Les nano-oscillateurs à transfert de spin (STNO) sont des candidats prometteurs pour la réalisation de composants radiofréquence (RF) intégrés, du à leur taille nanométrique, l'importante gamme de fréquences de base qu'ils peuvent couvrir, ainsi qu'à leur accordabilité autour de ces fréquences de base. Le signal RF est obtenu grâce à l'effet de transfert de spin (STT) qui donne lieu à une oscillation non-linéaire de l'aimantation dans un élément magnétorésistif. Jusqu'ici, ces excitations ont été examinées dans le cadre d'une couche magnétique isolée, c'est-à-dire sans prendre en compte le couplage entre couches. Cependant, nombreux aspects du spectre d'excitation ne peuvent pas être expliqués si l'on considère une couche isolée. Dans cette thèse nous nous attacherons à répondre à la question importante du couplage dynamique entre couches magnétiques dans un nanopilier magnétorésistif, afin de développer une meilleure compréhension des spectres d'excitation, et en particulier la dépendance en courant et champ magnétique appliqué des caractéristiques du pic d'émission, telles que la largeur de raie et la fréquence. Une première étude est réalisée pour un système composé de deux couches ferromagnétiques, couplées entre elles par le couplage RKKY (ce système est appelé un ferrimagnétique synthétique (SyF)). Le couplage induit des différences importantes dans la dépendance en courant de la fréquence par rapport aux excitations d'une couche isolée. Ces différences sont expliquées par l'important couplage dynamique RKKY. Une seconde étude prend en considération une interaction plus complexe, ayant lieu dans un nano-pilier STNO standard basé sur jonctions tunnel ou vannes de spin. Ce dispositif est composé d'un SyF ainsi que d'une couche libre(FL) magnétique, séparés par une fine couche métallique ou isolante. Pour ce système, en plus du couplage dynamique RKKY propre au SyF, nous prenons en compte le couplage dynamique généré par le champ dipolaire ainsi que le spin-torque mutuel (MSTT) entre la couche libre et le SyF. Ce couplage multiple donne lieu à deux signatures distinctes. La première est l'apparition d'un « saut » dans le spectre d'excitation dû à l'hybridation des modes SyF and FL dans le régime atténué. Le second est dû à l'interaction entre les excitations en régime entretenu, éventuellement via leurs composantes harmoniques, avec les excitations en régime atténué. Cette interaction donne lieu à des discontinuités dans la dépendance fréquence – champ, ce lorsque les excitations FL sont prédominantes. Il est intéressant de noter que cela mène à des régions ou la largeur de raie est diminuée. De plus, lorsque les excitations SyF sont prédominantes, la largeur de raie est diminuée par rapport aux cas ou les excitations FL sont prédominantes. Partant de ces observations, nous proposons une structure plus complexe, où un seconde couche de type SyF remplace la couche libre. Les résultats obtenus par une combinaison d'expériences, de simulations numériques et d'analyse analytique, montrent le rôle important des interactions dynamiques dans un nano-pilier. Ils ouvrent de nouvelles voies pour la conception de configurations STNO qui mèneront à des améliorations des performances du signal ainsi synthétisé. / Spin-torque nano-oscillators (STNOs) are promising candidates for integrated radiofrequency (RF) components due to their nanoscale size, the large range of base frequencies that can be covered, as well as the large achievable tuning ranges around the base frequency. The RF signal is obtained due to the spin transfer torque (STT) generating a non-linear magnetization oscillation in a magnetoresistive device. In the past, these excitations were investigated using the picture of a single (or independent) layer. However, many features of the excitation spectra observed experimentally in nanopillar devices cannot be explained considering a single layer. In this thesis we address the important question on the dynamical coupling between the magnetic layers inside a magneto-resistive nanopillar device, to gain a better understanding of the excitation spectra, i.e. the dependence of the frequency and the linewidth on current and applied magnetic field. A first study is realized for a coupled system, composed by two ferromagnetic layers, coupled by the interlayer RKKY coupling (so called Synthetic Ferrimagnet SyF). Due to the coupling the frequency dependence versus current is different as compared to excitations of a single layer. This is explained by the strong dynamical RKKY coupling. A second study considers a more complex interaction, occurring within standard STNO nanopillar spin valves or tunnel junctions. They are composed by a SyF separated by a metallic or insulating spacer respectively from the single free layer (FL). For this system we take into account besides the RKKY coupling within the SyF, also the dynamical dipolar field coupling and the mutual spin torque (MSTT) between the SyF and the free layer. We find two definite signatures arising from this coupling. The first is a gap in the steady state excitation spectra that is due to the hybridization of the SyF and FL modes in the damped regime. The second is the possibility of the spintorque driven excitation or its harmonics with the damped modes leading to discontinuities in the frequency field dependence when the free layer is dominantly excited. Interestingly this leads to a region of reduced linewidth. Furthermore for SyF layer dominated excitations, the linewidth is lower than in the FL dominated excitations. From these observations we propose a more complex structure, composed by two SyF layers where the single FL is replaced by a SyF. The results obtained by a combination of experiments, numerical simulations and analytical analysis, demonstrate the important role of the dynamic interactions in nanopillar STNOs and provide routes for designing novel STNO configurations that should lead to improved microwave performances.

