Electrons not only have charges but also have spin. By utilizing the electron spin, the energy consumption of electronic devices can be reduced, their size can be scaled down and the efficiency of `read' and `write' in memory devices can be significantly improved. Hence, the manipulation of electron spin in electronic devices becomes more and more appealing for the advancement of microelectronics. In spin-based devices, the manipulation of ferromagnetic order parameter using electrical currents is a very useful means for current-driven operation. Nowadays, most of magnetic memory devices are based on the so-called spin transfer torque, which stems from the spin angular momentum transfer between a spin-polarized current and the magnetic order parameter. Recently, a novel spin torque effect, exploiting spin-orbit coupling in non-centrosymmetric magnets, has attracted a massive amount of attention. This thesis addresses the nature of spin-orbit coupled transport and torques in non-centrosymmetric magnetic semiconductors. We start with the theoretical study of spin orbit torque in three dimensional ferromagnetic GaMnAs. Using the Kubo formula, we calculate both the current-driven field-like torque and anti-damping-like torque. We compare the numerical results with the analytical expressions in the model case of a magnetic Rashba two-dimensional electron gas. Parametric dependencies of the different torque components and similarities to the analytical results of the Rashba two-dimensional electron gas in the weak disorder limit are described. Subsequently we study spin-orbit torques in two dimensional hexagonal crystals such as graphene, silicene, germanene and stanene. In the presence of staggered potential and exchange field, the valley degeneracy can be lifted and we obtain a valley-dependent Berry curvature, leading to a tunable antidamping torque by controlling the valley degree of freedom. This thesis then addresses the influence of the quantum spin Hall effect on spin orbit torque in nanoribbons with a hexagonal lattice. We find a dramatic modification of the nature of the torque (field like and damping-like component) when crossing the topological phase transition. The relative agnitude of the two torque components can be significantly modifies by changing the magnetization direction. Finally, motivated by recent experimental results, we conclude by investigating the features of spin-orbit torque in magnetic transition metal dichalcogenides. We find the torque is associated with the valley polarization. By changing the magnetization direction, the torque can be changed from a finite value to zero when the valley polarization decreases from a finite value to zero.

Magnetic random­access memory (MRAM) devices have been widely studied since the 1960s. During this time, the size of spintronic devices has continued to decrease. Conse quently, there is now an urgent need for new low­dimensional magnetic materials to mimic the traditional structures of spintronics at the nanoscale. We also require new effective mechanisms to conduct the main functions of memory devices, which are: reading, writ ing, and storing data. To date, most research efforts have focused on MRAM devices based on magnetic tun nel junction (MTJ), such as a conventional field­driven MRAM and spin­transfer torque (STT)­MRAM devices. Consequently, many efforts are currently focusing on new alterna tives using different techniques, such as spin­orbit torque (SOT) and magnetic skyrmions (a skyrmion is the smallest potential disruption to a uniform magnet required to obtain more effective memory devices). The most promising memory devices are SOT­MRAMs and skyrmion­based memories. This study investigates the magnetic properties of 1T­phase vanadium dichalcogenide (VXY) Janus monolayers, where X, Y= S, Se, or Te (i.e., monolayers that exhibit inversion symme try breaking due to the presence of different chalcogen elements). This study is developed along four directions: (I) the nature of the magnetism and the SOT effect of Janus mono layers; (II) the Dzyaloshinskii Moriya interaction (DMI); (III) investigation of stability en hancement by adopting practical procedures for industry; and (IV) study of the effect of a hexagonal boron nitride (h­BN) monolayer as an insulator on the magnetism of the VXY monolayer. This study provides a clear perspective for the next generation of memory de vices, such as SOT­MRAMs based on transition metal dichalcogenide monolayers.

