Le contrôle de l’aimantation sans application de champ magnétique externe est un domaine de recherche en plein essor, étant prometteur pour les applications technologiques d’enregistrement magnétique et de spintronique. En 2007, Stanciu et al. ont découvert la possibilité de retourner l’aimantation dans un film fait d’alliage ferrimagnétique de GdFeCo en utilisant des impulsions laser femtoseconde. Longtemps cantonné aux alliages de GdFeCo, ce retournement tout-optique s’avère un phénomène plus général, puisqu’il a été mesuré plus récemment dans une large variété de matériaux ferrimagnétiques et ferromagnétiques. Cette découverte a ainsi ouvert la voie à l’intégration de l’écriture tout-optique dans l’industrie des mémoires magnétiques. Néanmoins, l’ensemble des modèles théoriques expliquant le retournement tout-optique dans le GdFeCo ne semblent pas s’appliquer aux autres matériaux magnétiques, mettant ainsi en question l’unicité de l’origine microscopique de ce phénomène. Au cours de cette thèse, nous avons étudié la réponse aux impulsions laser femtoseconde des alliages ferrimagnétiques et des multicouches ferromagnétiques, dans l'objectif d'élucider divers aspects du mécanisme du retournement optique. Nous avons élucidé expérimentalement les paramètres magnétiques gouvernant le retournement tout-optique. Nous avons montré que l’observation du retournement tout-optique nécessite des domaines magnétiques plus grands que la taille du faisceau laser pendant le processus de refroidissement, un critère qui est commun à la fois aux matériaux ferrimagnétiques et ferromagnétiques. En outre, nous nous sommes intéressés à l’intégration du retournement tout-optique dans des dispositifs de spintronique. Grâce à une caractérisation temporelle de l’aimantation dans des croix de Hall via l’effet Hall extraordinaire, nous avons distingué entre deux types de mécanismes du retournement optique. Le premier type est un retournement purement thermique obtenu avec une impulsion unique dans les alliages ferrimagnétiques de GdFeCo, tandis que le deuxième type est un retournement cumulative et à deux régimes dans les alliages ferrimagnétiques de TbCo et les multicouches ferromagnétiques de Co/Pt. Ce dernier consiste en une formation indépendante de l’hélicité de multidomaines magnétiques suivie d'une ré-aimantation dépendante de l'hélicité sur plusieurs dizaines de millisecondes. / The control of magnetization without external magnetic fields is an emergent field of research due to the prospect of impacting many technological applications such as magnetic recording and spintronics. In 2007, Stanciu et al. discovered an intriguing new possibility to switch magnetization in a ferrimagnetic GdFeCo alloy film using femtosecond laser pulses. This all-optical switching of magnetization had long been restricted to GdFeCo alloys, though it turned out to be a more general phenomenon for a variety of ferromagnetic and ferromagnetic materials. This discovery paved the way for an integration of the all-optical writing in storage industries. Nevertheless, the theoretical models explaining the switching in GdFeCo alloys films do not appear to apply in the other materials, thus questioning the uniqueness of the microscopic origin of all-optical switching. In this thesis, we have investigated the response of femtosecond laser pulses in ferrimagnetic alloys and ferromagnetic multilayers to the action of femtosecond laser pulses, in order to elucidate several aspects of the all-optical switching mechanism. We have experimentally studied the magnetic parameters governing the all-optical switching. We showed that the observation of all-optical switching requires magnetic domains larger than the laser spot size during the cooling process; such a criterion is common for both ferrimagnets and ferromagnets. Furthermore, we have investigated the integration of all-optical switching in spintronic devices via the anomalous Hall effect. Through a time-dependent electrical investigation of the magnetization in Hall crosses, we distinguished between two types of all-optical switching mechanisms. The first type is the single-pulse helicity-independent switching in ferrimagnetic GdFeCo alloy films as shown in previous studies, whereas the second is a two regimes helicity-dependent switching in both ferrimagnetic TbCo alloys and ferromagnetic Co/Pt multilayers. The latter consists in a step-like helicity-independent multiple-domain formation followed by a helicity-dependent remagnetization on several tens of milliseconds.

