Magnetic Skyrmions Research Articles

Plasmonic skyrmions with bound states in the continuum

Plasmonic skyrmions are a subject of significant interest due to their potential applications in optics, photonics, and electromagnetic wave technology. These skyrmions are created by the interference of surface plasmon polaritons or spoof localized surface plasmons (SLSP), offering new possibilities for controlling light–matter interactions and structuring light. In this study, we have demonstrated the generation of both electric and magnetic skyrmions simultaneously using a rotational symmetric SLSP spiral meta-structure lattice, operating across a broad range from microwave to terahertz frequencies. By implementing them into a resonance configuration of the quasi-bound state in the continuum through symmetry breaking, we enhance the Q factor and fields, resulting in highly sensitive sensing performance. The SLSP metasurface enables tunable plasmonic skyrmions controlled by the incident polarization. Our findings have potential applications in highly sensitive sensing, filtering, modulation, and communication.

Manipulating Skyrmion Motion on a Nanotrack with Varied Material Parameters and Tilted Spin Currents

Magnetic skyrmions are topological quasiparticles with nanoscale size and high mobility, which have potential applications in information storage and spintronic devices. The manipulation of skyrmion’s dynamics in the track is an important topic due to the skyrmion Hall effect, which can deviate the skyrmions from the preferred direction. We propose a new model based on the ferromagnetic skyrmion, where the skyrmion velocity can be well controlled by adjusting the direction of the current. Using this design, we can avoid the annihilation of the skyrmion induced by the skyrmion Hall effect, which is confirmed by our micromagnetic simulation based on Mumax3. In the meantime, we increase the average velocity of the skyrmion by varying the intrinsic material parameters in the track, where the simulations agree well with our analytical results based on the Thiele equation. Finally, we give a phase diagram of the output of the skyrmion in the T-type track, which provides some practical ways for design of logic gates by manipulating crystalline anisotropy through the electrical control.

Stabilization and racetrack application of asymmetric Néel skyrmions in hybrid nanostructures

Magnetic skyrmions, topological quasiparticles, are small stable magnetic textures that possess intriguing properties and potential for data storage applications. Hybrid nanostructures comprised of skyrmions and soft magnetic material can offer additional advantages for developing skyrmion-based spintronic and magnonic devices. We show that a Néel-type skyrmion confined within a nanodot placed on top of a ferromagnetic in-plane magnetized stripe produces a unique and compelling platform for exploring the mutual coupling between magnetization textures. The skyrmion induces an imprint upon the stripe, which, in turn, asymmetrically squeezes the skyrmion in the dot, increasing their size and the range of skyrmion stability at small values of Dzyaloshinskii–Moriya interaction, as well as introducing skyrmion bi-stability. Finally, by exploiting the properties of the skyrmion in a hybrid system, we demonstrate unlimited skyrmion transport along a racetrack, free of the skyrmion Hall effect.

Direct observation of Néel-type skyrmions and domain walls in a ferrimagnetic DyCo3 thin film

Isolated magnetic skyrmions are stable, topologically protected spin textures that are at the forefront of research interests today due to their potential applications in information technology. A distinct class of skyrmion hosts are rare earth - transition metal (RE-TM) ferrimagnetic materials. To date, the nature and the control of basic traits of skyrmions in these materials are not fully understood. We show that for an archetypal ferrimagnetic material DyCo3 that exhibits a strong perpendicular anisotropy, the ferrimagnetic skyrmion size can be tuned by an external magnetic field. Moreover, by taking advantage of the high spatial resolution of scanning transmission X-ray microscopy (STXM) and utilizing a large x-ray magnetic linear dichroism (XMLD) contrast that occurs naturally at the RE resonant edges, we resolve the nature of the magnetic domain walls of ferrimagnetic skyrmions. We demonstrate that through this method one can easily discriminate between Bloch and Néel type domain walls for each individual skyrmion. For all isolated ferrimagnetic skyrmions, we observe that the domain walls are of Néel-type. This key information is corroborated with results of micromagnetic simulations and allows us to conclude on the nature of the Dzyaloshinskii-Moriya interaction (DMI) which concurs to the stabilisation of skyrmions in this ferrimagnetic system. Establishing that an intrinsic DMI occurs in RE-TM materials will also be beneficial towards a deeper understanding of chiral spin texture control in ferrimagnetic materials.