Visant des applications en électronique moléculaire, de nouveaux édifices moléculaires combinant quatre dithiényléthènes (DTEs) et une porphyrine ont été synthétisés par de nouvelles voies de synthèse. Les études en solution par photochimie ou spectroélectrochimie des édifices tetraDTEs-porphyrine métallée ou base livre ont montré l’efficacité (62-88 pourcent) des photochromes en tant qu’interrupteurs et la possibilité de lire l’état des DTEs par la fluorescence de la porphyrine. Pour mieux comprendre les interactions électroniques DTEs- porphyrines, des édifices plus simples, bis(porphyrine)DTE, déjà partiellement étudiés dans la littérature, ont été explorés. L’ensemble de ces études à mis en évidence l’impact des métaux sur la capacité photochromique des édifices dithiényléthène – porphyrine. / The main purpose of this thesis was the synthesis of new molecular structures, combining dithienylethenes and porphyrins, for applications in molecular electronics and spintronic. Metallated and free base porphyrins bearing four DTEs were characterizes and studied in solution by photochemistry and spectroelectrochemistry. The efficiency of switching of our systems reach 88 percent and strong quenching porphyrin’s fluorescence was reported. New synthetic pathways were investigated to prepare a broader scope of tetraDTEs-porphyrin derivatives. Moreover, to better understand the electronic communications between DTEs and porphyrins, a simpler bis(porphyrin)DTE molecular structure was synthetized and studied. These studies demonstrated showed the important role of the metal on photochromic behavior of dithienylethene – porphyrin architectures.

Le sujet de cette thèse concerne la spintronique moléculaire. Des jonctions tunnel magnétiques formées par une barrière tunnel moléculaire (monocouche auto-assemblée) insérée entre deux électrodes métalliques ferromagnétiques ont été étudiées. Afin de fabriquer les dispositifs, un procédé de greffage des molécules sur des substrats ferromagnétiques a été mis en place et une technique de lithographie a été développée pour définir des jonctions de taille submicronique. L’ensemble de ce travail expérimental a permis l’obtention de jonctions non court-circuitées et mesurables, où le transport électronique est bien du transport par effet tunnel.Les mesures de magnétotransport de ces échantillons ont amené des résultats intéressants et nouveaux : les jonctions, dont la barrière est formée par des alcanes-thiols, présentent de la magnétorésistance à température ambiante, allant jusqu’à 12%. Une partie de la thèse s’attelle à la compréhension des différents comportements magnétorésistifs observés : un modèle de barrière tunnel à deux niveaux est proposé pour les décrire.La dernière partie du travail présente des résultats préliminaires obtenus lorsque la barrière moléculaire est formée par des molécules aromatiques ou commutables et met en évidence des phénomènes nouveaux par rapport au cas précédent.L’ensemble des résultats prouve le fonctionnement de jonctions tunnel magnétiques à base de monocouches moléculaires à température ambiante et ouvre la voie à l’utilisation de molécules plus complexes pour une électronique de spin moléculaire multifonctionnelle. / This thesis work enters within the molecular spintronic fields. Magnetic tunnel junctions based on molecular self assembled monolayers have been investigated. The devices structure is a molecular monolayer inserted between two ferromagnetic electrodes.A process to graft molecules on a ferromagnet’s surface and a lithography technique have been developed to define the junctions. This experimental work has led to non short-circuited and measurable junctions, in which an electronic tunnel transport has been demonstrated.Interesting and new results have been found out from magnetoresistance measurement of the samples: junctions made with alkanes-thiols barrier have shown magnetoresistance signal at room temperature (up to 12%). In order to explain the magnetoresistive behaviour, a simple model where the barrier is discribed by two levels has been proposed.The last part of the thesis reports preliminary results obtained when the barrier is made of aromatic molecules or switchable molecules and it points out new phenomenons compared to the alkanes case.The overall work proves that devices made from magnetic tunnel junctions with self-assembled monolayers work at room temperature. It is then possible to consider switchable molecules to build multifunctional molecular spintronics devices.