This work has involved the study of the magnetic behaviour of small magnetic nanoparticle systems. Due to the reduced size of magnetic nanoparticles they present distinctive properties, such as size and surface effects, that have been analysed in this work, as well as the effect of interactions in such systems. The samples chosen for the study were magnetite particles in the form of a ferrofluid and Co nanoclusters in a nonmagnetic matrix of Cu. Both systems present very narrow particle size distributions, which facilitates the interpretation of the data. The samples have been subjected to basic characterisation, which includes the determination of the distribution of magnetic particle sizes using the magnetisation curves at room temperatures, TEM microscopy and X-ray diffraction, in the case of the ferrofluid samples. For the nanoclusters, a time of flight spectrometer has been used to obtain the number of atoms per cluster. Many of the measurements have been performed at low temperatures, where thermal effects are minimised. For such measurements the samples have been frozen in a zero applied field, so that they have a random distribution of magnetic moments prior to the measurement. The energy barrier distributions have been calculated via the temperature decay of remanence (TDR). From this study, an effective anisotropy constant has been calculated. For the study of the interactions, surface and size effects, magnetisation, susceptibility (ZFC), remanence and delta-M curves, as well as the time dependence of magnetisation have been studied. The attempt frequency of the different particle size systems has been calculated using different techniques. The basic magnetic behaviour can be explained on the basis of the Neel blocking model. It has been found that the systems with the smaller particles have significant surface effects, which are enhanced at lower temperatures. Interactions, which are weak due to the low concentration of magnetic material in the samples (<10%), have been found to be overall demagnetising and the evolution of the magnetic properties with dilution has been explained. As is the case for the surface effects, interaction effects are stronger at low temperatures due the reduction of thermal effects. The experimental results have been compared with calculations from a Montecarlo model for fine particles, which includes the effects of concentration, anisotropy, particle size and temperature.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / CoFeB thin films were produced using the technique of magnetronsputtering. Six samples were produced magnetic thin films with two different thicknesses, using different settings for shading masks. For each film were made two measurements of magnetization, a parallel to the easy axis of magnetization, and the other perpendicular to the axis. The films produced with shading masks non-magnetic show a deteriorated magnetic configuration compared to those produced with magnetic material with a cut. Different thicknesses of the films provide different values in the coercive field of the sample. The thicker films have an increase in the coercive field. With these results there is a possibility of making tunnel junctions measures aimed tunnel magneto-resistance with CoFeB films as an electrode. / Filmes finos de CoFeB foram produzidos utilizando a técnica de magnetron sputtering. Foram produzidas seis amostras de filmes finos magnéticos com duas espessuras diferentes, utilizando diferentes configurações para máscaras de sombreamento. Para cada filme foram feitas duas medidas de magnetização, uma paralela ao eixo fácil de magnetização, e outra perpendicular ao eixo. Os filmes produzidos com máscaras de sombreamento não magnéticas apresentam uma configuração magnética deteriorada em comparação aos produzidos com máscaras de material magnético com um corte. Diferentes espessuras dos filmes proporcionam diferentes valores do campo coercivo da amostra, para os filmes mais espessos temos um aumento no campo coercivo. Com esses resultados há uma possibilidade de confecção de junções túnel visando medidas de magneto resistência túnel com filmes de CoFeB como eletrodo.

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.

Les travaux présentés dans ce manuscrit consistent à élaborer par la méthode céramique des oxydes ferromagnétiques de type pérovskite et à étudier leurs propriétés physiques (structurales, magnétiques, magnétocaloriques..). Nous avons commencé ce travail par la synthèse de séries de manganites à base de praséodyme ( Pr0.6-xEuxSr0.4MnO3 et Pr0.6-xErxSr0.4MnO3 ) et de lanthane ( La0.6Sr0.2Ba0.2-x□xMnO3 et (La0.6Sr0.2Ba0.2MnO3)1-x /(Co2O3)x ) en utilisant la méthode solide-solide à haute température. Les échantillons élaborés ont été caractérisés par diffractométrie de poudre RX. Les diffractogrammes obtenues ont été affinés par la méthode Rietveld en utilisant le logiciel Fullprof. L'affinement structural a montré que les manganites synthétisés se présentent sous forme de phases pures avec des raies fines et intenses sans phases parasites et cristallisent dans des structures déformées.Des mesures magnétiques (M(T) et M(H)) ont permis d'obtenir des informations sur le comportement magnétique à basse température, les transitions magnétiques et l'évaluation de l'aimantation à saturation. L'aimantation en fonction de la température montre que ces manganites