Platform for braiding Majorana modes with magnetic skyrmions

Platform for braiding Majorana modes with magnetic skyrmions

A Reversed Inverse Faraday Effect

The inverse Faraday effect is a magneto‐optical process allowing the magnetization of matter by an optical excitation carrying a non‐zero spin. In particular, a right circular polarization generates a magnetization in the direction of light propagation and a left circular polarization in the opposite direction to this propagation. Herein, it demonstrates that by manipulating the spin density of light, i.e., its polarization, in a plasmonic nanostructure, a reversed inverse Faraday effect is generated. A right circular polarization will generate a magnetization in the opposite direction of the light propagation, a left circular polarization in the direction of propagation. Also, it demonstrates that this new physical phenomenon is chiral, generating a strong magnetic field only for one helicity of the light, the opposite helicity producing this effect only for the mirror structure. This new optical concept opens the way to the generation of magnetic fields with unpolarized light, finding application in the ultrafast manipulation of magnetic domains and processes, such as spin precession, spin currents, and waves, magnetic skyrmion or magnetic circular dichroism, with direct applications in data storage and processing technologies.

Electrically-Excited Motion of Topological Defects in Multiferroic Materials

Topological magnetic defects in multiferroic materials acquire an electric charge or dipole moment due to the inverse Dzyaloshinskii-Moriya mechanism. This magnetoelectric coupling makes possible to excite large-amplitude collective motion of topological magnetic textures with an oscillating electric field. Here, I discuss electric excitation of a polar optical mode in a vortex-antivortex crystal and the electrically-induced spin precession in a magnetic skyrmion, which gives rise to rotation of skyrmions around each other and a translational motion of skyrmion-antiskyrmion pairs. The electric manipulation of magnetic topological defects in Mott insulators can find applications in magnetoelectric memory and logical devices.

Conducting Multiferroics

A review is given on the recent developments for the physics of multiferroics, i.e., the physics of broken time-reversal (\(\mathcal{T}\)) and inversion (\(\mathcal{P}\)) symmetries, extended to conducting systems. It includes (i) emergent inductance in spiral magnets, (ii) nonreciprocal nonlinear resistivity, (iii) superconducting multiferroics, and (iv) skyrmions in chiral magnets.

Estimating Patterns of Classical and Quantum Skyrmion States

In this review we discuss the latest results concerning development of the machine learning algorithms for characterization of the magnetic skyrmions that are topologically-protected magnetic textures originated from the Dzyaloshinskii-Moriya interaction that competes Heisenberg isotropic exchange in ferromagnets. We show that for classical spin systems there is a whole pool of machine approaches allowing their accurate phase classification and quantitative description on the basis of few magnetization snapshots. In turn, investigation of the quantum skyrmions is a less explored issue, since there are fundamental limitations on the simulation of such wave functions with classical supercomputers. One needs to find the ways to imitate quantum skyrmions on near-term quantum computers. In this respect, we discuss implementation of the method for estimating structural complexity of classical objects for characterization of the quantum skyrmion state on the basis of limited number of bitstrings obtained from the projective measurements.

Instability of Magnetic Skyrmion Strings Induced by Longitudinal Spin Currents.

It is well established that spin-transfer torques exerted by in-plane spin currents give rise to a motion of magnetic skyrmions resulting in a skyrmion Hall effect. In films of finite thickness or in three-dimensional bulk samples the skyrmions extend in the third direction forming a string. We demonstrate that a spin current flowing longitudinally along the skyrmion string instead induces a Goldstone spin wave instability. Our analytical results are confirmed by micromagnetic simulations of both a single string as well as string lattices, suggesting that the instability eventually breaks the strings. A longitudinal current is thus able to melt the skyrmion string lattice via a nonequilibrium phase transition. For films of finite thickness or in the presence of disorder a threshold current will be required, and we estimate the latter assuming weak collective pinning.

High field magnetic transport measurements of FeGe thin plates

Magnetic skyrmions have garnered considerable attention due to their topological properties and potential applications in information storage. These unique structures can be found in chiral magnets, including well-known compounds like MnSi and FeGe with a B20-type crystal structure. In this study, we utilized Lorentz transmission electron microscopy to investigate the influence of magnetic skyrmions on the Hall effect in FeGe under low magnetic fields. Additionally, we examined the magnetoresistance (MR) and Hall effect of FeGe under a high magnetic field of 28 T. Our findings reveal distinct mechanisms governing the MR at low and high temperatures. Notably, the anomalous Hall effect plays a significant role in the Hall resistivity observed at low magnetic fields. Meanwhile, the contribution of the skyrmion-induced topological Hall signal in the FeGe is ignorable. Furthermore, by employing a two-carrier model and fitting the carrier concentration of FeGe under high magnetic fields, we demonstrate a transition in the dominant carrier type from electrons to holes as the temperature increases. These results contribute to a deeper understanding of the intrinsic magnetic properties of FeGe.

