Skyrmion Spin Texture Research Articles

Quantifying the topology of magnetic skyrmions in three dimensions.

Magnetic skyrmions have so far been treated as two-dimensional spin structures characterized by a topological winding number. However, in real systems with the finite thickness of the device material being larger than the magnetic exchange length, the skyrmion spin texture extends into the third dimension and cannot be assumed as homogeneous. Using soft x-ray laminography, we reconstruct with about 20-nanometer spatial (voxel) size the full three-dimensional spin texture of a skyrmion in an 800-nanometer-diameter and 95-nanometer-thin disk patterned into a 30× [iridium/cobalt/platinum] multilayered film. A quantitative analysis finds that the evolution of the radial profile of the topological skyrmion number is nonuniform across the thickness of the disk. Estimates of the micromagnetic energy densities suggest that the changes in topological profile are related to nonuniform competing energetic interactions. Our results provide a foundation for nanoscale metrology for spintronics devices using topology as a design parameter.

Unveiling the influence of fixed layer polarization on skyrmion nucleation and dynamics in circular geometry

Chiral magnetic skyrmion spin textures promise to be the next generation of energy-efficient computing. An effective way of skyrmion creation is essential to use as an information carrier. We report the creation of a skyrmion in a circular spin valve nanostructure using spin-polarized currents. This research investigates skyrmion nucleation under perpendicular and vortex fixed layer polarizations. The impact of Dzyaloshinskii-Moriya interaction (DMI), magnetic anisotropy, and saturation magnetization on the nucleation process has been studied for the perpendicular case. Depending on fixed layer polarization, we observed a trade-off between current density and pulse width for skyrmion nucleation. Vortex polarization enables faster nucleation at higher currents, while perpendicular polarization facilitates lower current thresholds at the expense of longer pulses. Further skyrmion dynamics have been explored under the sinusoidal magnetic fields.

Wavelength-tuned transformation between photonic skyrmion and meron spin textures

Topological spin textures, among which skyrmions and merons are typical examples, have with their swirling vectorial structures triggered enormous interest in physical systems including elementary particles and magnetic materials. Manipulating their symmetry and topology is important for understanding the mechanisms that underlie their topological phase transformation as well as offering tunable degrees of freedom to encode information, which has already been demonstrated in magnetic materials. Recently, the photonic counterparts of skyrmions and merons were constructed in a 2D wave system with deep-subwavelength features promising for optical sensing, imaging, and information decoding. However, their experimental realization relied on stringent excitation conditions that only support a single spin texture type on a specific structure. Here, we demonstrate for the first time the transformation between photonic skyrmion and meron spin lattices on the same metallic meta-surface having a well-designed structural period. We show experimentally the wavelength-tuned symmetry transformation of the photonic spin lattices, which are also found to be robust against disorder in the structure to a certain degree. This work provides new insights into controlling the electromagnetic field symmetry and topology, as well as in developing applications in spin optics and topological photonics.

Bimeron in a ferromagnetic spin-1 Bose–Einstein condensate

The spin degrees of freedom in multicomponent Bose–Einstein condensates allow for various types of topological excitations, such as skyrmions and knots. Systems with high spins offer a broader range of internal symmetries. For instance, in spin-1 Bose–Einstein condensates, one can observe not only the two-dimensional (2D) skyrmion spin texture but also the bimeron spin texture. However, it remains challenging to experimentally create or stabilize a bimeron and identify the conditions that facilitate its formation. In this context, our research predicts a novel form of topological structure known as the domain wall bimeron in ferromagnetic spin-1 Bose–Einstein condensates by manipulating the magnetic field. Unlike conventional 2D skyrmions, this unique bimeron exhibits a non-axisymmetric spin texture and features complex off-axis vortices whose cores are connected by domain walls. This work significantly contributes to the study of 2D topological excitations in quantum matter with high spins.

