Polarity-controllable magnetic skyrmion filter

The skyrmion generator is one of the indispensable components for the future functional skyrmion devices,but the process of generating skyrmion cannot avoid mixing with other magnetic textures,such as skyrmionium and nested skyrmion bags.These mixed magnetic textures will inevitably lead to the blockage of skyrmion transport and even the distortion of data information.Therefore,the design of an efficient skyrmion filter is of great significance for the development of skyrmion-based spintronic devices.In this work,a skyrmion filter scheme is proposed,and the high-efficiency filtering function is demonstrated by micromagnetic simulations.The results show that the filtering effect of the scheme depends on the structure geometry and the spin current density that drives the skyrmion.Based on this scheme,the polarity of the filtered skyrmion can be controlled by switching the magnetization state at the output end,and the“cloning”of the skyrmion can be realized by geometric optimization of the structure.We believe that in the near future,the skyrmion filter will become one of the important components of skyrmion-based spintronic devices in the future.

应力调控下基于Skyrmion的晶体管特性

研究了不同应力、磁各向异性常数对电流驱动Skyrmion的动力学特性影响。随着施加应力增大,能够产生稳定Skyrmion的各向异性常数取值范围的上下限增大,即外加应力影响电流驱动时Skyrmion运行的稳定性。设计了一种宽-窄-宽型Skyrmion基晶体管,通过调控应力和各向异性常数来调控晶体管的开关特性。仿真结果表明,当极化电流密度、各向异性常数变化时,晶体管导通状态对应的应力范围也随之变化。研究结果为设计Skyrmion基晶体管及相关逻辑功能器件提供了新思路。

电流驱动RKKY耦合作用下Skyrmion的动力学特性研究

研究了不同RKKY铁磁耦合强度对Skyrmion振荡及进动特性的调控。研究结果表明,T层和B层的Skyrmion都可以在一定条件下达到稳定振荡,即Skyrmion进动呈现圆形轨迹;同一时刻,T层Skyrmion进动位置要超前B层,且耦合强度的大小会改变上下两层Skyrmion进动半径差值。此外,文章还研究了不同驱动电流对Skyrmion动力学特性的影响,研究结果表明驱动电流可以对起始振荡时间、稳定振荡半径等动力学特性进行有效调控。

Tunable dispersion relations manipulated by strain in skyrmion-based magnonic crystals

We theoretically investigate the propagation characteristics of spin waves in skyrmion-based magnonic crystals. It is found that the dispersion relation can be manipulated by strains through magneto-elastic coupling. Especially, the allowed bands and forbidden bands in dispersion relations shift to higher frequency with strain changing from compressive to tensile,while shifting to lower frequency with strain changing from tensile to compressive. We also confirm that the spin wave with specific frequency can pass the magnonic crystal or be blocked by tuning the strains. The result provides an advanced platform for studying the tunable skyrmion-based spin wave devices.

Frequency combs based on magnon-skyrmion interaction in magnetic nanotubes

Within the magnonics community,there has been a lot of interests in the magnon–skyrmion interaction.Magnons and skyrmions are two intriguing phenomena in condensed matter physics,and magnetic nanotubes have emerged as a suitable platform to study their complex interactions.We show that magnon frequency combs can be induced in magnetic nanotubes by three-wave mixing between the propagating magnons and skyrmion.This study enriches our fundamental comprehension of magnon–skyrmion interactions and holds promise for developing innovative spintronic devices and applications.This frequency comb tunability and unique spectral features offer a rich platform for exploring novel avenues in magnetic nanotechnology.

Skyrmion motion induced by spin-waves on magnetic nanotubes

We investigate the skyrmion motion driven by spin waves on magnetic nanotubes through micromagnetic simulations.Our key results include demonstrating the stability and enhanced mobility of skyrmions on the edgeless nanotube geometry,which prevents destruction at boundaries—a common issue in planar geometries.We explore the influence of the damping coefficient,amplitude,and frequency of microwaves on skyrmion dynamics,revealing a non-uniform velocity profile characterized by acceleration and deceleration phases.Our results show that the skyrmion Hall effect is significantly modulated on nanotubes compared to planar models,with specific dependencies on the spin-wave parameters.These findings provide insights into skyrmion manipulation for spintronic applications,highlighting the potential for high-speed and efficient information transport in magnonic 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.

Dynamic properties of the magnetic skyrmion driven by electromagnetic waves with spin angular momentum and orbital angular momentum

We theoretically studied the dynamic properties of the skyrmion driven by electromagnetic(EM)waves with spin angular momentum(SAM)and orbital angular momentum(OAM)using micromagnetic simulations.First,the guiding centers of the skyrmion driven by EM waves with SAM,i.e.,left-handed and right-handed circularly polarized EM waves,present circular trajectories,while present elliptical trajectories under linear EM waves driving due to the superposition of oppositely polarized wave components.Second,the trajectories of the skyrmion driven by EM waves with OAM demonstrate similar behavior to that driven by linearly polarized EM waves.Because the wave vector intensity varies with the phase for both linearly polarized EM waves and EM waves with OAM,the angular momentum is transferred to the skyrmion non-uniformly,while the angular momentum is transferred to the skyrmion uniformly for left-handed and right-handed circularly polarized EM driving.Third,the dynamic properties of the skyrmion driven by EM waves with both SAM and OAM are investigated.It is found that the dynamic trajectories exhibit more complex behavior due to the contributions or competition of SAM and OAM.We investigate the characteristics of intrinsic gyration modes and frequency-dependent trajectories.Our research may provide insight into the dynamic properties of skyrmion manipulated by EM waves with SAM or OAM and provide a method for controlling skyrmion in spintronic devices.

