Creation Of Skyrmions Research Articles

Slow steady flow of a skyrmion lattice in a confined geometry probed by narrow-band resistance noise

Using resistance fluctuation spectroscopy, we observe current-induced narrow-band noise (NBN) in the magnetic skyrmion-lattice phase of micrometer-sized MnSi. The NBN appears only when electric-current density exceeds a threshold value, indicating that the current-driven motion of the skyrmion lattice triggers the NBN. The observed NBN frequency is 10-10$^4$ Hz at $\sim$10$^{9}$ A/m$^{2}$, implying a skyrmion steady flow velocity of 1-100 $\mu$m/s, 3-5 orders of magnitude slower than previously reported. The temperature evolution of the NBN frequency suggests that the steady flow entails thermally activated processes, which are most likely due to skyrmion creation and annihilation at the sample edges. This scenario is qualitatively supported by our numerical simulations considering boundary effects, which reveals that the edges limit the steady flow of skyrmions, especially at low temperatures. We discuss a mechanism that dramatically slows the skyrmion steady flow in a microfabricated specimen.

Voltage-controlled magnetic skyrmions in magnetic tunnel junctions

We investigated the fully electrical manipulation of magnetic skyrmions in combination with magnetic tunnel junctions. The high output voltage due to the tunneling magnetoresistance effect allows us to detect the single skyrmion, and its size can be identified around 220 nm in diameter. We also demonstrate skyrmion creation and annihilation by applying nano-second pulsed voltage, which offers the fully electrical memory function based on the magnetic skyrmion.

Influence of the dipolar interaction in the creation of skyrmions in coupled nanodisks

Abstract In the present work, using micromagnetic simulation, we show that the dipolar magnetic interaction effect plays a very important role in the process of creation of skyrmions in a coupled system of nanodisks. First, we have determined the magnetic ground state in an isolated nanodisk for different values of perpendicular uniaxial anisotropy (PUA) and Dzyaloshinskii-Moriya interaction (DMI). Next, we have applied perpendicular pulse polarized current densities (J) and found that it is possible to create a metastable Neel skyrmion from a nanodisk whose ground state is a single magnetic domain. From these results, we obtained a phase diagram of polarized current intensity vs. time of application of the current pulses, for different values of parameters such as PUA, DMI, and distance between the nanodisks. Our results show that, depending on the separation distance between the nanodisks, the current density required to create a skyrmion changes, due to the dipolar magnetic interaction.

Creation and Destruction of Skyrmions via Electrical Modulation of Local Magnetic Anisotropy in Magnetic Thin Films

Magnetic skyrmions are topologically protected spin textures that are promising as carriers of information for computing and storage beyond CMOS-based systems. An electrostatic approach to creating a skyrmion can be more energy efficient than those based on a driving current. In this work, formation and dissolution of N\'eel skyrmions are examined theoretically via electrical modulation of magnetic anisotropy, in both ferromagnetic and antiferromagnetic structures. Simulations clearly illustrate the feasibility of the mechanism, enabling local control for versatility in applications. Also noteworthy is that the dynamical processes involved are much faster in antiferromagnetic materials.

Boundary twists, instabilities, and creation of skyrmions and antiskyrmions

We formulate and study the general boundary conditions dictating the magnetization profile in the vicinity of an interface between magnets with dissimilar properties. Boundary twists in the vicinity of an edge due to Dzyaloshinskii-Moriya interactions have been first discussed in [Wilson et al., Phys. Rev. B 88, 214420 (2013)] and in [Rohart and Thiaville, Phys. Rev. B 88, 184422 (2013)]. We show that in general case the boundary conditions lead to the magnetization profile corresponding to the N\'eel, Bloch, or intermediate twist. We explore how such twists can be utilized for creation of skyrmions and antiskyrmions, e.g., in a view of magnetic memory applications. To this end, we study various scenarios how skyrmions and antiskyrmions can be created from interface magnetization twists due to local instabilities. We also show that a judicious choice of Dzyaloshinskii-Moriya tensor (hence a carefully designed material) can lead to local instabilities generating certain types of skyrmions or antiskyrmions. The local instabilities are shown to appear in solutions of the Bogoliubov-de-Gennes equations describing ellipticity of magnon modes bound to interfaces. In one considered scenario, a skyrmion-antiskyrmion pair can be created due to instabilities at an interface between materials with properly engineered Dzyaloshinskii-Moriya interactions. We use micromagnetics simulations to confirm our analytical predictions.