In the past two decades, the interest in materials with strong spin-orbit coupling has attracted substantial attention because of the novel physical mechanisms they display and their potential for applications. The interface displaying large spin-orbit coupling has been recognized as a powerful platform to investigate the spin transport in ferromagnetic, antiferromagnetic, and non-magnetic materials, as well as their interfaces. Besides its rich physics, the related applications are also worth studying. The current-induced spin-orbit-torque arising from angular momentum transfer from the lattice to the spin system has substantial potential in recent state-of-art spin-orbit torque magnetic random access memory. In this dissertation, we have been interested in better understanding and characterizing the spin-orbit torque and spin Hall transport in various heterostructures of interest. We used the second harmonic method to determine the magnitude of the spin currents generation and transmission in Cu-Au alloy and Ir-Mn compound, respectively. We also characterized the device performance in selected heterostructures displaying either perpendicular MgO-based tunnel magnetoresistance or unusual surface states. Finally, we used these properties to approach spin-orbit torque magnetic random access memory through designing, fabricating, and characterizing the devices that focused on current-induced spin-orbit-torque magnetization switching.

One of the most remarkable achievements of modern condensed matter physics is the discovery of topological phases of matter. Materials in a non-trivial topological phase or the topological insulators can be distinguished by their unique electronic and transport properties which are indifferent to different types of perturbations and thus open new routes towards the dissipationless transport. Explaining their properties requires proper involvement of relativistic approach as well as topological analysis. Among different classes of topological insulators, the Z2 topological insulators have drawn special attention due to their strong spin-orbit coupling which makes them a promising candidate for spintronics application, especially for magnetic memory devices. Due to their inherent strong spin-orbit coupling, they provide an efficient way to manipulate electronic spin with an applied electric field via spin orbit torque. The topological insulators have been found to be far more superior in manipulating the magnetic order parameter of a ferromagnet compared to the conventional heavy metals like platinum or tantalum. Another milestone in magnetic memory devices is marked by the introduction of antiferromagnetic memory devices which has not drawn any attention for long time as they cannot be controlled by an applied magnetic field. Recently it has been found that in case of a non-centrosymmetric antiferromagnet, the magnetic order parameter can be manipulated by with spin-orbit torque which also have been verified experimentally. The advantages of antiferromagnetic devices over ferromagnetic devices are that they allow faster switching speed and they are immune to an external magneticfield which are two highly solicited properties for next generation spintronic devices. This thesis is focused on understanding the transport properties in topologically nontrivial materials and their interface with different magnetic material. We use simplified continuum model as well as tight binding models to capture the salient features of these systems. Using non-equilibrium Green's function we explore their transport properties as well as spin-charge conversion mechanism. Our finding would provide a better understanding of these new class of materials and thus would be instrumental to discover new mechanisms to manipulate their properties.

Dans cette thèse, nous présentons la conception et les performances de capteurs magnétiques basés sur les technologies AMR et GMR en vue d'une utilisation dans la navigation. Afin d'obtenir une sensibilité et une linéarité optimales à champ nul, le design des capteurs utilise des barberpoles et un pont de wheatstone pour l'un et prend avantage des différentes anisotropies et couplages pour l'autre. Les capteurs sont fabriqués par pulvérisations cathodiques et photolithographies et leurs performances en terme de sensibilité, de linéarité et de bruit sont testées et comparées. La conception de gradiomètres nécessaires à la navigation magnéto-inertielle repose sur l'utilisation de magnétorésistances placées aux extrémités du capteur reliées par un pont de WheatStone dont la sortie est proportionnelle au gradient du champ magnétique. Des concentrateurs de Flux destinés à amplifier le champ afin d'améliorer la sensibilité sont conçus et testés sur les capteurs. Un concentrateur de flux destiné à amplifier un champ ayant la forme du gradient est proposé comme amélioration des gradiomètres. Des solutions innovantes pour le swithching d'anisotropie et la compensation d'offset utilisant l'effet Hall de spin et le couplage spin-transfer sont étudiées. Dans cette optique, un dispositif de mesure du couplage spin-transfer appelé spin-torque-bridge est conçu et utilisé pour étudier l'effet Hall de spin et le spin transfer dans différentes multi-couches. / In this work, we present the conception and the performances of AMR and GMR-based sensors for navigation applications In order to obtain the best sensitivity and linearity at zero field, the design include barberpoles and a Wheatstone bridge for the first and takes advantage of the different anisotropies and coupling for the other. The sensor are fabricated by magnetron sputtering and photolithography and their performances in terms of sensitivity, linearity and noise are compared. The concept of gradiometers used for magneto-inertial navigation is based on the utilisation of magnetoresistances placed at each extremity of the sensor connected by a wheatstone bridge whose output is proportional to the gradient of the magnetic field. Flux concentrators designed to amplify the incoming field in order to improve the sensitivity are fabricated and tested on the sensors. A flux concentrator designed to amplify a magnetic gradient is proposed as an improvement of the gradiometers. Innovant solutions for low consumption anisotropy switching and offset compensation are being experimented using spin Hall effect and spin transfer torque with adjacent Pt and AuW layers. In that regard a spin torque measurement device: the spin-torque-bridge, is designed and used to study the spin hall effect and the spin transfer couplings in different multi layers.

