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Chirinos, Villaruel Fernando. "Precesión de los equinoccios de la tierra." Universidad Nacional de Ingeniería. Programa Cybertesis PERÚ, 2008. http://cybertesis.uni.edu.pe/uni/2008/chirinos_vf/html/index-frames.html.
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Benyahia, Dhygham Alkoudsi. "Análisis de los efectos de la insuficiente consistencia dinámica de las teorías de precesión y nutación IAU2006 e IAU2000." Doctoral thesis, Universidad de Alicante, 2019. http://hdl.handle.net/10045/99671.
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En esta memoria se ha estudiado, por primera vez y de manera exhaustiva, los efectos de la inconsistencia dinámica que está bien demostrado que existe entre las teorías oficiales de precesión y nutación, denominadas IAU2006 e IAU2000 respectivamente. Se ha cuantificado sus efectos en el análisis de datos de observación reales así como las interacciones que se inducen entre las dos componentes de las desviaciones del polo celeste, clasificadas de manera tradicional como precesión y nutación dependiendo fundamentalmente de su origen y de la longitud de su periodo. Se ha examinado los tres modelos de correcciones de las series de nutación que fueron propuestos a principios de 2018 por Escapa y Capitaine para su consideración por la comunidad científica y por los órganos correspondientes de la Unión Astronómica Internacional. Dichas correcciones se basan en todos los resultados previos obtenidos independientemente por ambos autores y sus respectivos colaboradores con el fin de lograr la consistencia dinámica completa de ambas teorías. La terna de modelos de corrección analizada incluye el antiguo modelo recogido en la Convenciones del IERS que ignora los efectos debidos a la diferencia de los valores de referencia de la oblicuidad de la eclíptica y la velocidad de precesión en longitud usados en las teorías IAU2000 e IAU2006, denominado (a) en el documento de trabajo citado de Escapa y Capitaine, otro modelo denominado (c) en este documento, que es el único que proporciona consistencia dinámica completa, así como un modelo simplificado, el (b) que está formado solo por todos los términos seculares mixtos del (c). Se ha realizado experimentos numéricos extendidos a diferentes intervalos de tiempo. Uno coincide con el periodo de ajuste de la teoría IAU2006, aproximadamente 1984-2003 ya que la versión en vigor se publicó en 2003. Otros se extienden hasta casi la actualidad, de 1984 a septiembre de 2018 y también de 1990 a 2018 si se suprimen las observaciones de la época inicial del VLBI que adolecían de mayores errores. Finalmente, se analizan los tres modelos en intervalos temporales de mayor longitud para los que no existen observaciones VLBI, por lo que en este caso los métodos no pueden ponderarse con los pesos asociados a las observaciones. La metodología fundamental procede analizando los residuales de las series originales y ajustando a cada modelo polinomios con grados que varían desde el 1 hasta el 5, ya que este último es el grado de los polinomios del modelo IAU2006, cuya obtención se basa en procedimientos empíricos. En los ajustes se emplean métodos de mínimos cuadrados ponderados, se calculan los valores de las desviaciones cuadráticas medias o WRMS de los datos originales y los residuales de cada ajuste, así como los errores formales de los coeficientes: También se realizan pruebas de significación estadística tanto para los ajustes individuales como para la comparación de modelos regresivos encajados, aunque suelen tener escaso poder de discriminación ya que casi siempre resultan muy significativas. Se obtiene que, aunque el efecto de las tres correcciones de la teoría de nutación sobre la varianza de los residuales es pequeño, pues solo reducen los valores del WRMS en pocos microsegundos de arco cuando se aplican a las series oficiales de datos de observación del IERS, existe un efecto sobre el propio modelo de precesión que podría denominarse “indirecto” y que es de mucha mayor trascendencia. Puede este resumirse diciendo que, mientras que la presencia de los términos seculares mixtos en los modelos de corrección implica que no pueden aproximarse por polinomios de grados hasta el 5 (salvo el idénticamente nulo) en intervalos de tiempo suficientemente largos, aunque insignificantes cuando se comparan con los 25.000 años del periodo de la precesión, como por ejemplo dos siglos, en cambio en intervalos de tiempo más cortos, particularmente en todos aquellos comprendidos dentro del periodo de observaciones VLBI dichos polinomios son capaces de aproximar las correcciones con gran precisión. Así, en el intervalo de ajuste 1984-2003, los polinomios absorben más del 90% de la varianza de los tres modelos estudiados. La cifra disminuye al aumentar la longitud del intervalo, de modo que en el periodo 1984-2018 baja al 60% y en las simulaciones del intervalo 1990-2030 solo absorbe un 50% de la varianza, de modo que el modelo IAU2006 se va deteriorando de manera clara aunque no alarmante y sus efectos serán más notorios en poco más de diez años. La consecuencia más directa de este hecho es que los coeficientes del actual modelo IAU2006 contienen contribuciones espurias que no se deben realmente al movimiento de precesión sino a la falta de aplicación de las correcciones del modelo de nutación en el periodo de ajuste 1984-2003. Los experimentos muestran también que ya es posible determinar correcciones al modelo de precesión que no se limiten a calcular desviaciones y tendencias, y que pueden efectuarse ajustes de los datos de la serie IERS EOP 14 C04 hasta los grados tercero y cuarto de manera que todos los coeficientes son muy significativos y el WRMS de los residuales de dichos ajustes es menor que para el grado 1. Además, los resultados para polinomios de grado superior al dos son más estables en cuanto a los valores de sus coeficientes. Por otra parte, los órdenes de magnitud de los términos de grados 0 y 1 son similares a los obtenidos en otros estudios de las series de datos que limitan su análisis al grado 1, por supuesto sin contemplar efectos de las correcciones aquí analizadas.
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Belda, Santiago. "Contributions to the Earth Monitoring by Space Geodesy Methods." Doctoral thesis, Universidad de Alicante, 2015. http://hdl.handle.net/10045/50535.
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Baenas, Tomás. "Contribuciones al estudio analítico del movimiento de rotación de una Tierra deformable." Doctoral thesis, Universidad de Alicante, 2014. http://hdl.handle.net/10045/40903.
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Esta memoria para optar al grado de doctor por la Universidad de Alicante, en el área de Matemática Aplicada, se divide en ocho capítulos. En los cuatro primeros se lleva a cabo una revisión del problema de la rotación de la Tierra rígida y elástica, en formalismo hamiltoniano, y se introducen algunas alternativas originales a la construcción de ciertos elementos de la teoría. En los otros cuatro se presentan nuevas contribuciones al estudio analítico de los movimientos de precesión y nutación de la Tierra deformable. Estas se pueden agrupar en dos categorías: la obtención de fórmulas que describen el movimiento de rotación del eje de figura, que completan las existentes hasta la fecha, y la extensión del formalismo para hacerlo compatible con modelos reológicos de la Tierra más generales.
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Gilmore, Keith. "Precession Damping in Itinerant Ferromagnets." Thesis, Montana State University, 2007. http://etd.lib.montana.edu/etd/2007/gilmore/GilmoreK1207.pdf.
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Precession damping in metallic ferromagnets had been assumed to result from the spinorbit interaction. While several theories of spin-orbit damping had been postulated, no convincing numerical comparisons to data existed. We selected one promising theory and performed first-principles numerical calculations of damping for bulk iron, cobalt, and nickel. Comparison of minimal calculated and measured damping rates demonstrated a 70 % agreement for nickel, 60 % for iron, and 40 % for cobalt. We then relaxed the initial constraint of a universal electron-lattice scattering rate by allowing the scattering rate to be spin dependent. The spin dependent lifetime ratio was equated to the ratio of the spin resolved density of states at the Fermi level. This modification improved the agreement to 95 % for nickel, 70 % for iron, and 47 % for cobalt. With this level of agreement, we next constructed a simple effective field explanation for the damping process. As the magnetization rotates, the energy of the spin system gets pushed out of equilibrium and this excitation is quenched by electron-lattice scattering. The energy of the spin system changes by two mechanism: the energies of the states change and transitions to excited states occur. The first mechanism had previously been described within the effective field picture as producing a breathing of the Fermi surface. As the magnetization precesses, the spin-orbit energy of each state changes leading to expansions and contractions of the Fermi surface that are periodic with the precession. To expand this metaphor, we have dubbed the second effect of transitions to excited states as a bubbling of the Fermi sea. In this picture, individual electrons across the Fermi surface undergo larger excitations. Finally, we investigated the dependence of the damping rate on the density of states and the spin-orbit coupling parameter. We found that the damping due to the breathing effect was roughly proportional to the density of states while damping from the bubbling terms correlated strongly with the density of states squared. By tuning the spin-orbit parameter we found that the breathing terms were proportional to the spin-orbit parameter cubed while the bubbling terms went as the spin-orbit parameter squared.
