电流调控磁涡旋的极性和旋性

提出了一种特殊自旋阀结构, 其极化层(钉扎层)磁矩沿面内方向, 自由层磁矩成磁涡旋结构. 自由层在形状上设计成左右两边厚度不同的阶梯形圆盘. 微磁学模拟研究发现, 通过调控所施加的高斯型脉冲电流的大小、方向和脉冲宽度, 可以实现磁涡旋的不同旋性、不同极性的组态控制. 分析了该结构中电流调控磁涡旋旋性和极性的物理原因和微观机理.     

Current-driven resonant excitation of magnetic vortices

A magnetic vortex core in a ferromagnetic circular nanodot has a resonance frequency originating from the confinement of the vortex core. By the micromagnetic simulation including the spin-transfer torque, we show that the vortex core can be resonantly excited by an ac (spin-polarized) current through the dot and that the resonance frequency can be tuned by the dot shape. The resistance measurement under the ac current successfully detects the resonance at the frequency consistent with the simulation.     

Reliable control of magnetic vortex chirality in asymmetrically optimized magnetic nanodisk

Magnetic vortex has attracted attention in the field of information storage because their topological spin structures with chiral bistable states. If the vortex core polarity and vortex circulation sense can be controlled simultaneously in a nanodisk, which will be more beneficial to realize the multi-bit ultrahigh density storage. In this paper, a reliable control scheme for magnetic vortex chirality is proposed by optimizing the structure of Pac-Man-like nanodisk. The results show that the polarity and circulation of the vortex can be controlled simultaneously by changing the direction of the global magnetic field, and even the chiral states of the vortex can be determined by detecting the stray field distribution on the surface of the nanodisk. The optimized Pac-Man-like nanodisk provide an experimental method for the control and detection of magnetic vortex chirality, which will be beneficial to the realization of multi-bit magnetic storage or magnetic logic technology in the future.     

Electric polarization of magnetic textures: New horizons of micromagnetism

A common scenario of magnetoelectric coupling in multiferroics is the electric polarization induced by spatially modulated spin structures. It is shown in this paper that the same mechanism works in magnetic dielectrics with inhomogeneous magnetization distribution: the domain walls and magnetic vortexes can be the sources of electric polarization. The electric field driven magnetic domain wall motion is observed in iron garnet films. The electric field induced nucleation of vortex state of magnetic nanodots is theoretically predicted and numerically simulated. From the practical point of view the electric field control of micromagnetic structures suggests a low-power approach for spintronics and magnonics.     

矩形磁性纳米点动力学反磁化过程的微磁学研究

采用微磁学模拟方法研究了初始态为C形磁结构的矩形CoFe纳米点在方波脉冲场作用下的动力学反磁化过程.研究发现,随着脉冲场强的增强,磁体的反磁化模式发生了改变.当场强较弱时反磁化过程通过畴壁移动-单涡旋的形成和移动来完成;当场强较大时反磁化过程模式转变为畴壁移动-双涡旋的形成与移动;在更强的场强下反磁化过程通过畴壁的移动-多涡旋的形成与湮没来实现.由于反磁化模式随场强的变化而改变,反磁化时间随场强的增大出现振荡变化现象. 关键词： 动力学反磁化过程 反磁化时间 微磁学模拟     

Influence of Dzyaloshinskii-Moriya interaction on the magnetic vortex reversal in an off-centered nanocontact geometry

The influence of Dzyaloshinskii-Moriya interaction (DMI) on the vortex reversal driven by an out-of-plane spin-polarized current in an off-centered nanocontact structure is investigated. The simulation results show that DMI plays a vital role in vortex core reversal, including reversal current density, reversal velocity and reversal time. Under the influence of DMI, magnetic vortices still reverse polarity through the nucleation and annihilation of vortex and anti-vortex, with some peculiar characteristics. These results open up new possibilities for the application of magnetic vortex-based spin-transfer encryption nano-storage.     

Rectified vortex motion in an Nb film with a spacing-graded array of holes

Superconducting Nb thin films with a spacing-graded array of holes were prepared by electron beam lithography. Two films with different hole gradients were fabricated. The ac-driven vortices were investigated in Nb superconductors with a spacing-graded array of holes. The measurements revealed pronounced rectified voltage when the vortex lattice is driven by an ac injected current. The rectified voltage is mainly caused by the strength of the vortex–vortex interaction. The rectified motion of a vortex is affected by the pinning potential of the spacing-graded array and the applied magnetic field. The vortex–vortex interaction strength changes the effective pinning landscape of the vortices and an asymmetric potential is formed. Vortices depin easily from high concentration to low concentration of pinning sites. In both samples, the ac-driven vortices exhibit a variety of dynamical responses and the rectified voltage is tunable with the applied magnetic field.     