Shape-influenced non-reciprocal transport of magnetic skyrmions in nanoscale channel

Skyrmions, with their vortex-like structures and inherent topological protection, play a pivotal role in developing innovative low-power memory and logic devices. The efficient generation and control of skyrmions in geometrically confined systems are crucial for the development of skyrmion-based spintronic devices. In this study, we focus on investigating the non-reciprocal transport behavior of skyrmions and their interactions with boundaries of various shapes. The shape of the notch structure in the nanotrack significantly affects the dynamic behavior of magnetic skyrmions. Through micromagnetic simulation, the non-reciprocal transport properties of skyrmions in nanowires with different notch structures are investigated in this work.

另一种Skyrmion的稳定项和内部结构

在“Ｓｋｙｒｍｅ模型中的一个问题及其简单的讨论”的基础上提出了两个问题：一是对Ｈｉｇｇｓ场形成的Ｓｋｙｒｍｉｏｎ其拉氏量中的Ｓｋｙｒｍｅ项可否用矢量介子作用项代替？另一个是对Ｈｉｇｇｓ场形成的Ｓｋｙｒｍｉｏｎ可否引进内部结构？对前一个问题找到了用规范矢量玻色子Ｚ０的作用作为Ｈｉｇｇｓ形成的Ｓｋｙｒｍｉｏｎ的稳定项；对后一间题接线性σ模型的方法引进了Ｈｉｇｇｓ形成的Ｓｋｙｒｍｉｏｎ的内部结构．获得这两个模型的方程组，并借助于别人的结果，得知这些方程组的某些信息．

Lorentz transmission electron microscopy for magnetic skyrmions imaging

Magnetic skyrmions have interesting properties,including their small size,topological stability,and extremely low threshold current for current-driven motion.Therefore,they are regarded as promising candidates for next-generation magnetic memory devices.Lorentz transmission electron microscopy(TEM)has an ultrahigh magnetic domain resolution(~2 nm),it is thus an ideal method for direct real-space imaging of fine magnetic configurations of ultra-small skyrmions.In this paper,we describe the basic principles of Lorentz-TEM and off-axis electron holography and review recent experimental developments in magnetic skyrmion imaging using these two methods.

Voltage-controllable magnetic skyrmion dynamics for spiking neuron device applications

Voltage-controlled magnetic skyrmions have attracted special attention because they satisfy the requirements for well-controlled high-efficiency and energy saving for future skyrmion-based neuron device applications.In this work,we propose a compact leaky-integrate-fire(LIF)spiking neuron device by using the voltage-driven skyrmion dynamics in a multiferroic nanodisk structure.The skyrmion dynamics is controlled by well tailoring voltage-induced piezostrains,where the skyrmion radius can be effectively modulated by applying the piezostrain pulses.Like the biological neuron,the proposed skyrmionic neuron will accumulate a membrane potential as skyrmion radius is varied by inputting the continuous piezostrain spikes,and the skyrmion radius will return to the initial state in the absence of piezostrain.Therefore,this skyrmion radius-based membrane potential will reach a definite threshold value by the strain stimuli and then reset by removing the stimuli.Such the LIF neuronal functionality and the behaviors of the proposed skyrmionic neuron device are elucidated through the micromagnetic simulation studies.Our results may benefit the utilization of skyrmionic neuron for constructing the future energy-efficient and voltage-tunable spiking neural networks.

Zero-field skyrmions in FeGe thin films stabilized through attaching a perpendicularly magnetized single-domain Ni layer

A numerical study reports that the zero-field skyrmions in Fe Ge thin films are stabilized when a Fe Ge layer is exchange coupled to a single-domain Ni layer,which has been magnetized perpendicularly.Due to the small thickness,an easy-plane anisotropy in the Fe Ge layer is taken into account,and the skyrmion-crystal state is favored to appear for low anisotropies and intermediate Fe Ge/Ni interlayer exchange couplings,and finally transformed from a labyrinth-like and into an out-ofplane uniform state for the large couplings or into an in-plane state for the high anisotropies.Furthermore,the maximum skyrmion charge number is bigger for the periodic and fixed boundary conditions with an out-of-plane magnetization;on the contrary,the Bloch-type skyrmions can be frozen and stabilized for the larger couplings on the fixed boundary with an in-plane magnetization,similar to the experimental results of the magnetic-field-induced skyrmions.Finally,the skyrmion charge number and diameter both decrease if the nonmagnetic defects exist,and the skyrmion centers are prone to being captured by defect sites.This work evidences that the ensembles of homochiral skyrmions stabilized in the multilayers fabricated by well-established technologies present a roadmap to design new classes of the materials that can host skyrmions.

