This thesis presents the theoretical studies of chiral magnetic structures, which exist or are affected by antisymmetric Dzyaloshinskii-Moriya interactions. The theoretical approach is based on the phenomenological model of ferromagnetic materials lacking inversion symmetry. Equilibrium magnetic states are described as static structures in the micromagnetic low temperature limit with a fixed magnitude of the magnetization. The studies are focused on two cases: (i) magnetization structures that are affected by chiral exchange so that a particular chirality of these structures is selected, and (ii) novel solitonic states that are called chiral Skyrmions and only exist because of the chiral exchange. Vortex states in magnetic nanodisks provide the simplest example of a handed magnetization structure, where effects of the chiral couplings may become noticeable. A chiral exchange here favours one chirality of such a vortex state over the other. This effect can stem from surface-induced or other defect-related chiral Dzyaloshinskii-Moriya exchange. The different chiral versions of the vortex states are shown to display strong dependencies on the materials properties of such nanodisks. Within a micromagnetic model for these effects, numerical calculations of the shape, size, and stability of the vortices in equilibrium as functions of magnetic field and the material and geometrical parameters provide a general analysis of the influence of the broken mirror symmetry caused by the surface/interfaces or structural defect on their properties. The Dzyaloshinskii-Moriya interactions impose differences in the energies and sizes of vortices with different chirality: these couplings can considerably increase sizes of vortices with one sense of rotation and suppress vortices with opposite sense of rotation. Torsions related to lattice defects can cause similar to the surface-induced chiral couplings. A general phenomenological magneto-elastic formulation for this torsional chirality selection is given. It is applied to calculate similar effects on vortex states in magnetic disks with a screw dislocation at their center. In systems with strong chiral exchange the magnetic equilibrium states themselves become chiral twisted structures. The most interesting structures in this context are the two-dimensional solitonic states that are now known as chiral Skyrmions. The properties and stability of multiply twisted states composed of these particle-like units are the subject of the second part of this thesis. These states compete with the well known onedimensionally modulated helical states in non-centrosymmetric magnetic systems. Studies of modulated states in cubic helimagnets have shown, that in absence of additional effects, the only thermodynamically stable state is a cone helix. Uniaxial distortions, that can be caused by uniaxial stresses in the bulk samples or arise due to surface effects in thin films, suppress the helical states and stabilize Skyrmion lattices in a broad range of thermodynamical parameters. Using the phenomenological theory for modulated and localized states in chiral magnets, the equilibrium parameters of the Skyrmion and helical states have been derived as functions of applied magnetic field and induced uniaxial anisotropy. These results show that due to a combined effect of induced uniaxial anisotropy and an applied magnetic field, Skyrmion lattices can be formed as thermodynamically stable states. The theoretical results provide a comprehensive description of the evolution of modulated states in an applied magnetic field depending on type of anisotropy. The cases of a uniaxial anisotropy of easy axis and easy plane type with fields applied along its axis are investigated in detail. Existence of Skyrmion-lattice states in the easy axis case as thermodynamic field-induced phase is demonstrated. The results explain recent observation of Skyrmion lattices by magnetic Lorentz microscopy in thin foils of cubic chiral magnets. In systems with easy plane type of anisotropy, Skyrmion states do not form thermodynamic phases in applied fields along the axis. However, distorted Skyrmion phases can exist in fields applied perpendicularly to the axis. In this configuration of anisotropy axis and fields, both the helical states and the Skyrmions display elliptical distortions. The investigated micromagnetic model maps out the basic helical and Skyrmionic states expected to exist in cubic and nearly cubic chiral magnets.