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Oleynikov, Peter. "Exploiting reciprocal space : electron diffraction, textures and precession /." Stockholm : Institutionen för fysikalisk kemi, oorganisk kemi och strukturkemi, Stockholms universitet, 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-791.
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Rolfe, Daniel James. "Accretion flow and precession phenomena in cataclysmic variables." Thesis, n.p, 2001. http://library7.open.ac.uk/abstracts/page.php?thesisid=52.
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Derakhshan, Jamal Jon. "Innovations Involving Balanced Steady State Free Precession MRI." Cleveland, Ohio : Case Western Reserve University, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=case1247256364.
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Thesis (Ph.D.)--Case Western Reserve University, 2009<br>Title from PDF (viewed on 19 August 2009) Department of Biomedical Engineering Includes abstract Includes bibliographical references Available online via the OhioLINK ETD Center
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Nobes, David Scott. "The generation of large-scale structures by jet precession /." Title page, abstract, and contents only, 1997. http://web4.library.adelaide.edu.au/theses/09PH/09phn7444.pdf.
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White, Thomas Ashley. "Structure solution using precession electron diffraction and diffraction tomography." Thesis, University of Cambridge, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.611748.
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Xu, Jing. "Modeling and simulation of spin transport and precession in silicon." Access to citation, abstract and download form provided by ProQuest Information and Learning Company; downloadable PDF file, 69 p, 2008. http://proquest.umi.com/pqdweb?did=1601517971&sid=9&Fmt=2&clientId=8331&RQT=309&VName=PQD.
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Larwood, John Dail. "The tidally induced warping, precession and truncation of accretion discs." Thesis, Queen Mary, University of London, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.268037.
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Vautrin, Christopher. "Electronic spin precession in all solid state magnetic tunnel transistor." Thesis, Université de Lorraine, 2017. http://www.theses.fr/2017LORR0075/document.
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Ce travail porte sur la précession du spin d’électrons chauds polarisés en spin. Celle-ci est induite par le champ d’échange d’une couche mince ferromagnétique dans une structure multicouche. La précession du spin électronique a déjà été mesurée dans des matériaux ferromagnétiques mais uniquement pour des électrons qui possèdent une énergie supérieure à 4eV au-dessus du niveau de Fermi. L’objectif premier de cette thèse est de mesurer la précession du spin de l’électron pour des faibles énergies, comprises entre 0.7eV et 2eV au-dessus du niveau de Fermi. Pour ce faire, un transistor tunnel magnétique comportant trois couches magnétiques avec les aimantations qui pointent dans les trois directions de l’espace doit être construit. Les électrons sont injectés à basse énergie grâce à une jonction tunnel. Une diode Schottky (interface entre du Cu et du Si) filtre en énergie les électrons incidents, permettant uniquement aux électrons balistiques de contribuer au courant mesuré dans le semi-conducteur. Le premier travail a consisté à obtenir une couche magnétique exhibant une anisotropie perpendiculaire. Ainsi, nous avons réussi à faire croître une multicouche de Co et Ni sur une diode Schottky qui possède une anisotropie perpendiculaire jusqu’à 5 répétitions. Le deuxième travail réalisé dans cette thèse était d’optimiser le magnéto-courant d’une la vanne de spin. En effet, le magnéto-courant détermine la sensibilité de notre transistor tunnel magnétique. Nous avons notamment démontré ici que le magnéto-courant augmente avec le nombre de répétitions de la multicouche [Co/Ni], pour atteindre quasiment le maximum de 100% théoriquement prédit dans une vanne de spin à aimantations croisées. Enfin, le troisième travail de cette thèse résidait dans l’étude de la précession du spin de l’électron dans différents matériaux ferromagnétiques. Cet effet a été mis en évidence ici pour des couches à aimantation planaire composée de Co, de CoFeB, ainsi que pour un alliage de CoAl et ceci en fonction de leur épaisseur<br>This work is about polarised hot electrons spin precession. This phenomenon is induced by the exchange field of a ferromagnetic thin film in a multilayer structure. The electronic spin precession has already been measured in ferromagnetic materials, but only for electrons whose energy is more than 4eV over the Fermi level. The initial aim of this PhD work is to measure the electron spin precession for weak energies, between 0.7 eV and 2eV over the Fermi level. In order to achieve that, a magnetic tunnel transistor composed of three magnetic layers with their magnetisations directions perpendicular to each other has to be elaborated. The electrons are injected at low energy by means of a tunnel junction. A Schottky diode (interface between Cu and Si) filters the incident electrons by their energies, which enables only ballistic electrons to contribute to the measured current in the semi-conductor. The first task consisted in obtaining a magnetic layer showing perpendicular magnetic anisotropy. We succeeded in growing cobalt nickel multilayers exhibiting a perpendicular magnetic anisotropy up to five repetitions. The second part of the job carried out during this PhD was to optimise the magneto-current of a spin valve. Indeed, it determines the magnetic tunnel transistor sensitivity. We have demonstrated that the magneto-current increases with the number of repetitions of the [Co/Ni] multilayer up to a maximum of nearly a hundred percent, which is the maximum theoretically predicted in a spin valve with crossed magnetisations. Eventually, the third task of this PhD was the study of the electron spin precession in various ferromagnetic materials. This effect has been evidenced here for thin layers with in-plane magnetisations composed of Co, CoFeB, and also for a CoAl alloy depending on the thickness of the layers
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Marino, Michael A. "Precession damping and axial velocity control of a lightweight reentry vehicle." Thesis, Massachusetts Institute of Technology, 1989. http://hdl.handle.net/1721.1/41239.
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Mendoza, Michael A. "Water Fat Separation with Multiple-Acquisition Balanced Steady-State Free Precession MRI." BYU ScholarsArchive, 2013. https://scholarsarchive.byu.edu/etd/4304.
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Magnetic resonance imaging (MRI) is an important medical imaging technique for visualizing soft tissue structures in the body. It has the advantages of being noninvasive and, unlike x-ray, does not rely on ionizing radiation for imaging. In traditional hydrogen-based MRI, the strongest measured signals are generated from the hydrogen nuclei contained in water and fat molecules.Reliable and uniform water fat separation can be used to improve medical diagnosis. In many applications the water component is the primary signal of interest, while the fat component represents a signal which can obscure the underlying pathology or other features of interest. In other applications the fat signal is the signal of interest. There currently exist many techniques for water fat separation. Dixon reconstruction techniques take multiple images acquired at select echo times with specific phase properties. Linear combinations of these images produce separate water and fat images. In MR imaging, images with high signal-to-noise ratio (SNR), that can be generated in a short time, are desired. Balanced steady-state free precession (bSSFP) MRI is a technique capable of producing images with high SNR in a short imaging time but suffers from signal voids or banding artifacts due to magnetic field inhomogeneity and susceptibly variations. These signal voids degrade image quality. Several methods have been developed to remove these banding effects. The simplest methods combine images across multiple bSSFP image acquisitions. This thesis describes a technique in water fat separation I developed which combines the advantages of bSSFP with Dixon reconstruction in order to produce robust water fat decomposition with high SNR in a short imaging time, while simultaneously reducing banding artifacts which traditionally degrade image quality. This algorithm utilizes four phased-cycled bSSFP acquisitions at specific echo times. Phase sensitive post-processing and a field map are used to prepare the data and reduce the effects of field inhomogeneities. Dixon reconstruction is then used to generate separate water and fat images.
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NOIR, JEROME. "Ecoulements d'un fluide dans une cavite en precession : approches numerique et experimentale." Université Joseph Fourier (Grenoble), 2000. http://www.theses.fr/2000GRE10217.
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Nous savons aujourd'hui que le champ magnetique terrestre est cree au sein du noyau par des processus dynamo. Pour pouvoir comprendre la creation et l'evolution de ce champ nous devons connaitre la dynamique de la partie liquide du noyau terrestre. Le soleil et la lune exercent sur la terre un couple de gravite moyen entrainant un mouvement de precession de l'axe de rotation de notre planete. Comment le noyau liquide reagit-il a ce forcage luni-solaire ? pour pouvoir aborder cette question nous avons considere un modele physique simple constitue d'une cavite remplie d'eau dont nous imposons la vitesse de rotation et la vitesse de precession. La premiere partie de ce travail porte sur une approche numerique du probleme dans le cas particulier d'un conteneur spherique, nous avons pu ainsi etudier le role de la viscosite. Nous montrons en particulier que l'ecoulement laminaire se superposant a la rotation solide du fluide est controle par une couche limite visqueuse localisee au voisinage de la paroi. Une etude detaillee nous a permis d'identifier des lois de comportement asymptotiques pour la vitesse. Dans une seconde partie nous presentons l'experience de precession ciceron au cours de laquelle nous nous sommes attaches d'une part a decrire le mouvement de rotation solide du fluide, d'autre part a caracteriser les differentes transitions de l'ecoulement secondaire vers la turbulence. Nous proposons a l'issue de cette etude experimentale que la principale source d'instabilite soit de type cisaillante. Bien que nos connaissances ne soient pas encore suffisantes pour pouvoir decrire la dynamique du noyau liquide induite par la precession, nous pouvons penser d'apres nos resultats que l'ecoulement n'est certainement pas laminaire. D'autres etudes, notamment experimentales, sont envisagees pour poursuivre ce travail.