Skyrmion transport driven by pure voltage generated strain gradient

The magnetic skyrmion transport driven by pure voltage-induced strain gradient is proposed and studied via micromagnetic simulation.Through combining the skyrmion with multiferroic heterojunction,a voltage-induced uniaxial strain gradient is adjusted to move skyrmions.In the system,a pair of short-circuited trapezoidal top electrodes can generate the symmetric strain.Due to the symmetry of strain,the magnetic skyrmion can be driven with a linear motion in the middle of the nanostrip without deviation.We calculate the strain distribution generated by the trapezoidal top electrodes pair,and further investigate the influence of the strain intensity as well as the strain gradient on the skyrmion velocity.Our findings provide a stable and low-energy regulation method for skyrmion transport.

磁性Skyrmions信息存储器的微磁学模拟

为了解决当前存储信息技术的发展问题,人们发现了一种新型的高密度信息载体——磁性Skyrmions.实现对磁性Skyrmions生成和输运的微观调控是完成其器件化过程中至关重要的一步.本文基于微磁学模拟方法,通过OOMMF软件首先在纳米盘模型中详细研究了自旋极化电流注入对磁性Skyrmions生成的影响.在局域电流注入过程中,调节自旋极化电流密度、电流注入时间和注入电流面积,得到了不同注入电流面积下阈值电流密度以及注入时间对Skyrmions拓扑数的影响;在全局电流注入中,实现了两种手性Skyrmions的生成,并讨论了模型尺寸对于Skyrmions拓扑数和尺寸的影响,对于低能耗、高密度新型Skymions信息存储器的研发提供了理论指导.

SKYRMIONS IN GROSS-PITAEVSKII FUNCTIONALS

In Bose-Einstein condensates （BECs）, skyrmions can be characterized by pairs of linking vortex rings coming from two-component wave functions. Here we construct skyrmions by studying critical points of Gross-Pitaevskii functionals with two-component wave functions. Using localized energy method, we rigorously prove the existence, and describe the configurations of skyrmions in such BECs.

Control of Hall angle of Skyrmion driven by electric current

Skyrmions are very promising for applications in spintronics and magnetic memory.It is desired to manipulate and operate a single skyrmion.Here we report on the thermal effect on the motion of current-driven magnetic Skyrmions in magnetic metal.The results show that the magnon current induced by the thermal gradient acts on Skyrmions via magnonic spin-transfer torque,an effect of the transverse and longitudinal Skyrmions drift velocities,thus leading to the effective manipulation of the Hall angle through the ratio of thermal gradient to electric current density,which can be used as a Skyrmion valve.

Dynamics of magnetic skyrmions

We review the recent progress on the magnetic skyrmions in chiral magnetic materials. The magnetic skyrmion is a topological spin configuration with localized spatial extent, which could be thought of as an emergent rigid particle, owing to its particular topological and chiral properties. Static skyrmionic configurations have been found in various materials with different transport and thermodynamic properties. The magnetic skyrmions respond to externally applied fields in a very unique way, and their coupling to other quasiparticles in solid-state systems gives rise to the emergent electrodynamics.Being not only theoretically important, the magnetic skyrmion is also very promising to be the information carrier in next generation spintronic devices.

Realization of artificial skyrmion in CoCrPt/NiFe bilayers

Skyrmion, as a quasi-particle structure, has attracted much attention due to its potential applications in future spintronic devices. Artificial skyrmion structure has aroused great interest as it can be stabilized at room temperature, without needing to incorporate materials with Dzyaloshinskii–Moriya interaction(DMI) into it. In this paper, it is found that the artificial skyrmion structure can be realized in CoCrPt/NiFe bilayers by micromagnetic simulations. The critical magnetic field of the core decreases as the diameter of the NiFe soft magnetic layer increases. The artificial skyrmion has excellent topological protection, and the critical magnetic field of plane is about 76 mT(760 Oe, 1 Oe = 79.5775 A·m-1). The external magnetic field plays a key role in determining the core diameter of the skyrmion, and the artificial skyrmion can be realized in CoCrPt/Cu/CoCrPt/NiFe four-layer with a diameter of 13 nm.

Skyrmion crystals in pseudo-spin-1/2 Bose–Einstein condensates

Exact two-dimensional solutions are constructed for the pseudo-spin-1/2 Bose-Einstein condensates, which are de- scribed by the coupled nonlinear Gross-Pitaevskii equations where the intra-and inter-species coupling constants are as- sumed to be equal. The equations are decoupled by means of re-combinations of the nonlinear terms of the hyperfine states according to the spatial dimensions. The stationary solutions form various spin textures which are identified as skyrmion crystals. In a special case, a crystal of skyrmion-anti-skyrmion pairs is formed in the soliton limit.