Skyrmion is a topologically protected spin texture excited in magnetic thin films. The radii of skyrmions are typically 10-100 nm. Because of the size, the skyrmion is expected to be a candidate for memory and novel-device usages. To realize the futuristic devices that will be using the skyrmion circuit, the tracks which guide the motion of skyrmions are needed. The tracks patterned with differences in the magnetic-anisotropy energy are well-paved without a potential pocket, whereas the tracks carved out of magnetic films have the potential pockets at corners due to the demagnetizing field. Therefore, the tracks patterned with the magnetic anisotropy plays a key role in making the skyrmion circuits. The experiment along this idea has been conducted for the hub and bent tracks. However, we have little known the motion of skyrmions in these tracks. This work aims to identify the forces acting between skyrmions and walls of the tracks. The static force on a skyrmion can be expressed as minus the gradient of the potential energy caused by the magnetic-anisotropy undulation. The potential can be estimated numerically, modeling the shape of skyrmions with their radii and domain wall widths. We find that the forces depend not only on the distance from the wall but also on the shape of skyrmions. We have also performed micromagnetic simulations where the Magnus force and the acceleration by the magnetic-anisotropy gradient are taken into account as well as the force by the walls. The simulation results show good agreement with those calculated from the modeled skyrmions.
We study the modification of micromagnetic configurations in nanostructures, due to the Dzyaloshinskii-Moriya interaction (DMI) that appear at the interface of an ultrathin film. We show that this interaction leads to new micromagnetic boundary conditions that bend the magnetization at the edges. We explore several cases of ultrathin film nanostructures that allow analytical calculations (1D systems, domain walls, cycloids and skyrmions), compare with fully numerical calculations, and show that a good physical understanding of this new type of micromagnetics can be reached. We particularly foc…
Large skyrmion bubbles in confined geometries of various sizes and shapes are investigated, typically in the range of several micrometers. Two fundamentally different cases are studied to address the role of dipole-dipole interactions: (I) when there is no magnetic material present outside the small geometries and (II) when the geometries are embedded in films with a uniform magnetization. It is found that the preferential position of the skyrmion bubbles can be controlled by the geometrical shape, which turns out to be a stronger influence than local variations in material parameters. In addi…
We have employed spin-polarized scanning tunneling microscopy and Monte-Carlo simulations to investigate the effect of lateral confinement onto the nanoskyrmion lattice in Fe/Ir(111). We find a strong coupling of one diagonal of the square magnetic unit cell to the close-packed edges of Fe nanostructures. In triangular islands this coupling in combination with the mismatching symmetries of the islands and of the square nanoskyrmion lattice leads to frustration and triple-domain states. In direct vicinity to ferromagnetic NiFe islands, the surrounding skyrmion lattice forms additional domains. …
Nanoscale magnetic skyrmions have interesting static and transport properties that make them candidates for future spintronic devices. Control and manipulation of the size and behavior of skyrmions is thus of crucial importance. Using a Ginzburg-Landau approach, we show theoretically that skyrmions and skyrmion lattices can be stabilized by a spatial modulation of the uniaxial magnetic anisotropy in a thin film of centro-symmetric ferromagnet. Remarkably, the skyrmion size is determined by the ratio of the exchange length and the period of the spatial modulation of the anisotropy, at variance …
The discovery of magnetic skyrmion bubbles in centrosymmetric magnets has been receiving increasing interest from the research community, due to the fascinating physics of topological spin textures and its possible applications to spintronics. However, key challenges remain, such as how to manipulate the nucleation of skyrmion bubbles to exclude the trivial bubbles or metastable skyrmion bubbles that usually coexist with skyrmion bubbles in the centrosymmetric magnets. Here, we report having successfully performed this task by applying spatially geometric confinement to a centrosymmetric frust…