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Li, Jing. "Oblique Hanle effect in silicon spin transport devices." Access to citation, abstract and download form provided by ProQuest Information and Learning Company; downloadable PDF file, 53 p, 2009. http://proquest.umi.com/pqdweb?did=1885755961&sid=6&Fmt=2&clientId=8331&RQT=309&VName=PQD.
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Moraes, Tiago Bueno de. "Precessão Livre no Estado Estacionário com alternância de fase para RMN em alta e baixa resolução." Universidade de São Paulo, 2016. http://www.teses.usp.br/teses/disponiveis/76/76132/tde-14102016-091634/.
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A aplicação de uma sequência de pulsos com tempo de repetição muito menor que os tempos de relaxação Tp << T2; T1, faz com que a magnetização atinja um estado estacionário descrito por H.Y. Carr como Estado Estacionário em Precessão Livre, Steady State Free Precession (SSFP). Nessa condição, o sinal é composto pela complexa sobreposição das componentes FID e eco. Sequências tipo SSFP são utilizadas na aquisição rápida de sinais, resultando em uma boa razão sinal ruído (s/r) em curto intervalo de tempo, porém introduzem fortes anomalias de fase e amplitude devido a complexa interação das componentes que formam o estado estacionário. Neste trabalho, desenvolvemos sequências de pulsos tipo SSFP para RMN em alta e baixa resolução com alternância e incremento de fase. Em alta resolução desenvolvemos as sequências SSFPdx e SSFPdxdt com incremento de fase linear e quadrático respectivamente. Os resultados mostram que espectros de núcleos com baixa sensibilidade podem ser obtidos com mesma razão s/r em menor tempo experimental e as sequências desenvolvidas removem as anomalias espectrais. Em baixa resolução, os resultados mostram que a introdução de alternâncias de fase na Continuous Wave Free Precession (CWFP) possibilita a remoção da dependência da sequência com o offset de frequência e com o tempo entre pulsos. Além disso, mostramos que a sequência CP-CWFPx-x com ângulo de refocalização pequeno (5° a 10°) possibilita a estimativa rápida do tempos de relaxação longitudinal. Apresentamos também resultados dos estudos e desenvolvidos no estágio de pesquisa no exterior, onde as sequências de pulsos no estado estacionário – DECPMG e Split 180° – foram estudas numericamente e implementadas nos sistemas magnéticos compactos: mini-Halbach e MOUSE-NMR. Por fim, são apresentados resultados com os métodos de processamento de dados Krylov Basis Diagonalization Method (KBDM) e a Transformada Inversa de Laplace aplicados na análise de sinais SSFP. Resultados mostram que KBDM é uma ferramenta útil no processamento de dados em alta e baixa resolução, tanto na obtenção de espectros como na determinação da distribuição dos tempos de relaxação.<br>The application of a pulse sequence with repetition time much smaller than the relaxation times, Tp << T2; T1, causes the magnetization to reach a steady state, described by H. Y. Carr as a Steady State Free Precession (SSFP). In this condition, the signal is composed of the complex overlapping of the FID and eco components. SSFP type sequences are used in fast acquisition of NMR signals, resulting in a good signal to noise ratio (s/r) in a short time interval, however, they introduce phase and amplitude anomalies due to the complex interaction between the components of the steady state. In this work, we develop SSFP type pulse sequences for NMR in high and low resolution, with alternation and increment of phase. In high resolution, we develop SSFPdx and SSFPdxdt sequences, with linear and quadratic phase increment respectively. Results show that the low sensitivity nuclei spectra can be obtained with the same s/r ratio in smaller experimental time, about an order of magnitude, and the developed sequences can remove the spectral anomalies. In low resolution, the results show that the introduction of a phase alternation in the Continuous Wave Free Precession (CWFP) allows the elimination of the dependence of the sequence with the offset frequency and the time between pulses. Besides, we show that the CP-CWFPx-x sequence with a small refocalization angle (5° to 10°) allows the fast estimative of the longitudinal relaxation time in a single experiment. The results of the studies conducted during an international research internship are also presented. Steady state pulse sequences – DECPMG and Split 180° – were studied and implemented in compact magnetic systems: mini-Halbach and MOUSE-NMR. Finally, the results of the application of the Krylov Basis Diagonalization Method (KBDM) and the Inverse Laplace Transform for the analysis of SSFP signals are presented. The results show that KBDM is a useful tool in data processing for low and high resolution, both for obtaining spectra and determining the relaxation times distribution.
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Gyngell, M. L. "The application of steady-state free precession (SSFP) in 2d-Ft NMR imaging." Thesis, University of Nottingham, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.376522.
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Li, Fang. "An Analysis of the Linked-pulse in Steady-state Free Precession in MRI." PDXScholar, 1994. https://pdxscholar.library.pdx.edu/open_access_etds/4769.
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The steady-state free precession (SSFP) is one type of the fast scanning technique in MRI. So far most of its analysis are concentrated on the gradient echo SSFP (GR SSFP), very few paper~discuss the spin echo (SSFP (SE SSFP), and they are usually based on the simplified the hard pulse assumption. The advantage of the SE SSFP is that it can refocus the dephasing caused by the magnetic field inhomogeniety, which is the disadvantage of the GR SSFP. Also the hard pulse model can provide very limited information. The purpose of this paper is to establish the soft pulse model for both GR SSFP and SE SSFP. By using the spinor method to describe the interaction between the RF pulse, magnetic field, and the spin's magnetization, we create the steady state equations of the GR SSFP and SE SSFP, and give their analytical solutions. Because the SE SSFP's mathematical model is very complicated, we introduce a new concept, the linked-pulse, to simplify the problem, and provide the valuable results. Based on both traditional hard pulse model and our soft pulse model, we did a series of simulations, and compared both results. First of all, the soft pulse model can provide the slice profile and gradient effects, which is impossible for the hard pulse model. Second, in both models, the signal intensities are all depended on the Tl/T2 ratio, which is the characterization of the SSFP image. Third, we also observed how the pulse shape and the flip angles affect the slice profile and the signal intensity. In conclusion, the soft pulse model can give more information than hard pulse model can, such as slice profile and gradient effects, etc., provide more aspects for analyzing the SSFP image.
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Liu, Yue. "Precession Electron Diffraction Assisted Characterization of Deformation in α and α+β Titanium Alloys". Thesis, University of North Texas, 2015. https://digital.library.unt.edu/ark:/67531/metadc804946/.
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Ultra-fine grained materials with sub-micrometer grain size exhibit superior mechanical properties when compared with conventional fine-grained material as well as coarse-grained materials. Severe plastic deformation (SPD) techniques have been shown to be an effective way to modify the microstructure in order to improve the mechanical properties of the material. Crystalline materials require dislocations to accommodate plastic strain gradients and maintain lattice continuity. The lattice curvature exists due to the net dislocation that left behind in material during deformation. The characterization of such defects is important to understand deformation accumulation and the resulting mechanical properties of such materials. However, traditional techniques are limited. For example, the spatial resolution of EBSD is insufficient to study materials processed via SPD, while high dislocation densities make interpretations difficult using conventional diffraction contrast techniques in the TEM. A new technique, precession electron diffraction (PED) has gained recognition in the TEM community to solve the local crystallography, including both phase and orientation, of nanocrystalline structures under quasi-kinematical conditions. With the assistant of precession electron diffraction coupled ASTARÔ, the structure evolution of equal channel angular pressing processed commercial pure titanium is studied; this technique is also extended to two-phase titanium alloy (Ti-5553) to investigate the existence of anisotropic deformation behavior of the constituent alpha and beta phases.
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Ngassa, Tankeu Yvan Georges. "Affinement de structure par PEDT (Precession Electron Diffraction Tomography) dans des échantillons d’intérêt minéralogique." Thesis, Lille 1, 2018. http://www.theses.fr/2018LIL1R079.