Strain-mediated voltage-controlled switching of magnetic skyrmions in nanostructures

Magnetic skyrmions are swirling spin structures stabilized typically by the Dyzaloshinskii-Moriya interaction. The existing control of magnetic skyrmions has often relied on the use of an electric current, which may cause overheating in densely packed devices. Here we demonstrate, using phase-field simulations, that an isolated Néel skyrmion in a magnetic nanodisk can be repeatedly created and deleted by voltage-induced strains from a juxtaposed piezoelectric. Such a skyrmion switching is non-volatile, and consumes only ~0.5 fJ per switching which is about five orders of magnitude smaller than that by current-induced spin-transfer-torques. It is found that the strain-mediated skyrmion creation occurs through an intermediate vortex-like spin structure, and that the skyrmion deletion occurs though a homogenous shrinkage during which the Néel wall is temporarily transformed to a vortex-wall. These findings are expected to stimulate experimental research into strain-mediated voltage control of skyrmions, as well as other chiral spin structures for low-power spintronics.

Rapid creation and reversal of skyrmion in spin-valve nanopillars

Abstract Skyrmions are nanoscale and topologically protected objects, which are promising candidates for information carriers of magnetic storage devices. To this end, the rapid creation and manipulation of a single skyrmion is required. In this work, a spin-valve device with a single-skyrmion reference layer is considered. Using micromagnetic simulations, we find that a skyrmion is likely to nucleate in the free layer as a picosecond current pulse is applied. In addition, we also demonstrate that the skyrmion in the free layer can be reversed by injecting an alternating current. It is promising that such a study can benefit the development of skyrmion-based storage devices.

Duplication, Collapse, and Escape of Magnetic Skyrmions Revealed Using a Systematic Saddle Point Search Method

Various transitions that a magnetic Skyrmion can undergo are found in calculations using a method for climbing up the energy surface and converging onto first order saddle points. In addition to collapse and escape through a boundary, the method identifies a transition where the Skyrmion divides and forms two Skyrmions. The activation energy for this duplication process can be similar to that of collapse and escape. A tilting of the external magnetic field for a certain time interval is found to induce the duplication process in a dynamical simulation. Such a process could turn out to be an important avenue for the creation of Skyrmions in future magnetic devices.

Creation of Magnetic Skyrmion Bubble Lattices by Ultrafast Laser in Ultrathin Films.

Magnetic skyrmions are topologically nontrivial spin textures which hold great promise as stable information carriers in spintronic devices at the nanoscale. One of the major challenges for developing novel skyrmion-based memory and logic devices is fast and controlled creation of magnetic skyrmions at ambient conditions. Here we demonstrate controlled generation of skyrmion bubbles and skyrmion bubble lattices from a ferromagnetic state in sputtered ultrathin magnetic films at room temperature by a single ultrafast (35 fs) laser pulse. The skyrmion bubble density increases with the laser fluence, and it finally becomes saturated, forming disordered hexagonal lattices. Moreover, we present that the skyrmion bubble lattice configuration leads to enhanced topological stability as compared to isolated skyrmions, suggesting its promising use in data storage. Our findings shed light on the optical approach to the skyrmion bubble lattice in commonly accessible materials, paving the road toward the emerging skyrmion-based memory and synaptic devices.

In Situ Electric Field Skyrmion Creation in Magnetoelectric Cu2OSeO3.

Exploiting additional degrees of freedom in solid-state materials may be the most-promising solution when approaching the quantum limit of Moore's law for the conventional electronic industry. Recently discovered topologically nontrivial spin textures, skyrmions, are outstanding among such possibilities. However, the controlled creation of skyrmions, especially by electric means, remains a pivotal challenge in technological applications. Here, we report that skyrmions can be created locally via electric field in the magnetoelectric helimagnet Cu2OSeO3. Using Lorentz transmission electron microscopy, we successfully write skyrmions in situ from a helical-spin background. Our discovery is highly coveted because it implies that skyrmionics can be integrated into modern field effect transistor based electronic technology, in which very low energy dissipation can be achieved and, hence, realize a large step forward toward its practical applications.

