磁性斯格明子的研究现状和展望

磁性斯格明子是具有拓扑保护性质的纳米尺度涡旋磁结构.斯格明子主要存在于非中心对称的手性磁性材料以及界面镜面对称性破缺的磁性薄膜材料中.因具有实空间的非平庸拓扑性,磁性斯格明子展现出丰富新奇的物理学特性,例如拓扑霍尔效应,新兴电磁动力学等,为研究拓扑自旋电子学提供了新的平台.另一方面,由于其具有尺寸小,高稳定性和易操控的特性,磁性斯格明子在未来高密度,低能耗,非易失性计算和存储器件中也具有潜在应用.现阶段的研究已经初步发现一系列磁斯格明子材料,并证明能够通过电流操控室温下稳定的磁性斯格明子,但是室温下单个斯格明子的精确产生、湮灭以及探测在实验上仍具有挑战性.本文阐述了磁性斯格明子的基础理论以及动力学研究现状,并对现有的斯格明子材料和斯格明子的产生,湮灭以及探测方法进行了总结,最后还对未来磁性斯格明子的物理理论研究以及应用发展中的挑战和机遇进行了讨论.     

磁斯格明子拓扑特性及其动力学微磁学模拟研究进展

具有非平庸拓扑性的新型磁结构斯格明子,由于其拓扑稳定性、尺寸小、低电流驱动等方面的显著优势,有望应用于自旋电子学储存器件.拓扑和凝聚态物理学的结合,使得斯格明子展现出很多有趣的拓扑物理现象,吸引了众多的研究兴趣,同时这些性质也是其电流驱动下动力学特点的重要影响因素.本文从斯格明子的拓扑物理学基础及其自旋电子学器件应用相关动力学两个方面介绍了相关研究进展.在拓扑物理基础方面,介绍了斯格明子的拓扑霍尔效应、斯格明子霍尔效应以及自旋轨道转矩等拓扑性质,由此讨论了斯格明子的动力学性质及其计算方法;在动力学方面,从非均匀电流驱动生成斯格明子、电流驱动下的稳定输运、产生湮灭过程的人工控制几个赛道存储应用关心的问题简要地介绍了相关微磁学模拟研究最新进展.     

磁性斯格明子的多场调控研究

斯格明子(skyrmion)的概念最早是由英国的粒子物理学家Tony Skyrme提出,它被用来描述粒子的一个状态,是一种拓扑孤立子.磁性斯格明子是一种具有拓扑行为的新型磁结构,其空间尺寸为纳米量级,空间距离从纳米到微米量级可调;其存在温度涵盖从低温、室温到高温的宽温区;其材料体系不仅包括早期发现的低温区B20型中心对称破缺的铁磁体和螺旋磁有序的弱铁磁材料,也包括近期发现的室温及以上的中心对称六角结构磁性MnNiGa金属合金和磁性薄膜/多层膜体系.利用磁性斯格明子的拓扑磁结构可以实现类似于自旋阀或者磁性隧道结中的自旋转移矩效应,即外加电流可以驱动斯格明子,其临界电流密度比传统翻转磁性多层膜体系中磁矩的电流密度(一般为10~7A/cm~2)要低5个数量级,约为10~2A/cm~2,该临界值远低于硅基半导体技术中沟道电流密度的上限,在未来的磁信息技术中具有广泛的应用前景.本综述简单介绍了磁性斯格明子的发展历程,归纳总结了磁性斯格明子的材料体系,介绍了观察磁性斯格明子的实验手段,重点介绍了多场(磁场、电流、温度场)调控作用下中心对称MnNiGa合金和Pt/Co/Ta磁性多层膜体系中磁性斯格明子的产生、消失以及外场调控演变等动态行为.     

纳米结构中磁斯格明子的原位电子全息研究

斯格明子(skyrmion)磁序结构与晶体微观结构的关联是新型功能磁材料和器件研发的重要问题.本文利用微纳加工技术制备了形状、尺寸均可控的磁纳米结构,通过电子全息术观察定量地分析了斯格明子磁序结构,确定了材料晶格缺陷和空间受限效应对斯格明子磁结构形成和稳定机制的影响,系统地分析了斯格明子基元的磁功能与材料微结构的关联.文中主要探讨了两个问题:1)斯格明子在磁纳米结构中的空间受限效应.重点研究斯格明子磁序随外磁场和温度变化的演变规律,探索其演变过程的拓扑属性和稳定性;2)晶格缺陷对斯格明子磁结构的影响,重点考察晶界原子结构手性反转对斯格明子磁序的影响.这些研究结果可为研发以磁斯格明子为基元的磁信息存储器及自旋电子学器件提供重要实验基础.     