Manipulating vortices in F=2 Bose-Einstein condensates through magnetic field and spin-orbit coupling

We investigate the vortex structures excited by Ioffe-Pritchard magnetic field and Dresselhaus-type spin-orbit coupling in F=2 ferromagnetic Bose-Einstein condensates. In the weakly interatomic interacting regime, an external magnetic field can generate a polar-core vortex in which the canonical particle current is zero. With the combined effect of spin-orbit coupling and magnetic field, the ground state experiences a transition from polar-core vortex to Mermin-Ho vortex, in which the canonical particle current is anticlockwise. For fixed spin-orbit coupling strengths, the evolution of phase winding, magnetization, and degree of phase separation with magnetic field are studied. Additionally, with further increasing spin-orbit coupling strength, the condensate exhibits symmetrical density domains separated by radial vortex arrays. Our work paves the way to explore exotic topological excitations in high-spin systems.     

Systematic study of magnetization reversal in square Fe nanodots of varying dimensions in different orientations

Ferromagnetic nanoparticles can be used for data storage, spintronics, and other applications. Especially vortex states are often suggested to be used to store information. Due to the shape anisotropy dominating in nanoparticles, magnetization reversal processes can be expected to depend not only on the dimensions, but also on the orientation with respect to the external magnetic field. While several papers evaluate magnetization dynamics, including vortex precessions, in round nanodots, square nanodots are less often investigated. Here we report on different magnetization reversal processes found in micromagnetic simulations of square Fe nanodots with lateral dimensions between 100 nm and 500 nm and thicknesses between 10 nm and 50 nm. Choosing magnetic field orientations parallel to one of the square edges and under 45^°, seven different reversal mechanisms were found, most of them including a single-vortex state, while in some cases two, three or more vortex-antivortex pairs were found. The ground state, i.e. the magnetic state at vanishing external magnetic field, was often a single-vortex state, making the nanodot with the respective dimensions suitable for data storage applications. The stability of this state, i.e. the field range over which it existed, depended strongly on the lateral dimensions and the dot thickness and was largest for small lateral dimensions and large thicknesses.     

利用扫描透射X射线显微镜观测磁涡旋结构

利用上海光源软X射线谱学显微光束线站(STXM)并结合X射线的磁圆二色效应, 我们对方形、圆形和三角形的Ni₈₀Fe₂₀薄膜微结构中的磁涡旋结构进行了定量实验观测, 并利用同步辐射光源的元素分辨特性, 分别在Fe和Ni的L₃吸收边对涡旋磁结构进行了观测. 我们还对磁涡旋中磁矩的分布进行了定量分析, 发现实验结果与微磁学模拟结果完全符合.     

Edge-soliton-mediated vortex-core reversal dynamics

We report an additional reversal mechanism of magnetic vortex cores in nanodot elements driven by currents flowing perpendicular to the sample plane, occurring via dynamic transformations between two coupled edge solitons and bulk vortex solitons. This mechanism differs completely from the well-known switching process mediated by the creation and annihilation of vortex-antivortex pairs in terms of the associated topological solitons, energies, and spin-wave emissions. Strongly localized out-of-plane gyrotropic fields induced by the fast motion of the coupled edge solitons enable a magnetization dip that plays a crucial role in the formation of the reversed core magnetization. This work provides a deeper physical insight into the dynamic transformations of magnetic topological solitons in nanoelements.     

带辅助磁场等离子体断路开关的数值模拟

 利用全电磁网格粒子方法模拟了外加磁场对等离子体断路开关（POS）断路性能的影响，给出了电压倍增系数与外加磁场的关系曲线。数值模拟表明，外加轴向磁场线圈必须放在同轴型POS阴极的同侧才能显著改善开关的断路性能；当外加角向磁场时，内电极为阴、阳极的同轴型POS的断路性能都得到了不同程度的改善。随着外加磁场的增大，电压倍增系数将达到饱和。     

Field-controllable injection of virtual magnetic domain wall in discrete magnetic nanodot chains

Periodic injection behaviors of virtual magnetic domain wall (VDW) have been systematically investigated in asymmetrically shaped nanodot chains by means of micromagnetic simulations. Systematic investigation on a controllable VDW injection has been carried out. We demonstrate that precise control of VDW injection is achievable by using different nanodot shapes as well as by changing alternating magnetic field (AC field) profiles. The VDW position can be tuned by adjusting AC field frequency and amplitude. Field-controllable periodic VDW injection phenomenon is found to be sustainable over wide ranges of phase diagram spanned by AC field frequency and amplitude.     

Nanodomain wall film structure and its magnetic characterization

In this letter, we report on a nanodomain wall thin-film structure and its fabrication. The core unit of this structure consists of a magnetic nanodot layer sandwiched between a magnetically free layer and a pinned layer. When the magnetizations of the free layer and the pinned layer are unparallel, a nanodomain wall is formed in the magnetic nanodot. Based on this concept, a nanodomain wall film structure with a Ni/Al₂O₃ nanodot layer is prepared. Since the free and pinned layers are coupled through magnetic nanodots, a displacement of free layer M–H loop from zero field is observed. By measuring the displacement field of the free layer, the nanodomain wall energy is estimated.     