Les nano-oscillateurs spintroniques présentent des propriétés remarquables en termes de détection radiofréquence. Leurs tailles nanométriques, leur fonctionnement à température ambiante et leurs compatibilité CMOS en font des candidats sérieux pour apporter la fonction d’analyse spectrale instantanée dans des systèmes embarqués. Les travaux de cette thèse portent sur les propriétés de détection des STNOs à base de vortex magnétiques. Un des effets conférant aux STNOs la possibilité de détecter un signal rf est l’effet diode de spin. Une source rf permet de créer le signal à détecter. Lorsque la fréquence du courant rf injecté dans le STNO correspond à sa fréquence de résonance, une tension de rectification est créée à ses bornes. La mesure de cette tension par un simple voltmètre permet alors de déterminer la présence d’un courant rf. L’étude de l’évolution de la fréquence de résonance en fonction du rayon du STNO, du courant dc et du champ magnétique a mis en avant la possibilité de choisir la fréquence de résonance et de l’accorder avec ces paramètres. Dans une perspective applicative, cette propriété est essentielle pour allouer un STNO à une fréquence spécifique à détecter. De plus, la taille nanométrique des STNOs permettent d’envisager un système composé d’un réseau de milliers, voire de millions de STNOs contenues sur une puce fonctionnant à température ambiante. Cependant plusieurs problématiques se posent. La sensibilité du STNOs à un signal rf extérieur doit permettre de déterminer l’état d’occupation d’un canal de fréquence par une simple mesure de la tension ou par un comparateur de tension. Cela nécessite une variation de tension de l’ordre de la dizaine de mV. L’effet diode de spin ne permettant pas d’atteindre de telle variation, un autre effet, mesuré pour la première fois à l’Unité Mixte de Physique CNRS/Thales, appelé expulsion de vortex magnétique, est étudié. Ce phénomène a lieu quand le cœur de vortex franchit les bords du STNO lors de sa dynamique induite par transfert de spin. Grâce à cet effet, l’amplitude de la variation de tension peut atteindre jusqu’à 25 mV dans les STNOs caractérisés dans le cadre de nos travaux. De plus, ce phénomène est également accordable en fréquence. Dans une perspective applicative, un réseau de STNO doit être crée afin d’allouer un STNO à une gamme de fréquence spécifique et ainsi couvrir une large bande de fréquence. La répartition du courant rf vers tous les STNOs est donc une problématique à laquelle nous avons apporté une solution. L’excitation du cœur de vortex par un champ rf nous permet en effet d’exciter un grand nombre de STNO grâce à une ligne inductive lithographiée au-dessus des STNOs. La possibilité d’expulser le cœur de vortex, dans ces conditions, a été démontré. Nous avons alors étudié la dynamique du cœur de vortex induite par un champ rf lors de l’expulsion. Ces études temporelles et fréquentielles nous ont non seulement apporté des informations sur le temps de détection d’un signal rf par le STNO mais aussi sur son aimantation dans le régime d’expulsion. De plus, l’accord en fréquence du STNO est possible même lors de l’excitation du cœur de vortex par un champ rf. Enfin, ces études nous ont permis de mettre en place, étape après étape une preuve de concept démontrant la faisabilité de la détection rf avec des nano-oscillateurs spintronique. Les différentes études du phénomène d’expulsion du cœur de vortex alliées à un travail technique de conception et de fabrication considérable a permis de converger vers