Skyrmion Annihilation Research Articles

Strain-controlled switching of magnetic skyrmioniums in ultrathin nanodisks

Magnetic skyrmions and skyrmioniums have garnered significant attention due to their distinctive topologically nontrivial spin structures. Gaining a deep understanding of the magnetization dynamics of these structures and their interconversion processes is essential for fully leveraging their potential in magnetic storage technology. Here, the dynamics of strain-controlled generation and annihilation of skyrmions and skyrmioniums are investigated using phase field simulation methods. It is discovered that tensile strain can induce the transformation of a single domain into skyrmions and skyrmioniums, which can still exist stably after the strain is released. Notably, skyrmioniums demonstrate robust stability within a specific strain window of −0.2% to 0.5%. Beyond this, escalating the compressive strain magnitude induces a phase transition from skyrmioniums to skyrmions, culminating in a direct collapse to a single-domain state at a critical compressive strain of −0.8%. This study reveals that strain can effectively control a variety of topological magnetic domain structures and achieve their interconversion, providing guidance for the design of low-power, nonvolatile, multi-state spin storage devices.

Stability and evolution of skyrmionium and skyrmions in a spherical shell

The study of magnetic structures, particularly those with curved geometries such as spherical shells, has obtained significant interest due to their potential applications in data storage, spintronics, and other advanced technologies. However, the effects of material parameters, geometric dimensions, and magnetic fields on the equilibrium and induced behaviors of skyrmions remain largely unresolved. Here, based on micromagnetic simulations, we firstly investigate the influence of spherical shell dimensions, magnetic anisotropy, exchange interaction, and Dzyaloshinskii–Moriya interaction on the magnetic states of spherical shells. We find that curvature effects become more pronounced with increasing thickness and decreasing radius, providing evidence for the role of curvature-induced DMI-like interactions in skyrmion formation. Additionally, we observe that applying a magnetic field to the spherical shell induces behaviors similar to those in disks, including the topological transition between skyrmionium and skyrmion states, the annihilation of skyrmions, and polarity reversal. Our study aims to advance the understanding of magnetic phenomena in curved geometries and contribute to the development of novel magnetic devices.

Energy-efficient synthetic antiferromagnetic skyrmion-based artificial neuronal device

Magnetic skyrmions offer unique characteristics such as nanoscale size, particle-like behavior, topological stability, and low depinning current density. These properties make them promising candidates for next-generation spintronics-based memory and neuromorphic computing. However, one of their distinctive features is their tendency to deviate from the direction of the applied driving force that may lead to the skyrmion annihilation at the edge of nanotrack during skyrmion motion, known as the skyrmion Hall effect (SkHE). To overcome this problem, synthetic antiferromagnetic (SAF) skyrmions that having bilayer coupling effect allows them to follow a straight path by nullifying SkHE making them alternative for ferromagnetic (FM) counterpart. This study proposes an integrate-and-fire (IF) artificial neuron model based on SAF skyrmions with asymmetric wedge-shaped nanotrack having self-sustainability of skyrmion numbers at the device window. The model leverages inter-skyrmion repulsion to replicate the IF mechanism of biological neuron. The device threshold, determined by the maximum number of pinned skyrmions at the device window, can be adjusted by tuning the current density applied to the nanotrack. Neuronal spikes occur when initial skyrmion reaches the detection unit after surpassing the device window by the accumulation of repulsive force that result in reduction of the device’s contriving current results to design of high energy efficient for neuromorphic computing. Furthermore, work implements a binarized neuronal network accelerator using proposed IF neuron and SAF-SOT-MRAM based synaptic devices for national institute of standards and technology database image classification. The presented approach achieves significantly higher energy efficiency compared to existing technologies like SRAM and STT-MRAM, with improvements of 2.31x and 1.36x, respectively. The presented accelerator achieves 1.42x and 1.07x higher throughput efficiency per Watt as compared to conventional SRAM and STT-MRAM based designs.

