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.

Magnetic vortex gyration mediated by point-contact position

Micromagnetic simulation is employed to study the gyration motion of magnetic vortices in distinct permalloy nanodisks driven by a spin-polarized current. The critical current density for magnetic vortex gyration, eigenfrequency, trajectory, velocity and the time for a magnetic vortex to obtain the steady gyration are analyzed. Simulation results reveal that the magnetic vortices in larger and thinner nanodisks can achieve a lower-frequency gyration at a lower current density in a shorter time. However, the magnetic vortices in thicker nanodisks need a higher current density and longer time to attain steady gyration but with a higher eigenfrequency. We also find that the point-contact position exerts different influences on these parameters in different nanodisks, which contributes to the control of the magnetic vortex gyration. The conclusions of this paper can serve as a theoretical basis for designing nano-oscillators and microwave frequency modulators.

Nonadiabatic spin-torque and damping effects on vortex core switching triggered by a single current pulse

Switching the orientation of a vortex core by spin-polarised pulse current introduces a promising concept for the reliable addressing of a single nanodisc element inside dense arrays. In this paper, micromagnetic simulations are employed to study the vortex core switching behaviour excited by a short in-plane Gaussian current pulse. We find that both the switching mechanism and the switching time are not sensitive to changes in the phenomenological parameters of spin-torque nonadiabaticity and Gilbert damping. The switching time, however, strongly depends on the current strength. In addition, we have theoretically predicted the parameter range of current pulses to achieve a single switching event.

Multi-segmented nanowires for vortex magnetic domain wall racetrack memory

A vortex domain wall's(VW) magnetic racetrack memory's high performance depends on VW structural stability,high speed, low power consumption and high storage density. In this study, these critical parameters were investigated in magnetic multi-segmented nanowires using micromagnetic simulation. Thus, an offset magnetic nanowire with a junction at the center was proposed for this purpose. This junction was implemented by shifting one portion of the magnetic nanowire horizontally in the x-direction(l) and vertically(d) in the y-direction. The VW structure became stable by manipulating magnetic properties, such as magnetic saturation(M_(4)) and magnetic anisotropy energy(K_(u)). In this case, increasing the values of M_(4) ≥ 800 kA/m keeps the VW structure stable during its dynamics and pinning and depinning in offset nanowires,which contributes to maintenance of the storage memory's lifetime for a longer period. It was also found that the VW moved with a speed of 500 m/s, which is desirable for VW racetrack memory devices. Moreover, it was revealed that the VW velocity could be controlled by adjusting the offset area dimensions(l and d), which helps to drive the VW by using low current densities and reducing the thermal-magnetic spin fluctuations. Further, the depinning current density of the VW(J_(d)) over the offset area increases as d increases and l decreases. In addition, magnetic properties, such as the M_(4) and K_(u),can affect the depinning process of the VW through the offset area. For high storage density, magnetic nanowires(multisegmented) with four junctions were designed. In total, six states were found with high VW stability, which means three bits per cell. Herein, we observed that the depinning current density(J_(d)) for moving the VW from one state to another was highly influenced by the offset area geometry(l and d) and the material's magnetic properties, such as the M_(4) and K_(u).

Micromagnetic Investigation of Microwave Permeability of Magnetic Artificial Spin Ice

The complex permeability of the artificial spin ice array at different magnetic states is calculated using the micromagnetic simulation method. It is observed that the permeability spectra are dependent on the magnetization distributions of the array. The dependence of the permeability spectrum on the applied magnetic field strength and on the spacing gap between the neighboring elements is also investigated. Depending on the initial magnetization saturating direction, the permeability spectrum exhibits different sensitivity to the strength of the external applied magnetic field and to the spacing distance between the comprising elements of the artificial spin ice array.

Faster vortex core switching with lower current density using three-nanocontact spin-polarized currents in a confined structure

We perform micromagnetic simulations on the switching of magnetic vortex core by using spin-polarized currents through a three-nanocontact geometry. Our simulation results show that the current combination with an appropriate current flow direction destroys the symmetry of the total effective energy of the system so that the vortex core can be easier to excite,resulting in less critical current density and a faster switching process. Besides its fundamental significance, our findings provide an additional route to incorporating magnetic vortex phenomena into data storage devices.

Spin torque nano-oscillators with a perpendicular spin polarizer

We present an overview in the understanding of spin-transfer torque(STT) induced magnetization dynamics in spintorque nano-oscillator(STNO) devices. The STNO contains an in-plane(IP) magnetized free layer and an out-of-plane(OP) magnetized spin polarizing layer. After a brief introduction, we first use mesoscopic micromagnetic simulations,which are based on the Landau–Lifshitz–Gilbert equation including the STT effect, to specify how a spin-torque term may tune the magnetization precession orbits of the free layer, showing that the oscillator frequency is proportional to the current density and the z-component of the free layer magnetization. Next, we propose a pendulum-like model within the macrospin approximation to describe the dynamic properties in such type of STNOs. After that, we further show the procession dynamics of the STNOs excited by IP and OP dual spin-polarizers. Both the numerical simulations and analytical theory indicate that the precession frequency is linearly proportional to the spin-torque of the OP polarizer only and is irrelevant to the spin-torque of the IP polarizer. Finally, a promising approach of coordinate transformation from the laboratory frame to the rotation frame is introduced, by which the nonstationary OP magnetization precession process is therefore transformed into the stationary process in the rotation frame. Through this method, a promising digital frequency shift-key modulation technique is presented, in which the magnetization precession can be well controlled at a given orbit as well as its precession frequency can be tuned with the co-action of spin polarized current and magnetic field(or electric field) pulses.