Cette thèse porte sur l’étude de la génération, la propagation et la détection d’ondes de spin dans des nanostructures et microstructures élaborées à partir de couches ultra-minces (quelques nanomètres d’épaisseur) de Y₃Fe₅O₁₂ (YIG). Ce travail se trouve à l’interface entre deux thématiques du magnétisme : la magnonique et la spintronique. Grâce aux effets spin-orbite dans des microstrutures YIG|Pt, il a été possible d’étudier et de manipuler la dynamique d’aimantation du YIG, un matériau utilisé de longues dates sous forme de films épais ou billes pour ses très faibles pertes magnétiques. Ce travail ouvre la voie au développement de circuits magnoniques submicroniques soit pour le traitement des signaux hyperfréquences pour les applications télécom soit pour la réalisation de circuits logiques dans la perspective du remplacement de la technologie CMOS (beyond-CMOS). Ce travail repose sur une expertise dans la croissance de films de YIG développée au laboratoire. Les couches ultra-minces de YIG ont été élaborées par ablation laser pulsée. Pour les meilleurs films ayant une épaisseur de 20 nm, la constante d’amortissement de Gilbert caractérisant les pertes des films, estimée par résonance ferromagnétique, est typiquement de α=3x10⁻ 4. Cette avancée cruciale sur l’aspect matériau a ouvert au début de ma thèse un champ de possibilités pour la réalisation et l’étude de dispositifs magnoniques. En effet, la diminution des épaisseurs a permis d’ouvrir le YIG au domaine de la micro/nanofabrication, levant ainsi un verrou technologique vieux de plusieurs décennies. Nous avons donc pu montrer par des mesures inductives et optiques que la propagation d’ondes de spin dans des guides d’onde de YIG de 20 nm d’épaisseur pouvait être faite sur plusieurs dizaines de microns. Prouvant que la structuration des films de YIG n’altère pas la propagation des ondes de spin ouvrant la voie vers la réalisation de circuits magnoniques plus complexes. En structurant ces films de YIG pour obtenir des cristaux magnoniques, il est possible de générer une modulation spatiale du potentiel vu par les ondes de spin, se traduisant par l’apparition de bande interdite (ou gap) dans la transmittance de fréquences. L’étude de la propagation des ondes de spin dans un cristal a montré l’apparition d’un gap par des mesures BLS, accompagnée par une augmentation de l’atténuation pour la longueur d’onde de Bragg. Pour la première fois dans des films ultra-minces de YIG, ce gap montre la possibilité de réaliser une fonctionnalité de filtrage fréquentiel. La preuve de concept a été validée pour un cristal magnonique adapté pour l’intégration à des dispositifs magnoniques. Afin de manipuler et exciter la dynamique d’aimantation du YIG, nous avons dans une deuxième partie réalisée des microstructures à base de bicouche YIG|Pt. L’injection d’un courant électrique dans le Pt donne naissance, grâce à l’effet Hall de spin, à une accumulation de spin qui se couple à l’interface avec l’aimantation du YIG et permet ainsi d’exercer un couple de transfert de spin (STT) et de générer une dynamique d’aimantation du YIG. Nous avons mis en évidence la modulation d’un facteur cinq de la longueur d’atténuation des ondes de spin se propageant dans une piste YIG|Pt grâce à l’amplification des ondes de spin par STT. Ce contrôle efficace de l’atténuation s’avère très intéressant pour le transport d’information porté par les ondes de spin, afin d’amplifier ou supprimer les ondes de spin et donc sélectionner l’information transmise. Par ailleurs, au-delà d’un courant critique d’injection, nous avons pu observer des auto-oscillations de l’aimantation du YIG à la fois dans des plots ou des pistes. Ce résultat confirme la possibilité d’exciter