Based on the findings of stripe skyrmions and the metastability of a state of an arbitrary number of skyrmions, precisely controlled manipulation of stripe skyrmions in pre-designed structures and mutual transformation between helical states and skyrmion crystals (SkXs) are demonstrated in chiral magnetic films. As a proof of the concept, we show how to use patterned magnetic fields and spin-transfer torques (STTs) to generate nematic and smectic stripe phases, as well as "UST" mosaic from three curved stripe skyrmions. Cutting one stripe into many pieces and coalescing several skyrmions into …
Magnetic skyrmions are topologically protected states that are very promising for the design of the next generation of ultralow-power electronic devices. In this letter, we propose a magnetic tunnel junction based spin-transfer torque diode with a magnetic skyrmion as ground state and a perpendicular polarizer patterned as nanocontact for a local injection of the current. The key result is the possibility to achieve sensitivities (i.e. detection voltage over input microwave power) larger than 2000V/W for optimized contact diameters. Our results can be very useful for the identification of a ne…
Quantum confinement of graphene Dirac-like electrons in artificially crafted nanometer structures is a long sought goal that would provide a strategy to selectively tune the electronic properties of graphene, including bandgap opening or quantization of energy levels However, creating confining structures with nanometer precision in shape, size and location, remains as an experimental challenge, both for top-down and bottom-up approaches. Moreover, Klein tunneling, offering an escape route to graphene electrons, limits the efficiency of electrostatic confinement. Here, a scanning tunneling mic…
Magnetic skyrmions are promising candidates as elementary nanoscale bits in logic-in-memory devices, intrinsically merging high density memory and computing capabilities. Here we exploit the dynamics of skyrmions interacting with anisotropy energy barriers patterned by ion irradiation to design programmable logic gates. Using micromagnetic simulations with experimental parameters, we show that a fine tuning of the barrier height and width allows the selective tunneling of skyrmions between parallel nanotracks triggered by skyrmion-skyrmion interaction. This can be leveraged to design skyrmion …
We consider the interplay between magnetic skyrmions in an insulating thin film and the Dirac surface states of a 3D topological insulator (TI), coupled by proximity effect. The magnetic texture of skyrmions can lead to confinement of Dirac states at the skyrmion radius, where out of plane magnetization vanishes. This confinement can result in charging of the skyrmion texture. The presence of bound states is robust in an external magnetic field, which is needed to stabilize skyrmions. It is expected that for relevant experimental parameters skyrmions will have a few bound states that can be tu…
Magnetic skyrmions are topologically protected stable magnetization configurations, which are expected to be a promising candidate as information carrier, while defect is inevitable and plays an important role on the stabilization and movement of skyrmion. In this paper, we investigated the influence of a point defect and a ring defect on the stabilization and dynamics of skyrmion in the nanodisk, where the defect are acquired by local modification magnetic material parameters. Considering the combined action of geometry confinement and pinning effect, we demonstrate the preferred skyrmion pos…
We have imaged Néel skyrmion bubbles in perpendicularly magnetised polycrystalline multilayers patterned into 1 μm diameter dots, using scanning transmission x-ray microscopy. The skyrmion bubbles can be nucleated by the application of an external magnetic field and are stable at zero field with a diameter of 260 nm. Applying an out of plane field that opposes the magnetisation of the skyrmion bubble core moment applies pressure to the bubble and gradually compresses it to a diameter of approximately 100 nm. On removing the field the skyrmion bubble returns to its original diameter via a hyste…
Magnetic skyrmions are promising candidates for logic-in-memory applications, intrinsically merging high density non-volatile data storage with computing capabilities, owing to their nanoscale size, fast motion, and mutual repulsions. However, concepts proposed so far suffer from reliability issues as well as inefficient conversion of magnetic information to electrical signals. In this paper, we propose a logic-in-memory device which exploits skyrmion confinement and channeling using anisotropy energy barriers to achieve reliable data storage and synchronous shift in racetracks combined with c…