Shape-manipulated spin-wave eigenmodes of magnetic nanoelements

The magnetization dynamics of nanoelements with tapered ends have been studied by micromagnetic simulations.Several spin-wave modes and their evolutions with the sharpness of the element ends are characterized. The edge mode localized in the two ends of the element can be effectively tuned by the element shape. Its frequency increases rapidly with the tapered parameter h and its localized area gradually expands toward the element center, and it finally merges into the fundamental mode at a critical tapered parameter h0. For nanoelements with h 〉 h0, the edge mode is completely suppressed. The standing spin-wave modes mainly in the internal area of the element are less affected by the element shape.The shifts of their frequencies are small and they display different tendencies. The evolution of the spin-wave modes with the element shape is explained by considering the change of the internal field.

基于能量守恒的空间旋转目标消旋力矩计算方法

针对空间旋转目标电磁消旋技术,提出一种计算空间旋转目标消旋力矩的新方法。首先,介绍了电磁消旋的常用计算方法;然后,介绍了能量守恒法的基本思想,从系统能量守恒的角度出发,从整体考虑电磁消旋过程中各物理场之间的相互作用,提出了一种基于能量守恒的消旋力矩计算方法,并通过球壳模型消旋力矩的计算过程对能量守恒法进行阐述。最后,进行了地面消旋实验,对比了实验结果和有限元法、电磁张量法和能量守恒法的计算结果。实验及仿真结果表明,本文提出的基于能量守恒的空间旋转目标消旋力矩计算方法有效。

Micromagnetic modeling of magnetization dynamics driven by spin-transfer torque in magnetic nanostructures

Magnetization orientation of a nanoscale ferromagnet can be manipulated by an electric current via spin-transfer torque(STT) effect,which holds great promise in the applications of non-volatile magnetic random access memory(MRAM) and spintorque oscillators.We review the fundamental mechanism and experimental progress of the STT effect.Then,different formula of STT torque has been classified,which can be added to the conventional Landau-Lifshitz-Gilbert equation.After that,we show some simulation results that mainly concern the STT-driven vortex dynamics,magnetization oscillations excited by a perpendicular polarizer,and the detail dynamics by in-plane and out-of-plane dual spin polarizers.

Dynamic nucleation of domain-chains in magnetic nanotracks

Most of the reported observations are about the dynamic properties of individual domain-walls in magnetic nanowires,but the properties of multiple stripe-domains have rarely been investigated.Here,we demonstrate a simple but efficient scenario for multiple domains injection in magnetic nanowires.The domain-chains(DCs),a cluster of multiple domains,can be dynamically generated with tunable static properties.It is found that the number of domains in a single DC can be dynamically adjusted by varying the frequency of microwave field(MF)and the period of spin-polarized current(SPC)intensity.The static properties of the DCs,i.e.,its length,spacing,and period between neighboring DCs,can be dynamically controlled by regulating the frequency of MF and the intensity of SPC.We have also discussed the possibility of using domain-chains as information carries,which provides a meaningful approach for flexible multi-bit information storage applications.

自旋波辅助对电流驱动磁畴壁移动的影响及其物理机理

通过微磁学模拟方法研究了长直线磁性纳米带中,自旋波辅助电流驱动磁畴壁移动的相关规律及其物理机制。结果显示,在小电流情形下,自旋波辅助能有效增高电流驱动磁畴壁移动速度,其移动速度随自旋波振幅的增大近似线性增大,随自旋波频率的增高呈现多峰非单调增大;但在大电流情形下,自旋波辅助对电流驱动磁畴壁移动速度的影响较小。进一步研究发现,施加的自旋波不仅驱动磁畴壁移动还能有效增强磁畴壁的垂直磁化强度,其垂直磁化强度的增幅随施加电流的增大而减小;另一方面,电流驱动磁畴壁移动的速度正比于磁畴壁的垂直磁化强度,因此自旋波辅助在小电流情形下能显著增大磁畴壁移动的速度,而大电流情形下,其增幅较小。

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.