Micromagnetic simulation of magnetic vortex cores in circular permalloy disks: Switching behavior in external magnetic field

Switching behaviors of magnetic vortex cores under external magnetic field in submicron circular permalloy disks have been systematically studied by using micromagnetic simulations. Simulation results show that the vortex core is stable in out-of-plane field even when it is located at the edge of the disk. The out-of-plane switching field H_sw is strongly dependent on the thickness of the disk. The core polarity and the vortex chirality can be modulated simultaneously on purpose by using a tilted field far smaller than the out-of-plane switching field H_sw. Moreover, it is found that the core polarities in asymmetric disks do not follow the direction of the z projection of the external saturation field.     

Role of a uniform magnetic field on the energy spectrum of a single donor in a core/shell spherical quantum dot

Using the variational approach within the framework of the effective-mass approximation (EMA), the binding energy of a centred hydrogenic donor impurity in a CdSe/ZnTe core/shell spherical quantum dot (CSSQD) in the presence of an external magnetic field was investigated. In this model, we have taken into account the effect of the radial dependence of the dielectric constant and of the electron effective mass. Our numerical results show a remarkable influence of the nanodot spatial parameters and of the external magnetic field strength on the shallow donor binding energy.     

Vortex chains induced by anisotropic spin-orbit coupling and magnetic field in spin-2 Bose-Einstein condensates

We investigate the anisotropic spin-orbit coupled spin-2 Bose-Einstein condensates with Ioffe-Pritchard magnetic field. With nonzero magnetic field, anisotropic spin-orbit coupling will introduce several vortices and further generate a vortex chain. Inside the vortex chain, the vortices connect to each other, forming a line along the axis. The physical nature of the vortex chain can be explained by the particle current and the momentum distribution. The vortex number inside the vortex chain can be influenced via varying the magnetic field. Through adjusting the anisotropy of the spin-orbit coupling, the direction of the vortex chain is changed, and the vortex lattice can be triggered. Moreover, accompanied by the variation of the atomic interactions, the density and the momentum distribution of the vortex chain are affected. The realization and the detection of the vortex chain are compatible with current experimental techniques.     

磁涡旋极性翻转的局域能量

针对坡莫合金纳米圆盘中的单个磁涡旋结构,采用微磁学模拟研究了磁涡旋极性翻转过程中的局域能量密度.磁涡旋的极性翻转通过与初始涡旋极性相反的涡旋与反涡旋对的生成,以及随后发生的反涡旋与初始涡旋的湮没来实现.模拟结果显示当纳米圆盘样品中局域能量密度的最大值达到一临界值时,磁涡旋将会实现极性翻转,其中交换能起主导作用.基于涡旋极性翻转过程中出现的三涡旋态结构,应用刚性磁涡旋模型对局域交换能量密度进行了理论分析.通过刚性磁涡旋模型得到的磁涡旋极性翻转所需的局域交换能量密度的临界值与模拟结果符合得较好.     

Magnetic vortices and thermoelectric effect in a hollow superconducting cylinder

The question of a surface barrier which determines the behavior of a vortex in a hollow superconducting cylinder of finite thickness in an external magnetic field is discussed, taking into account magnetic flux quantization in the cavity. The behavior of magnetic vortices in a hollow superconductor in the presence of a thermoelectric current is also considered. Pairs of magnetic vortices with opposite magnetic field orientations (vortex-antivortex pairs) are generated by this current near T _c. The thermoelectric current drives the antivortex (the vortex with oppositely directed field) out of the cylinder, whereas the vortex is ejected into the cavity and remains on the inside cylinder surface as a current. The number of magnetic flux quanta trapped inside the cylinder increases by one. The relation of this mechanism to the “giant” thermoelectric effect in hollow superconductors is discussed. Zh. éksp. Teor. Fiz. 111, 2175–2193 (June 1997)     

Features of the effect of the Josephson medium parameters on vortex formation and critical current

The response of an intergranular Josephson junction to displacements of an Abrikosov vortex in a superconducting polycrystal is studied theoretically. The vortex filament in the vicinity of the junction excites a tunnel current in the junction and also generates a Josephson vortex with which it merges upon emergence at the surface of the junction. It is shown that the process of the Josephson vortex formation passes through a stage of overcoming a potential barrier, whose height depends on the distance between the Abrikosov vortex and the junction, as well as on the effective thickness of the junction, which is determined by the characteristic grain size, grain anisotropy, and the intensity of the intergranular coupling. The magnetic field dependence of the critical current of the intergranular Josephson junction is determined for various grain and intergranular parameters, as well as for the triangular and square configurations of the Abrikosov vortex lattice. The results indicate that a high degree of texturing in the grain size, anisotropy, and intensity of intergranular coupling is very important for obtaining high critical currents in pure polycrystalline materials.