Study Of The Structure And Dynamics Of Magnetic Skyrmions

Magnetic skyrmions are particle-like objects described by localized solutions of non-linear partial differential equations. Up until a few decades ago, it was believed that magnetic skyrmions only existed in condensed matter as short-term excitations that would quickly collapse into linear singularities. The contrary was proven theoretically in 1989 and evidentially in 2009. It is now known that skyrmions can exist as long-living metastable configurations in low-symmetry condensed matter systems with broken mirror symmetry, increasing the potential applications possible. Magnetic Skyrmions and their Applications delves into the fundamental principles and most recent research and developments surrounding these unique magnetic particles. Despite achievements in the synthesis of systems stabilizing chiral magnetic skyrmions and the variety of experimental investigations and numerical calculations, there have not been many summaries of the fundamental physical principles governing magnetic skyrmions or integrating those concepts with methods of detection, characterization and potential applications. Magnetic Skyrmions and their Applications delivers a coherent, state-of-the-art discussion on the current knowledge and potential applications of magnetic skyrmions in magnetic materials and device applications. First the book reviews key concepts such as topology, magnetism and materials for magnetic skyrmions. Then, charactization methods, physical mechanisms, and emerging applications are discussed. Covers background knowledge and details the basic principles of magnetic skyrmions, including materials, characterization, statics and dynamics Reviews materials for skyrmion stabilization including bulk materials and interface-dominated multilayer materials Describes both well-known and unconventional applications of magnetic skyrmions, such as memristors and reservoir computing

Magnetic textures known as skyrmions promise new breakthroughs in memory, logic, and neuromorphic applications. Skyrmions have been found in a variety of material systems, yet there existed no experimental evidence of a material that could simultaneously host them at room temperature and also allow for their reproducible current-induced nucleation and motion. One main goal of this thesis is to fill this gap and demonstrate all the aforementioned properties in the introduced here [Pt/CoFeB/MgO]15 thin film heterostructures, consisting of a perpendicularly magnetized ferromagnetic layer (M), a heavy metal (H), and a symmetry-breaking spacer layer (S). Here, I developed, fabricated, and characterized the [Pt/CoFeB/MgO]15 multilayers with an extremely low density of pinning centers, which enable not only a fully reproducible skyrmion motion but also a clean study of the skyrmion nucleation process. By using X-ray microscopy, I performed the imaging of various magnetic textures in these multilayers and studied their current-induced generation and motion as a function of applied field and temperature. Finally, another goal of this work is to establish a direct link between the properties of these [H/M/S][subscript N]-type materials and the structure of magnetic textures that they can host. The energetics of such systems is understood very poorly due to the very complex multilayer stray fields and up until now, most of their analysis involved the exclusive use of micromagnetic simulations. Here, I develop an alternative theoretical approach by calculating all the stray field interactions analytically, which enables the prediction of the exact structure and dynamics of magnetic domain walls, domains, and skyrmions. Thesis

The energy cost associated with modern information technologies has been increasing exponentially over time, stimulating the search for alternative information storage and processing devices. Magnetic skyrmions are solitonic nanometer-scale quasiparticles whose unique topological properties can be thought of as that of a Mobius strip. Skyrmions are envisioned as information carriers in novel information processing and storage devices with low power consumption and high information density. As such, they could contribute to solving the energy challenge. In order to be used in applications, isolated skyrmions must be thermally stable at the scale of years. In this work, their stability is studied through two main approaches: the Kramers' method in the form of Langer's theory, and the forward flux sampling method. Good agreement is found between the two methods. We find that small skyrmions possess low internal energy barriers, but are stabilized by a large activation entropy. This is a direct consequence of the existence of stable modes of deformation of the skyrmion. Additionally, frustrated exchange that arises at some transition metal interfaces leads to new collapse paths in the form of the partial nucleation of the corresponding antiparticle, as merons and antimerons.