末端形状对磁纳米薄膜自旋波模式特性的影响

基于微磁学模拟方法研究了磁纳米膜末端形状对自旋波模式特性的影响。获得了多种不同的局域化、量子化自旋波模式特性,以及末端形状对自旋波模式特性的调制规律。通过裁剪纳米膜的末端形状可有效调控边缘模式自旋波特性,存在一临界裁剪参数h0。当裁剪度h<h0时,随着h的增大,边缘模式频率快速增加,局域于磁薄膜两末端的磁振荡区域向中央扩展。当h≥h0时,局域于磁薄膜两末端磁振荡在磁体中央合并为一致振荡模式自旋波,边缘模式被抑制。薄膜末端边缘形状对别的自旋波模式特性影响较小。最后,基于近似色散关系理论模型对研究结果进行了解释。

纳米复合永磁材料的研究进展

纳米复合永磁材料兼具硬磁性相的高矫顽力和软磁性相的高饱和磁化强度,磁性能优异。本文综述了纳米复合永磁材料的磁性相晶粒间的交换耦合作用、矫顽力机制、微磁学模拟等理论方面的研究进展,同时论述了成分、添加元素、制备工艺对材料微观结构和磁性能的影响,着重介绍了最有应用价值的快淬法磁粉制备工艺。

自旋极化电流辅助的超快反磁化特性研究

基于微磁学模拟方法研究了椭圆纳米薄膜的超快反磁化特性。提出一种利用弱直流自旋极化电流辅助的场驱动进动反磁化新机制。新机制可以有效扩宽脉冲持续时间窗口,在宽的脉冲持续时间范围内可实现进动磁反转而无需精确控制;也可有效抑制磁反转后长时间磁振荡,缩短有效反磁化时间。时间窗口的扩宽和进动磁反转后磁振荡的衰减依赖于自旋极化电流密度。磁反转后的磁振荡由多种不同的自旋波模式构成,自旋转移力矩可以调制磁体有效阻尼,加速磁能量的耗散。

切边纳米铁磁盘对中磁涡旋旋性的磁场调控

铁磁纳米盘中的磁涡旋态因稳定性高,并且其面内磁化的旋转方向具有天然的二向性(顺时针(CW)和逆时针(CCW)),可以作为信息存储的一个比特单元而成为最近研究的热点.基于磁涡旋旋性的信息存储要求人们能够独立地控制磁涡旋的旋转方向.从旋性的角度考虑,在一对纳米盘中可能出现四种磁涡旋基态,即(CCW,CCW),(CCW,CW),(CW,CCW)和(CW,CW).本文通过引入厚度不同且切边的纳米磁盘对,并对其施加面内磁场来实现对四种涡旋基态的独立控制,并通过微磁学模拟证明了这种方法的可行性.

具有倾斜极化层的自旋阀结构中磁翻转以及磁振荡模式的微磁模拟

基于自由层与钉扎层均为垂直磁各向异性的自旋阀结构,采用微磁学模拟与傅里叶分析相结合的技术,研究了极化层磁矩小角度倾斜情形下自由层磁矩的进动翻转特性.通过沿样品垂直膜面方向同时施加电流与磁场,观察到自由层磁矩垂直膜面方向分量的平均值随磁场的演化翻转曲线中出现了多个凹槽.模拟研究结果表明:在一定的电流范围内,凹槽出现的位置与电流大小无关;而在固定的应用电流下,凹槽出现的位置将会受到样品厚度的影响;在凹槽区域内,非一致进动模式、自旋驻波模式、局域自旋波模式等多种磁振荡模式被激发.通过傅里叶分析,得到了各种磁振荡模式的频谱,频谱中的频率分布体现出了倍频以及间谐波的频率特性.

椭圆纳米盘中磁涡旋结构的方位角自旋波模式

本文针对坡莫合金椭圆形盘中的磁涡旋结构,采用微磁学模拟与傅里叶分析相结合的技术研究了磁涡旋自旋波的本征激发模式.通过沿样品短轴方向施加一面内方向的脉冲磁场,观察到一系列方位角自旋波模式.观察到的自旋波模式具有两重对称性,可以通过C2群理论来进行类型的划分.此外,自旋波模式的频率随着方位角指标的变化而线性增加.模拟结果显示样品的平均交换能量密度明显的高于平均静磁能量密度;局域交换能量密度主要集中在涡核初始位置,而局域静磁能量密度主要分布在长轴附近.交换作用对受限于铁磁薄膜椭圆盘中的单个涡旋态的能量要起主导作用,从而导致方位角自旋波模式频率随着方位角指标的增加而增加.

矩形磁纳米点的磁反转及动力学特性研究

基于微磁学模拟方法研究了矩形纳米点的磁反转和动力学特性.获得了矩形纳米点的磁反转过程、自旋波本征动力学特性,以及两者间的内在联系.矩形纳米点的磁反转过程从边缘畴壁的失稳开始,伴随着边缘畴壁移出纳米点外微磁结构从"Flower"态转变为"C"态,随着"C"态边缘畴向磁体中央扩展形成大角畴壁,最后伴随着畴壁移出纳米点外实现反转.磁反转过程中微磁结构相变总是伴随着某种自旋波模式的软化;对称的边缘模式(S-EM)模式软化诱发微磁结构从"Flower"态向"C"态演化,在反转场附近,局域于Y形畴壁的Y-D畴壁模式自旋波的软化激发矩形纳米点不可逆磁反转.