Observation of skyrmion bubbles in multilayer [Pt/Co/Cu]n using spin-polarized STM

Magnetic multilayers are a promising platform for storage and logic devices based on skyrmion spin textures, due to the large materials phase space for tuning properties. Epitaxial superlattice structures of [Pt/Co/Cu]n thin films were grown by molecular beam epitaxy at room temperature. Spin-polarized scanning tunneling microscopy (SP-STM) of these samples was used to probe the connection between surface structure and skyrmion morphology with nanoscale spatial resolution. Irregular-shaped skyrmion bubbles were observed, with effective diameters from 20 to 200 nm that are much larger than the nanoscale grain structure of the surface topography. Nucleation, annihilation, and motion of skyrmion bubbles could be driven using the stray field of the ferromagnetic tip in repeated imaging, and spin-polarized current/voltage pulses. Our detailed comparison of STM topography and differential conductance images shows that there are no surface defects or inhomogeneities at length scales that could account for the range in skyrmion bubble size or shape observed in the measurements.

Electric field manipulation of spin chirality and skyrmion dynamic.

The Dzyaloshinskii-Moriya interaction (DMI) is an antisymmetric exchange interaction that stabilizes spin chirality. One scientific and technological challenge is understanding and controlling the interaction between spin chirality and electric field. In this study, we investigate an unconventional electric field effect on interfacial DMI, skyrmion helicity, and skyrmion dynamics in a system with broken inversion symmetry. We design heterostructures with a 3d-5d atomic orbital interface to demonstrate the gate bias control of the DMI energy and thus transform the DMI between opposite chiralities. Furthermore, we use this voltage-controlled DMI (VCDMI) to manipulate the skyrmion spin texture. As a result, a type of intermediate skyrmion with a unique helicity is created, and its motion can be controlled and made to go straight. Our work shows the effective control of spin chirality, skyrmion helicity, and skyrmion dynamics by VCDMI. It promotes the emerging field of voltage-controlled chiral interactions and voltage-controlled skyrmionics.

Spin texture and chiral coupling of circularly polarized dipole field

Abstract We show that a circularly polarized electric dipole harbors a near-field concentrated wave which orbits around with an energy flux significantly larger (five orders of magnitudes at ∼1 nm radial distance) than far-field radiation. This near-field wave is found to carry transverse spins and reveal skyrmion spin texture (Néel-type). By performing electromagnetic analysis and numerical simulation, we demonstrate chiral extraction of a near-field rotational energy flux: the confined energy flow is out-coupled to surface plasmons on metal surface, whose curvature is designed to provide orbital angular momentum matched to spin angular momentum of dipole field, that is, to facilitate spin–orbit interaction. Strong coupling occurs with high chiral selectivity (∼113) and Purcell enhancement (∼17) when both linear and angular momenta are matched between dipole field and surface plasmons. Existence of a high-intensity energy flux in the deep-bottom near-field region (r ∼ 1 nm) opens up an interesting avenue in altering fundamental properties of dipole emission. For example, extracting ∼1% of this flux would result in enhancing spontaneous emission rate by ∼1000 times.

Skyrmion helicity: Quantization and quantum tunneling effects

We derive the quantization of magnetic helicity in the solid-state and demonstrate tunable macroscopic quantum tunneling, coherence, and oscillation for a skyrmion spin texture stabilized in frustrated magnets. We also discuss the parameter space for the experimental realization of quantum effects. Typically, for a skyrmion of 5 nm radius, quantum tunneling between two macroscopic states with distinct helicities occurs with an inverse escape rate within seconds below 100 mK, and an energy splitting in the MHz regime. Feasibility of quantum tunneling of an ensemble of magnetic spins inspires new platforms for quantum operations utilizing topologically protected chiral spin configurations.