We demonstrate the controlled generation and annihilation of (anti)skyrmions with tunable chirality in magnetic heterostructures by means of micromagnetic simulations. By making use of magnetic (anti)vortices in patterned ferromagnetic layer, we stabilize full lattices of (anti)skyrmions in an underlying skyrmionic thin film in a reproducible manner. The stability of the (anti)skyrmion depends on the polarization of the (anti)vortex, whereas their chirality is given by those of the (anti)vortices. Furthermore, we demonstrate that the core coupling between the (anti)vortices and (anti)skyrmions…
Magnetic skyrmions are topological solitons that exhibit an increased stability against annihilation, and can be displaced with low current densities, making them a promising candidate as an information carrier. In order to demonstrate a viable skyrmion-based memory device, it is necessary to reliably and reproducibly nucleate, displace, detect, and delete the magnetic skyrmions. While the skyrmion displacement and detection have both been investigated in detail, much less attention has been dedicated to the study of the sub-ns dynamics of the skyrmion nucleation process. Only limited studies …
Magnetic skyrmions are chiral spin structures that have recently been observed at room temperature (RT) in multilayer thin films. Their topological stability should enable high scalability in confined geometries - a sought-after attribute for device applications. While umpteen theoretical predictions have been made regarding the phenomenology of sub-100 nm skyrmions confined in dots, in practice their formation in the absence of an external magnetic field and evolution with confinement remain to be established. Here we demonstrate the confinement-induced stabilization of sub-100 nm RT skyrmion…
Devising magnetic nanostructures with spatially heterogeneous Dzyaloshinskii-Moriya interaction (DMI) is a promising pathway towards advanced confinement and control of magnetic skyrmions in potential devices. Here we discuss theoretically how a skyrmion interacts with a heterochiral interface using micromagnetic simulations and analytic arguments. We show that a heterochiral interface deflects the trajectory of ferromagnetic (FM) skyrmions, and that the extent of such deflection is tuned by the applied spin-polarized current and the difference in DMI across the interface. Further, we show tha…
We report the results of simulations of magnetic skyrmion stability in ultrathin magnetic multilayer nanodots with interfacial Dzyaloshinskii-Moriya exchange interaction (DMI). We found that in presence of the lateral confinement the magnetostatic energy significantly influences the skyrmion stability and leads to stabilization of large radius skyrmion even at low values of the DMI strength, in addition to small radius skyrmion stabilized by DMI. In particular, stabilization of the skyrmion state with two different radii (bi-stability) is found in dipolarly-coupled (Pt/Co/Ir)$_{n}$ circular na…
Magnetic skyrmions are particle-like chiral spin textures found in a magnetic film with out-of-planeanisotropy and are considered to be potential candidates as information carriers in next generationdata storage devices. Despite intense research into the nature of skyrmions and their dynamic prop-erties, there are several key challenges that still need to be addressed. In particular, the outstandingissues are the reproducible generation, stabilization and confinement of skyrmions at room tempera-ture. Here, we present a method for the capture of nanometer sized magnetic skyrmions in an arrayof…
A model for a spin-torque nano-oscillator based on the self-sustained oscillation of a magnetic skyrmion is presented. The system involves a circular nanopillar geometry comprising an ultrathin film free magnetic layer with a strong Dzyaloshinkii-Moriya interaction and a polariser layer with a vortex-like spin configuration. It is shown that spin-transfer torques due to current flow perpendicular to the film plane leads to skyrmion gyration that arises from a competition between geometric confinement due to boundary edges and the vortex-like polarisation of the spin torques. A phenomenology fo…