une solution qui constitue un point de départ vers le développement d’un détecteur d’occupation de spectre spintronique large bande, tenant sur une puce et fonctionnant à température ambiante. / Spintronic nano-oscillators have remarkable properties in terms of radio frequency detection. Their nanoscale sizes, room temperature operation, and CMOS compatibility make them serious candidates for providing instantaneous spectral analysis in embedded systems. This thesis concerns the detection properties of magnetic vortex-based STNOs. One of the effects conferring on STNOs the possibility of detecting a rf signal is the spin diode effect. An rf source is used to create the signal to be detected. When the rf current frequency injected into the STNO corresponds to its resonant frequency, a rectification voltage is created at its terminals. The measurement of this voltage by a simple voltmeter makes possible to determine the rf current presence. The evolution study of the resonance frequency as a function of the STNO radius, the dc current and the magnetic field has highlighted the possibility of choosing the resonant frequency and tuning it with these parameters. From an application point of view, this property is essential for allocating an STNO to a specific frequency to be detected. Furthermore, the STNO nanometric allows us to envisage a network of thousands, even millions of STNOs contained on a chip operating at ambient temperature. However, several problems arise. The STNO sensitivity to an external rf signal must allow to determine the occupancy state of a frequency channel by a simple measurement of the voltage or with a voltage comparator. This requires a voltage variation of ten mV order. The spin diode effect doesn’t allow to achieve such variation. Another effect, measured for the first time at the Unité Mixte de Physique CNRS/Thales, called magnetic vortex expulsion, is studied. This phenomenon occurs when the vortex core crosses the STNO edges during its spin transfer induced dynamics. Thanks to this effect, the voltage amplitude variation can reach up to 25 mV in the STNOs characterized during this thesis. Moreover, this phenomenon can be tuned. From an application perspective, a network of STNOs must be created in order to allocate an STNO to a specific frequency range and thus cover a broad frequency band.The rf current distribution to all STNOs is therefore a problem to which we have brought a solution. The excitation of the vortex core by a rf field allows us to excite a large number of STNO thanks to an inductive line lithographed above the STNOs. The possibility of expelling the vortex core under these conditions has been demonstrated. We then studied the vortex core dynamics induced by an rf field during the expulsion. A time and frequency domain studies not only provided us detection time information of an rf signal by the STNO but also on its magnetization in the expulsion regime. Moreover, the STNO frequency tuning is possible even when the vortex core is excited by an rf field. Finally, these studies enabled us to implement step by step a proof of concept demonstrating the rf detection feasibility with spintronic nano-oscillators. The various studies of vortex core expulsion combined with a considerable technical work of design and manufacture finally allowed us to converge towards a solution that constitutes a starting point towards the development of a broadband spintronic spectrum occupancy detector, contained on a chip and operating at room temperature.