Second-Harmonic Response in Magnetic Nodal-Line Semimetal Fe3GeTe2

We experimentally investigate second-harmonic transverse voltage response to ac electrical current for a magnetic nodal-line semimetal Fe3GeTe2 (FGT). For zero magnetic field, the observed second-harmonic voltage behaves as a square of the longitudinal current, as it should be expected for nonlinear Hall effect. The magnetic field behavior is found to be sophisticated: while the first-harmonic response shows the known anomalous Hall hysteresis in FGT, the second-harmonic Hall voltage is characterized by the pronounced high-field hysteresis and flat (B-independent) region with curves touching at low fields. The high-field hysteresis strongly depends on the magnetic field sweep rate, so it reflects some slow relaxation process. For the lowest rates, it is also accomplished by multiple crossing points. Similar shape of the second-harmonic hysteresis is known for skyrmion spin textures in nonlinear optics. Since skyrmions have been demonstrated for FGT by direct visualization techniques, we can connect the observed high-field relaxation with deformation of the skyrmion lattice. Thus, the second-harmonic Hall voltage response can be regarded as a tool to detect spin textures in transport experiments.

Ultrafast microscopy of a twisted plasmonic spin skyrmion

We report a transient plasmonic spin skyrmion topological quasiparticle within surface plasmon polariton vortices, which is described by analytical modeling and imaging of its formation by ultrafast interferometric time-resolved photoemission electron microscopy. Our model finds a twisted skyrmion spin texture on the vacuum side of a metal/vacuum interface and its integral opposite counterpart in the metal side. The skyrmion pair forming a hedgehog texture is associated with co-gyrating anti-parallel electric and magnetic fields, which form intense pseudoscalar E·B focus that breaks the local time-reversal symmetry and can drive magnetoelectric responses of interest to the axion physics. Through nonlinear two-photon photoemission, we record attosecond precision images of the plasmonic vectorial vortex field evolution with nanometer spatial and femtosecond temporal (nanofemto) resolution, from which we derive the twisted plasmonic spin skyrmion topological textures, their boundary, and topological charges; the modeling and experimental measurements establish a quantized integer photonic topological charge that is stable over the optical generation pulse envelope.

Isolated skyrmion, skyrmion lattice and antiskyrmion lattice creation through magnetization reversal in Co/Pd nanostructure

Skyrmion and antiskyrmion spin textures are axisymmetric inhomogeneous localized objects with distinct chirality in magnetic systems. These spin textures are potential candidates for the next generation energy-efficient spintronic applications due to their unique topological properties. Controlled and effective creation of the spin textures is required to use in conventional and neuromorphic computing applications. Here we show by micromagnetic simulations creating an isolated skyrmion, skyrmion lattice and antiskyrmion lattice through the magnetization reversal in Co/Pd multilayer nanostructure using spin-polarized current. The spin textures' stability depends on the spin-polarized current density, current pulse width, and Dzyaloshinskii–Moriya interaction (DMI). Antiskyrmions are evolved during the formation of a single skyrmion and skyrmion lattice. Skyrmion and antiskyrmion lattices together are observed for lower pulse width, 0.05 ns. Our micromagnetic studies suggest that the two distinct lattice phases' evolution could help to design the topological spin textures-based devices.

Threshold behaviors of direct and Hall currents in topological spin-Hall effect

We study spin-dependent direct and Hall conductivities in the threshold region of Fermi energy, ɛF=2J, where J is the exchange integral between the conduction electron spins and the skyrmion spin texture. For ɛF at the threshold value and above the spin-down electrons are allowed to exist. We find the two in the direct and four narrow peaks in the Hall conductivities for Fermi energies slightly below the threshold value. The found effects are dramatic because the electric current changes by approximately eight times in the narrow range of gate voltages (∼4 meV). The values of the peaks strongly depend on skyrmion size. For small and very large skyrmion sizes the peak amplitudes are small compared to the conductivity absolute values. At the skyrmion radius a=6nm and very light conduction electrons, m∗∼10−2me, the extrema are the most pronounced. The temperature evolution reveals the strong smearing effect where the peak-wise behavior completely disappears at room temperatures. Spin transistor could be considered for possible applications where in the narrow region of gate voltage the sharp conductivity change occurs.