中心对称的阻挫磁体中斯格明子直径的调节

在带有垂直各向异性的二维三角晶格磁体中,当同时存在最近邻铁磁性和第三近邻反铁磁性交换作用时,垂直于膜面施加外磁场会使体系内自旋沿着非共面的方向排列,甚至出现拓扑稳定的斯格明子自旋结构.基于蒙特卡罗模拟方法,本文研究了在该二维阻挫磁体中,竞争性交换作用和外磁场对斯格明子直径的影响.与常规非中心对称的手性磁体中的斯格明子性质类似,外磁场会磁化斯格明子外围自旋而减小斯格明子直径.但是,磁体中反铁磁性交换作用的增强会整体压缩斯格明子.本文结合自旋波理论和蒙特卡罗模拟,首次量化了此类阻挫磁体中斯格明子的直径.结果表明:在弱的反铁磁性交换作用磁体中,斯格明子直径随磁场增大而快速线性减小;随着反铁磁性交换作用的增大,斯格明子直径随外磁场增大的减小变得相对平缓,但在强磁场下也会造成斯格明子直径的加速减小;随着反铁磁性交换作用的增强,斯格明子在不同外磁场下的直径的最大值和中值均从逐渐减小到渐趋稳定,而直径的最小值则从快速减小到表现出很大的涨落.这些现象都可以通过分析斯格明子在不同交换作用和外磁场下的构型和磁能变化加以解释.该项工作阐明了在中心对称的阻挫磁体中斯格明子直径的可调节性,不仅完善了我们对斯格明子本身物理机理的认识,同时也为发展基于斯格明子的新一代存储和逻辑器件提供了理论支撑.     

磁畴壁手性和磁斯格明子的拓扑性表征及其调控

磁性斯格明子由于拓扑的保护性,具有很高的稳定性和较小的临界驱动电流,有望应用于未来的赛道存储器件中.而在中心对称体系,由于偶极作用的各向同性,磁泡的拓扑性和螺旋度都呈现出多样性的特征.其中非平庸的磁泡即等同于磁性斯格明子.我们通过近期实验结果,结合微磁学模拟的方法,发现在中心对称体系中磁斯格明子的拓扑性会受到体系垂直各向异性的调控.另外在加磁场的演变过程中,会很大程度上依赖于基态畴的畴壁特性.磁场的倾斜或者一定的面内各向异性也会改变磁斯格明子的形态.通过对材料的基态磁结构及磁各向异性的调节,辅助以面内分量的控制,可以对基态磁畴、进而对磁斯格明子的拓扑性实现调控.这对磁斯格明子在电流驱动存储器件中的应用具有重要意义.     

磁性斯格明子晶格的磁弹现象与机理

近年来,人们在一些具有手性相互作用的磁性体材料及薄膜中成功观测到具有非平凡拓扑性质的二维自旋结构,称作磁性斯格明子.在大部分情况下,磁性斯格明子自发地聚集成一种晶格结构,称作斯格明子晶格.孤立的斯格明子由于其奇特的拓扑性质以及优异的电流驱动性质等"局域化特征"受到人们的广泛关注.与此相对,斯格明子晶格作为一种新颖的宏观磁性相,可能与材料固有的多场耦合性质发生相互作用进而引发许多奇特的宏观物理现象乃至新性质.在此范畴内,人们发现由于手征磁体内禀的磁弹耦合,斯格明子晶格不但对材料的力学性质产生影响,而且在外力作用下自身具备"层展的弹性性质".本文对相关现象进行梳理,并基于一种针对B20族手征磁体磁弹耦合效应普遍适用的热力学唯象模型,逐一简述对于不同类型的磁弹现象如何建模分析,进而给出其中一部分现象的实验与理论结果比对.最后,对这一领域的发展提出几个可供进一步探索的方向.     