Structure and stripe domain of melt-spun HoMn6Sn6-xGax ribbons

Topological magnetic skyrmions are expected to become a new generation of information carrier units, and exploring new magnetic skyrmion host materials and their magnetic domain structures is critical. This paper prepared HoMn6Sn6-xGax ribbons by melt-spinning and annealing and studied the effects of the second phase and doping Ga on the structure and magnetic domain of the ribbons. The results show that doped Ga only replaces the Sn sites in the HoMn6Sn6 phase, and the maximum substitution rate of Ga in the HoMn6Sn6 phase is close to 1/6. The unequal substitution of crystal positions causes the lattice to bear considerable compressive stress and negative strain. With an increase in doped Ga content, the lattice constant and cell volume of the HoMn6Sn6-xGax phase gradually decrease, and the substitution positions also change from mainly occupying Sn(2c) sites to occupying Sn(2e) and Sn(2d) sites simultaneously. Doping Ga enhances the preferential orientation in the grains, Ga occupying Sn(2d) sites enhances 〈009〉 orientation, and occupying Sn(2e) sites contributes to the enhancement in 〈204〉 orientation. Whether doped with Ga or not, the ribbons have stable stripe domains with HoMn6Sn6 structure. Increasing Ga content from x = 0.3 to x = 1.6 decreases the spin reorientation temperature, Curie temperatures, and the difference between the two temperatures.

Skyrmion-based racetrack multilevel data storage device manipulated by pinning

Magnetic skyrmions are particle-like topological protected spin structures that can be driven by electric current. We demonstrate a single skyrmion-based multilevel data storage device that could also work as the potential artificial synapse for neuromorphic computing. Four pinning sites are designed to situate the skyrmion in selective regions, whose resistances rise owing to the anisotropic magnetoresistance (AMR) induced by the skyrmion. Skyrmion’s position could denote the data levels when the device works as a multilevel data storage device. Pinning sites’ positions represent the synaptic weights when it functions as the artificial synapse. Multilevel data storage device’s data level change or the synapse’s potentiation and depression is realized by the electric current applied parallel to the track. Our work shows that the AMR could help to detect a single skyrmion without the magnetic tunnel junction. Moreover, the skyrmion manipulated by pinning provides an approach for precise control and helps realize skyrmion-based in-memory computing.

Some spin textures relevant to magnetic moments in the octonion spaces

The paper aims to apply the octonions to explore the contribution of some influence factors to magnetic moments, revealing the connection between the influence factors and spin texture. Maxwell was the first to introduce the quaternions to describe the electromagnetic theory. The subsequent scholars utilized the quaternions and octonions to study the gravitational and electromagnetic theories, and so forth. The paper adopts the octonions to research the gravitational and electromagnetic fields, including the octonion’s angular momentum, torque, force, and so on. When the octonion force is equal to zero under some circumstances, it is able to achieve eight equations independent of each other. In particular, one of eight independent equations reveals the interrelation between the second-torque and the divergence of magnetic moments. One of its deductions is that the directions, magnitudes, and frequencies of some terms are capable of impacting the orientation and vortex of magnetic moments, altering the frequency of magnetic vortex clusters. It means that the spin textures are relevant to some external influence factors. Some terms may have an influence on the arrangements of spin textures. The study will be helpful for understanding the physical properties of magnetic skyrmions and merons.

Magnetic skyrmions: materials, manipulation, detection, and applications in spintronic devices

Abstract Magnetic skyrmions are vortex-like spin configurations that possess nanometric dimensions, topological stability, and high controllability through various external stimuli. Since their first experimental observation in helimagnet MnSi in 2009, magnetic skyrmions have emerged as a highly promising candidate for carrying information in future high-performance, low-energy-consumption, non-volatile information storage, and logical calculation. In this article, we provide a comprehensive review of the progress made in the field of magnetic skyrmions, specifically in materials, manipulation, detection, and application in spintronic devices. Firstly, we introduce several representative skyrmion material systems, including chiral magnets, magnetic thin films, centrosymmetric materials, and Van der Waals materials. We then discuss various methods for manipulating magnetic skyrmions, such as electric current and electric field, as well as detecting them, mainly through electrical means such as the magnetoresistance effect. Furthermore, we explore device applications based on magnetic skyrmions, such as track memory, logic computing, and neuromorphic devices. Finally, we summarize the challenges faced in skyrmion research and provide future perspectives.