Influence of Material Parameters on the Stability of Artificial Skyrmion in Hard/Soft Magnetic Bilayers

Herein, the influence of material parameters on the core diameter of the artificial skyrmions in hard magnetic/soft magnetic bilayer thin film is investigated. It is found that the perpendicular magnetic anisotropy (PMA) constant and the interlayer exchange constant play a key role in the realization of artificial skyrmions. Through simulation, the PMA constant of the hard magnetic materials should be between 1.0 × 105 and 4.5 × 105 J m−3. Therefore, some rare earth materials with high magnetic anisotropy or other materials with low magnetic anisotropy do not meet the requirement. The initial skyrmion spin texture is Bloch‐type artificial skyrmion in the hard/soft magnetic bilayer system. In the presence of 1.0–1.5 × 105 J m−3 PMA constant and 0 J m−1 interlayer exchange constant, the Néel‐type artificial skyrmions are formed in the hard/soft magnetic bilayer system. The size of the hard magnetic layer material has little effect on the stability of skyrmions, so the stability of skyrmions should mainly be adjusted with the PMA of the hard magnetic materials or by changing the material types.

Emergence of Nontrivial Spin Textures in Frustrated Van Der Waals Ferromagnets.

In this work, first principles ground state calculations are combined with the dynamic evolution of a classical spin Hamiltonian to study the metamagnetic transitions associated with the field dependence of magnetic properties in frustrated van der Waals ferromagnets. Dynamically stabilized spin textures are obtained relative to the direction of spin quantization as stochastic solutions of the Landau–Lifshitz–Gilbert–Slonczewski equation under the flow of the spin current. By explicitly considering the spin signatures that arise from geometrical frustrations at interfaces, we may observe the emergence of a magnetic skyrmion spin texture and characterize the formation under competing internal fields. The analysis of coercivity and magnetic hysteresis reveals a dynamic switch from a soft to hard magnetic configuration when considering the spin Hall effect on the skyrmion. It is found that heavy metals in capped multilayer heterostructure stacks host field-tunable spiral skyrmions that could serve as unique channels for carrier transport. The results are discussed to show the possibility of using dynamically switchable magnetic bits to read and write data without the need for a spin transfer torque. These results offer insight to the spin transport signatures that dynamically arise from metamagnetic transitions in spintronic devices.

Structural analysis of high-pressure phase for skyrmion-hosting multiferroic Cu2OSeO3

Cu2OSeO3 is known as a unique example of insulating multiferroic compounds with skyrmion spin texture, which is characterized by the chiral cubic crystal structure at ambient pressure. Recently, it has been reported that this compound shows pressure-induced structural transition with large enhancement of magnetic ordering temperature Tc. In the present study, we have investigated the detailed crystal structure in the high pressure phase, by combining the synchrotron X-ray diffraction experiment with the diamond anvil cell and the analysis based on the genetic algorithm. Our results suggest that the original pyrochlore Cu network is sustained even after the structural transition, while the orientation of SeO3 molecule as well as the position of oxygen in the middle of Cu tetrahedra are significantly modified. The latter features may be the key for the reported enhancement of Tc and associated stabilization of skyrmion phase at room temperature.

Magnetic Skyrmions in a Hall Balance with Interfacial Canted Magnetizations.

Magnetic skyrmions are attracting interest as efficient information-storage devices with low energy consumption, and have been experimentally and theoretically investigated in multilayers including ferromagnets, ferrimagnets, and antiferromagnets. The 3D spin texture of skyrmions demonstrated in ferromagnetic multilayers provides a powerful pathway for understanding the stabilization of ferromagnetic skyrmions. However, the manipulation mechanism of skyrmions in antiferromagnets is still lacking. A Hall balance with a ferromagnet/insulating spacer/ferromagnet structure is considered to be a promising candidate to study skyrmions in synthetic antiferromagnets. Here, high-density Néel-type skyrmions are experimentally observed at zero field and room temperature by Lorentz transmission electron microscopy in a Hall balance (core structure [Co/Pt]n /NiO/[Co/Pt]n ) with interfacial canted magnetizations because of interlayer ferromagnetic/antiferromagnetic coupling between top and bottom [Co/Pt]n multilayers, where the Co layers in [Co/Pt]n are always ferromagnetically coupled. Micromagnetic simulations show that the generation and density of skyrmions are strongly dependent on interlayer exchange coupling (IEC) and easy-axis orientation. Direct experimental evidence of skyrmions in synthetic antiferromagnets is provided, suggesting that the proposed approach offers a promising alternative mechanism for room-temperature spintronics.