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Depuis quelques années, une méthode de diffraction électronique utilisant la précession, la tomographie et le calcul dynamique des intensités diffractées permet la détermination des structures à l’échelle nanométrique. Nous présentons ici l’application de cette méthode à l’affinement de spinelles et d’orthopyroxènes ferromagnésiens, pour leur utilisation comme géothermomètre. Il s’agit de tester la sensibilité de la méthode pour la détermination des taux d’occupation, variables avec la température, des cations Fe2+ ou Fe3+ et Mg2+ sur les sites cristallographiques spécifiques des structures analysées. Les échantillons étudiés ont été synthétisés en laboratoire (magnésioferrite (MgFe2)O4 et hercynite (FeAl2)O4) ou recueillis à l’état naturel (orthopyroxène (Mg1.4Fe0.6)Si2O6). La sensibilité de la méthode est évaluée en fonction de plusieurs paramètres : technique de préparation des échantillons pour leur analyse en microscopie électronique, composition chimique, méthodologie et paramètres d’affinement. Dans le cas de nos échantillons de spinelle, il a été établi que les lames minces obtenues par faisceau d’ions focalisés (FIB) n’étaient pas adaptées, contrairement aux échantillons obtenus par simple broyage mécanique. Les affinements dynamiques ont pu alors être menés avec succès à la condition d’imposer la composition chimique des cristaux au cours de la procédure d’affinement. La précision obtenue permet alors leur utilisation pour les applications en géoscience. Dans le cas des pyroxènes, les échantillons obtenus par FIB sont exploitables mais les affinements réalisés lorsque la composition est fixée sont également plus fiables que ceux obtenus lorsqu’elle ne l’est pas<br>Since a few years, Precession Electron Diffraction Tomography (PEDT) coupled to dynamical calculations of diffracted intensities has been used to allow the structure solving and refinement at the nanoscale in a Transmission Electron Microscope (TEM). The method is here applied to the structure refinement of ferromagnesian spinels and orthopyroxenes for their use as geothermometers. The aim is to test the sensitivity of the method for the measurement of the occupancy rate, depending on temperature, of iron and magnesium cations on specific crystallographic sites of the structure. Studied samples are either synthetic (magnesioferrite (MgFe2)O4 and hercynite (FeAl2)O4) or natural (orthopyroxene (Mg1.4Fe0.6)Si2O6). The sensitivity of the method is tested as a function of various parameters such as the sample-thinning technique used for TEM observations, the sample chemical composition, the refinement methodology and parameters. In the case of our spinels, it is shown that samples as obtained by Focused Ion Beam (FIB) are not appropriated, contrary to samples as obtained by simple mechanical grinding. Structure refinements have then been successfully conducted, provided the chemical composition of the sample is fixed during the procedure. The final accuracy is then far sufficient to use the refinement results for geosciences applications. Concerning pyroxenes, FIB samples are suitable for the study. Refinement results are nevertheless more accurate once the chemical composition is imposed during the procedure
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Giuly, Richard James. "Design and testing of a new conical beam precession system for high-throughput electron tomography." Diss., Connect to a 24 p. preview or request complete full text in PDF format. Access restricted to UC campuses, 2008. http://wwwlib.umi.com/cr/ucsd/fullcit?p1457449.
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Thesis (M.S.)--University of California, San Diego, 2008.<br>Title from first page of PDF file (viewed November 13, 2008). Available via ProQuest Digital Dissertations. Vita. Includes bibliographical references (p. 163-164).
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Bullock, Matthew R. "Angular momentum precession, acquisition maneuver and attitude estimation design for SILA, a university micro-satellite project." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp04/mq22117.pdf.
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Bullock, Matthew R. (Matthew Robert) Carleton University Dissertation Engineering Mechanical and Aerospace. "Angular momentum precession, acquisition maneuver and attitude estimation design for SILA, a university micro satellite project." Ottawa, 1997.
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Chadwick, Angus. "Hippocampal theta sequences : from phenomenology to circuit mechanisms." Thesis, University of Edinburgh, 2016. http://hdl.handle.net/1842/22887.
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The hippocampus is a brain structure involved in episodic memory and spatial cognition. Neuronal activity within the hippocampus exhibits intricate temporal patterning, including oscillatory and sequential dynamics, which are believed to underlie these cognitive processes. In individual cells, a temporal activity pattern called phase precession occurs which leads to the organisation of neuronal populations into sequences. These sequences are hypothesised to form a substrate for episodic memory and the representation of spatial trajectories during navigation. In this thesis, I present a novel theory of the phenomenological properties of these neuronal activity sequences. In particular, I propose that the sequential organisation of population activity is governed by the independent phase precession of each cell. By comparison of models in which cells are independent and models in which cells exhibit coordinated activity against experimental data, I provide empirical evidence to support this hypothesis. Further, I show how independent coding affords a vast capacity for the generation of sequential activity patterns across distinct environments, allowing the representation of episodes and spatial experiences across a large number of contexts. This theory is then extended to account for grid cells, whose activity patterns form a hexagonal lattice over external space. By analysing simple forms of phase coding in populations of grid cells, I show how previously undocumented constraints on phase coding in two dimensional environments are imposed by the symmetries of grid cell firing fields. To overcome these constraints, I propose a more complex phenomenological model which can account for phase precession in both place cells and grid cells in two dimensional environments. Using insights from this theory, I then propose a biophysical circuit mechanism for hippocampal sequences. I show that this biophysical circuit model can account for the proposed phenomenological coding properties and provide experimentally testable predictions which can distinguish this model from existing models of phase precession. Finally, I outline a scheme by which this biophysical mechanism can implement supervised learning using spike time dependent plasticity in order to learn associations between events occurring on behavioural timescales. The models presented in this thesis challenge previous theories of hippocampal circuit function and suggest a much higher degree of flexibility and capacity for the generation of sequences than previously believed. This flexibility may underlie our ability to represent spatial experiences and store episodic memories across a seemingly unlimited number of distinct contexts.
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Girotti, Paolo. "Study of the systematics on the precession frequency measurement at the Muon g-2 experiment at Fermilab." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2019. http://amslaurea.unibo.it/18039/.
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Il momento magnetico anomalo del muone a_mu = (g-2)/2 è una delle quantità conosciute con più precisione nella fisica, sia sperimentalmente che teoricamente. L'alto livello di accuratezza permette l'uso della misura di a_mu come test del Modello Standard (MS). In particolare, qualsiasi deviazione dal valore teorico del MS potrebbe essere dovuto a contributi fisici Oltre il Modello Standard (BSM). Il notevole risultato ottenuto dall'esperimento E821 nel 2001 presso BNL, con un valore misurato di a_mu = 11 659 208.0(6.3)x10^-10 e una precisione complessiva di 0.54 ppm, ci ha lasciati con una differenza di 3.4 sigma rispetto alla predizione del modello standard. Il nuovo esperimento Muon g-2 (E989) presso Fermilab (FNAL) mira a ridurre l'errore sperimentale di un fattore 4, con un obiettivo finale di 0.14 ppm, utilizzando lo stesso anello di accumulazione e migliorando le incertezze statistiche e sistematiche. In circa 2 anni di presa dati sarà raggiunto un numero di decadimenti di muone 21 volte maggiore rispetto a E821, ottenendo un'incertezza statistica di 0.1 ppm. Al momento di questa scrittura, l'esperimento è operativo e ha già collezionato due volte la statistica totale di E821. Molti miglioramenti riguardanti i rivelatori implicano una riduzione dell'errore sistematico da 180 ppb a 70 ppb per la misura di w_a, la frequenza di precessione anomala del muone. In particolare, il nuovo Sistema di Calibrazione Laser ridurrà l'errore sistematico dovuto alle fluttuazioni del guadagno dei fotomoltiplicatori al silicio (SiPM) da 120 ppb a 20 ppb. Questa tesi descrive l'esperimento e poi discute due analisi effettuate da me utilizzando il Sistema di Calibrazione Laser. La prima è una sincronizzazione dei 1296 rivelatori al livello del nanosecondo, descritta nel capitolo 5. La seconda è uno studio sulla risposta del guadagno dei rivelatori a segnali che sono distanti meno di 100 ns, oltre alla valutazione del suo effetto nella misura di w_a, descritto nel capitolo 6.
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Wienands, Joshua Nikolai. "Coils, fields and xenon : towards measuring xenon spin precession in a magnetic field for the UCN collaboration." Thesis, University of British Columbia, 2016. http://hdl.handle.net/2429/59080.
