of slightly above 0.2 eV going from bi- to the brd-position. A vibrational analysis showed that switching of the Fe atom is likely, when perturbed out of equilibrium in the brd- and bru- positions
Bertin, Alexandre. "Geometrical frustration and quantum origin of spin dynamics". Thesis, Université Grenoble Alpes (ComUE), 2015. http://www.theses.fr/2015GRENY014/document.
Pełny tekst źródłaThis Phd thesis focuses on the study of magnetically frustrated compounds where magnetic ions lie at the vertices of a corner-sharing tetrahedra network: the pyrochlore compounds. The two series of chemical formula R2M2O7, where R is a lanthanide and M=Ti, Sn, are of peculiar interest since they display a large variety of exotic magnetic ground states. First, we have studied the crystal-electric-field acting at the rare earth within the Stevens approximation where only the ground state multiplet is considered. A single set of parameters for each families of interest has been determined through a global analysis including several inelastic neutron scattering spectra of various compounds. Then, we have characterised with a large panel of techniques the low temperature physical properties of Nd2Sn2O7. This compound enters a long-range magnetic order at transition temperature Tc=0.91 K with an ``all-in-all-out'' spin configuration. A persistence of spin dynamics has been found in the ordered phase, ascribed to one-dimensional spin loops excitations. Anomalously slow paramagnetic spin fluctuations are also reported. Finally, we have brought information on the two proposed ground states of the widely studied compound Tb2Ti2O7: first, a Jahn-Teller transition is claimed to occur at low temperatures but no broadening of the Bragg peaks is seen down to T=4 K precluding premises of a structural transition. Secondly, this compound could be a realisation of a quantum spin-ice but no definitive evidence of a magnetisation plateau is found down to T=20 mK
Sarvezuk, Paulo Willian Carvalho. "Estudo do sistema quase unidimensional AxA'1-xNb2O6 (A e A'=Ni, Fe e Co) : preparação e caracterização das propriedades e estruturas magnéticas". reponame:Biblioteca Digital de Teses e Dissertações da UFRGS, 2011. http://hdl.handle.net/10183/49345.
Pełny tekst źródłaEste trabalho é dedicado à investigação das propriedades estruturais e magnéticas destes compostos de estrutura ortorrômbica (A = metais magnéticos) ANb2O6 que chamaram nossa atenção como um sistema de Ising quase-1D. Esse comportamento magnético original de baixa dimensionalidade é resultado das interações magnéticas ao longo das cadeias de átomos magnéticos quase uni-dimensional, e ainda pelas fracas interações antiferromagnética entre as cadeias. Quando esses compostos são ordenadas todas essas interações dentro e entre as cadeias leva a uma ordem global antiferromagnético. Nossa investigação é baseada em uma caracterização sistemática da série de compostos AxA’1-xNb2O6 (A et A’ = Ni, Fe e Co), por diferentes tipos de medidas complementares, tais como: difração de raios X à temperatura ambiente, difração de nêutrons acima e abaixo da temperatura de ordenamento magnético, medidas magnéticas: evolução térmica das curvas de magnetização e isoterma de magnetização sob variação de campo aplicado. Além disso, medidas de calor específico e espectroscopia Mössbauer foram realizadas em algumas amostras selecionadas. Nós encontramos interessantes resultados, para diferentes concentrações, com diferentes comportamentos nos sistemas pseudobinários Fe-Co, Ni-Fe ou Co-Ni, que podem permanecer no estado paramagnético até temperaturas muito baixas ou ordenarse dependendo da série analisada. Nossas medidas mostram que a temperatura de ordenamento magnético e vetores de propagação diferem substancialmente, dependendo da composição, por esses sistemas serem caracterizados por uma competição entre as interações magnéticas que estão envolvidas em uma rede triangular intermoleculares. Este estudo mostra que a desordem catiônicos Fe/Co induz uma redução substancial em ambas as interações dentro das; e entre as; cadeias, refletindo a tendência de se opor à criação de uma ordem de longo alcance magnético. Da mesma forma, o sistema NixFe1-xNb2O6 não mostra ordenamento magnético nem em medidas de até 400 mk realizadas para x = 0,2.
This work is dedicated to the investigation of structural and magnetic properties of these compounds with orthorhombic structure (A = magnetic metals) ANb2O6 that caught our attention as a system of quasi-1D Ising. This unique magnetic behavior is the result of low-dimensional magnetic interaction along the chains of magnetic atoms almost uni-dimensional, and also by weak antiferromagnetic interactions between the chains. When these compounds are ordered all these interactions between and within the chain leads to a global antiferromagnetic order. Our research is based on a systematic characterization of the series of compounds AxA'1-xNb2O6 (A et A' = Ni, Fe and Co) by differents types of complementary measures, such as X-ray diffraction at room temperature, neutron diffraction above and below the magnetic ordering temperature, magnetic measurements: thermal evolution of magnetization curves and isotherm magnetization variation in applied field. In addition, specific heat measurements and Mössbauer spectroscopy were performed on some selected samples. We found interesting results depending on the concentration, different behaviors on these pseudo binary systems Ni-Fe, Fe-Co, or Co-Ni, which can remain in the paramagnetic state up to very low temperatures or to order depending on the series analyzed. Our measurements show that the magnetic ordering temperature and propagation vectors differ substantially depending on the composition, for these systems are characterized by a competition between magnetic interactions that are involved in a triangular lattice intermolecular. This study shows that disorder cationic Fe / Co induced a substantial reduction in both the interactions within and between them; chains, reflecting the tendency to oppose the creation of a long-range magnetic order. Likewise, the system NixFe1-xNb2O6 dont shows magnetic ordering even at measures taken up to 400 mk for x = 0.2.
Leonardi, Valentin. "Modélisation dynamique et suivi de tumeur dans le volume rénal". Thesis, Aix-Marseille, 2014. http://www.theses.fr/2014AIXM4056/document.
Pełny tekst źródłaThis Ph.D. thesis deals with the 3D dynamic modeling of the kidney and tracking a tumor of this organ. It is in line with the KiTT project (Kidney Tumor Tracking) which gathers researchers from different fileds: geometric modeling, radiology and urology. This work arised from the tendency of nowadays surgical gestures to be less and less invasive (HIFU, coelioscopy). Its goal is to result in a totally non-invasive protocol of kidney tumors eradication by transmitting ultrasound waves through the skin without breaking in it. As the kidney presents motions and deformations during the breathing phase, the main issue is to know the kidney and tumor positions at any time in order to adjust the waves accordingly