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. / 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.

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. / 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.

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 / 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

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. / 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.

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. / 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.

No description available.

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.

Precession photography was used to determine the lattice parameters, the crystal system, the space group and the structure of an artificial Sr-labradorite of composition: Ab27, SrAn73. C= 7.107Å, Ɣ= 90 degrees, β= 115.834 degrees. The crystal system is monoclinic, space group= C2/ m and structure is albite type, reflections being restricted to the 'a' type. The abundance, lithophile characteristics and appropriate ionic radii of elements in Groups IA and IIA are the factors governing their presence as cations of feldspars in nature. The structures of feldspars are discussed as a function of the relative proportion of cations of a charge of +1 and +2. The crystal symmetry (i.e. monoclinicity or triclinicity) is discussed as a function of the ionic radius of the cation. / Thesis / Bachelor of Science (BSc)

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.

No description available.