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In this thesis I present my work on building a set of magnetic coils for the purpose of performing nuclear magnetic resonance (NMR) on Boltzmann polarized protons in water, and on hyperpolarized ¹²⁹Xe. The coils were designed to be used as a method for testing the degree of polarization achieved in ¹²⁹Xe, and for testing the capability of an in-house developed continuous wave (CW) ultraviolet (UV) laser to drive a 2-photon transition in ¹²⁹Xe. This laser will be used to measure the precession frequency of ¹²⁹Xe in a magnetic field, in order to precisely measure the magnitude of that field. This work is being done for the ultra-cold neutron (UCN) collaboration’s flagship experiment: to measure the neutron electric dipole moment (EDM). Previous neutron EDM experiments have only found an upper limit, and have been limited in precision largely because of systematic errors in the magnetic field strength measurement. These experiments, such as the one performed at Institut Laue-Langevin (ILL), which has given us the current lowest limit, used ¹⁹⁹Hg as a co-magnetometer. The UCN EDM experiment will add ¹²⁹Xe in addition to the ¹⁹⁹Hg, to make a dual co-magnetometer. By using multiple species of atoms in the measurement, systematic effects can be greatly reduced. I have characterized the coils that I built by performing NMR on protons in water. I measured the inhomogeneity in the B₀ field, across the sample container, to be 18.9±0.9 μT. It turns out that the homogeneity of the B₀ field can be improved significantly, and it will likely be necessary to do so in order to perform similar experiments on hyperpolarized ¹²⁹Xe. I also found the T₁ time of water in this setup to be 2.7±0.2 s.<br>Science, Faculty of<br>Graduate
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Slawig, Anne [Verfasser], and Herbert [Gutachter] Köstler. "Reconstruction methods for the frequency-modulated balanced steady-state free precession MRI-sequence / Anne Slawig ; Gutachter: Herbert Köstler." Würzburg : Universität Würzburg, 2018. http://d-nb.info/1161344284/34.
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Shimada, Kotaro. "Non-contrast-enhanced Hepatic MR Angiography with True Steady-state Free-precession and Time Spatial Labeling Inversion Pulse." Kyoto University, 2011. http://hdl.handle.net/2433/142053.
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Venâncio, Tiago. "\"Novas aplicações da precessão livre em onda contínua em ressonância magnética nuclear de baixa e alta resolução\"." Universidade de São Paulo, 2006. http://www.teses.usp.br/teses/disponiveis/75/75131/tde-18012007-093344/.
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Neste trabalho foi desenvolvido um estudo sobre a precessão livre de onda contínua (CWFP-continuous wave free precession), uma condição especial da precessão livre no estado estacionário (SSFP), e algumas aplicações. Os resultados experimentais, e de simulações, mostraram que este sinal é atingido após a passagem por dois estágios iniciais. O primeiro é dependente da não homogeneidade do campo magnético, relacionado a razão Tp/T2*. O segundo, o qual é chamado de estado quasi-estacionáio, é dependente ambos os tempos de relaxação do sistema em estudo, T1 e T2. Este segundo estágio é responsável por levar a magnetização do estado de equilíbrio térmico a um estado verdadeiramente estacionário. Tendo como informação a amplitude da magnetização no estado de equilíbrio térmico e no estado verdadeiramente estacionário, e também do decaimento do sinal durante o estado quasi-estacionário, é possível determinar, rápida e simultaneamente, ambos os tempos de relaxação do sistema, fazendo um único experimento. Esse método oferece aplicações interessantes para o estudo de processos dinâmicos, propostas também neste trabalho. Foi possível verificar que o sinal de CWFP pode ser utilizado para acompanhar cinética de reações, e também variações de viscosidade do meio, por exemplo, em reações de polimerização, associadas à mobilidade molecular em função de mudanças estruturais. Um método para avaliar a condutividade térmica de elastômeros foi proposto, com o fornecimento de resultados quantitativos muito próximos daqueles encontrados na literatura, e que utilizam outros métodos. A dependência do sinal de CWFP com a freqüência de offset também permitiu realizar um estudo, com aplicação em RMN de alta resolução, sobre a supressão de sinal de solvente. Os resultados demonstraram que é necessário fazer alguns ajustes na largura de pulsos, juntamente com o ciclo de fases, para minimizar as anomalias de intensidade e de fase dos sinais no espectro transformado. Essa técnica foi aplicada em espectroscopia in vivo, a qual permite resolver, por exemplo, problemas como a determinação de açúcares em frutas, onde o sinal é bastante próximo do intenso sinal da água.<br>In this work a study of the continuous wave free precession (CWFP), a special condition of the steady-state free precession (SSFP), and some applications, was developed. The experimental results, together with simulated ones, have shown that the CWFP signal is attained after two previous stages. The first one is dependent on the non-homogeneity of the magnetic field, related to Tp/T2* ratio. The second, which is called quasi-stationary state, is dependent of both relaxation times, T1 and T2. This second stage allows leading the magnetization in the thermal equilibrium to a truly stationary state. The information of the signal amplitude in the thermal equilibrium and in the steady state, and also the signal decaying during the quasistationary state, allows the fast and simultaneous determination of the relaxation times, performed in a single experiment. This method offers interesting applications for studying dynamical processes, also proposed in this work. It was possible to verify that the CWFP signal can be used to monitor kinetics of reactions, which variations of viscosity are involved, for example, in polymerization reactions, always associated to structural changes. A method for evaluating thermal conductivity in elastomers was proposed, with results in accordance to the literature, which uses other methods. The dependence of the CWFP signal with the offset frequency has permitted to study an application to the High-Resolution NMR, about the solvent suppression. The results demonstrated that it is necessary to perform some adjustments in the pulse width, together the phase cycle, to minimize some phase and intensity anomalies of the frequency-domain signal. This technique was applied in in vivo spectroscopy, which can solve, for example, problems with the determination of the sugar content in fresh fruits, where the sugar signal is very close to the intense signal of the water.
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Mantsis, Damianos F. "Atmospheric Response to Orbital Forcing and 20th Century Sea Surface Temperatures." Scholarly Repository, 2011. http://scholarlyrepository.miami.edu/oa_dissertations/597.
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This study investigates modes of atmospheric variability in response to changes in Earth's orbit and changes in 20th century sea surface temperatures (SST). The orbital forcing is manifested by a change in obliquity and precession, and changes the distribution of the top-of-atmosphere insolation. A smaller obliquity reduces the the annual insolation that the poles receive and increases the annual insolation in the tropics. As the meridional insolation gradient increases, the zonal mean atmospheric-ocean circulation increases. The resulting climate also has a reduced global mean temperature due to the effect of climate feedbacks. This cooling can be attributed to a reduced lapse rate, increased cloud fraction. reduced water vapor in the atmosphere, and an increase in the surface albedo. A change in the precession, as the perihelion shifts from the winter to the summer solstice, causes a strengthening as well as an expansion of the N. Pacific summer subtropical anticyclone. This anticyclonic anomaly can be attributed to the weakening of the baroclinic activity, but also represents the circulation response to remote and local diabatic heating. The remote diabatic heating is associated with monsoonal activity in the SE Asia and North Africa. Regarding the 20th century SST forcing, it is represented by a multidecadal variability in the inter-hemispheric SST difference. This change in the SST causes a latitudinal shift in the ascending branch of the Hadley cell and precipitation in the tropics, as well as an increase in the atmospheric meridional heat transport from the warmer to the colder hemisphere.
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Schneider, Gerald Manfred. "Structures and turbulence characteristics in a precessing jet flow /." Title page, contents and summary only, 1996. http://web4.library.adelaide.edu.au/theses/09PH/09phs358.pdf.
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Freitag, Sonja-Marie [Verfasser], and Dirk [Akademischer Betreuer] Blondin. "Kontrastmittelfreie balanced Steady-State Free Precession 3D-MR-Angiographie an Nieren- und Nierentransplantatarterien / Sonja-Marie Freitag. Gutachter: Dirk Blondin." Düsseldorf : Universitäts- und Landesbibliothek der Heinrich-Heine-Universität Düsseldorf, 2012. http://d-nb.info/1029350167/34.
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Maurer, Andrew Porter. "Contributions of self-motion information and theta phase precession to the spatial metric of the dorsal and middle hippocampus." Diss., The University of Arizona, 2008. http://hdl.handle.net/10150/193990.