Topological charge analysis of dynamic process of transition to Néel-type skyrmion: Role of domain wall skyrmions

Magnetic skyrmions are intriguing topological spin textures that promise future high-density spintronic devices. The creation of magnetic skyrmions has been understood based on the energetics of skyrmions, but the detailed dynamic process of the skyrmion creation remains unclear. Here topological evolution in conversion from uneven domains to Néel-skyrmions is investigated using micromagnetic simulations. We find that, rather than the overall topological charge, annihilation of novel topological defects, i.e., recently suggested domain wall skyrmions, dominantly govern the skyrmion creation process. Also, the topological charge evolution is interpreted in terms of the number and the combination of such topological defects.     

Rotating skyrmions of the (2 + 1)-dimensional Skyrme gauge model with a Chern-Simons term

The (2 + 1)-dimensional Skyrme gauge model with a Chern-Simons term is considered. The presence of the Chern-Simons term makes possible the existence of two-dimensional skyrmions in this model, which carry magnetic flux and have an electric charge and a nonzero angular momentum. It is shown that the model also admits the existence of two-dimensional skyrmions with a nonzero phase frequency of rotation. Due to the nontrivial topological properties of the model, the magnetic flux, the electric charge, and the angular momentum of a two-dimensional rotating skyrmion turn out to be interrelated. Analytic and numerical investigations of the properties of rotating two-dimensional skyrmions are carried out.     

Non-volatile multi-state magnetic domain transformation in a Hall balance

High performance of the generation, stabilization and manipulation of magnetic skyrmions prompts the application of topological multilayers in spintronic devices. Skyrmions in synthetic antiferromagnets (SAF) have been considered as a promising alternative to overcome the limitations of ferromagnetic skyrmions, such as the skyrmion Hall effect and stray magnetic field. Here, by using the Lorentz transmission electron microscopy, the interconversion between the single domain, labyrinth domain and skyrmion state can be observed by the combined manipulation of electric current and magnetic field in a Hall balance (a SAF with the core structure of [Co/Pt]₄/NiO/[Co/Pt]₄ showing perpendicular magnetic anisotropy). Furthermore, high-density room temperature skyrmions can be stabilized at zero field while the external stimulus is removed and the skyrmion density is tunable. The generation and manipulation method of skyrmions in Hall balance in this study opens up a promising way to engineer SAF-skyrmion-based memory devices.     

The relation between the radii and the densities of magnetic skyrmions

Compared with the traditional magnetic bubble, a skyrmion has a smaller size, and better stability and therefore is considered as a very promising candidate for future memory devices. When skyrmions are manipulated, erased and created, the density of skyrmions can be varied, however the relationship between the radii and the densities of skyrmions needs more exploration. In this paper, we study this problem both theoretically and by using the lattice simulation. The average radius of skyrmions as a function of material parameters, the strength of the external magnetic field and the density of skyrmions is obtained and verified. With this explicit function, the skyrmion radius can be easily predicted, which is helpful for the future study of skyrmion memory devices.     

Progress and challenges in magnetic skyrmionics

Magnetic skyrmions are two-dimensional localized topological spin-structures characterized by the skyrmion number that measures the number of times of spins wrapping the Bloch sphere. Skyrmions behave like particles under an external stimulus and are promising information carriers. Skyrmions can exist as an isolated object as well as skyrmion condensates in crystal structures, helical/conical states, mazes or irregular stripy states with emergent electromagnetic fields. Thus, skyrmions provide a nice platform for studying fundamental physics, other than its applications in spintronics. In this perspective, we briefly review some recent progress in the field and present an outlook of the fundamental challenges in device applications.     

Dense skyrmion crystal stabilized through interfacial exchange coupling: Role of in-plane anisotropy

A Monte Carlo simulated-annealing algorithm was used to study the magnetic state in an in-plane helimagnet layer on triangular lattice that exchange couples to an underlayer with strong out-of-plane anisotropy. In the single helimagnet layer with in-plane anisotropy (K), the formation of labyrinthlike domains with local spin spirals, instead of parallel stripes, is favored, and these domains rapidly transform into dense skyrmion crystals with increasing interfacial exchange coupling (J′), equivalent to a virtual magnetic field, and finally evolve to an out-of-plane uniform state at large enough J′. Moreover, with increasing K, the skyrmion crystal state can vary from regular 6-nearest-neighboring circular skyrmion arrangement to irregular squeezed skyrmions with less than 6 nearest neighbors when the in-plane anisotropy energy is higher than the interfacial exchange energy as the skyrmion number is maximized. Finally, we demonstrated that the antiferromagnetic underlayer cannot induce skyrmions while the chirality inversion can be achieved on top of an out-of-plane magnetization underlayer with 180◦ domain walls, supporting the experimental findings in FeGe thin film. This compelling advantage offers a fertile playground for exploring emergent phenomena that arise from interfacing magnetic skyrmions with additional functionalities.     