Room-Temperature Skyrmions at Zero Field in Exchange-Biased Ultrathin Films

We demonstrate that magnetic skyrmions with a mean diameter around 60 nm can be stabilized at room temperature and zero external magnetic field in an exchange-biased Pt/Co/NiFe/IrMn multilayer stack. This is achieved through an advanced optimization of the multilayer stack composition in order to balance the different magnetic energies controlling the skyrmion size and stability. Magnetic imaging is performed both with magnetic force microscopy and scanning Nitrogen-Vacancy magnetometry, the latter providing unambiguous measurements at zero external magnetic field. In such samples, we show that exchange bias provides an immunity of the skyrmion spin texture to moderate external magnetic field, in the tens of mT range, which is an important feature for applications as memory devices. These results establish exchange-biased multilayer stacks as a promising platform towards the effective realization of memory and logic devices based on magnetic skyrmions.

Low-energy magnons in the chiral ferrimagnet Cu2OSeO3 : A coarse-grained approach

We report a comprehensive neutron scattering study of low energy magnetic excitations in the breathing pyrochlore helimagnetic Cu2OSeO3. Fully documenting the four lowest energy magnetic modes that leave the ferrimagnetic configuration of the "strong tetrahedra" intact ( meV), we find gapless quadratic dispersion at the point for energies above 0.2 meV, two doublets separated by 1.6(2) meV at the R point, and a bounded continuum at the X point. Our constrained rigid spin cluster model relates these features to Dzyaloshinskii-Moriya (DM) interactions and the incommensurate helical ground state. Combining conventional spin wave theory with a spin cluster form factor accurately reproduces the measured equal time structure factor through multiple Brillouin zones. An effective spin Hamiltonian describing complex anisotropic intercluster interactions is obtained.

Real-space imaging of confined magnetic skyrmion tubes

Magnetic skyrmions are topologically nontrivial particles with a potential application as information elements in future spintronic device architectures. While they are commonly portrayed as two dimensional objects, in reality magnetic skyrmions are thought to exist as elongated, tube-like objects extending through the thickness of the host material. The study of this skyrmion tube state (SkT) is vital for furthering the understanding of skyrmion formation and dynamics for future applications. However, direct experimental imaging of skyrmion tubes has yet to be reported. Here, we demonstrate the real-space observation of skyrmion tubes in a lamella of FeGe using resonant magnetic x-ray imaging and comparative micromagnetic simulations, confirming their extended structure. The formation of these structures at the edge of the sample highlights the importance of confinement and edge effects in the stabilisation of the SkT state, opening the door to further investigation into this unexplored dimension of the skyrmion spin texture.

Scaling, rotation, and channeling behavior of helical and skyrmion spin textures in thin films of Te-doped Cu2OSeO3.

Topologically nontrivial spin textures such as vortices, skyrmions, and monopoles are promising candidates as information carriers for future quantum information science. Their controlled manipulation including creation and annihilation remains an important challenge toward practical applications and further exploration of their emergent phenomena. Here, we report controlled evolution of the helical and skyrmion phases in thin films of multiferroic Te-doped Cu2OSeO3 as a function of material thickness, dopant, temperature, and magnetic field using in situ Lorentz phase microscopy. We report two previously unknown phenomena in chiral spin textures in multiferroic Cu2OSeO3: anisotropic scaling and channeling with a fixed-Q state. The skyrmion channeling effectively suppresses the recently reported second skyrmion phase formation at low temperature. Our study provides a viable way toward controlled manipulation of skyrmion lattices, envisaging chirality-controlled skyrmion flow circuits and enabling precise measurement of emergent electromagnetic induction and topological Hall effects in skyrmion lattices.