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This dissertation explores the relationships between self-motion, place field size, and theta phase precession with a primary focus on providing insight into the interactions between theta phase precession and place field size. The present data indicate that place field size increases along the dorsal to ventral axis of the hippocampus. Pyramidal neurons of the middle hippocampus, with larger place fields, exhibit a lower intrinsic burst frequency compared to dorsal place cells. Moreover, the firing rate of neurons in the middle hippocampus show a weaker relationship with running velocity compared with cells in the dorsal CA1 region suggesting that there is a decrease in the self-motion input to this region. By defining place fields as phase shifts up to, but not exceeding 360 degrees, the rate of phase precession is found to significantly correlate with place field size. Moreover, this definition revealed that approximately 10% of the pyramidal neurons will have place fields that overlap in space. Applying this critereon to interneurons reveals that a subset shows a similar spatial metric to those of pyramidal cells, inheriting the activity profiles and spike-phase relationships of the pyramidal cells that they are putatively monosynaptically coupled to. Finally, a reliable reconstruction of the look-ahead phenomenon provides preliminary evidence that suggests an increase in place field size as velocity increases.The results are presented to imply that the influence of the self-motion signals is graded along the dorsal-ventral axis of the hippocampus. These self-motion signals are capable of influencing the neuronal spike times of both pyramidal cells and interneurons on short-time scales of a theta cycle or less. Despite these short-time scale spike timing control mechanisms, preliminary data is presented that the influence of self-motion information with velocity is not enough to maintain a fixed place field size.
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Gao, Ying Gao. "Development of Quantitative Fast Imaging with Steady-State Free Precession (FISP) Techniques for High Field Preclinical Magnetic Resonance Imaging." Case Western Reserve University School of Graduate Studies / OhioLINK, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=case1475150834919997.
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Zhao, Peng. "A STUDY OF SURFACE ACOUSTIC WAVE AND SPIN PRECESSION USING AN ULTRAFAST LASER FOR LOCALIZED ELASTIC AND MAGNETIC PROPERTY MEASUREMENT." The Ohio State University, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=osu1367591435.
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Huxter, John Richard. "Theta phase precession and the temporal representation of space : environmental and behavioural contributions to place cell firing in the rat." Thesis, University College London (University of London), 2004. http://discovery.ucl.ac.uk/1446611/.
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Place cells" fire when a rat enters a particular region of the environment - a rate code for space. As the rat traverses the "place field", the place cell fires earlier in each successive cycle of the theta EEG oscillation - a temporal code, known as "phase precession". Simultaneous recordings were made of place cell action potentials and the local EEG in rats shuttling between the ends of a stationary treadmill for food reward. Probe trials involved 1) changing the height of the walls bounding the ends of the treadmill, 2) compressing the treadmill by bringing the end walls together, and 3) changing the speed at which the treadmill moved. The effects of natural variability in running speed and firing rate were also investigated. Compressing the treadmill compressed place fields, with a proportional increase in phase precession slope. Phase precession was unaffected by other manipulations or natural variability in running speed and firing rate. The moving treadmill produced shifts in place field positions, suggesting that path integration based information plays a direct role in determining where place cells fire. O'Keefe and Recce (1993) proposed that the interference pattern of two oscillatory inputs produced phase precession. Alternatively it has been suggested (Harris et al., 2003; Buzsaki et al., 2002) that increased depolarisation as the rat approaches the centre of the field allows the cell to fire earlier in the theta cycle, where it would normally be inhibited. Here, the rate and temporal codes were shown to be dissociable, suggesting that a depolarisation model is insufficient to explain phase precession. Firing phase probably encodes information about position in space, with the rate of visual change in the environment determining the rate of one of the oscillators in a dual oscillator model. Firing rate is free to encode other variables, such as running speed.
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SASSIER, ERIC. "Magnetometres a precession libre de noyaux d'3he et a jonction josephson. Contribution a la realisation d'une reference de champ magnetique." Caen, 2000. http://www.theses.fr/2000CAEN2033.
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L'etalonnage de magnetometres absolus a l'aide d'un banc de calibration fonctionnant dans le champ terrestre suppose l'utilisation de magnetometres de reference. La sonde a 3he developpee au ciril exploite la precession libre du noyau de cet atome, et en fournit une fiable pour les champs de l'ordre de grandeur du champ terrestre. Les magnetometres josephson fraunhofer meissner (jfm) a haute temperature critique (htc) sont susceptibles d'en fournir une pour les champ faibles. Les deux premieres parties de ce memoire sont chacune consacree a l'un de ces capteurs, dont nous etudions d'abord la reponse au champ magnetique, et dont nous contribuons ensuite a l'optimisation. Concernant les jfm, nous presentons une methode de simulation de l'effet de structures concentratrices carrees, debouchant sur quelques principes generaux d'optimisation. Nous proposons enfin une methode de mise en uvre adaptee au capteur. Concernant la sonde a 3he, nous etudions le comportement frequentiel de la resistance apparente des bobines de capture du signal de precession. Nous abordons ensuite le probleme de l'adaptation en bruit de l'amplificateur et proposons l'utilisation d'un etage preamplificateur a squid htc fonctionnant sans blindage magnetique, avec une bobine d'entree en fil de cuivre. La troisieme partie presente les performances des dispositifs realises d'apres les analyses precedentes et l'integration de ces magnetometres au sein d'un banc de calibration operationnel. Les performances du demonstrateur d'amplificateur a squid realise sont conformes aux previsions. Nous obtenons egalement une resolution de 15 pt r m s avec un amplificateur plus classique. Nous obtenons d'autre part un jfm exhibant une sensibilite de 550 v/t et une resolution de 600 ft/hz 1 / 2 pour f>500 hz. Nous avons enfin realise une structure de bobines carrees (2 2 m) permettant de compenser le champ terrestre a 20 nt pres sur un volume de 1 dm 3, et presentons un protocole de calibration utilisant cette structure.
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Accioly, Artur Difini. "Estudo do efeito de transferência de spin." reponame:Biblioteca Digital de Teses e Dissertações da UFRGS, 2011. http://hdl.handle.net/10183/29508.
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A ideia de transferência de spin, como forma de controle da magnetização, foi introduzida independentemente por Slonczewski e por Berger em 1996. Desde então, esse efeito tem sido alvo de inúmeras pesquisas, em especial pela possibilidade de aplicações em memorias magnéticas não voláteis e em osciladores de alta frequência. Devido _a complexidade do problema, a grande maioria das pesquisas teóricas sobre o assunto _e baseada em resultados numéricos. Porém, esses métodos podem dificultar a visualização das influências individuais dos diferentes termos envolvidos. Para isso, seria melhor a utilização de métodos analíticos, o que nos motiva a buscar por esses resultados. Nesse trabalho, apresentamos uma revisão sobre a teoria básica do efeito de transferência de spin e da dinâmica da magnetização. São revistas as principais equações que descrevem o comportamento da magnetização, as equações de Landau-Lifshitz e de Landau-Lifshitz-Gilbert, e comparadas suas componentes quando da inclusão do termo de transferência, analisando a melhor forma de incluir esse termo. É destacada a contribuição dada pelo termo de transferência na frequência de precessão da magnetização, que aparece ao se utilizar a equação de Landau-Lifshitz-Gilbert. Após essa revisão dos conceitos base, são buscadas soluções analíticas para a dinâmica da magnetização da camada livre de um sistema nanopilar em tricamada. Quatro casos são analisados: primeiro um sistema sem anisotropias e sem a inclusão do campo de Oersted, no segundo caso é incluído um termo de anisotropia e no terceiro novamente um sistema sem anisotropias, mas com a inclusão do campo de Oersted. Todas essas análises são feitas em uma aproximação de macrospin. Por último, uma aproximação de microspin com campo de Oersted. Nos três primeiros casos, é possível obter resultados analíticos e simular os resultados. São estimados o tempo de reversão e a frequência de precessão estável.<br>The idea of spin transfer as a way to control magnetization was introduced independently by Slonczewski and Berger in 1996. Since then, this e ect has been the subject of numerous studies, especially for potential applications in nonvolatile magnetic memories and high-frequency oscillators. Due to the complexity of the problem, the vast majority of theoretical research on this subject is based on numerical results. However, these methods might not display the in uences of individual terms involved. For this, it would be better to use analytical methods, which motivates us to search for these results. In this paper, we review the basic theory of spin transfer e ect and of magnetization dynamics. We review the main equations that describe the behavior of magnetization, the Landau-Lifshitz and Landau-Lifshitz-Gilbert equations, and compare its components when inserting the spin torque term, analyzing the best way to include this term. The contribution of spin transfer on magnetization precession frequency, which appears when using the Landau-Lifshitz- Gilbert equation, is emphasized. After this review of basic concepts, analytical solutions for magnetization dynamics of the free layer in a tri-layer nanopillar are searched. Four cases are analyzed: rst a system without anisotropy and without the inclusion of the Oersted eld, in the second case an anisotropy term is considered and in the third case, again a system without anisotropy, but with the inclusion of Oersted eld. All these analisys are done in a macrospin approximation. Finally, a microspin approach including Oersted eld. In the rst three cases, it is possible to obtain analytical results and simulate these results. Reversal time and stable precession frequency values are estimated.