présentent des transitions ferromagnétiques - paramagnétiques et que leurs températures de Curie diminuent sous l'effet de la substitution dans le cas des composés à base du praséodyme et sous l'effet de l'introduction des lacunes dans le système basé sur le lanthane. Les isothermes M (H) confirment le comportement ferromagnétique à basses températures des échantillons étudiés. A partir de ces mesures et en utilisant les relations de Maxwell, on a déterminé les variations d'entropie magnétique ∆Sm et on a évalué l'effet magnétocalorique présent dans ces matériaux. Via la connexion entre la chaleur spécifique et l'aimantation, on a déterminé la variation de la chaleur spécifique ∆Cp dans tous ces échantillons en exploitant les résultats ∆Sm. Nos résultats confirment que les grandeurs caractéristiques de l'effet magnétocalorique sont très sensibles au champ magnétique appliqué, d'où l'étude de leur dépendance en champ magnétique présente un très grand intérêt. Cette dépendance en champ magnétique de la variation d'entropie magnétique peut être exprimée selon une loi de puissance de type ∆Sm ~ a (µ0H)n où n est appelé exposant local. Cette étude permet donc d'une part, d'identifier les matériaux qui se comportent de façon similaire et les voies d'amélioration de ces propriétés et elle constitue, d'autre part, un outil intéressant permettant d'extrapoler ces propriétés dans des conditions non accessibles au laboratoire. / The studies presented in this manuscript deal with the synthesis and characterization of ferromagnetic perovskite oxides. Four material systems have been described in this work ( Pr0.6-xEuxSr0.4MnO3 et Pr0.6-xErxSr0.4MnO3 ) et lanthanum ( La0.6Sr0.2Ba0.2-x□xMnO3 et (La0.6Sr0.2Ba0.2MnO3)1-x /(Co2O3)x ) . Our samples have been synthesized using the solid-state reaction method at high temperatures. Rietveld refinement of the X-ray diffraction patterns using Fullprof program shows that all our samples are single phase and crystallize in the distorted structures. Magnetic measurements show that all our samples exhibit a paramagnetic–ferromagnetic transition with decreasing temperature. The Curie Temperature TC shifts to lower values with increasing substitution in the Pr0.6-x(Eu or Er)xMnO3 system and under the effect of barium deficiency in the La0.6Sr0.2Ba0.2-x□xMnO3 system. From the magnetization isotherms at different temperatures, magnetic entropy change ∆Sm and relative cooling power RCP have been evaluated. By means of the connection between the specific heat and the magnetization was determined the variation of the specific heat ΔCp in these samples using the results ΔSm. Our results confirm that the characteristic values of the magnetocaloric effect are very sensitive to the applied magnetic field, where the study of their dependence on magnetic field has a very great interest. For fixed temperatures, the magnetic field dependence of magnetic entropy change ∆Sm is accounted for by the n exponent, which may be derived by a numerical fitting to the formula ∆Sm ~ a (µ0H)n where a is a constant. This study allows one hand, identify materials that behave similarly and ways to improve these properties and it is, on the other hand, a useful tool to extrapolate these properties under conditions not accessible in the laboratory.

Un magnétomètre microSQUID basse température couplé à une antenne micro-onde a été utilisé pour sonder la dynamique du retournement assisté de l'aimantation d'une nanoparticule ferromagnétique. Grâce au développement d'une technique de mesure originale, basée notamment sur le contrôle de l'amplitude et de la phase du champ micro-onde, nous avons pu mettre en évidence les bassins d'attraction liés aux modes de précession présents dans la dynamique de l'aimantation. Il devient possible de contrôler le retournement de l'aimantation selon que l'amplitude et la phase du champ AC sont judicieusement choisis dans l'un ou l'autre des bassins d'attraction. Nous avons pu mettre en évidence un fait contre-intuitif où le retournement est bloqué par une phase "mal" choisie alors que l'amplitude est largement suffisante pour retourner le système avec une autre phase. De plus, nous avons pu utiliser la sensibilité des bassins d'attraction aux paramètres gouvernant la dynamique de l'aimantation pour déterminer expérimentalement une valeur de la constante d'amortissement de Gilbert α. C'est d'ailleurs la première mesure de la constante d'amortissement sur une nanoparticule unique. / A low temperature microSQUID magnetometer coupled to a microwave antenna was used to probe the assisted switching of the magnetization of a ferromagnetic nanoparticle. Using an original measurement protocol based on the control of the microwave amplitude and phase, we studied the basins of attraction of the precessional modes of the magnetization dynamics. The switching of the magnetization can thus be controlled by choosing an amplitude and phase in one particular basin. In particular, we evidenced the counter intuitive fact that the magnetization switching can be prevented by choosing a “wrong” phase, even though the amplitude is high enough to switch the system with another phase. Moreover, we used the basin's sensitivity to the parameters governing the magnetization dynamics to determine a value for the Gilbert's damping constant α. This is the first measurement of the damping constant on an individual nanoparticle.

