arXiv subjects

X. S. Wang

Publications and source records attributed to X. S. Wang.

At least 19 recordsLinked to original sources

arXiv2026

Propagation, generation, and utilization of topologically trivial magnetic solitons in magnetic nanowires

Magnetic solitons are nonlinear, local excitations in magnetic systems. In this study, we theoretically and numerically investigate the properties and generation of one-dimensional (1D) topologically trivial magnetic solitons in ferromagnetic nanowires. An approximate analytical soliton solution described by two free parameters is validated by comparing with the micromagnetic simulation. Across an interface between two media of different anisotropy, the reflection and refraction of a soliton are highly nonlinear that are different from the linear spin waves. A pair of magnetic solitons that propagate in opposite directions can be generated by alternately applying magnetic field or spin-polarized current pulses of opposite directions to at least two successive regions. Each soliton falls into a soliton solution that can be controlled by the generation process. These magnetic solitons can be used to drive domain wall motion over a certain distance determined by the soliton magnitude, allowing for discrete manipulation of domain walls compatible with the digital nature of information technology. Our findings pave the way for the application of topologically trivial solitons in spintronics.

cond-mat.mes-hall↗

arXiv2025

Breaking the Sabatier Principle by Dynamic Adsorption-Desorption Decoupling in Electrocatalytic Hydrogen Evolution

The Sabatier principle establishes a fundamental trade-off in heterogeneous electrocatalysis.In the hydrogen evolution reaction (HER), this trade-off is manifested by the coupling of Volmer step, which requires strong hydrogen adsorption, with the Heyrovsky/Tafel step, which favors facile desorption, thus giving rise to the classical volcano relationship and limiting activity even at $ΔG=0$. Here, we demonstrate a ferroelectric platform with dynamic tunability -- monolayer GeS$_2$ decorated with transition metal atoms as a proof-of-concept -- where polarization-driven surface electronic reconstruction enables real-time modulation of intermediate binding strength, thereby breaking the Sabatier constraint. Reversible control of hydrogen adsorption allows strong H binding to accelerate the Volmer step, followed by weakened adsorption to promote the Heyrovsky/Tafel step.This dynamic adsorption-desorption decoupling not only surpasses the volcano limit to achieve unprecedented HER activity, but also establishes a general paradigm for designing adaptive electrocatalysts capable of reconfiguring under operating conditions.

cond-mat.mtrl-sci↗

arXiv2025

Electronic origin of stability of 2D 1H-phase Janus transition metal dichalcogenides and beyond

Janus transition metal dichalcogenides (JTMDs) monolayers have emerged as a new paradigm to broaden the family of two-dimensional (2D) materials. Despite numerous theoretical predictions of JTMDs, their experimental realization remains scarce, most probably due to intrinsic structural fragility. We identify a dependence of the structural stability of 1H-phase JTMDs on the transition metal group, with Group-VIB-based monolayers exhibiting robust stability, as evidenced by the successful synthesized MoSSe and WSSe. The group-dependent stability arises from the competition between metal-ligand ionic bonding and ligand-ligand covalent bonding, as well as the high-energy d-electron orbital splitting. We propose an electron configuration that describes the interactions of electrons near the Fermi level to correlate the stability, and introduce an electron compensation strategy to stabilize certain unstable JTMDs systems. Guided by the electronic origin of stability, we predict a family of stable 2D Janus transition metal halides with intrinsic ferromagnetic valley properties. This work bridges the gap between electronic structure and stability predictions, and extends the design rules for synthesizing 2D Janus materials.

cond-mat.mtrl-sci↗

arXiv2024

Stabilization and dynamics of magnetic antivortices in a nanodisk with anisotropic Dzyaloshinskii-Moriya interaction

We theoretically investigate the antivortex stabilized by anisotropic Dzyaloshinskii-Moriya interaction (DMI) in nanodisks. It is remarkably found that the antivortex remains stable even when the nanodisk radius is reduced to 15 nm, owing to the short-range nature of the DMI. We also investigate the antivortex dynamics under a static in-plane magnetic field, which shows that the displacement of the antivortex core depends on its vorticity and helicity, providing a fundamental basic for distinguishing different vortex types. Additionally, spin-polarized currents can trigger a self-sustained gyration of the antivortex at low current densities, while inducing polarity switching at high current densities. Our findings offer valuable insights into the DMI role in stabilizing topological solitons and their potential applications in spin-torque nano-oscillators and magnetic memories.

cond-mat.mes-hall↗

arXiv2024

Field-free ultrafast magnetization reversal of a nanodevice by a chirped current pulse via spin-orbit torque