This brief reviews current research on magnetic skyrmions, with emphasis on formation mechanisms, observation techniques, and materials design strategies. The response of skyrmions, both static and dynamical, to various electromagnetic fields is also covered in detail. Recent progress in magnetic imaging techniques has enabled the observation of skyrmions in real space, as well as the analysis of their ordering manner and the details of their internal structure. In metallic systems, conduction electrons moving through the skyrmion spin texture gain a nontrivial quantum Berry phase, which provides topological force to the underlying spin texture and enables the current-induced manipulation of magnetic skyrmions. On the other hand, skyrmions in an insulator can induce electric polarization through relativistic spin-orbit interaction, paving the way for the control of skyrmions by an external electric field without loss of Joule heating. Because of its nanometric scale, particle nature, and electric controllability, skyrmions are considered as potential candidates for new information carriers in the next generation of spintronics devices.

Magnetic skyrmionics is an advanced and active research field, which involves fundamental physics, the creation of efficient next-generation high-density information devices, the formation and manipulation of nanometer-size skyrmions in devices, and the development of compatible materials at room temperature. The magnetic skyrmions found in magnetic materials exhibit spiral magnetism. This book presents a basic overview of magnetic skyrmions along with current research on magnetic skyrmions, emphasizing formation mechanisms and materials design strategies. This book is suitable for an interdisciplinary audience of undergraduates, graduates, engineers, scientists, and researchers in the development of the next generation of spintronic devices.

Abstract: We review the recent progress on the magnetic skyrmions in chiral magnetic materials. The magnetic skyrmion is a topological spin configuration with localized spatial extent, which could be thought of as an emergent rigid particle, owing to its particular topological and chiral properties. Static skyrmionic configurations have been found in various materials with different transport and thermodynamic properties. The magnetic skyrmions respond to externally applied fields in a very unique way, and their coupling to other quasiparticles in solid-state systems gives rise to the emergent electrodynamics. Being not only theoretically important, the magnetic skyrmion is also very promising to be the information carrier in next generation spintronic devices.

This book presents both experimental and theoretical aspects of topology in magnetism. It first discusses how the topology in real space is relevant for a variety of magnetic spin structures, including domain walls, vortices, skyrmions, and dynamic excitations, and then focuses on the phenomena that are driven by distinct topology in reciprocal momentum space, such as anomalous and spin Hall effects, topological insulators, and Weyl semimetals. Lastly, it examines how topology influences dynamic phenomena and excitations (such as spin waves, magnons, localized dynamic solitons, and Majorana fermions). The book also shows how these developments promise to lead the transformative revolution of information technology.

This book focuses on the characterisation of 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 grid of nearly millimetre-level correlation length, while the size of a single skyrmion is 3–100 nm. This is a very challenging range of length scales (spanning 5 orders of magnitude from tens of nm to mm) for magnetic characterisation techniques, and, to date, extensive information on this fascinating, magnetically ordered state has remained elusive. In response, this work develops novel resonant elastic x-ray scattering (REXS) techniques, which allow the magnetic structure, including the long-range order and domain formation, as well as microscopic skyrmion parameters, to be measured across the full range of length scales. Most importantly, using circular dichroism in REXS, the internal structure of a given skyrmion, the topological winding number, and the skyrmion helicity angle can all be unambiguously determined. These new techniques are applicable to many materials systems, and allow us to retrieve information on modulated spin structures, multiferroic order, spin-density-waves, and other forms of topological magnetic order.

Topological solitons occur in many nonlinear classical field theories. They are stable, particle-like objects, with finite mass and a smooth structure. Examples are monopoles and Skyrmions, Ginzburg-Landau vortices and sigma-model lumps, and Yang-Mills instantons. This book is a comprehensive survey of static topological solitons and their dynamical interactions. Particular emphasis is placed on the solitons which satisfy first-order Bogomolny equations. For these, the soliton dynamics can be investigated by finding the geodesics on the moduli space of static multi-soliton solutions. Remarkable scattering processes can be understood this way. The book starts with an introduction to classical field theory, and a survey of several mathematical techniques useful for understanding many types of topological soliton. Subsequent chapters explore key examples of solitons in one, two, three and four dimensions. The final chapter discusses the unstable sphaleron solutions which exist in several field theories.