Efficient generation and deterministic annihilation of a single skyrmion via pure localized heating

A method for achieving rapid generation and annihilation of skyrmions is to apply local heating. However, the mechanism underlying heating-induced skyrmion formation is poorly understood, and achieving deterministic thermal excitation remains a major challenge. In this study, we utilized micromagnetic simulations to generate and annihilate individual skyrmions in a two-dimensional homogeneous ferromagnetic film with Dzyaloshinskii–Moriya interactions using a localized heating method without the assistance of an external magnetic field. By introducing pinning into the uniformly magnetized ferromagnetic background, the energy difference between the initial state and the skyrmion state is reduced, and the efficiency of generating skyrmions through local heating is improved. Additionally, deterministic annihilation of skyrmions can be achieved by exploiting the peculiarity that the energy of the skyrmion state is greater than that of the ground state. Based on this work, a practical application of skyrmions as a new type of information storage unit is proposed using a purely thermal approach.

Manipulating topological charge of nested skyrmion bags by microwave magnetic fields

Nested skyrmion bags, as magnetic solitons with arbitrary integer topological charges (Q), hold potential for applications in data encoding. A crucial issue is the local manipulation of skyrmions within nested skyrmion bags to control the total Q. In this study, we explore different possible ground states and resonant excitation spectra of nested skyrmion bags through micromagnetic simulations. More importantly, we demonstrate that the manipulation of the Q of nested skyrmion bags can be achieved by using microwave magnetic fields, i.e., the inner, middle, and outer skyrmions within the nested skyrmion bags are selectively excited by using the diverse out-of-plane excitation modes. By calculating the energy of skyrmions, we further analyze the relationship between the annihilation of skyrmions at different positions and the out-of-plane microwave magnetic fields. Our findings present a promising approach for manipulating the Q of nested skyrmion bags, potentially advancing their application in storage and logic devices.

Hysteresis-free voltage gating of the skyrmion

Magnetic skyrmions, which exhibit Brownian motion in solids, are considered good candidates as information carriers in devices, such as Brownian computers. Voltage control of skyrmions is essential for the ultralow power consumption of such devices. However, the gate operation must be realized with hysteresis-free voltage effects that are independent of ion migration for high-speed devices. In this study, we manipulated the skyrmion diffusion in a Ta|Co-Fe-B|Ta|MgO stacking structure by fabricating a device with a gate introducing an out-of-plane electrical field. Using feedback control, we rectified skyrmion diffusion in one direction, with the number of skyrmions passing through the gate wire from left to right N→ = 28 and from right to left N← = 43. Devices comprising Ta|Co-Fe-B|Pt|MgO junctions were fabricated, and a change in the density of skyrmions was observed upon the application of an out-of-plane electrical field. The creation or annihilation of skyrmions was dependent on the sign of the applied voltage. Furthermore, the skyrmions exhibited no hysteresis during the voltage sweep. Subsequently, the voltage dependence of the hysteresis loops in magneto-optical Kerr signals corresponding to the M–H curve was measured. However, no change was observed, nor was there any change in the saturated magnetization or perpendicular magnetic anisotropy. This result implied the voltage control of the Dzyaloshinskii–Moriya interaction.

Annihilation mechanisms for interacting skyrmions in magnetic nanowire

Abstract Magnetic skyrmions are considered potential candidates for spintronics-based memory and logic devices. For achieving high-density and high-speed devices, it is essential to study their interactions. In this paper, the interaction, dynamics, and annihilation mechanisms of Néel skyrmions in nanowire confinement under the influence of spin-transfer torque (STT) and edge forces have been studied. Initially isolated, two Néel skyrmions are brought into proximity, leading to distinct interaction scenarios characterized by varying current densities. We explore the impact of these interactions on skyrmion trajectories, size evolution, and annihilation phenomena. Our findings reveal the interplay of skyrmion-skyrmion repulsive forces, edge effects, and the influence of STT, shedding light on the rich dynamics of these topological magnetic textures. Furthermore, we unveil the distinct annihilation mechanisms of the leading and trailing skyrmions under different forces, providing valuable insights into the fundamental physics of skyrmion behavior in confined geometries.

Regulating magnetic skyrmions in multiferroic monolayer MnOBr.