We investigated the magnetization reversal of a perpendicularly magnetized nanodevice using a chirped current pulse (CCP) via spin-orbit torques (SOT). Our findings demonstrate that both the field-like (FL) and damping-like (DL) components of SOT in CCP can efficiently induce ultrafast magnetization reversal without any symmetry-breaking means. For a wide frequency range of the CCP, the minimal current density obtained is significantly smaller compared to the current density of conventional SOT-reversal. This ultrafast reversal is due to the CCP triggering enhanced energy absorption (emission) of the magnetization from (to) the FL- and DL-components of SOT before (after) crossing over the energy barrier. We also verified the robustness of the CCP-driven magnetization reversal at room temperature. Moreover, this strategy can be extended to switch the magnetic states of perpendicular synthetic antiferromagnetic (SAF) and ferrimagnetic (SFi) nanodevices. Therefore, these studies enrich the basic understanding of field-free SOT-reversal and provide a novel way to realize ultrafast SOT-MRAM devices with various free layer designs: ferromagnetic, SAF, and SFi.

cond-mat.mes-hall↗

arXiv2022

Field-driven side-by-side magnetic domain wall dynamics in ferromagnetic nanostrips

There has been a plethora of studies on domain wall dynamics in magnetic nanostrips, mainly because of its versatile non-linear physics and potential applications in data storage devices. However, most of the studies focus on out-of-plane domain walls or in-plane head-to-head (tail-to-tail) domain walls. Here, we numerically study the field-driven dynamics of in-plane side-by-side domain walls in ferromagnetic strips, which can be stable in the presence of an in-plane easy-axis anisotropy transverse to the strip. The domain walls move in a rigid-body manner at low field, and show complex Walker breakdown behavior at high field. We observe a multi-step Walker breakdown through vortex nucleation in wide strips. In the presence of Dzyaloshinskii-Moriya interaction (DMI), the first Walker breakdown field first decreases then increases with interfacial DMI, while keeps increasing with bulk DMI. These findings complement the current understanding on domain wall dynamics.

cond-mat.mes-hall↗

arXiv2021

Type-II Weyl Excitation in Vortex Arrays

Weyl-like magnon excitations in ordered magnets have attracted significant recent attention. Despite of the tantalizing physics and application prospects, the experimental observation of Weyl magnons is still challenging owing to their extraordinarily high frequency that is not accessible to the microstrip antenna technique. Here we predict gigahertz Weyl excitations in the collective dynamics of dipolar-coupled magnetic vortices arranged in a three-dimensional stacked honeycomb lattice. It is found that the inversion symmetry breaking leads to the emergence of the type-II Weyl semimetal (WSM) state with tilted dispersion. We derive the full phase diagram of the vortex arrays that support WSMs with both single and double pairs of Weyl nodes, and the topological insulator phase. We observe robust arc surface states in a dual-segment fashion due to the tilted nature of type-II WSMs. Our findings uncover the low-frequency WSM phase in magnetic texture based crystals that are indispensable for future Weyltronic applications.

cond-mat.mes-hall↗

arXiv2021

Fast magnetization reversal of a magnetic nanoparticle induced by cosine chirp microwave field pulse

We investigate the magnetization reversal of single-domain magnetic nanoparticle driven by the circularly polarized cosine chirp microwave pulse (CCMP). The numerical findings, based on the Landau-Lifshitz-Gilbert equation, reveal that the CCMP is by itself capable of driving fast and energy-efficient magnetization reversal. The microwave field amplitude and initial frequency required by a CCMP are much smaller than that of the linear down-chirp microwave pulse. This is achieved as the frequency change of the CCMP closely matches the frequency change of the magnetization precession which leads to an efficient stimulated microwave energy absorption (emission) by (from) the magnetic particle before (after) it crosses over the energy barrier. We further find that the enhancement of easy-plane shape anisotropy significantly reduces the required microwave amplitude and the initial frequency of CCMP. We also find that there is an optimal Gilbert damping for fast magnetization reversal. These findings may provide a pathway to realize the fast and low-cost memory device.

cond-mat.mes-hall↗

arXiv2021

Thermally assisted magnetization reversal of a magnetic nanoparticle driven by a down-chirp microwave field pulse

It has been shown that a single-domain magnetic nanoparticle can be effectively switched by a linear down-chirp microwave field pulse (DCMWP) in zero temperature limit. However, finite temperature is ubiquitous in practice. Here, we study the effect of finite temperature on the DCMWP-induced magnetization reversal based on the stochastic Landau-Lifshitz-Gilbert equation. It is found that any one of the three controlling parameters of a DCMWP, i.e. the amplitude, chirp rate, or initial frequency, decreases with increasing temperature while the other two are fixed. The maximal temperature at which the reversal can happen increases with enlarging the system size. These phenomena are related to the facts that the energy barrier induced by anisotropy increases with the system volume, and the effective magnetization decreases with temperature. We also provide a set of optimal parameters for practical realization of our proposal. These findings may provide a way to realize low-cost and fast magnetization reversal with a wide operating temperature.