Les systèmes hybrides supraconducteur-ferromagnétique présentent souvent de nouveaux phénomènes physiques, et pourraient également être utiles pour concevoir de nouvelles mémoires non-volatiles et haute densité pour les circuits supraconducteurs. Cette thèse étudie deux types différents d'hybrides SF, chacun suivant une approche possible de dispositif mémoire, en se focalisant sur les aspects fondamentaux. L'un porte sur l'effet de proximité dans des hétérostructures d'oxydes. Dans celles-ci, des corrélations triplet apparaissent, qui sont à la fois supraconductrices et polarisées en spin. Elles permettraient d'utiliser des effets de la spintronique comme la GMR, mais sont également très sensibles aux propriétés d'interface. Nous les avons étudiées dans des tricouches SFS d'oxydes, par des mesures de conductance. Celles-ci montrent des oscillations, en partie liées à ces états triplets. Nous observons également que les effets d'interface affectent les propriétés électroniques du ferromagnetique, en particulier lorsque cette couche est mince. Un autre genre d'interaction se produit par les champs de fuite provenant des structures de domaines. Des propositions théoriques récentes ont suggéré que de petites structures en tourbillon appelées skyrmion peuvent interagir avec la supraconductivité par ce mécanisme. Nous avons étudié ce couplage dans des bicouches, dans lesquelles les propriétés de transport sont dominées par la dynamique des vortex supraconducteurs. Nous avons vu une augmentation du courant critique en présence de skyrmions comme de domaines. Celles-ci créent également un effet Hall inhabituel dans l'état supraconducteur. La plupart de ces propriétés peuvent être expliquées qualitativement par la dynamique et le mouvement guidé des vortex. / Superconductor-ferromagnet hybrid systems often bring about new physics and may as well be useful to design new non-volatile, high-density memory devices for superconducting electronics.In this thesis, we study two different types of SF hybrids, each following a possible approach to memory devices, but focusing on fundamental aspects. One is about the proximity effect in oxide heterostructures. In these, triplet correlations appear, that are both superconducting and spin-polarized. These enable using effects from spintronics like GMR, but are also very dependent on interface properties. We investigated these in SFS oxide trilayers by conductance measurements. These showed oscillations which may, in part, be related to these triplet states. We also observed that interface effects affect the electronic properties of the ferromagnet, especially when that layer is thin. Another type of interaction occurs through stray magnetic fields from the domain structures. Recent theoretical proposals suggested that small swirling spin textures called skyrmions could similarly interact with superconductivity through this mechanism. We investigated such coupling in bilayers, in which the superconducting vortex dynamics dominate the transport properties. We found that the presence of skyrmions and domains alike enhances the critical current. It also leads to an unusual Hall effect in the superconducting state. Most of these properties can be explained qualitatively in terms of vortex pinning and guided motion.

Dans cette thèse nous étudions théoriquement et numériquement les solitons magnétiques et leurs transitions topologiques. Dans une première partie, nous trouvons une solution en 3 dimensions appelée Point de Bloch qui vient de la minimisation de l'énergie d'échange, de l'énergie de Landau et de l'énergie dipolaire. Les oscillations autour du point de Bloch sont trouvées et quantifiées pour étudier le rôle des fluctuations quantiques dans sa stabilité.Dans une deuxième partie, nous regardons l'évolution d'un système ferromagnétique avec des textures de topologie non-triviale, couplé à des électrons itinérants qui interagissent avec la texture au moyen de leurs spins. Ce système physique est modelé avec l'équation de Landau-Lifshitz-Gilbert couplée à l'équation de Schrödinger des électrons quantiques. Des transitions topologiques sont observées et mises dans un cadre général. De la grande quantité des transitions topologiques observées, nous distinguons les différents rôles que jouent les électrons selon le régime et l'ensemble de paramètres. Les ordres de grandeur temporels et spatiales des transitions topologiques montrent l'importance des effets quantiques ainsi que des effets de discrétisation du problème. / In this thesis we study the magnetic solitons and its topological transitions, both theoretically and numerically. In the first part, we find a particular configuration of what is denominated the Bloch Point, a three-dimensional solution of the Free Energy minimization with exchange, Landau and dipolar terms. Oscillations around the Bloch point are found and quantized in order to understand the role of quantum fluctuations over its stability.In the second part, we look at the evolution of a system coupling ferromagnetic textures with nontrivial topology, with itinerant electrons. The interaction between the magnetic texture and the electrons is understood by means of spin-torque phenomena. This physical system is modeled with the equation Landau-Lifshitz-Gilbert equation coupled with Schrödinger equation for quantum electrons. Topological transitions are observed and understood in a general framework that unifies older works done in a more classical context. Among the large amount of topological transitions observed, we can distinguish the different roles played by electrons depending on parameters. The orders of magnitude of time and space in the topological transition events show the importance of quantum effects as well as the fundamental role of discretization.