Ordered creation and motion of skyrmions with surface acoustic wave

Magnetic skyrmions with a well-defined spin texture have shown unprecedented potential for various spintronic applications owning to their topologically non-trivial and quasiparticle properties. To put skyrmions into practical technology, efficient manipulation, especially the inhibition of skyrmion Hall effect (SkHE) has been intensively pursued. In spite of the recent progress made on reducing SkHE in several substituted systems, such as ferrimagnets and synthetic antiferromagnets, the organized creation and current driven motion of skyrmions with negligible SkHE in ferromagnets remain challenging. Here, by embedding the [Co/Pd] multilayer into a surface acoustic wave (SAW) delay line where the longitudinal leaky SAW is excited to provide both the strain and thermal effect, we experimentally realized the ordered generation of magnetic skyrmions. The resultant current-induced skyrmions movement with negligible SkHE was observed, which can be attributed to the energy redistribution of the system during the excitation of SAW. Our findings open up an unprecedentedly new perspective for manipulating topological solitons, which could possibly trigger the future discoveries in skyrmionics and spin acousto-electronics.

A magnetic skyrmion diode based on potential well inducting effect

Magnetic skyrmions have great potential in the application of spintronic devices due to their stable topologically protected spin configuration. To meet the needs of spintronic device design, it is necessary to manipulate the movement of the magnetic skyrmions. Here we propose a skyrmion diode based on potential well induced skyrmion motion through theoretical calculations. The potential well is generated by the voltage-controlled magnetic anisotropy (VCMA) gradient. By utilizing the induction of the potential well as well as the skyrmion Hall effect (SkHE), the velocity and trajectory of the skyrmions can be controlled and the forward pass and reverse cutoff functions of diode-like devices have been realized. Furthermore, we report the dynamics of current-driven skyrmions in a racetrack with locally applied VCMA. Under the influence of the SkHE, the difference in dynamic behavior between forward and reverse motion of the skyrmions is obvious, and the potential well can produce different pinning, depinning and annihilating effects on forward and reverse moving skyrmions. Our results can be beneficial for the design and development of magnetic skyrmion diodes.

Analytic theory for Néel skyrmion size, accounting for finite film thickness

The energy of a Néel skyrmion in a magnetic film with interfacial DMI is developed analytically, with the finite thickness of the film taken into account. Many theories for the energy of magnetic skyrmions use models that are only accurate in the limit that the film thickness vanishes. One theory [Büttner et al. Sci. Rep. 8, 4464 (2018)] provides exceptionally accurate expressions for the skyrmion energy for finite thickness films, but does not yield an analytic solution for the skyrmion size, plus it contains contributions which are found using fitting methods. Here, we approximate the demagnetizing energy of a skyrmion by examining the form of magnetostatic Green’s function expressions for the demagnetizing fields. Then, analytic expressions for the skyrmion radius and the skyrmion domain wall width are derived in the limit of zero applied field. Our methods are rigorously compared to other calculation methods and compare well to numerical calculations and experimental results, both with and without an applied magnetic field. The approximations that allow for an analytic result lead to a loss of accuracy for the skyrmion radius of up to 20% but this loss in accuracy may be offset by the simplicity of the developed physical expressions. Finite thickness is important to consider for skyrmions existing in multilayer stacks with total thickness of a few nanometers.

Stability and asymptotic interactions of chiral magnetic skyrmions in a tilted magnetic field

Using a general framework, interaction potentials between chiral magnetic solitons in a planar system with a tilted external magnetic field are calculated analytically in the limit of large separation. The results are compared to previous numerical results for solitons with topological charge \pm 1±1. A key feature of the calculation is the interpretation of Dzyaloshinskii-Moriya interaction (DMI) as a background SO(3)SO(3) gauge field. In a tilted field, this leads to a U(1)U(1)-gauged version of the usual equation for spin excitations, leading to a distinctive oscillating interaction profile. We also obtain predictions for skyrmion stability in a tilted field which closely match numerical observations.

Implementation of the Toffoli and Peres reversible logic gates using magnetic skyrmions in operational gates

Micromagnetic simulations have been used to create reversible logic gates that utilize magnetic skyrmions as input and output signals. The geometrical design of the logic devices consists of a two-dimensional assembly of ferromagnetic nanotracks, resembling rectangular grooves. Drawing an analogy to the billiard-ball model, this study takes into account elastic collisions between magnetic skyrmions and introduces the concept of control skyrmion. The application of spin-polarized currents to the system induces the motion of magnetic skyrmions along the nanotracks. The synchronized movement of magnetic skyrmions across the operational logic gates, required for their interaction, is achieved by incorporating multiple triangular magnetic notches along the nanotracks. Short polarized current pulses are then applied through the logic gates to facilitate this process. We have successfully implemented the XOR/AND and Full Adder logic gates as well as the reversible Toffoli and Peres logic gates using skyrmion-based architectures for computing. Our results offer valuable guidelines for leveraging magnetic skyrmions as digital signals in skyrmion-based computing systems.