cond-mat.mes-hall↗

arXiv2020

Bosonic Bott Index and Disorder-Induced Topological Transitions of Magnons

We investigate the role of disorder on the various topological magnonic phases present in deformed honeycomb ferromagnets. To this end, we introduce a bosonic Bott index to characterize the topology of magnon spectra in finite, disordered systems. The consistency between the Bott index and Chern number is numerically established in the clean limit. We demonstrate that topologically protected magnon edge states are robust to moderate disorder and, as anticipated, localized in the strong regime. We predict a disorder-driven topological phase transition, a magnonic analog of the "topological Anderson insulator" in electronic systems, where the disorder is responsible for the emergence of the nontrivial topology. Combining the results for the Bott index and transport properties, we show that bulk-boundary correspondence holds for disordered topological magnons. Our results open the door for research on topological magnonics as well as other bosonic excitations in finite and disordered systems.

cond-mat.mes-hall↗

arXiv2020

Current-Driven Dynamics of Magnetic Hopfions

Topological magnetic textures have attracted considerable interest since they exhibit new properties and might be useful in information technology. Magnetic hopfions are three-dimensional (3D) spatial variations in the magnetization with a non-trivial Hopf index. We find that in ferromagnetic materials, two types of hopfions, Bloch-type and Néel-type hopfions, can be excited as metastable states in the presence of bulk and interfacial Dzyaloshinskii-Moriya interactions, respectively. We further investigate how hopfions can be driven by currents via spin-transfer torques (STTs) and spin-Hall torques (SHTs). Distinct from 2D ferromagnetic skyrmions, hopfions have a vanishing gyrovector. Consequently, there are no undesirable Hall effects. Néel-type hopfions move along the current direction via both STT and SHT, while Bloch-type hopfions can be moved either transverse to the current direction by SHT or parallel to the current direction by STT. Our findings open the door to utilizing hopfions as information carriers.

cond-mat.mes-hall↗

arXiv2019

Thermal gradient driven domain wall dynamics

The issue of whether a thermal gradient acts like a magnetic field or an electric current in the domain wall (DW) dynamics is investigated. Broadly speaking, magnetization control knobs can be classified as energy-driving or angular-momentum driving forces. DW propagation driven by a static magnetic field is the best-known example of the former in which the DW speed is proportional to the energy dissipation rate, and the current-driven DW motion is an example of the latter. Here we show that DW propagation speed driven by a thermal gradient can be fully explained as the angular momentum transfer between thermally generated spin current and DW. We found DW-plane rotation speed increases as DW width decreases. Both DW propagation speed along the wire and DW-plane rotation speed around the wire decrease with the Gilbert damping. These facts are consistent with the angular momentum transfer mechanism, but are distinct from the energy dissipation mechanism. We further show that magnonic spin-transfer torque (STT) generated by a thermal gradient has both damping-like and field-like components. By analyzing DW propagation speed and DW-plane rotation speed, the coefficient ( \b{eta}) of the field-like STT arising from the non-adiabatic process, is obtained. It is found that \b{eta} does not depend on the thermal gradient; increases with uniaxial anisotropy K_(||) (thinner DW); and decreases with the damping, in agreement with the physical picture that a larger damping or a thicker DW leads to a better alignment between the spin-current polarization and the local magnetization, or a better adiabaticity.

cond-mat.mes-hall↗

arXiv2018

Current-induced skyrmion motion on magnetic nanotubes

Magnetic skyrmions are believed to be the promising candidate of information carriers in spintronics. However, the skyrmion Hall effect due to the nontrivial topology of skyrmions can induce a skyrmion accumulation or even annihilation at the edge of the devices, which hinders the real-world applications of skyrmions. In this work, we theoretically investigate the current-driven skyrmion motion on magnetic nanotubes which can be regarded as "edgeless" in the tangential direction. By performing micromagnetic simulations, we find that the skyrmion motion exhibits a helical trajectory on the nanotube, with its axial propagation velocity proportional to the current density. Interestingly, the skyrmion's annular speed increases with the increase of the thickness of the nanotube. A simple explanation is presented. Since the tube is edgeless for the tangential skyrmion motion, a stable skyrmion propagation can survive in the presence of a very large current density without any annihilation or accumulation. Our results provide a new route to overcome the edge effect in planar geometries.