Les skyrmions magnétiques sont des enroulements bidimensionnels et nanométriques de la configuration de spin, pouvant être stabilisés dans certains matériaux magnétiques soumis à l’interaction d’échange antisymétrique Dzyaloshinskii-Moriya. Ils présentent une topologie non-triviale et s’annoncent peut-être comme étant les plus petites configurations magnétiques pouvant être réalisées. Très récemment, des skyrmions magnétiques ont pu être stabilisés à température ambiante grâce à la conception de multicouches magnétiques brisant la symétrie d’inversion selon la direction verticale. Suite à cette avancée, l’objectif central de cette thèse est la compréhension et la maîtrise des multiples propriétés physiques des skyrmions hébergés dans ces systèmes multicouches. Pour aborder cet objectif, un modèle original est décrit puis employé, permettant la prédiction des profils adoptés par les skyrmions multicouches. Ce modèle numérique est très générique, n’utilisant que la symétrie cylindrique des skyrmions afin de simplifier la détermination des interactions magnétostatiques. Ce modèle est ensuite étendu afin de pouvoir approximer la stabilité thermique des skyrmions, ce qui constitue un élément clé dans leur obtention expérimentale. Une seconde dimension de ce travail consiste en l’étude expérimentale de la manipulation électrique des skyrmions multicouches, démontrant la possibilité de trois fonctionnalités centrales que sont leur nucléation par courants locaux, leur déplacement sous courant de spin et leur détection électrique individuelle par tension transverse. Le troisième aspect de ma thèse est l’étude des propriétés physiques influençant le déplacement des skyrmions dans les multicouches magnétiques. Un comportement d’ancrage sur des défauts est mis en évidence expérimentalement et est analysé à l’aide d’une modélisation micromagnétique. Un des résultats importants de ce travail est aussi la prédiction d’une chiralité hybride dans les configurations magnétiques de certaines multicouches, qui est ensuite démontrée expérimentalement par des mesures au synchrotron. Les conséquences attendues de cette chiralité hybride sur le déplacement des skyrmions sont étudiées pour permettre l’optimisation des multicouches, aboutissant à l’observation expérimentale de la propagation de skyrmions de 50 nm de rayon à des vitesses atteignant environ 40 m/s. La dernière partie de cette thèse vise à mettre à profit ces avancées théoriques et expérimentales afin de parvenir à réduire la taille des skyrmions à température ambiante. Après avoir analysé l’impact des interactions dipolaires sur la stabilité des skyrmions, il est entrepris d’optimiser les matériaux et la périodicité des couches. Je m’intéresse aussi à la conception expérimentale de textures magnétiques dont l’aimantation est compensée au sein de structures multicouches appelées antiferromagnétiques synthétiques, dont je montre qu’elles peuvent héberger des skyrmions antiferromagnétiques à température ambiante. Ce résultat final ouvre de nouvelles perspectives vers l’obtention de skyrmions à la fois mesurant moins de 10 nm et très mobiles, qui pourraient être utilisés dans la conception de composants de calcul et de stockage d’information plus compacts et plus efficaces. / Magnetic skyrmions are nanoscale two-dimensional windings in the spin configuration of some magnetic materials subject to the Dzyaloshinskii-Moriya antisymmetric exchange interaction. They feature a non-trivial topology and show promise to be the smallest achievable magnetic textures. Very recently, magnetic skyrmions have been successfully stabilised up to room temperature by leveraging on the design of magnetic multilayer systems breaking the vertical inversion symmetry. Following up on this achievement, the main objective of this thesis is the understanding and the control of the various physical properties of skyrmions hosted by such multilayer systems. As a first approach to this objective, an original model allowing to predict the profiles adopted by multilayer skyrmions is described and then employed. This numerical model is very generic, as it exploits only the cylindrical symmetry of