électriquement la dynamique d’aimantation du YIG par STT. Une étude rigoureuse de ce régime a été effectuée dans des microdisques YIG|Pt pour déterminer le comportement des auto-oscillations et imager les modes d’ondes de spin excités dans le YIG. / The aim of this thesis is to study the generation, propagation and detection of spin waves in nanostructures and microstrutures based on ultrathin (a few nanometers thickness) Y₃Fe₅O₁₂ (YIG) films. This work is at the interface between two fields of magnetism: magnonics and spintronics. Thanks to spin-orbit effects in YIG|Pt microstructures, it has been possible to study and manipulate YIG magnetization dynamic, a material known and used for a long time as thick films or spheres due to its very low magnetic losses. This work opens the path towards the development of submicronic magnonic circuits either for processing radiofrequency signals of for the realization of spin waves logic devices for a future beyond-CMOS technology. Prior to the present work, a significant efforts have been made in the lab to grow epitaxial nanometer thick YIG films by pulsed laser deposition (PLD). It was possible to reduce the film thickness down to a few nanometers while preserving excellent magnetic properties. For the best YIG films having a thickness of 20 nm, ferromagnetic resonance measurements yield a Gilbert magnetic damping of α=3x10⁻ 4 . This value is comparable to micrometer thick YIG films grown by liquid phase epitaxy (LPE). This important step forward on the material aspect opened new possibilities for the realization of magnonic devices that can have a large impact on the ICT industry. Indeed, microfabrication of YIG is now possible thanks to the advent of high quality nanometer thick YIG films. Thus, we have observed the propagation of spin waves in 20-nm thick, 2.5 µm wide YIG waveguides over large distances using inductive and optical detection. Spin-wave propagation characteristics are not affected by microstructuration opening the path to the reliable design of complex magnonic circuits.By structuring YIG films to obtain magnonic crystals, it is possible to generate spatial modulation of the potential seen by spin waves, resulting in the appearance of gaps in the transmittance in frequency. To do so, magnonic crystals implemented in form of microscopic waveguides whose width is periodically varied, were fabricated. The study of spin-wave propagation showed the appearance of a gap accompanied by an increase of the spin-wave attenuation length due to Bragg reflection. For the first time in ultrathin YIG films, this gap shows the possibility to realize radiofrequency filtering. In order to manipulate and excite YIG magnetization dynamics, we have designed YIG|Pt microstructures either stripes or microdisks. Thanks to the spin Hall effect, an electrical current passing in Pt generates a transverse spin accumulation coupled at the interface to the YIG’s magnetization making it possible to exert spin transfer torque (STT). We have highlighted an efficient modulation, by a factor of five, of the spin-wave attenuation length. This control on the decay constant proves to be very interesting for the transport of information using spin waves as data carriers, in order to be able to amplify or suppress spin waves and to select transmitted information. In addition, beyond a critical current, we have induced auto-oscillations of YIG magnetization, either in stripes of microdisks, confirming the possibility to electrically excite YIG magnetization dynamics using STT. A rigorous study of this nonlinear regime has been carried out in YIG|Pt microdisks to determine auto-oscillations behavior and to observe directly dynamic modes excited in YIG.