Reciprocal space tomography of 3D skyrmion lattice order in a chiral magnet

It is commonly assumed that surfaces modify the properties of stable materials within the top few atomic layers of a bulk specimen only. Exploiting the polarization dependence of resonant elastic X-ray scattering to go beyond conventional diffraction and imaging techniques, we have determined the depth dependence of the full 3D spin structure of skyrmions-that is, topologically nontrivial whirls of the magnetization-below the surface of a bulk sample of Cu2OSeO3 We found that the skyrmions change exponentially from pure Néel- to pure Bloch-twisting over a distance of several hundred nanometers between the surface and the bulk, respectively. Though qualitatively consistent with theory, the strength of the Néel-twisting at the surface and the length scale of the variation observed experimentally exceed material-specific modeling substantially. In view of the exceptionally complete quantitative theoretical account of the magnetic rigidities and associated static and dynamic properties of skyrmions in Cu2OSeO3 and related materials, we conclude that subtle changes of the materials properties must exist at distances up to several hundred atomic layers into the bulk, which originate in the presence of the surface. This has far-reaching implications for the creation of skyrmions in surface-dominated systems and identifies, more generally, surface-induced gradual variations deep within a bulk material and their impact on tailored functionalities as an unchartered scientific territory.

Overview and outlook of magnetic skyrmions

Magnetic skyrmions are topologically protected nano-scale spin textures. They normally exist in chiral magnets and magnetic thin films with broken inversion symmetry. The size of skyrmion ranges from 1 nm to several hundred nanometers, depending on the material parameters. The spins of skyrmion wrap around the unit sphere exactly once, thus facilitating the unit topological charge of a skyrmion. Due to their non-trivial topology, skyrmions exhibit exotic physics such as the topological Hall effect (THE) and the emergent electrodynamics. Skyrmions show particle-like dynamics and can be driven with ultra-low current density. Furthermore, they can be created, annihilated, manipulated and detected by all-electric methods, making skyrmion a promising candidate for next-generation information storage and processing technologies. On the other hand, combining skyrmions with superconductors and topological insulators may also lead to intriguing physics and applications such as the topological quantum computing. Over the past few years, the creation, annihilation and detection of skyrmion at room temperature have already been demonstrated, but the precise control of single skyrmion with size below 10 nm is still a challenge. In this paper, we first review the fundamental physics of skyrmion, from its topology to its emergent dynamics. Physical mechanisms of the Dzyaloshinskii-Moriya interaction, the emergent electrodynamics and the THE are discussed. Then the skyrmion material systems, including chiral magnets, magnetic thin films, artificial skyrmion systems, frustrated magnets, bi-skyrmion materials and antiskyrmion materials, are comprehensively summarized. The optimizations of materials and potential new skyrmion materials are also proposed for different material systems. Methods of creating, annihilating and detecting skyrmions, which also cover potential application methods other than electrical methods, are discussed from both theoretical and experimental point of view. The energy efficiencies and reliabilities of different creation and annihilation methods and the sensitivities of different detection methods are still unclear, these current bottlenecks and possible avenues towards skyrmion-based spintronics are described. Finally, we address some possible future directions of skyrmion research, such as the antiferromagnetic skyrmion and skyrmions in topological insulators, which may lead to the discovery of peculiar topological quantum physics and materials.

Multi-field control on magnetic skyrmions

The concept of skyrmion is proposed by Tony Skyrme, a British particle physicist, to describe a state of particles as a topological soliton. Magnetic skyrmion is a novel spin structure with topological behavior, whose size is on a nanometer scale. The space between skyrmions is tunable from a few nanometers to micrometer. Magnetic skyrmion can be stable in a large temperature range, from lower temperatures, to room temperature, and even to higher temperature. The materials with magnetic skyrmions include not only low temperature B20-type ferromagnets with centrosymmetry breaking and weak ferromagnets with helical magnetic ordering, but also the hexagonal MnNiGa alloy and ferromagnetic multilayers over room temperature. By using topological spin structure of skyrmions, an electrical current can be applied to driving or flipping the skyrmions, similar to the spin transfer torque effect in spin-valves and magnetic tunnel junctions. The critical current density is on the order of 102 A/cm2, which is five orders lower than that in magnetic multilayered structures such as 107 A/cm2. This critical value is much lower than the channel current density in Si-based semiconductor technology, thus leading to great potential applications in the future magnetic information devices. In this review paper, we first introduce the discovery, a brief development history of magnetic skyrmions. Then, we summarize the materials with skyrmion spin structures, focusing on the key physical properties. Finally, we mention the recent progress of the multi-field (such as magnetic field, electrical current, and temperature) control on magnetic skyrmions in hexagonal MnNiGa alloy and Pt/Co/Ta magnetic multilayers, together with the creation, annihilation, and dynamic behavior of skyrmions.