<p>        The last decade has witnessed an explosive growth in highly data-centric applications such as Internet of Things (IoT) and Artificial Intelligence (AI). Such applications demand highly efficient data storage and processing, especially when the systems operate under high energy/resource constraints, such as in intermittent-powered systems or edge AI platforms. Therefore, at the hardware level, high storage capacity along with low power operations has become more crucial than ever. Although conventional silicon-based complementary metal-oxide semiconductor (CMOS) has brought great prosperity to the semiconductor industry to date, enabling high-performance computing, increasing leakage energy and low cell density hinder their ability to sustain their benefits at scaled nodes and meet the demands of emerging data-intensive workloads. On the other hand, emerging non-volatile memories (NVMs) have gained much attention due to their distinct advantages over CMOS, such as zero leakage, high density, and non-volatility. However, they suffer from issues associated with high write power, endurance and/or variability. Thus, there is a need for new memory technologies that offer high density, low power and high-performance attributes to meet the data storage and efficiency demands of the new workloads. Furthermore, such technological advances need to be supported by architectural innovations. Despite hardware advances, the energy efficiency gains in traditional von-Neumann architectures are limited by power-hungry data movements between memory and processor, also known as the memory bottleneck. To alleviate this issue, in-memory computing (IMC) has emerged as a promising technique, wherein certain computations are executed within a memory macro, thus reducing processor-memory transactions. Along similar lines, incorporating non-volatile storage in logic state elements, such as flip-flops, has gained much attention for intermittently-powered systems, wherein the state of the processor is efficiently backed-up in the local non-volatile memory in the event of a power failure. Such techniques enabling logic-memory synergy reduce compute, storage, and/or communication costs and thus can be highly promising for future computing platforms. However, existing techniques for logic-memory fusion suffer from key design bottlenecks that need to be mitigated via extensive technology-circuit-architecture co-design. In this dissertation, we address some of the issues associated with data storage and processing by exploring spin-based low-power non-volatile devices, their memory applications, and logic-memory coupling enabled by their unique technological attributes. </p> <p>      We propose spin-based devices that employ the valley-spin Hall (VSH) effect in monolayer transition metal dichalcogenides (TMDs), such as tungsten di-selenide (WSe2). With the unique features of WSe2, the proposed devices are designed to have an integrated back-gate, enabling control of the charge and spin currents in 2D TMD channel. This design leads to an access-transistor-less compact layout in memory arrays. The generated spin currents diverge into opposite directions with out-of-plane spins, allowing for the coupling of WSe2 with perpendicular magnetic anisotropy (PMA) magnets. This enables low-power write operations and facilitates differential logic encoding within a single device. Additionally, we utilize inter-layer exchange-coupling mediated by FeCo-oxide and Ta layers to electrically isolate but magnetically couple the PMA free layers. This configuration benefits read performance by achieving low series resistance in the read path. To ensure reliable inter-layer coupling and the functionality of the proposed devices, we perform micromagnetic OOMMF simulations and extensively investigate the impact of process variations on the exchange-coupled PMA free layers. From the simulations, we conclude that the proposed design is resilient to potential process variations arising from misalignment of the PMA free layers and reductions in exchange-coupling strength. Based on the proposed devices, we explore circuit designs for logic and memory applications. </p> <p>      First, we propose VSH effect-based non-volatile flip-flops (VSH-NVFFs) using the proposed devices to introduce non-volatility in logic targeted for intermittently powered systems. The key challenge to design such systems is to enable energy-efficient data back-up in the event of power failure. In our design, we achieve high energy-efficiency via device-circuit co-design of VSH devices and NVFFs. We propose two flavors of NVFFs: NVFF-1 with a compact design and NVFF-2 targeted for lowering data restore energy. Compared to existing giant spin Hall (GSH) effect-based NVFFs, also known as spin-orbit torque or SOT-NVFFs, our NVFFs exhibit 68%-71%, 74%-75% and 55%-59% lower normal, back-up, and restore energies, respectively. Among the proposed VSH-NVFFs, NVFF-1 exhibits 8% lower operation energy than NVFF-2, while NVFF-2 exhibits 6% lower back-up energy and 11% lower restore energy. This result suggests that NVFF-1 is more suitable for systems with a smaller number of checkpointing operations (data back-up/restore), while NVFF-2 is beneficial for systems needing a larger number of checkpointing operations. Furthermore, by conducting Monte Carlo simulations, we confirm the reliable restore operation of the proposed NVFFs.