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Millot, Yannick. "Nouvelles applications des noyaux quadrupolaires en rmn : - gradient de champ electrique dans la zeolithe y ; - dynamique des spins : precession, nutation et rotation." Paris 6, 2001. http://www.theses.fr/2001PA066168.
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Cette these a pour objet l'etude de nouvelles applications des noyaux quadrupolaires en rmn. Nous avons quantifie le gradient de champ electrique (gce) dans la zeolithe y avec trois methodes. Deux d'entre elles utilisent le 1 3 1xe, celui-ci etant affecte par le ringing de la sonde, nous avons cherche une sequence d'impulsions capable de l'eliminer. Ces methodes ont ete appliquees sur des zeolithes y desaluminees avec vapeur d'eau, ainsi qu'aux zeolithes echangees aux lanthanes. La premiere methode se fonde sur la difference des deplacements chimiques du 1 3 1xe et du 1 2 9xe, cette difference est proportionnelle au carre du gce. La deuxieme methode est basee sur la nutation du 1 3 1xe ; nous avons mis au point un programme informatique, a l'aide duquel nous determinons la valeur du gce par ajustement de l'integrale de la raie avec son expression theorique. La troisieme methode utilise le mq-mas (multiple-quantum magic-angle-spinning) qui nous a permis de determiner le gce des differents environnements des atomes d'aluminium dans des zeolithes y desaluminees (avec et sans vapeur d'eau). A partir de l'etude approfondie de cette sequence, nous avons mis en evidence les liens existant entre le spin etudie, le cycle de phase et le traitement des donnees experimentales. De plus, nous proposons une nouvelle convention pour la graduation de l'axe f1 d'un spectre a deux dimensions et nous apportons une solution aux problemes de repliements des raies d'un spectre mq-mas. La rmn utilise constamment la rotation a travers la precession, la nutation, les changements de referentiel nous avons traite la rotation de facon classique et quantique, et plus particulierement la matrice rotation de wigner qui peut etre utilisee lorsque l'interaction quadrupolaire est exprimee sous forme de tenseurs spheriques irreductibles. Nous donnons les differentes formes de cette matrice en fonction des conventions choisies.
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Casula, V. (Victor). "Quantitative magnetic resonance imaging methods for evaluation of articular cartilage in knee osteoarthritis:free-precession and rotating-frame relaxation studies at 3 Tesla." Doctoral thesis, Oulun yliopisto, 2016. http://urn.fi/urn:isbn:9789526213521.
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Abstract Osteoarthritis (OA) is a common chronic condition that reduces the quality of life of millions of individuals worldwilde. In OA, the progressive degradation of articular cartilage (AC) and bone can cause joint pain and disability. Currently there is no effective treatment available for OA besides gross joint replacement surgery. This is largely due to the lack of accurate biomarkers to test potential drugs and treatments that could stop or reverse the disease progression. The primary tools in use for OA diagnostics are inadequate in the early stage of the disease and merely allow its late manifestations to be visualized. Quantitative MRI (qMRI) has demonstrated the ability to distinguish degenerated from intact AC prior to radiographic changes. Moreover, so-colled rotating frame relaxation time parameters are sensitive to the slow molecular motion domain, relevant for clinical applications. T1ρ and T2ρ relaxation time measurements using adiabatic spin-lock pulses (AdT1ρ and AdT2ρ) have shown superior sensitivity to cartilage degeneration in vitro over conventional qMRI parameters. This thesis aimed to compare the differences that exist between established qMRI methods for AC against arthroscopic evaluation as well as optimize and validate AdT1ρ and AdT2ρ mapping of cartilage in vivo. The findings showed that T1 and T2 relaxation time and delayed gadolinium enhanced MRI of cartilage (dGEMRIC) were able to discriminate among different arthroscopic grades of cartilage lesions. However, arthroscopic findings and qMRI parameters were not correlated, supporting the viewpoint that qMRI may be able to elicit more comprehensive information on the quality of remaining cartilage tissue as compared to diagnostic arthroscopy, which detects cartilage loss. AdT1ρ and AdT2ρ mapping of cartilage in vivo were successfully implemented and validated on a clinical MRI system. AdT1ρ and AdT2ρ were able to overcome limitations of the existing methods (continuous wave spin-lock approach), which are challenging to implement on clinical settings for their susceptibility to field inhomogeneity and relatively high power deposition in tissue. The association of AdT1ρ and AdT2ρ with cartilage and bone marrow lesions and osteophytes was demonstrated. The findings indicate the utility of AdT1ρ and AdT2ρ mapping as potential biomarkers for evaluation of early cartilage degeneration in OA research and clinical applications<br>Tiivistelmä Nivelrikko on tavallinen krooninen sairaus, joka huonontaa miljoonien ihmisten elämänlaatua ympäri maailman. Nivelrikossa nivelruston ja luun asteittain etenevä rappeuma voi aiheuttaa nivelkipua ja liikuntakyvyttömyyttä. Tällä hetkellä nivelrikkoon ei ole muuta tehokasta hoitokeinoa kuin tekonivelleikkaus. Tämä johtuu suuremmaksi osaksi tarkkojen biomarkkereiden puutteesta, joiden avulla voitaisiin kokeilla mahdollisia nivelrikon pysäyttäviä tai tautia parantavia lääkkeitä tai hoitokeinoja. Nykyisin nivelrikon diagnostiikassa käytetyimmät menetelmät ovat riittämättömiä sairauden alkuvaiheessa, ja ne tunnistavat vain sairauden loppuvaiheet. Kvantitatiivisilla magneettikuvausmenetelmillä pystytään erottamaan rappeutunut ja ehjä nivelrusto toisistaan ennen röntgenkuvissa havaittavia muutoksia. Nk. pyörivän koordinaatiston relaksaatioaikaparametrit ovat herkkiä havaitsemaan hidasta molekyyliliikettä, jolla on merkitystä kliinisissä sovelluksissa. Kokeellisissa olosuhteissa T1ρ- ja T2ρ-relaksaatioaikojen mittaaminen käyttäen adiabaattisia spin-lukkopulsseja (AdT1ρ ja AdT2ρ) on osoittautunut erityisen tarkaksi nivelruston rappeutumisen toteamiseksi verrattuna perinteisiin kvantitatiivisiin magneettikuvausmenetelmiin. Tämän väitöskirjan tavoitteena oli vertailla vakiintuneita nivelrikon kvantitatiivisia magneettikuvausmenetelmiä sekä optimoida ja validoida AdT1ρ- ja AdT2ρ-menetelmien käyttöä ihmisen ruston tutkimisessa. Tulokset osoittivat, että T1- ja T2-relaksaatioajat sekä varjoainetehosteinen nivelruston magneettikuvaus pystyivät erottelemaan niveltähystyksessä määritettyjä nivelrikon eri asteita. Kuitenkaan niveltähystyslöydökset ja kvantitatiiviset magneettikuvausparametrit eivät korreloineet viitaten kvantitatiivisen magneettikuvauksen kykyyn osoittaa tarkemmin jäljellä olevan ruston laatua verrattuna niveltähystykseen, jossa arvioidaan ruston puutosta. Nivelruston AdT1ρ- ja AdT2ρ-relaksaatioaikamittaukset onnistuttiin suorittamaan ja validoimaan vapaaehtoisilla koehenkilöillä kliinisellä magneettikuvauslaitteella. AdT1ρ ja AdT2ρ eivät kärsineet samoista puutteista kuin tavanomaiset jatkuva-aaltoiset spin-lukkomenetelmät, joka ovat herkkiä kenttäepähomogeenisuuksille ja aiheuttavat suuremman radiotaajuusaltistuksen. Tutkimuksessa osoitettiin AdT1ρ- ja AdT2ρ-mittausten ja luuödeemin sekä osteofyyttien välinen yhteys. Tulokset osoittivat, että AdT1ρ- ja AdT2ρ-mittaukset ovat potentiaalisia biomarkkereita varhaisen nivelruston rappeuman havaitsemiseksi tutkimus- ja kliinisessä käytössä
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Kienzle, Melina [Verfasser]. "Darstellung und Quantifizierung der Myelonbewegung mittels MRT unter Einsatz von cine true fast imaging with steady-state Precession-Sequenzen und Phasenkontrast / Melina Kienzle." Ulm : Universität Ulm. Medizinische Fakultät, 2012. http://d-nb.info/1019563362/34.
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Mirijanian, James Julian. "Techniques to Characterize Vapor Cell Performance for a Nuclear-Magnetic-Resonance Gyroscope." DigitalCommons@CalPoly, 2012. https://digitalcommons.calpoly.edu/theses/724.