Research progress on topological properties and micro-magnetic simulation study in dynamics of magnetic skyrmions

Skyrmions, as a nontrivial topological magnetic structure, have the advantages of topological stability, small size and low driving electrical current, showing potential applications in spintronic memory device. There are several mechanisms for skyrmion formation in magnets. One major mechanism is, in chiral-lattice ferromagnets, the competition between the Dzyaloshinskii-Moriya and ferromagnetic exchange interactions, due to the lack of spatial inversion symmetry. The combination of topology and condensed physics demonstrates various new topological phenomena of skyrmions, which also determine their dynamics. In this review, recent progress on the topological physics foundation of Skyrmions, as well as their dynamics of application in spintronics devices, is reviewed. The topological physics foundations of skyrmions is introduced. Firstly, the structure of skyrmions, which shows a special nontrivial topology in the real space, is presented accompanied with the formation of skyrmions caused by Dzyaloshinskii Moriya interactions in chiral magnets. Secondly, due to the importance of the describable method of the topology of a skyrmion, the topological charge, that characterize the topology, as well as the calculation method are introduced. Also, the arising topological stability is discussed here. Then, the typical topological effects arising from the topology of a skyrmion, including topological Hall effect and the skyrmion Hall effect are reviewed. The next is the introduction of the helical and the spiral spin configuration, the alternatives for Bloch and Nal type skyrmions respectively, which show up under lower external magnetic field with the same interaction. Also the phase transition of the helical/spiral state to skyrmions and the Monte Carlo method to simulate the spin configuration of a chiral magnet are introduced. At last, the spin orbital torque and the spin transfer torque, that describe the driven effect of a skyrmion by an electrical current or a thermal field, are reviewed. The consequence dynamics of skyrmions, the Landau-LifshitzGilbert equation, are also introduced. The recent progress of typical dynamics of skyrmions on several concerned problems in practical applications are reviewed. The applications in spintronics memory require skyrmions have steady transportation driven by electrical current and controllable creation and annihilation process. Firstly, skyrmion can be generated by the spatial nonuniform electric current with a certain geometry constrain. Especially for the Nal type skyrmion, nonuniformity of the spin orbital torque, come from the non-uniform electric current, play an important role in the skyrmion generation process. Secondly, skyrmion moves with a perpendicular velocity under an electrical current, because of the skyrmion Hall effect. So the elimination of skyrmion Hall effect is practically concerned to make the transportation steady. The anti-ferromagnetic skyrmion and antiferromagnetic coupled skyrmion bilayer are found with no skyrmion Hall effect by have two opposite component cancel out. Finally, with topological stability, skyrmions are hard to convert from and to a nontrivial topological spin configuration at low temperature. So the manipulation of skyrmion creation and annihilation are discussed accompanied with their difference of Bloch and Nal type skyrmiom.

Creation and Annihilation of Skyrmions in the Frustrated Magnets with Competing Exchange Interactions

In triangular-lattice magnets, the coexistence of third-neighbor antiferromagnetic and nearest-neighbor ferromagnetic exchange interactions can induce rich magnetic phases including noncoplanar skyrmion crystals. Based on Monte Carlo simulation, we studied the dependence of magnetic phase transition on exchange interaction strength. Under the consideration of uniaxial anisotropy and magnetic field both perpendicular to the film plane, a large antiferromagnetic exchange interaction induces a high frustration. When the value of antiferromagnetic exchange interaction is one and a half times larger than the ferromagnetic one, a magnetic phase composed of canting spin stripes, never observed in the chiral magnets, forms. Interestingly, different canting spin stripes along three 120 degree propagation directions may coexist randomly in a magnetic phase, attesting that the canting spin stripes are three-fold degenerate states akin to helices and the multiple state of canting spin stripes is a circular configuration with zero skyrmion charge number. Moreover, skyrmions and antiskyrmions can be observed simultaneously in the configuration at the low temperature nearly close to 0 K, and their configuration and diameter properties are discussed. Finally, the mechanisms of skyrmion creation and annihilation are properly interpreted by comparing exchange and Zeeman energy terms.