</p> <p>      Secondly, we design memory arrays using the proposed devices to gain benefits over previously proposed VSH effect-based memory designs, in which read currents flow through a highly resistive 2D TMD channel, degrading read performance. For read operations, our memory array requires a read access transistor. By sharing the read access transistor per word, we minimize the area overhead in our memory array design. The area of our bit-cell is comparable to a previously proposed VSH memory, despite the inclusion of an additional read access transistor. Additionally, with the electrical isolation of the read and write paths in our design, we achieve improvements in read performance, with reductions of 39%-42% and 36%-46% in read time and energy, respectively. However, this improvement comes at the cost of write performance, with a 1.7X and 2.0X increase in write time and energy, respectively. We also achieve a 1.1X-1.3X larger sense margin (SM) and a 1.2X-1.3X improvement in read disturb margin (RDM). Furthermore, by increasing the size of the read access transistor in our memory array, we can further improve the SM by up to 1.5X-1.6X with only a 7%-12% area increase. Our design can be particularly useful for applications that involve frequent reads and few writes, such as neural accelerators.</p> <p>      We further expand our exploration of VSH effect-based devices for implementing IMC. As XNOR-based binary neural networks (BNNs) have shown immense promise for resource-intensive AI edge systems, their implementation has been explored using SRAMs and emerging NVMs. However, these designs typically need two bit-cells (2T-2R) to encode signed weights, resulting in an area overhead. Therefore, we address this issue by proposing a compact and low-power IMC technique for XNOR-based dot products. Our approach utilizes the VSH effect in monolayer WSe2 to design XNOR bit-cells that feature an access-transistor-less compact layout and differential weight encoding in a single device (XNOR-VSH). We co-optimize the proposed VSH device and the memory arrays to enable efficient in-memory dot product computations between signed binary inputs and signed binary weights. The compactness of the proposed XNOR-VSH array leads to 4.8%-9.0% lower compute latency and 36.6%-62.5% lower compute energy, along with 49.3%-64.4% smaller area compared to spin-transfer torque magnetic RAM (STT-MRAM) and SOT-MRAM based XNOR-arrays.</p> <p>      Lastly, we explore the modeling and design of voltage-controlled spintronic devices, which have shown remarkable potential for ultra-low-power and high-speed operation empowered by magnetoelectric (ME) materials. The proposed ME device utilizes a monolayer WSe2 channel placed on top of a Cr2O3 ME dielectric, which are electrostatically controlled by top and bottom gates. To capture the electrostatics in 2D TMD and the gate-voltage-dependent ME effect, we establish a modeling framework using a distributed capacitive network. This framework self-consistently accounts for the interactions between the various components. We verify the functionality of the proposed model by simulating the proposed device, and show how it can capture the device characteristics.</p>

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Neste trabalho, apresentamos estudos sobre o magnetotransporte em um sistema de bicamadas com uma rede de antipontos triangulares em campos magnéticos baixos sob a aplicação de campos elétricos externos, que são produzidos por voltagens de porta. A bicamada é feita em poços quânticos largos (wide quantum well) de alta densidade eletrônica, formado em heteroestruturas semicondutoras de AlxGa1xAs=GaAs. Oscila- ções magneto-inter-sub-banda (MIS) são observadas em poços quânticos largos de alta densidade eletrônica com duas sub-bandas ocupadas. Estas são originadas pelo espalhamento inter-sub-bandas e tem um máximo para campos magnéticos B que satisfazem a condição de alinhamento entre os leques dos níveis de Landau de cada sub-banda. Oscila- ções de comensurabilidade são observadas na magnetoresistência que é sensível ao arranjo do potencial dos antipontos. A aplicação de campos elétricos faz diminuir o número de oscilações na magnetoresistência para campos magnéticos compreendidos entre 0; 1T e 0; 4T, observando-se uma transição das oscilações MIS aos efeitos de comensurabilidade. Aplicando voltagens de porta podemos variar a amplitude do potencial dos antipontos. / In this work, we present studying about magnetotransport in a bilayer system with triangular antidot lattices in low magnetic elds under the application of external electric eld. The bilayer forms inside a wide quantum well of high electron density in semiconductor heterostructures formed by AlxGa1xAs=GaAs. Magneto-inter-subband (MIS) oscillations are observed in a wide quantum wells of high electron density with two subbands occupied, and they are caused by intersubband scattering and have a maximum for a magnetic eld B that satises the alignment condition between the staircase of Landau level. Commensurability oscillations are observed in magnetoresistance, which is sensitive to the potential of antidot arrangements. The application of electric elds decrease the number of oscillations in the magnetoresistance for magnetic elds between 0; 1T and 0:4T, showing a transition of MIS oscillations to commensurability oscillations. We varied the amplitude of the potential of the antidots applying dierent gate voltages.