L’auto-oscillateur à transfert de spin est un nano système dont la preuve de concept fut donnée au début des années 2000. Il possède un fort potentiel applicatif dans le domaine des sources microonde à fréquence variable. Ses avantages sont : une fréquence émise dans la gamme RF (f ≈ 200 MHz – 20 GHz), une haute accordabilité grâce à la non linéarité et la possibilité de s’intégrer avec les procédés de l’électronique traditionnelle. Néanmoins à cause de sa taille, l’oscillateur est fortement perturbé par l’agitation thermique et il faut chercher différentes stratégies pour augmenter sa cohérence spectrale. Une solution prometteuse est la synchronisation mutuelle d’oscillateur. Afin de définir les meilleurs schémas de synchronisation, il est important d’étudier et de caractériser la synchronisation à une source externe.Ce travail, d’ordre théorique, traite la synchronisation d’un oscillateur spintronique à un signal microonde. L’équation de mouvement de l’aimantation (équation de Landau-Lifshitz-Gilbert-Slonczewski) est transformée en une équation d’amplitude complexe, qui permet de traiter les différents mécanismes de synchronisation de manière unifiée. En particulier, nous introduisons les forçages canoniques de l’oscillateur, transformés des couples {m x ui, m x m x ui}i=x,y,z et discutons leurs propriétés de synchronisation en fonction des paramètres non-linéaires. Dans le cas où l’oscillateur est fortement non isochrone (|| >> 1), il est possible de simplifier le problème pour traiter la combinaison de forçages canoniques, le régime transitoire, le régime de synchronisation sous température et sous fort signal externe. Les études analytiques sont confrontées aux études numériques et expérimentales pour un oscillateur à base d’une jonction tunnel. / The spin-transfer torque oscillator is a nano-sized system which could be part of the next generation of microwave current controlled oscillator. It combines several advantages as a frequency generation in the microwave range (f ≈ 200 MHz – 20 GHz), high tunability thanks to non-linearity and being fully compatible with CMOS technology. However, the oscillator is strongly affected by thermal fluctuations as every nano-sized system, which strongly degrades its spectral coherency. To solve this issue, the most promising strategy is to mutually synchronize an array of oscillators. In order to find the best synchronization schemes, one has to better understand the synchronization of the oscillator to an external source.The present work is essentially theoretical and consists to study the synchronization of a spintronic oscillator to a microwave external signal. The dynamic equation of the magnetization (Landau-Lifshitz-Gilbert-Slonczewski equation) is transformed in a simplified complex amplitude equation, which allows to address the different synchronizations mechanisms in a unified way. In particular, we introduce the canonical forcing function of the oscillator by transforming the torques {m x ui, m x m x ui} i=x,y,z and then discuss their synchronization properties. If the oscillator is strongly non-isochronous (|| >> 1), it becomes possible to address the combination of forcing functions, study of the transient regime, study of synchronization under thermal noise or under strong amplitude external signal. Analytical studies are compared with numerical simulations and experiments on magnetic tunnel barrier oscillators.

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The ferrites are a wide class of ferrimagnetic materials with great technological importance and are studied extensively for many decades. Their remarkable magnetic characteristics make it an element often used in induction transformers by his high magnetic permeability, high values of saturation magnetization and low coercivity, resulting in smaller losses in their hysteresis loops. In this work were characterized pure nickel ferrite (NiFe2O4), a soft magnetic material where the magnetic properties depend on the nature of the ions, their charges and their distribution among the tetrahedral and octahedral sites. Heat treatment and sintering time were emphasized. In order to improve one of the most important parameters for such an application, the saturation magnetization, new samples were doped with zinc at various concentrations (Ni1-xZnxFe2O4). Two new series were prepared starting from the higher value of the previous series (Ni0,6Zn0,4Fe2O4) with two new doping, indium and gadolinium. Finally, the values of saturation magnetization and initial permeability could be compared with samples of commercial cores, showing the potential of the compound to be used as the core of magnetic induction. / As ferritas formam uma ampla classe de materiais ferrimagnéticos com grande importância tecnológica e têm sido estudadas exaustivamente ao longo de décadas. Suas notáveis características magnéticas as tornam um elemento frequentemente utilizado em transformadores de indução por sua alta permeabilidade magnética, altos valores de magnetização de saturação e baixa coercividade, resultando em perdas menores observadas em seus ciclos de histerese. Neste trabalho foram caracterizadas ferritas de níquel pura (NiFe2O4), um material magnético mole onde as propriedades magnéticas dependem da natureza dos íons, suas cargas e sua distribuição entre os sítios tetraédricos e octaédricos. Nessa fase o tratamento térmico e o tempo de sinterização tiveram ênfase. A fim de melhorar um dos parâmetros mais importantes para tal aplicação, a magnetização de saturação, foram feitas novas amostras dopadas com zinco em várias concentrações (Ni1-xZnxFe2O4). Duas novas séries foram preparadas partindo-se do maior valor da série anterior (Ni0,6Zn0,4Fe2O4) com dois novos dopantes, o índio e o gadolínio. Finalmente, os valores de magnetização de saturação e permeabilidade inicial puderam ser comparados com amostras de núcleos comerciais, evidenciando a potencialidade do composto em ser utilizado como núcleo de indução magnética.