A microwave field-driven transistor-like skyrmionic device with the microwave current-assisted skyrmion creation

Magnetic skyrmion is a topologically protected domain-wall structure at nanoscale, which could serve as a basic building block for advanced spintronic devices. Here, we propose a microwave field-driven skyrmionic device with the transistor-like function, where the motion of a skyrmion in a voltage-gated ferromagnetic nanotrack is studied by micromagnetic simulations. It is demonstrated that the microwave field can drive the motion of a skyrmion by exciting the propagating spin waves, and the skyrmion motion can be governed by a gate voltage. We also investigate the microwave current-assisted creation of a skyrmion to facilitate the operation of the transistor-like skyrmionic device on the source terminal. It is found that the microwave current with an appropriate frequency can reduce the threshold current density required for the creation of a skyrmion from the ferromagnetic background. The proposed transistor-like skyrmionic device operated with the microwave field and current could be useful for building future skyrmion-based circuits.

Field-free deterministic ultrafast creation of magnetic skyrmions by spin-orbit torques.

Magnetic skyrmions are stabilized by a combination of external magnetic fields, stray field energies, higher-order exchange interactions and the Dzyaloshinskii-Moriya interaction (DMI). The last favours homochiral skyrmions, whose motion is driven by spin-orbit torques and is deterministic, which makes systems with a large DMI relevant for applications. Asymmetric multilayers of non-magnetic heavy metals with strong spin-orbit interactions and transition-metal ferromagnetic layers provide a large and tunable DMI. Also, the non-magnetic heavy metal layer can inject a vertical spin current with transverse spin polarization into the ferromagnetic layer via the spin Hall effect. This leads to torques that can be used to switch the magnetization completely in out-of-plane magnetized ferromagnetic elements, but the switching is deterministic only in the presence of a symmetry-breaking in-plane field. Although spin-orbit torques led to domain nucleation in continuous films and to stochastic nucleation of skyrmions in magnetic tracks, no practical means to create individual skyrmions controllably in an integrated device design at a selected position has been reported yet. Here we demonstrate that sub-nanosecond spin-orbit torque pulses can generate single skyrmions at custom-defined positions in a magnetic racetrack deterministically using the same current path as used for the shifting operation. The effect of the DMI implies that no external in-plane magnetic fields are needed for this aim. This implementation exploits a defect, such as a constriction in the magnetic track, that can serve as a skyrmion generator. The concept is applicable to any track geometry, including three-dimensional designs.

Skyrmion production on demand by homogeneous DC currents

Topological magnetic textures—like skyrmions—are major players in the design of next-generation magnetic storage technology due to their stability and the control of their motion by ultra-low currents. A major challenge to develop new skyrmion-based technologies is the controlled creation of magnetic skyrmions without the need of complex setups. We show how to create skyrmions and other magnetic textures in ferromagnetic thin films by means of a homogeneous DC current and without requiring Dzyaloshinskii–Moriya interactions. This is possible by exploiting a static loss of stability arising from the interplay of current-induced spin-transfer-torque and a spatially inhomogeneous magnetization, which can be achieved, e.g., by locally engineering the anisotropy, the magnetic field, or other magnetic interactions. The magnetic textures are created controllably and efficiently with a period that can be tuned by the applied current strength. We propose a specific experimental setup realizable with simple materials, such as cobalt based materials, to observe the periodic formation of skyrmions. We show that adding chiral interactions will not influence the basics of the generations but the consequent dynamics w.r.t. the stabilization of topological textures. Our findings allow for skyrmion production on demand in simple ferromagnetic thin films by homogeneous DC currents.

Controlled creation of nanometric skyrmions using external magnetic fields

To exploit nanometric magnetic skyrmions as information carriers in high-density storage devices, a method is needed that creates an intended number of skyrmions at specified places in the device preferably at a low energy cost. We theoretically propose that using a system with a fabricated hole or notch, the controlled creation of individual skyrmions can be achieved even when using an external magnetic field applied to the entire specimen. The fabricated defect turns out to work like a catalyst to reduce the energy barrier for the skyrmion creation.

Helical and skyrmion lattice phases in three-dimensional chiral magnets: Effect of anisotropic interactions

In this work, we study the magnetic orders of a classical spin model with anisotropic exchanges and Dzyaloshinskii-Moriya interactions in order to understand the uniaxial stress effect in chiral magnets such as MnSi. Variational zero temperature calculations demonstrate that various helical orders can be developed depending on the interaction anisotropy magnitude, consistent with experimental observations at low temperatures. Furthermore, the uniaxial stress induced creation and annihilation of skyrmions can be also qualitatively reproduced in our Monte Carlo simulations. Our work suggests that the interaction anisotropy tuned by applied uniaxial stress may play an essential role in modulating the magnetic orders in strained chiral magnets.