付記する学位プログラム名: 京都大学卓越大学院プログラム「先端光・電子デバイス創成学」 / 京都大学 / 新制・課程博士 / 博士(工学) / 甲第24891号 / 工博第5171号 / 新制||工||1987(附属図書館) / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 白石 誠司, 准教授 掛谷 一弘, 教授 小野 輝男, 教授 森山 貴広 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

This thesis experimentally demonstrates four approaches of frequency control of magnetic auto-oscillations in spin Hall nano-oscillators (SHNOs). The frequency can be changed in the GHZ-range by external magnetic fields as shown in this work. This approach uses large electromagnets, which is inconvenient for future applications. The nonlinear coupling between oscillator power and frequency can be used to control the latter one by changing the applied direct current to the SHNO. The frequency can be controlled over a range of several 100 MHz as demonstrated in this thesis. The first part of the experimental chapter demonstrates the synchronization (injection-locking) between magnetic auto-oscillations and an external microwave excitation. The additionally applied microwave current generates a modulation of the effective magnetic field, which causes the interaction with the auto-oscillation. Both synchronize over a range of several 100 MHz. In this range, the auto-oscillation frequency can be controlled by the external stimulus. An increase of power and a decrease of line width is achieved in the synchronization range. This is explained by the increased coherence of the auto-oscillations. A second approach is the synchronization of auto-oscillations to an alternating magnetic field. This field is generated by a freestanding antenna, which is positioned above the SHNO. The second part of the experimental chapter introduces a bipolar concept of SHNOs and its experimental demonstration. In contrast to conventional SHNOs, bipolar SHNOs generate auto-oscillations for both direct current polarities and both directions of the external magnetic field. This is achieved by combining two ferromagnetic layers in an SHNO. The combination of two different ferromagnetic materials is used to switch between two frequency ranges in dependence of the direct current polarity since it defines the layer showing auto-oscillations. This approach can be used to change the frequency in the GHz-range by switching the direct current polarity. / Diese Arbeit demonstriert experimentell vier verschiedene Methoden der Frequenzkontrolle magnetischer Auto-Oszillationen in Spin Hall Nano-Oszillatoren (SHNOs). Durch externe magnetische Felder kann die Frequenz im GHz-Bereich geändert werden, wie es in dieser Arbeit gezeigt wird. Dies erfordert jedoch große Elektromagneten, deren Nutzung für zukünftige Anwendungen der SHNOs nicht geeignet sind. Aufgrund der nichtlinearen Kopplung zwischen Oszillatorleistung und Oszillatorfrequenz, lässt sich letztere durch den Versorgungsstrom beeinflussen. Dies ist typischerweise in einem Bereich von mehreren 100 MHz möglich, wie es an mehreren Stellen dieser Arbeit gezeigt wird. Im ersten Abschnitt des Ergebnisteils wird die Synchronisation der magnetischen Auto-Oszillationen zu einer externen Mikrowellenanregung demonstriert. Der zusätzlich eingespeiste Mikrowellenstrom erzeugt eine Modulation des effektiven Magnetfelds, was zur Wechselwirkung mit den Auto-Oszillationen führt. Diese synchronisieren über eine Frequenzdifferenz von mehreren 100 MHz. In diesem Bereich lässt sich die Frequenz der Auto-Oszillation mit der äußeren Frequenz steuern. Innerhalb des Synchronisationsbereichs wird außerdem eine Erhöhung der Leistung und eine Verringerung der Linienbreite der Auto-Oszillationen festgestellt. Dies wird mit einer Erhöhung der Kohärenz der Auto-Oszillationen erklärt. Neben der zusätzlichen Einspeisung eines Mikrowellenstroms können die Auto-Oszillationen auch zu einem magnetischen Wechselfeld synchronisieren, welches von einer frei beweglichen Antenne erzeugt wird, die über dem SHNO positioniert wird. Im zweiten Abschnitt des Ergebnisteils wird ein bipolares Konzept eines SHNO vorgestellt und seine Funktionsfähigkeit experimentell nachgewiesen. Im Vergleich zu konventionellen SHNOs erzeugen bipolare SHNOs Auto-Oszillationen für beide Polaritäten des elektrischen Versorgungsstroms und beide Richtungen des externen Magnetfelds. Dies wird durch die Kombination zweier ferromagnetischer Lagen in einem SHNO erreicht. Die Kombination unterschiedlicher ferromagnetischer Materialien kann genutzt werden, um die Mikrowellenfrequenz in Abhängigkeit der Stromrichtung zu ändern, da diese bestimmt in welcher Lage die Auto-Oszillationen erzeugt werden können. Durch eine geeignete Materialkombination kann die Frequenz im GHz-Bereich geändert werden, wenn die Strompolarität umgekehrt wird.