multilayer skyrmions, in order to determine the magnetostatic interactions with less effort. This model is further extended in order to approximate the thermal stability of multilayer skyrmions, which is key to their experimental realisation. The next aspect of this thesis consists in the experimental study of the electrical manipulation of multilayer skyrmions, demonstrating three main functionalities that are nucleation by local currents, displacement under spin currents and individual detection by transverse voltage. The third aspect of my thesis is the study of the physical properties influencing the current-induced motion of skyrmions in magnetic multilayers. A pinning behaviour is evidenced experimentally and analysed relying on micromagnetic modelling. One of the important results of this work is also the prediction of hybrid chirality for some multilayer magnetic configurations, which is then demonstrated experimentally using a synchrotron technique. The impact of hybrid chirality on current-induced skyrmion motion is discussed and leads to the optimisation of the multilayer design, resulting in the experimental observation of motion for skyrmions below 50 nm in radius at velocities reaching around 40 m/s. The last part of this thesis aims at leveraging on these theoretical and experimental advances in order to reduce the size of skyrmions at room temperature. After the analysis of the impact of dipolar interactions on skyrmion stability, the engineering of the materials and of the layers periodicity is attempted. I also investigate experimentally the conception of magnetic textures with compensated magnetization in multilayer structures known as synthetic antiferromagnets, and show that they can host antiferromagnetic skyrmions at room temperature. This last result opens up new prospects for achieving room-temperature skyrmions combining size in the single-digit nm range and high mobility, potentially allowing applications towards energy-efficient computation and storage devices with a very dense integration.

Résumé : Nous étudions dans cette thèse plusieurs propriétés du gaz d’électrons bidimensionnel (GE2D) dans le graphène et la bicouche de graphène (BG). Nous commençons par étudier la nature des excitations à une particule du GE2D dans le graphène près des facteurs de remplissage entiers dans les niveaux de Landau N [pas égal à] 0. Nous utilisons une approche de type Hartree-Fock (HF) pour comparer l’énergie de l’excitation d’une paire électron-trou à celle d’une paire skyrmion (SK)-antiskyrmion (ASK). Dans le graphène, les excitations SK et ASK sont des excitations chargées avec une texture de spin et/ou de pseudospin de vallée qui est quantifiée topologiquement. Nos calculs montrent que les paires SK-ASK sont les excitations chargées de plus basse énergie jusqu’au niveau de Landau |N| = 3. Notre approche permet en plus de calculer le domaine de couplage Zeeman pour lequel les paires SK-ASK sont les excitations de plus basse énergie et de déterminer comment l’énergie de ces paires est modifiée par les corrections d’écrantage. Le diagramme de phase du GE2D dans la bicouche de graphène a fait l’objet d’intenses recherches théoriques et expérimentales [8, 13, 15, 16], mais jusqu’à maintenant, seuls les états uniformes ont été considérés. Nous adaptons notre approche HF à l’étude des états non uniformes pour montrer que le GE2D dans la BG à remplissage ν = −1 dans le niveau de Landau N = 0 subit une série de transitions de phase lorsqu’un champ électrique perpendiculaire à la BG est appliqué. Nous étudions tout particulièrement les phases comportant une texture de pseudospin orbital soit un cristal de skyrmions et une phase spirale. Nous calculons les modes collectifs de ces phases ainsi que leur absorption électromagnétique. Nous poursuivons ensuite avec une étude des phases cristallines autour de certains remplissages entiers dans la BG. Le GE2D dans la bicouche de graphène a principalement été étudié dans le niveau de Landau N = 0. Comme dernier problème, nous étudions le diagramme de phase lorsqu’un nombre entier de niveaux de Landau est occupé dans les niveaux supérieurs |N| > 0. Alors