The magnetization dynamics in nanostructures has been extensively studied in the last decades, and nanomagnetism has evolved significantly over that time, discovering new effects, developing numerous applications, and identifying promising new directions. This includes magnonics, an emerging research field oriented on the study of spin-wave dynamics and their applications. In this context, thin ferromagnetic films with perpendicular magnetic anisotropy (PMA) offer interesting opportunities to study spin waves, in particular, due to out-of-plane magnetization in remanence or at relatively weak …
We report on the effect of the lateral confinement and a perpendicular magnetic field on isolated room-temperature magnetic skyrmions in sputtered Pt/Co/MgO nanotracks and nanodots. We show that the skyrmions size can be easily tuned by playing on the lateral dimensions of the nanostructures and by using external magnetic field amplitudes of a few mT, which allow to reach sub-100 nm diameters. Our XMCD-PEEM observations also highlight the important role of the pinning on the skyrmions size and stability under an out-of-plane magnetic field. Micromagnetic simulations reveal that the effect of l…
In ferromagnetic multilayers, a single laser pulse with a fluence above an optical nucleation threshold can create magnetic skyrmions, which are randomly distributed over the area of the laser spot. However, in order to study the dynamics of skyrmions and for their application in future data technology, a controllable localization of the skyrmion nucleation sites is crucial. Here, it is demonstrated that patterned reflective masks behind a thin magnetic film can be designed to locally tailor the optical excitation amplitudes reached, leading to spatially controlled skyrmion nucleation on the n…
Semiconductor quantum dots (QDs) are being regarded as the primary unit for a wide range of advanced and emerging technologies including electronics, optoelectronics, photovoltaics and biosensing applications as well as the domain of q-bits based quantum information processing. Such QDs are suitable for several novel device applications for their unique property of confining carriers 3-dimensionally creating discrete quantum states. However, the realization of such QDs in practice exhibits serious challenge regarding their fabrication in array with desired scalability and repeatability as well…
Using the Finsler geometry modeling (FG) technique without spontaneous magnetic anisotropy, we numerically study the stability and morphology of geometrically confined skyrmions experimentally observed in nanodots. We find a confinement effect that stabilizes skyrmions for a low external magnetic field without mechanical stresses by decreasing the diameter of the cylindrical lattice and strain effects that cause the sky and vortex to emerge under the zero magnetic field. Moreover, the obtained MC data on the morphological changes are also consistent with the reported experimental data.
We study the quantum propagation of a Skyrmion in chiral magnetic insulators by generalizing the micromagnetic equations of motion to a finite-temperature path integral formalism, using field theoretic tools. Promoting the center of the Skyrmion to a dynamic quantity, the fluctuations around the Skyrmionic configuration give rise to a time-dependent damping of the Skyrmion motion. From the frequency dependence of the damping kernel, we are able to identify the Skyrmion mass, thus providing a microscopic description of the kinematic properties of Skyrmions. When defects are present or a magneti…
Magnetic skyrmions, vortex-like swirling spin structures with quantized topological number, observed in chiral magnets, are appealing for potential applications in spintronics because it is possible to control their motion with ultralow current density. In order to realise skyrmion-based spintronic devices, it is essential to understand the skyrmion motions in confined geometries. Here we show by micromagnetic simulations that the current-induced motion of skyrmions in the presence of geometrical boundaries is very different from that in an infinite plane. In a channel with finite width, the t…
Considerably different properties emerge in nanomaterials as a result of quantum confinement and edge effects. In this study, the electronic and magnetic properties of quasi zero dimensional silicene nanoflakes (SiNFs) are investigated using first principles calculations. Whilst the zigzag edged hexagonal SiNFs exhibit nonmagnetic semiconducting character, the zigzag edged triangular SiNFs are magnetic semiconductors. One effective method of harnessing the properties of silicene is hydrogenation owing to its reversibility and controllability. From bare SiNFs to half hydrogenated and then to fu…