cond-mat.mes-hall↗

arXiv2018

Wiggling skyrmion propagation under parametric pumping

We address the problem of how magnetic skyrmions can propagate along a guided direction by parametric pumping. As evidenced by our micromagnetic simulations, skyrmions can hardly be driven by either a static electric field or a static magnetic field alone. Although the magnetic anisotropy can be modified by an electric field, parametric pumping with an oscillating electric field can only excite the breathing modes. On the other hand, a static magnetic field can break rotational symmetry through the Zeeman interaction, but it cannot serve as an energy source for propelling a skyrmion. Here we found that the combination of a perpendicularly oscillating electric field and an in-plane static magnetic field can drive a skyrmion undergoing a wiggling motion along a well defined trajectory. The most efficient driving occurs when the frequency of the oscillating field is close to that of the breathing motion. The physics is revealed in a generalized Thiele equation where a net spin current excited by the parametric pumping can drive the skyrmion propagation through angular momentum transfer. Compared with other alternative proposals, our results open new possibilities for manipulating skyrmions in both metals and insulators with low-power consumption. The oscillating skyrmion motion can also be a microwave generator for future spintronic applications such as an nano-tool on a diamond Nitrogen-Vacancy center.

cond-mat.mes-hall↗

arXiv2018

Subnanosecond magnetization reversal of magnetic nanoparticle driven by chirp microwave field pulse

We investigate the magnetization reversal of single-domain magnetic nanoparticle driven by linear down-chirp microwave magnetic field pulse. Numerical simulations based on the Landau-Lifshitz-Gilbert equation reveal that solely down-chirp pulse is capable of inducing subnanosecond magnetization reversal. With a certain range of initial frequency and chirp rate, the required field amplitude is much smaller than that of constant-frequency microwave field. The fast reversal is because the down-chirp microwave field acts as an energy source and sink for the magnetic particle before and after crossing over the energy barrier, respectively. Applying a spin-polarized current additively to the system further reduces the microwave field amplitude. Our findings provide a new way to realize low-cost and fast magnetization reversal.

cond-mat.mes-hall↗

arXiv2018

Twisted skyrmion at domain boundaries and the method of image

We predict a novel twisted skyrmion structure at the boundary of two antiferromagnetically coupled magnetic domains with antiparallel magnetization directions. Through this intermediate state, skyrmions with opposite polarities can be freely switched between each other by spin-polarized electric currents. Based on these findings, we propose the concept of double-track skyrmion racetrack memory and logic gates where the binary data are represented by skyrmions with different polarities. The dynamics of skyrmion polarity reversal is theoretically studied. Using the method of image, we derive the analytical formula of the repulsive potential when a normal skyrmion approaches the domain boundary. A harmonic attractive potential well is obtained for the twisted skyrmion across the boundary. Micromagnetic simulations compare well with theoretical predictions. The method of image skyrmion proposed in this work can be used to deal with a large class of skyrmion-boundary interaction problems.

cond-mat.mes-hall↗

arXiv2018

Breaking the current density threshold in spin-orbit-torque magnetic random access memory

Spin-orbit-torque magnetic random access memory (SOT-MRAM) is a promising technology for the next generation of data storage devices. The main bottleneck of this technology is the high reversal current density threshold. This outstanding problem of SOT-MRAM is now solved by using a current density of constant magnitude and varying flow direction that reduces the reversal current density threshold by a factor of more than the Gilbert damping coefficient. The Euler-Lagrange equation for the fastest magnetization reversal path and the optimal current pulse are derived for an arbitrary magnetic cell. The theoretical limit of minimal reversal current density and current density for a GHz switching rate of the new reversal strategy for CoFeB/Ta SOT-MRAMs are respectively of the order of $10^5$ A/cm$^2$ and $10^6$ A/cm$^2$ far below $10^7$ A/cm$^2$ and $10^8$ A/cm$^2$ in the conventional strategy. Furthermore, no external magnetic field is needed for a deterministic reversal in the new strategy.

cond-mat.mes-hall↗

arXiv2018

A theory on skyrmion size and profile

A magnetic skyrmion is a topological object consisting of an inner domain, an outer domain, and a wall that separates the two domains. The skyrmion size and wall width are two fundamental quantities of a skyrmion that depend sensitively on material parameters such as exchange energy, magnetic anisotropy, Dzyaloshinskii-Moriya interaction, and magnetic field. However, there is no quantitative understanding of the two quantities so far. Here, we present general expressions for the skyrmion size and wall width obtained from energy considerations. The two formulas agree almost perfectly with simulations and experiments for a wide range of parameters, including all existing materials that support skyrmions. Furthermore, it is found that skyrmion profiles agree very well with the Walker-like 360\textdegree{} domain wall formula.

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