que l’état fondamental du GE2D dans le graphène pour ces mêmes niveaux est un ferroaimant de Hall quantique (FHQ) avec une symétrie SU(2) pour le spin (en l’absence de couplage Zeeman) et le pseudospin de vallée, le GE2D dans la BG a plutôt un comportement FHQ de type Ising avec une symétrie Z[indice inférieur 2] à champ électrique nul. Cette différence de comportement a une grande influence sur la nature des transitions de phase possibles ainsi que sur celle des excitations topologiques. // Abstract : In this thesis, we study several properties of the two-dimensional electron gas (2DEG) in graphene and bilayer graphene. We first study the nature of the single-particle excitations in graphene near integer filling factors in Landau levels (LLs) N [not equal to] 0. We use a Hartree-Fock approach to compare the energy of an electron-hole excitation pair with that of a Skyrmion-antiskyrmion pair. In graphene, Skyrmions are charged excitations with a topological quantized spin and/or valley pseudo-spin texture. We give the range of Zeeman coupling for which Skyrmion-antiskyrmion has the lowest energy up to LL N = 3. Then we discuss how screening corrections modifies these results. The phase diagram of the 2DEG in bilayer graphene had been studied previously by a number of authors [8, 13, 15, 16] but only uniform states had been considered. Extending the Hartree-Fock approach to non-uniform states, we show that at filling factor ν = −1 in LL N = 0, the 2DEG goes through a series of phase transitions as the bias from an external electric field between two layers is increased. We study a crystal phase with orbital SK textures and a spiral state with the orbital pseudospin rotating in space. We compute the collective mode of these phases and their signatures in electromagnetic absorption experiments. We finally extend the Hartree-Fock approach to study the crystal states with valley or orbital textures near integer filling factors. The research on the 2DEG in bilayer graphene has been focussed almost exclusively in LL N = 0. As our last problem, we study the phase diagram at quarter and half fillings of the quartet of states in LLs |N| > 0. While the ground state of the 2DEG in graphene in |N| > 0 is a valley and spin quantum Hall ferromagnet with SU(2) symmetry in the absence of Zeeman coupling, the ground state in bilayer graphene is an Ising quantum Hall ferromagnet with a Z[subscript 2] valley symmetry at zero bias. We note that this change has important consequences on the nature of the transport properties and the single-particle excitations at integer fillings.

A 2014 study by the US Department of Energy conducted at Lawrence Berkeley National Laboratory estimated that U.S. data centers consumed 70 billion kWh of electricity[1]. This represents about 1.8% of the total U.S. electricity consumption. Putting this in perspective 70 billion kWh of electricity is the equivalent of roughly 8 big nuclear reactors, or around double the nation's solar panel output[2]. Developing new memory technologies capable of reducing this power consumption would be greatly beneficial as our demand for connectivity increases in the future. One newly emerging candidate for an information carrier in low power memory devices is the magnetic skyrmion. This magnetic texture is characterized by its specific non-trivial topology, giving it particle-like characteristics. Recent experimental work has shown that these skyrmions can be stabilized at room temperature and moved with extremely low electrical current densities. This rapidly developing field requires new measurement techniques capable of determining the topology of these textures at greater speed than previous approaches. In this dissertation, I give a brief introduction to the magnetic structures found in Fe/Gd multilayered systems. I then present newly developed techniques that streamline the analysis of Lorentz Transmission Electron Microscopy (LTEM) data. These techniques are then applied to further the understanding of the magnetic properties of these Fe/Gd based multilayered systems. This dissertation includes previously published and unpublished co-authored material.