Two-dimensional multiferroic materials that exhibit both ferroelectricity and ferromagnetism provide a new platform for the discovery and regulation of magnetic skyrmions. In this study, we utilize first-principles calculations and Monte Carlo simulations to explore the properties and regulation of magnetic skyrmions in a novel multiferroic monolayer, MnOBr. MnOBr exhibits skyrmions without the need for an external magnetic field. Upon applying an external magnetic field, we found the disappearance of labyrinth domains and the formation of a periodic arrangement of the skyrmion lattice. By employing machine learning techniques, we depict a phase diagram of MnOBr under varying magnetic fields and biaxial strain, which provides a detailed depiction of phase transitions of spin textures in monolayer MnOBr. Furthermore, in MnOBr/CdClBr heterostructures, we demonstrate that the creation and annihilation of magnetic skyrmions can be controlled by switching the polarization direction of the Janus CdClBr. These findings show potential applications of MnOBr as a 2D magnetic skyrmion material in spintronic devices.

Creation and annihilation of artificial magnetic skyrmions with the electric field

Recent theory and experiments show that artificial magnetic skyrmions can be stabilized at room temperature without the need for the external magnetic field, casting strong potentials for the device applications. In this work, we study the electric field manipulation of artificial magnetic skyrmions imprinted by Co disks on CoPt multilayers utilizing the micromagnetic simulations. We find that the reversible annihilation and creation of skyrmions can be realized with the electric field via the strain mediated magnetoelastic coupling. In addition, we also demonstrate controllable manipulation of individual skyrmion, which opens a new platform for constructing magnetic field-free and low-energy dissipation skyrmion based media.

Skyrmion transport and annihilation in funnel geometries

Using atomistic simulations, we have investigated the transport and annihilation of skyrmions interacting with a funnel array under a current applied perpendicular to the funnel axis. We find that transport without annihilation is possible at low currents, when the motion is dominated by skyrmion–skyrmion interactions and skyrmions push each other through the funnel opening. Skyrmion annihilation occurs for higher currents when skyrmions in the upper half of the sample exert pressure on skyrmions in the bottom half of the sample due to the external current. Upon interacting with the funnel wall, the skyrmions undergo a size reduction that makes it easier for them to pass through the funnel opening. We find five phases as a function of the applied current and the size of the funnel opening: (i) pinned, (ii) transport without annihilation, (iii) transport with annihilation, (iv) complete annihilation, and (v) a reentrant pinning phase that only occurs for very narrow openings. Our findings provide insight into how to control skyrmion transport using funnel arrays by delineating regimes in which transport of skyrmions is possible as well as the conditions under which annihilation occurs.

Physical conversion and superposition of optical skyrmion topologies

Optical skyrmions are quasiparticles with nontrivial topological textures that have significant potential in optical information processing, transmission, and storage. Here, we theoretically and experimentally achieve the conversion of optical skyrmions among Néel, Bloch, intermediate skyrmions, and bimerons by polarization devices, where the fusion and annihilation of optical skyrmions are demonstrated accordingly. By analyzing the polarization pattern in Poincaré beams, we reveal the skyrmion topology dependence on the device, which provides a pathway for the study of skyrmion interactions. A vectorial optical field generator is implemented to realize the conversion and superposition experimentally, and the results are in good agreement with the theoretical predictions. These results enhance our comprehension of optical topological quasiparticles, which could have a significant impact on the transfer, storage, and communication of optical information.

Electric-field-induced formation and annihilation of skyrmions in a two-dimensional magnet

Electric-field-induced formation and annihilation of skyrmions in a two-dimensional magnet

Soliton motion induced along ferromagnetic skyrmion chains in chiral thin nanotracks

Using atomistic magnetic simulations we investigate the soliton motion along a pinned skyrmion chain containing an interstitial skyrmion. We find that the soliton can exhibit stable motion along the chain without a skyrmion Hall effect for an extended range of drives. Under a constant drive the solitons have a constant velocity. We also measure the skyrmion velocity–current curves and identify the signatures of different phases including a pinned phase, stable soliton motion, and quasi-free motion at higher drives where all of the skyrmions depin from the pinning centers and move along the rigid wall. In the quasi-free motion regime, the velocity is oscillatory due to the motion of the skyrmions over the pinning sites. For increasing pinning strength, the onset of soliton motion shifts to higher values of current density. We also find that for stronger pinning, the characteristic velocity–current shape is affected by the annihilation of single or multiple skyrmions in the drive interval over which the soliton motion occurs. Our results indicate that stable skyrmion soliton motion is possible and that the solitons could be used as information carriers instead of the skyrmions themselves for technological applications.