Magnetic Skyrmion Interacting with Optical Skyrmion
Waseda University（早稻田大学）；University of Shanghai for Science and Technology（上海理工大学）；The Hong Kong University of Science and Technology（香港科技大学）；The Chinese University of Hong Kong, Shenzhen（香港中文大学（深圳））；National University of Singapore（新加坡国立大学）
A new nano-lithography technique compatible with conducting oxide interfaces, which requires a single lithographic step with no additional amorphous layer deposition or etching, is presented. It is demonstrated on SrTiO3/LaAlO3 interface where a constriction is patterned in the electron liquid. We find that an additional back-gating can further confine the electron liquid into an isolated island. Conductance and differential conductance measurements show resonant tunneling through the island. The data at various temperatures and magnetic fields are analyzed and the effective island size is fou…
Confinement alters the energy landscape of nanoscale magnets, leading to the appearance of unusual magnetic states, such as vortices, for example. Many basic questions concerning dynamical and interaction effects remain unanswered, and nanomagnets are convenient model systems for studying these fundamental physical phenomena. A single vortex in restricted geometry, also known as a non-localized soliton, possesses a characteristic translational excitation mode that corresponds to spiral-like motion of the vortex core around its equilibrium position. Here, we investigate, by a microwave reflecti…
We theoretically investigate the dynamics of magnetic hedgehogs, which are three-dimensional topological spin textures that exist in common magnets, focusing on their transport properties and connections to spintronics. We show that fictitious magnetic monopoles carried by hedgehog textures obey a topological conservation law, based on which a hydrodynamic theory is developed. We propose a nonlocal transport measurement in the disordered phase, where the conservation of the hedgehog flow results in a nonlocal signal decaying inversely proportional to the distance. The bulk-edge correspondence …
Phase diagram of magnetic $S^3$ Skyrmions on three-dimensional lattices and the toroidal antiSkyrmion
TU Dresden（德累斯顿工业大学）；Institut für Theoretische Physik and Würzburg-Dresden Cluster of Excellence ct.qmat, TU Dresden（德累斯顿工业大学理论物理研究所与维尔茨堡-德累斯顿卓越集群ct.qmat）；Dipartimento di Fisica dell’Universitá di Pisa and INFN（比萨大学物理系与意大利国家核物理研究所）；Institute of Contemporary Mathematics, School of Mathematics and Statistics, Henan University（河南大学数学与统计学院当代数学研究所）；Department of Physics, Chemistry and Pharmacy, University of Southern Denmark（南丹麦大学物理、化学与药学系）；NEST, Scuola Normale Superiore（比萨高等师范学院NEST）
The Dzyaloshinskii-Moriya interaction (DMI) is a chiral interaction that favors formation of domain walls. Recent experiments and ab initio calculations show that there are multiple ways to modify the strength of the interfacially induced DMI in thin ferromagnetic films with perpendicular magnetic anisotropy. In this paper we reveal theoretically the effects of spatially varied DMI on the magnetic state in thin films. In such heterochiral 2D structures we report several emergent phenomena, ranging from the equilibrium spin canting at the interface between regions with different DMI, over parti…
The manipulation of magnetism with a gate voltage is expected to lead the way towards the realization of energy-efficient spintronics devices and high-performance magnetic memories. Exploiting magneto-ionic effects under micro-patterned electrodes in solid-state devices adds the possibility to modify magnetic properties locally, in a non-volatile and reversible way. Tuning magnetic anisotropy, magnetization and Dzyaloshinskii-Moriya interaction allows modifying at will the dynamics of non trivial magnetic textures such as skyrmions and chiral domain walls in magnetic race tracks. In this work,…
Nanomagnets form the building blocks for a gamut of miniaturized energy-efficient devices including data storage, memory, wave-based computing, sensors and biomedical devices. They also offer a span of exotic phenomena and stern challenges. The progress in the magnetization dynamics of single nanomagnets and one- and two-dimensional arrays of nanostructures in the form of dots, antidots, nanoparticles, binary and bicomponent structures and patterned multilayers have been presented. Progress in unconventional and new structures like artificial spin ice and three-dimensional nanomagnets and spin…