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第21598号 / 理博第4505号 / 新制||理||1647(附属図書館) / 京都大学大学院理学研究科化学専攻 / (主査)教授 小野 輝男, 教授 吉村 一良, 教授 島川 祐一 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Dans cette thèse, nous avons étudié la dynamique des parois de domaine sous courant dans des couches ferromagnétiques ultra-minces de type métal lourd/ métal ferromagnétique/ oxyde présentant un fort couplage spin-orbite. Dans ces systèmes, deux éléments liés au fort couplage spin-orbite et l'asymétrie structurelle d'inversion jouent un rôle clé sur la dynamique des parois : d'une part, l'amplitude des couples de spin-orbite (SOT) exercés sur la paroi lors de l'injection de courant; d'autre part, l'amplitude de l'interaction Dzyaloshinskii-Moriya qui stabilise la structure Néel interne de la paroi. L'objectif de ce travail a été de caractériser les couples agissant sur la paroi induits par le courant ainsi que l'amplitude de l'interaction DMI.Pour y parvenir, j'ai mis au point une nouvelle technique de mesure basée sur la mesure des déplacements nanométriques induits par le courant d'une paroi piégée dans un nanoplot découpé dans le matériau magnétique. Cette mesure quasi-statique permet de s'affranchir des difficultés liées à la modélisation de la dynamique des parois magnétique sous courant en présence de défauts.Par ailleurs, le dispositif a été conçu de façon à ce que le courant et le champ magnétique externe statique puissent être appliqués dans différentes directions orthogonales, ce qui permet séparer clairement les contributions des couples de transfert de spin (NA-STT) et de spin-orbite (DL-SOT).Les mesures ont permis de caractériser le couple exercé sur la paroi par le courant en fonction d'un champ magnétique planaire pour un empilement Pt/Co/AlOx et ceci pour quatre orientations champ/courant différentes. Les résultats permettent d'écarter l'hypothèse d'une structure de type Bloch bi-stable.Dans le cas où le courant est injecté au travers de la paroi, la comparaison des résultats avec le modèle aboutit à une valeur du couple NA-STT très faible. Nos mesures faites avec le champ magnétique planaire permettent de conclure à un champ SOT de 7,5+/-0,5 Oe pour 10 MA/m² en accord avec les résultats de couple publiés précédemment dans le cas d'une paroi de Néel. Si les deux configurations donnent de mesures de couple SOT similaire, elles ne permettent pas de conclure sur la valeur de DMI dans ce système. L'origine de ces valeurs contradictoires reste à expliquer. / In this thesis, we studied the current induced domain walls (DWs) dynamics in ultra-thin ferromagnetic films of heavy metal/ ferromagnetic metal/ oxide type with a high spin-orbit coupling. In these systems, two ingredients linked to the high spin-orbit coupling and the structural inversion asymmetry play a key role on the DWs dynamics: the amplitude of the spin-orbit torques (SOT) acting on the domain when a current is injected; and the amplitude of the Dzyaloshinskii-Moriya interaction (DMi) which stabilizes the Néel structure of the DW. The purpose of this work was to characterize the current induced torques acting on the DW and the amplitude of the DMi.For that purpose, I developed a new measurement technique relying on the measurement of current induced nanometer size motion of a DW, trapped inside a nanodot patterned in the magnetic material. This quasi-static measurement enables to avoid the difficulties related to the modelling of the DW dynamics in the presence of defects.Besides that, the device has been designed to enable different perpendicular directions for the current and the external magnetic field, which enable a clear measurement of spin transfer (NA-STT) and spin-orbit (DL-SOT) torques contributions.The measurements allowed the characterization of the torque exerted by the current on the DW with respect to a planar magnetic field for a Pt/Co/AlOx stack in 4 different couples of field/current directions. The results allow to exclude the hypothesis of a Bloch structure for the DW.In the case where the current is injected through the DW, the comparison between the results and the model leads to a very weak value for the NA-STT. Our measurements made with the planar magnetic field leads to a value of 7,5+/-0,5 Oe per 10 MA/m² for the DL-SOT, which is in agreement with previously published results in the case of a Néel DW. If both configurations lead to similar measurements for the SOT, they don't permit to conclude on the exact value of the DMi in this system. The origin of these contradictories values is still to be understood.