Spontaneous skyrmion conformal lattice and transverse motion during dc and ac compression

We use atomistic-based simulations to investigate the behavior of ferromagnetic skyrmions being continuously compressed against a rigid wall under dc and ac drives. The compressed skyrmions can be annihilated close to the wall and form a conformal crystal with both a size and a density gradient, making it distinct from conformal crystals observed previously for superconducting vortices and colloidal particles. For both dc and ac driving, the skyrmions can move transverse to the compression direction due to a combination of density and size gradients. Forces in the compression direction are converted by the Magnus force into transverse motion. Under ac driving, the amount of skyrmion annihilation is reduced and we find a skyrmion Magnus ratchet pump. We also observe shear banding in which skyrmions near the wall move up to twice as fast as skyrmions further from the wall. When we vary the magnitude of the applied drive, we find a critical current above which the skyrmions are completely annihilated during a time scale that depends on the magnitude of the drive. By varying the magnetic parameters, we find that the transverse motion is strongly dependent on the skyrmion size. Smaller skyrmions are more rigid, which interferes with the size gradient and destroys the transverse motion. We also confirm the role of the size gradient by comparing our atomistic simulations with a particle-based model, where we find that the transverse motion is only transient. Our results are relevant for applications where skyrmions encounter repulsive magnetic walls, domain walls, or interfaces.

Solid-State Lithium Ion Supercapacitor for Voltage Control of Skyrmions.

Ionic control of magnetism gives rise to high magnetoelectric coupling efficiencies at low voltages, which is essential for low-power magnetism-based nonconventional computing technologies. However, for on-chip applications, magnetoionic devices typically suffer from slow kinetics, poor cyclability, impractical liquid architectures, or strong ambient effects. As a route to overcoming these problems, we demonstrate a LiPON-based solid-state ionic supercapacitor with a magnetic Pt/Co40Fe40B20/Pt thin-film electrode which enables voltage control of a magnetic skyrmion state. Skyrmion nucleation and annihilation are caused by Li ion accumulation and depletion at the magnetic interface under an applied voltage. The skyrmion density can be controlled through dc applied voltages or through voltage pulses. The skyrmions are nucleated by single 60 μs voltage pulses, and devices are cycled 750000 times without loss of electrical performance. Our results demonstrate a simple and robust approach to ionic control of magnetism in spin-based devices.

Building Paraxial Optical Skyrmions Using Rational Maps

Optical paraxial skyrmions are associated with simple rational maps, which can be derived entirely from their topological structure. Such maps, used as seeds for experimental realizations, outline the rules for generating isolated skyrmions or multi-skyrmions and lay the mathematical foundation for the exploration of skyrmion nucleation and annihilation mechanisms in optics. More details can be found in article number 2200350 by Claire Cisowski, Calum Ross, and Sonja Franke-Arnold. Image credit: Claire Cisowski.

Steady-state skyrmion oscillations in a spin valve (SV) nanopillar based on hybrid polarizer: A skyrmion-based spin torque nano-oscillator (STNO)