This work focuses on characterising the chiral and topological nature of magnetic skyrmions in noncentrosymmetric helimagnets. In these materials, the skyrmion lattice phase appears as a long-range-ordered, close-packed lattice of nearly millimetre-level correlation length, while the size of a single skyrmion is 3-100 nm. This is a very challenging range of lengthscales (spanning 5 orders of magnitude from tens of nm to mm) for magnetic characterisation techniques. As a result, only three methods have been proven to be applicable for characterising certain aspects of the magnetic information: neutron diffraction, electron microscopy, and magnetic force microscopy. Nevertheless, none of them reveals the complete information about this fascinating magnetically ordered state. On the largest scale, the skyrmions form a three-dimensional lattice. The lateral structure and the depth profile are of importance for understanding the system. On the mesoscopic scale, the rigid skyrmion lattice can break up into domains, with the domain size about tens to hundreds of micrometers. The information of the domain shape, distribution, and the domain boundary is of great importance for a magnetic system. On the smallest scale, a single skyrmion has an extremely fine structure that is described by the topological winding number, helicity angle, and polarity. These pieces of information reveal the underlying physics of the system, and are currently the focus of spintronics applications. However, so far, there is no experimental technique that allows one to quantitatively study these fine structures. It has to be emphasised that the word 'quantitative' here means that no speculations have to be made and no theoretical modelling is required to assist the data interpretation -- what has been measured must be straightforward, and give a unique and unambiguous answer. Motivated by these questions, we developed soft x-ray scattering techniques that allow us to acquire much deeper microscopic information of the magnetic skyrmions -- reaching far beyond what has been possible so far. We will show that by using only one technique, all the information about the magnetic structure (spanning 5 orders of magnitude in length) can be accurately measured. The thesis is structured as follows: The key development is the Dichroism Extinction Rule, which is summarised in Chapter 6, and quintessentially summarises the thesis. In Chapter 1, the well-established theory for skyrmions is introduced, reconstructing the picture from single skyrmions to the skyrmion crystal. A few comments about the current characterisation techniques will be given. In Chapter 2, we will start with the largest lengthscale, the long-range-ordered skyrmion lattice phase. This is an intensely studied phase, mostly using neutron diffraction, and we will show that this piece of information can be equivalently (or actually even better) obtained using resonant x-ray diffraction. The theoretical foundation of this technique is also given. In Chapter 3, we will demonstrate imaging technique with which we were able to effectively map the skyrmion domains. The measurements also suggest a way to control the formation of skyrmion domains, which might be the key for enabling skyrmion-based device applications. Chapters 4 and 5 present the highlights of this work, in which we will show that using the dichroism extinction rule, the topological winding number and the skyrmion helicity angle can be unambiguously determined. In this sense, this technique is capable of accurately measuring the internal structure of single skyrmions.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第24890号 / 工博第5170号 / 新制||工||1987(附属図書館) / 京都大学大学院工学研究科機械理工学専攻 / (主査)教授 嶋田 隆広, 教授 平方 寛之, 教授 井上 康博 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

No description available.

Asymmetrically sandwiched ferromagnetic thin films display a large number of spin-orbit effects, including the Dzyaloschinsii-Moriya interaction (DMI), spin-orbit torques (SOT) and magnetoresistance (MR) effects. Their concurrence promises the implementation of interesting magnetic structures like skyrmions in future memory and logic devices. The complex interplay of various effects originating from the spin-orbit coupling and their dependencies on the microstructural details of the material system mandates a holistic characterization of its properties. In this PhD thesis, a comprehensive study of the spin-orbit effects in a chromium oxide/cobalt/platinum trilayer sample series is presented. The determination of the complete micromagnetic parameter set is based on a developed measurement routine that utilizes quasistatic methods. The unambiguous quantification of all relevant constants is crucial for the modeling of the details of magnetic structures in the system. In this context the necessity of a strict distinction of magnetic objects, that are stabilized by magnetostatics or the DMI, was revealed. Furthermore, a sample layout was developed to allow for the simultaneous quantification of the magnitudes of SOTs and MR effects from nonlinear magnetotransport measurements. In conjunction with a structural characterization, the dominating dependence of the effect magnitudes on microstructural details of the systems is concluded. Precisely characterized systems establish a solid groundwork for further investigations that are needed for viable skyrmion-based devices.