Orientador: Guillermo Gerardo Cabrera Oyarzun / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-09-24T19:09:49Z (GMT). No. of bitstreams: 1 Siqueira_EzequielCosta_D.pdf: 16155551 bytes, checksum: 43337169b3f9ac0ffbe444e3859ff790 (MD5) Previous issue date: 2010 / Resumo: Neste trabalho é estudado o transporte quântico em nanoestruturas híbridas compostas por pontos quânticos (PQ) duplos acoplados a eletrodos supercondutores (S) e ferromagnéticos (F). A primeira nanoestrutura, denotada por F - PQa - PQb - S consiste em dois PQs em série acoplados a um eletrodo ferromagnético e outro supercondutor. O segundo sistema, denotado por (F1, F2) - PQa - PQb - S consiste em dois PQs em série acoplados a dois eletrodos ferromagnéticos e um supercondutor. Através do método de funções de Green de não equilíbrio foram obtidas expressões para a corrente elétrica, condutância diferencial, densidade local de estados (LDOS) e a transmitância para energias inferiores ao gap supercondutor. Neste regime, o mecanismo de transmissão de carga é a reflexão de Andreev, a qual permite controlar a corrente através da polarização do ferromagneto. A presença de interações nos PQs é considerada usando um tratamento de campo médio. Para o sistema F - PQa - PQb - S, as interações tendem a localizar os elétrons nos PQs levando a um padrão assimétrico da LDOS reduzindo a transmissão através da nanoestrutura. Em particular, a interação intra PQ levanta a degenerescência de spin reduzindo o valor máximo da corrente devido ao desequilíbrio nas populações de spin up e spin down. Regiões de condutância diferencial negativa (CDN) aparecem em determinados valores do potencial aplicado, como resultado da competição entre o espalhamento Andreev e as correlações eletrônicas. Aplicando-se um potencial de gate nos pontos quânticos é possível sintonizar o efeito mudando a região onde este fenômeno ocorre. Para o sistema (F1, F2) - PQa - PQb - S observou-se que o sinal da magnetoresistência pode mudar de positivo para negativo mudando-se o sinal do potencial aplicado. Além disso é possível controlar a corrente no primeiro eletrodo mudando-se o valor do potencial no segundo ferromagneto. Este tipo de controle pode ser interessante do ponto de vista de aplicações desde que é um comportamento similar a um transistor. Na presença de interações nos PQs, observou-se novamente regiões de CDN para determinados valores do potencial aplicado mesmo para quando os ferromagnetos estão completamente polarizados. Desta forma, a interação entre supercondutividade e correlações eletrônicas permitiu observar fenômenos originais mostrando que este sistemas podem ser utilizados em aplicações tecnológicas futuras / Abstract: In this work we studied the quantum transport in two hybrid nanostructures composed of double quantum dots (DQD)s coupled to superconductor (S) and ferromagnetic (F) leads. The first nanostructure, denoted by F - QDa - QDb - S, is composed of a ferromagnet, two quantum dots, and a superconductor connected in series. In the second nanostructure, denoted by ( F1, F2) - QDa - Q Db - S, a second ferromagnetic lead is added and coupled to the first QD. By using the non-equilibrium Green's function approach, we have calculated the electric current, the differential conductance and the transmittance for energies within the superconductor gap. In this regime, the mechanism of charge transmission is the Andreev re°ection, which allows for a control of the current through the ferromagnet polarization. We have also included interdot and intradot interactions, and have analyzed their influence through a mean field approximation. For the F - QDa - QDb - S system the presence of interactions tend to localize the electrons at the double-dot system, leading to an asymmetric pattern for the density of states at the dots, and thus reducing the transmission probability through the device. In particular, for non-zero polarization, the intradot interaction splits the spin degeneracy, reducing the maximum value of the current due to different spin-up and spin-down densities of states. Negative differential conductance (NDC) appears for some regions of the voltage bias, as a result of the interplay of the Andreev scattering with electronic correlations. By