Current Driven Dynamics Of Magnetic Skyrmions

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 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

This book focuses on an increasingly important area of materials science and technology, namely, the fabrication and properties of artificial materials where slabs of magnetized materials are sandwiched between slabs of nonmagnetized materials. It includes reviews by experts on the theory and descriptions of the various experimental techniques such as those using nuclear or electron spin probes, as well as optical, X-ray or neutron probes. It also reviews potential applications such as the giant magnetoresistance, and one specialized preparation technique, the electrodeposition. The various chapters are tutorial in nature, making the subject accessible to nonspecialists, as well as useful to researchers in the field.

Magnetic skyrmions are nanometer-scale spin textures that enjoy topologically-protected stability and exhibit particle-like behavior. Their rich phenomenology has ignited a growing research interest. These features and their novel transport properties have also made them attractive candidates as information carriers in future high-density magnetic-storage and logic devices, as well as integral components of other spintronic applications. Achieving a high degree of control and manipulation of skyrmions is of immense importance to applications and involves understanding fundamental aspects of the dynamics of twisted spin textures with topological charge at the nanoscale. In chapter five, we have considered zero temperature quantum nucleation of a single skyrmion in magnetic ultrathin films with interfacial Dzyaloshinskii-Moriya interaction (DMI). While a uniform field stabilizes the ferromagnet, an opposing local magnetic field, generated by the tip of a local probe, drives the skyrmion nucleation. Using spin path integrals and a collective coordinate approximation, the tunneling rate from the ferromagnetic to the single skyrmion state is computed as a function of the tip's magnetization and height above the sample surface. Based on the relation between DMI coupling and skyrmion helicity, the latter must be included as an extra degree of freedom in chiral magnets with a spatially inhomogeneous DMI. In chapter six, an effective description of skyrmion dynamics for an arbitrary inhomogeneous DMI coupling is obtained. The resulting generalized Thiele's equation is a dynamical system for the center of mass position and helicity of the skyrmion. We fully characterize the effective dynamics of a single skyrmion in a particular case of engineered DMI coupling: half-planes with opposite-sign DMI. In chapter seven, a particle-based model was used to simulate current-driven magnetic skyrmions interacting with random quenched disorder. We show that the Magnus force combined with the random pinning produces an isotropic effective shaking temperature. Spectral analysis of the velocity noise fluctuations can be used to identify dynamical phase transitions and to extract information about the different dynamic phases. In chapter eight, avalanches of flux-driven magnetic skyrmions in systems with random quenched disorder were also simulated using a particle-based model. The distribution of the avalanche sizes and durations, the associated critical exponents, and the average avalanche shape, were studied for different pinning regimes and Magnus force strengths.

Magnetic Domain Walls in Bubble Materials covers the physics of domain walls in bubble domain materials. The book describes the microscopic origins and characteristics of the material parameters; the principles of domain statics and the Landau-Lifshitz equation, which is the basic equation of magnetization dynamics; and its physical significance. The text then discusses the experimental techniques, both static and dynamic, used in studying domain walls; the static internal structure of bubble-domain walls; the Bloch-wall dynamics based on one-dimensional solutions of the Landau-Lifshitz equation; and the wall-motion theory. The theory to low velocity phenomena in domain walls containing vertical Bloch; high-velocity radial and quasi-planar wall motions; and nonlinear bubble translation including the implications of the theory for bubble motion in devices, are also considered. The book further surveys special phenomena involving vibrations and wave motions of walls, and the effects of microwave-frequency fields on walls. Engineers and materials researchers involved in the development of practical bubble devices will find the book invaluable.

"The book reviews all the aspects of recent developments in research on skyrmions, from the presentation of the observation and characterization techniques to the description of physical properties and expected applications. It will be of great use for all scientists working in this field." – Albert Fert, 2007 Nobel Laureate in Physics (from the Foreword) A skyrmion is a tiny region of reversed magnetization – quasiparticles since they are not present except in a magnetic state, and also give rise to physics that cannot be described by Maxwell’s equations. These particles are fascinating subjects for theoretical and experimental studies. Moreover, as a new type of magnetic domain structure with special topological structures, skyrmions feature outstanding magnetic and transport properties and may well have applications in data storage and other advanced spintronic devices, as readers will see in this book. Chapters address the relationships between physical properties of condensed matter, such as the AB effect, Berry phase effect, quantum Hall effect, and topological insulators. Overall, it provides a timely introduction to the fundamental aspects and possible applications of magnetic skyrmions to an interdisciplinary audience from condensed matter physics, chemistry, and materials science.

This book discusses theoretical and experimental advances in metamaterial structures, which are of fundamental importance to many applications in microwave and optical-wave physics and materials science. Metamaterial structures exhibit time-reversal and space-inversion symmetry breaking due to the effects of magnetism and chirality. The book addresses the characteristic properties of various symmetry breaking processes by studying field-matter interaction with use of conventional electromagnetic waves and novel types of engineered fields: twisted-photon fields, toroidal fields, and magnetoelectric fields. In a system with a combined effect of simultaneous breaking of space and time inversion symmetries, one observes the magnetochiral effect. Another similar phenomenon featuring space-time inversion symmetries is related to use of magnetoelectric materials. Cross-coupling of the electric and magnetic components in these material structures, leading to the appearance of new magnetic modes with an electric excitation channel – electromagnons and skyrmions – has resulted in a wealth of strong optical effects such as directional dichroism, magnetochiral dichroism, and rotatory power of the fields. This book contains multifaceted contributions from international leading experts and covers the essential aspects of symmetry-breaking effects, including theory, modeling and design, proven and potential applications in practical devices, fabrication, characterization and measurement. It is ideally suited as an introduction and basic reference work for researchers and graduate students entering this field.

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.

In a new branch of physics and technology, called spin-electronics or spintronics, the flow of electrical charge (usual current) as well as the flow of electron spin, the so-called "spin current", are manipulated and controlled together. This book is intended to provide an introduction and guide to the new physics and applications of spin current.