The skyrmion-based spin torque nano-oscillators (STNOs) have received a lot of interest due to their prospects as microwave signal generators. These oscillators are accessible by achieving consistent circular spinning of the skyrmion in a circular disk, caused by the spin transfer torque (STT) effect. However, the Magnus force effect, that relates to the non-trivial topological part of the skyrmion and driven on by the higher current densities, leads the skyrmion to accumulate in the center or annihilate toward the edge of the device. To counterbalance the Magnus force, we propose a skyrmion-based STNO model built on a circular nanopillar-shaped spin valve having a hybrid polarizer, which is divided into two regions, each with a different polarization angle. Using Thiele equation-based analytical framework in parallel with micromagnetic simulations, we illustrate that the proposed hybrid polarizer system is intended to ensure steady circular motion of the skyrmion without skyrmion annihilation or accumulation, by strengthening the boundary-induced force. Moreover, we also investigate the current density and polarizer parameters effect on skyrmion oscillation frequency. The results show that the frequency that can be limited for a skyrmion-based STNO with a hybrid polarizer is up to 3.35 GHz, which is higher in terms of magnitude than that of the homogeneous polarizer-based STNO. Our findings provide an insight into the phenomenon of skyrmion dynamics in a circular nanopillar-shaped spin valve with hybrid polarizer, which may open up new avenues for overcoming the frequency limit of the skyrmion-based STNOs.

Building Paraxial Optical Skyrmions Using Rational Maps

A simple mathematical expression based on rational maps to describe all optical paraxial skyrmion known to date, including Néel‐type and Bloch‐type skyrmions, bimerons, and anti‐skyrmions, is introduced. This expression is derived solely from topological considerations and outlines the rules that fully polarized paraxial light fields must obey to qualify as optical skyrmions. It is shown that rational maps can be implemented experimentally by superposing a pair of orthogonally polarized Laguerre–Gaussian modes. Novel optical skyrmion fields, called multi‐skyrmions, are obtained upon generalizing the proposed expression, laying the foundation for the exploration of skyrmion nucleation and annihilation mechanisms in optics.

The influence of curved surfaces on the propagation of skyrmions in a magnetic racetrack

The purpose of the current study is to analyze the effect of curvature on magnetic skyrmions by micromagnetic simulations. It is divided into two parts: First, we studied the stability of skyrmions on CoPt cylindrical surfaces, analyzing the static conditions for the annihilation of skyrmions as a function of the radius and curvature (positive or negative) of the surface. Unlike planar geometry, the skyrmion can be deformed into an elliptical shape when the surface is curved. In the second part, we analyzed the skyrmion dynamics when a spin-polarized current density is applied to a CoPt racetrack with a geometric defect in the form of a cylindrical surface, intercalated in the racetrack. We studied the conditions for skyrmion annihilation and compared it with the static case on cylindrical surfaces for various values of the radius and width of the geometric defect and also for various intensities of spin-polarized current density. We analyzed the shape of the skyrmion by measuring its radius in two perpendicular directions and its dynamic behavior when going through of the geometric defect, measuring the position and velocities of the skyrmion’s center. Furthermore, we compared the velocity profile obtained in the simulation with the theoretical results obtained by the Thiele’s equation as a function of the radius and width of the geometrical defect, and we showed that skyrmion can be accelerated and its speed can be increased up to 14 times due to an induced force (repulsive and attractive) that arises in the geometric defect region. Our results are in agreement with the results using the Thiele’s equation.

Ferroelectrically tunable magnetic skyrmions in two-dimensional multiferroics.

Magnetic skyrmions are topologically protected entities that are promising for information storage and processing. Currently, an essential challenge for future advances of skyrmionic devices lies in achieving effective control of skyrmion properties. Here, through first-principles and Monte-Carlo simulations, we report the identification of nontrivial topological magnetism in two-dimensional multiferroics of Co2NF2. Because of ferroelectricity, monolayer Co2NF2 exhibits a large Dzyaloshinskii-Moriya interaction. This together with exchange interaction can stabilize magnetic skyrmions with the size of sub-10 nm under a moderate magnetic field. Importantly, arising from the magnetoelectric coupling effect, the chirality of magnetic skyrmions is ferroelectrically tunable, producing the four-fold degenerate skyrmions. When interfacing with monolayer MoSe2, the creation and annihilation of magnetic skyrmions, as well as phase transition between skyrmion and skyrmion lattice, can be realized in a ferroelectrically controllable fashion. A dimensionless parameter κ' is further proposed as the criterion for stabilizing magnetic skyrmions in such multiferroic lattices. Our work greatly enriches the two-dimensional skyrmionics and multiferroics research.