applying a gate voltage at the dots, one can tune the effect, changing the voltage region where this novel phenomenon appears. In the (F1, F2) - QDa - QDb - S, we have found that the signal of the magnetoresistance can be changed through the external potential applied in the ferromagnets. In addition, it is possible to control the current of the first ferromagnet (F1) through the potential applied in the second one (F2). This transistor-like behavior can be useful in technological applications. In the presence of interaction at the dots it was observed the NDC effect even when the electrodes were fully polarized. The results presented in this thesis show that the interplay between the superconductor correlation and electronic interactions can give rise to original effects which can be used in future technological applications / Doutorado / Física da Matéria Condensada / Doutor em Ciências

Cette thèse a pour but d’étudier la structure à l’échelle nanométrique et micrométrique de matériaux d’intérêt pour la spintronique organique, en se focalisant notamment sur deux aspects cruciaux pour la qualité des dispositifs : les interfaces molécules/métaux et les couches organiques. Pour pouvoir confronter nos résultats aux prédictions théoriques, nous avons utilisé des monocristaux métalliques et effectué des dépôts de molécules sous ultravide, permettant la réalisation d’échantillons de grande qualité. Nous avons concentré notre étude sur deux systèmes, l’un à base de C60 et de Cobalt et l’autre à base de molécules à transition de spin et d’Or. Pour élucider la structure de nos échantillons, nous avons réalisé in situ des mesures de microscopie à effet tunnel et de diffusion des rayons X, deux techniques complémentaires permettant respectivement l’obtention d’informations locales et globales sur le système. Les résultats obtenus ont été comparés à des calculs ab initio réalisés sur ces mêmes systèmes. Concernant le dépôt de molécules de C60 sur un substrat Co(0001), nous avons notamment pu mettre en évidence que le recuit de l’échantillon entraîne une transition structurale de l’interface, avec la création de lacunes dans le substrat sous chaque molécule, formant ainsi un réseau périodique. La couche moléculaire non recuite exhibe en outre une grande cristallinité. Le dépôt de Cobalt sur ce cristal moléculaire entraîne une contraction de ce dernier, due à la diffusion des atomes de Cobalt dans les sites interstitiels. Enfin, concernant les molécules à transition de spin [FeII (HB (3,5-(CH3)2Pz)3)2] déposées sur Au(111), nous avons mis en évidence une relation d’épitaxie inattendue entre le réseau moléculaire et le substrat / The aim of this thesis is to study at micrometric scale and nanoscale the structure of materials of interest for organic spintronics, focusing in particular on two crucial aspects to obtain good devices quality: molecular/metal interfaces and organic layers. In order to compare our results with theoretical predictions, we have used metallic single crystals and molecular deposition under ultra-high vacuum, allowing the obtention of high quality samples. We focused our study on two systems, one based on C60 and Cobalt and the other based on spin crossover molecules and Gold.To elucidate the structure of our samples, we used scanning tunneling microscopy and X-ray scattering, two techniques that are complementary, one probing the local organization and the other the global otganization of the system. The results obtained were compared to ab initio calculations carried out on the same systems. Regarding the deposition of C60 molecules on a Co (0001) substrate, we have been able to demonstrate that the annealing of the sample leads to a structural transition of the interface, with the creation of Cobalt vacancies under each molecules, forming a periodic network. Before annealing, the molecular layer also exhibits high crystallinity. The Cobalt deposition on this molecular crystal causes a contraction of the lattice, due to Cobalt diffusion into interstitial sites. Finally, concerning the [FeII (HB (3,5- (CH3) 2Pz) 3) 2] spin-crossover complex deposited on Au (111), we have demonstrated an unexpected epitaxial relationship between the molecular lattice and the substrate