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Research was performed to improve the procedures for testing performance parameters of vapor cells for a nuclear-magnetic-resonance gyroscope. In addition to summarizing the theoretical infrastructure of the technology, this research resulted in the development and successful implementation of new techniques to characterize gyro cell performance.
One of the most important parameters to measure for gyro performance is the longitudinal spin lifetime of polarized xenon atoms in the vapor cell. The newly implemented technique for measuring these lifetimes matches results from the industry standard method to within 3.5% error while reducing the average testing time by 76% and increasing data resolution by 54%. The vapor cell test methods were appended with new software to expedite the analysis of test data and to investigate more subtle details of the results; one of the two isotopes of xenon in the cells tends to exhibit troublesome second-order effects during these tests due to electric-quadrupole coupling, but now the added analysis capabilities can accurately extract relevant results from such data with no extra effort. Some extraneous lifetime measurement techniques were explored with less substantial results, but they provided useful insight into the complex workings of the gyro cell test system.
New criteria were established to define the signal to noise ratio on a consistent basis from cell to cell across various parameters such as cell volume, temperature, and vapor pressure. A technique for measuring gas pressures inside the sealed cells helped link cell performance to cell development processes. This led to informed decisions on filling and sealing methods that consistently yielded cells with better performance in the last few months of this work. When this research began, cells with xenon lifetimes over ten seconds were rare in our lab; by the end, anything under 30 seconds was a disappointment. Not only did the test procedures improve, but so did the parameters being tested, and quite significantly at that. At the same time, many new avenues for continued progress have been opened; the work presented here, while instrumental, is only the beginning.
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Huo, Changming. "A Bloch Sphere Animation Software using a Three Dimensional Java Simulator." University of Cincinnati / OhioLINK, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1249917839.
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Kawahara, Seiya. "Non-contrast-enhanced hepatic MR arteriography with balanced steady-state free-precession and time spatial labeling inversion pulse: optimization of the inversion time at 3 Tesla." Kyoto University, 2017. http://hdl.handle.net/2433/228217.
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Satogami, Naoe. "Visualization of external carotid artery and its branches: Non-contrast-enhanced MR angiography using balanced steady-state free-precession sequence and a time-spatial labeling inversion pulse." Kyoto University, 2011. http://hdl.handle.net/2433/142043.
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Frye, Christopher. "Modification to Einstein's field equations imposed by string theory and consequences for the classical tests of general relativity." Honors in the Major Thesis, University of Central Florida, 2013. http://digital.library.ucf.edu/cdm/ref/collection/ETH/id/852.
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String theory imposes slight modifications to Einstein's equations of general relativity (GR). In (4), the authors claim that the gravitational field equations in empty space, which in GR are just R [subscript greek letters mu nu ] = 0, should hold one extra term which is first order in the string constant [alpha'] and proportional to the Riemann curvature tensor squared. They do admit, however, that this simple modification is just schematic. In (1) the authors use modified equations which are coupled to the dilation field. We show that equations given in (4) do not admit an isotropic solution; justification of these equations would require sacrificing isotropy. We thus investigate the consequences of the coupled equations from (1) and the black-hole solution they give there. We calculate the additional perihelion precession of Mercury, the added deflection of photons by the sun, and the extra gravitational redshift which should be present if these equations hold. We determine that additional effects due to string theory in each of these cases are quite minuscule.<br>B.S.<br>Bachelors<br>Sciences<br>Physics
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Vidal, Jérémie. "Orbital forcings of a fluid ellipsoid. Inertial instabilities and dynamos." Thesis, Université Grenoble Alpes (ComUE), 2018. http://www.theses.fr/2018GREAU004/document.
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Les instabilités inertielles sont des instabilités fluides excitées au sein de modèles physiques simplifiés de planètes ou d'étoiles. Elles peuvent générer un champ magnétique dynamo. Ce sont donc des alternatives aux écoulements forcés par la convection thermo-chimique pour générer les champs magnétiques dans les noyaux liquides des planètes et les enveloppes fluides des étoiles. Cependant, ces modèles simplifiés questionnent la pertinence des résultats, qui sont ensuite extrapolés aux contextes géo- et astrophysique. D'un point de vue fondamental, de récentes études numériques et expérimentales, réalisées à grande ellipticité pour compenser l'importance des effets visqueux dans les modèles, semblent en désaccord avec les prédictions théoriques (valides dans la limite asymptotique d'une diffusion négligeable et à faible déformation). De plus, de nombreux effets physiques sont négligés dans les modèles. Par exemple, seules les orbites circulaires ont été considérées. Bien que généralement de faible amplitude, l'excentricité induit une dépendance temporelle dans le forçage orbital, ce qui pourrait générer de nouveaux phénomènes. Enfin, l'existence des instabilités inertielles dans les enveloppes fluides stablement stratifiées en densité, comme les zones radiatives des étoiles chaudes de masse intermédiaire (dont la masse est comprise entre une et huit masses solaires), reste incertaine. La génération de champs magnétiques dynamos dans ces enveloppes stratifiées permettrait de réconcilier les modèles avec les observations astronomiques. Lors de cette thèse, nous nous sommes attachés à rapprocher les modèles (théoriques, numériques ou expérimentaux) des contextes géo et astrophysique. Nous avons combiné les approches théoriques (analyses de stabilité locale et globale) et numériques (simulations non linéaires) afin d'étudier les effets des forc cages mécaniques de rotation dans un ellipsoïde fluide. Nous montrons que la dissipation en volume n'est en fait pas négligeable dans les expériences de laboratoire et les simulations numériques, contrairement aux régimes planétaires et stellaires. Nous montrons aussi que l’excentricité orbitale peut, via la variation temporelle des axes de l’ellipsoïde, générer des instabilités fluides pour dans une gamme de paramètres où elles n’étaient pas attendues. Enfin nous avons étudié la capacité dynamo de l'instabilité de marée, dans les enveloppes stablement stratifiées en densité des étoiles chaudes de masse intermédiaire. Environ 10~% de ces étoiles ont un champ magnétique de surface, dont l’origine reste énigmatique. Nous montrons que l’instabilité de marée peut générer des dynamos de grande échelle dans les enveloppes fluides stablement stratifiées. En particulier, ce mécanisme serait susceptible d’expliquer le champ magnétique de faible intensité des étoiles en rotation rapide similaires à Vega et déformées par un compagnon orbital<br>Inertial instabilities are fluid instabilities excited by mechanical forcings (e.g. tides, precession) in fluid bodies (e.g. planetary liquid cores or stellar envelopes) orbited by celestial companions. The nonlinear outcome of these instabilities can drive self-sustained, dynamo magnetic fields. Thus they could be an alternative to thermo-chemical convection to generate magnetic fields in geophysics and astrophysics. These instabilities have only been studied in idealised models, which challenges the extrapolation towards the relevant regimes in geophysics and astrophysics. Recent laboratory and numerical studies, performed in the achievable range of parameters (i.e. large deformations and overestimated diffusive effects), seem apparently not in agreement with theoretical predictions representative of celestial fluid bodies (i.e. extremely small deformations and vanishing diffusive effects). Several physical ingredients have been also neglected, such as the orbital eccentricity. This could drive additional tidal effects, as a result of the time-dependent forcing. Similarly, density variations have been largely neglected in these models. However, rotationally powered magnetic fields in stably stratified stellar envelopes could reconcile astronomical observations with dynamo models. In this thesis we have adopted more realistic models, by combining theoretical tools (linear stability analyses in unbounded and bounded fluids enclosed in ellipsoids) and numerical ones (direct numerical simulations) to study rotationally driven inertial instabilities. We show, with a linear stability analysis in bounded ellipsoidal geometry, that bulk diffusion cannot be neglected emph{a priori} compared to the boundary layer diffusion in laboratory experiments and simulations. This phenomena is not expected in celestial fluid bodies. We also show that an orbital eccentricity could generate additional instabilities in deformed bodies, for orbital configurations which were believed to be linearly stable. Finally, we have studied the dynamo capability of tidal flows in stably stratified fluid envelopes. These are idealised models of hot, intermediate-mass stars (i.e. with a mass ranging from one to eight solar masses). Approximatively 10~% of hot stars exhibit observable magnetic fields. We show that the tidal instability can drive dynamo magnetic fields of large wavelength in stably stratified fluids. Predictions obtained with this tidal model seem consistent with the ultra-weak magnetism of rapidly rotating, tidally deformed Vega-like stars
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Hesselbrock, Andrew J. "A PERTURBED MOON: SOLVING NEREID'S MOTION TO MATCH OBSERVED BRIGHTNESS VARIATIONS." Miami University / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=miami1344100992.
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