宽温域跨室温磁斯格明子材料的发现及器件研究

报道了阻挫型磁体Fe_3Sn_2单晶中宽温域跨室温磁斯格明子的发现及其"赛道型"微纳器件的初步探索.通过合金化设计和实验,突破晶体取向生长和克服包晶反应两个关键技术难关,制备出了高质量的Fe_3Sn_2单晶.原位洛伦兹电子显微镜结果表明,在该材料体系中,磁斯格明子具有多种拓扑结构,并可以在一定磁场下相互转化.基于高质量的Fe_3Sn_2单晶,利用聚焦离子束技术,进一步制备出了600 nm宽并具有磁斯格明子单链排列的"赛道性"微纳器件.实验结果表明,该单链磁斯格明子具有极高的温度稳定性:单个磁斯格明子的尺寸以及相邻两个磁斯格明子之间的距离可以在室温到630 K宽温区内保持不变.宽温域跨室温磁斯格明子材料Fe_3Sn_2的发现及单链"赛道型"微纳器件的成功制备,从材料和器件两个方面推进了磁斯格明子材料的实用化.     

Multi-types of Skyrmions in SU（N） Quantum Hall System

The skyrmions in SU（N） quantum Hall （QH） system are discussed. By analyzing the gauge field structure and the topological properties of this QH system it is pointed out that in the SU（N） QH system there can exist （N-1） types of skyrmion structures, instead of only one type of skyrmions. In this paper, by means of the Abelian projections according to the （N - 1） Cartan subalgebra local bases, we obtain the （N - 1） U（1） electromagnetic field tensors in the SU（N） gauge field of the QH system, and then derive （N - 1） types of skyrmion structures from these U（1） sub-field tensors. Furthermore, in light of the C-mapping topological current method, the topological charges and the motion of these skyrmions are also discussed.     

斯格明子电子学的研究进展

在过去的半个世纪中,微电子技术一直沿着著名的摩尔定律快速发展,当前已经达到单芯片可集成上百亿晶体管.然而随着晶体管尺寸的缩小,因量子效应所产生的漏电流及其所导致的热效应使得这一定律遇到瓶颈.自旋电子技术由于引入了电子自旋这一全新的自由度,将有望大幅度降低器件功耗,突破热效应枷锁.斯格明子是一种具有拓扑保护的类粒子自旋结构,有望成为下一代自旋电子信息载体,引起了从物理到电子领域的广泛关注.由于其特殊的拓扑性质,斯格明子具备尺寸小、结构稳定、驱动阈值电流小等诸多优点,室温下斯格明子的成核、输运及探测进一步验证了其广泛的应用潜力,由此诞生了研究相关器件及应用的斯格明子电子学.本综述从电子学角度首先介绍斯格明子的基础概念及发展现状、理论及实验研究方法,重点阐述斯格明子器件的写入、调控及读取功能,介绍了一系列具有代表性的新型信息器件;最后,结合斯格明子电子学现状分析了目前所面临的发展瓶颈以及未来的应用前景.     

Real-space observation of individual skyrmions in helimagnetic nanostripes

Controllable formation and manipulation of domain walls in one-dimensional(1D) nanostripes underpins a promising type of emergent spintronic device. Magnetic skyrmion is topologically stable whirlpool-like spin texture and is expected to replace familiar domain wall phenomena to build such devices, owing to its prominent features including small size,topological stability and the small critical current required to move it. It is thus essential to understand skyrmions' properties in such a nanostructured element. In this paper, we mainly give fundamental insight into this issue. Experimental achievements in the formation and stability of individual skyrmions in the nanostripe are outlined in detail.     

周期性应变调控斯格明子在纳米条带中的运动

斯格明子是一种拓扑稳定的手性自旋结构,凭借其在磁性赛道存储器和自旋电子器件方面的巨大应用潜力而受到研究人员的广泛关注.为了使斯格明子能够更好地应用于磁性赛道存储器,研究斯格明子在纳米条带中的运动行为就变得非常重要.本文主要研究了存在周期性应变的纳米条带中铁磁斯格明子和反铁磁斯格明子在电流驱动下的运动行为.研究结果表明:周期性应变使得驱动电流存在一个临界电流密度,只有当电流密度大于临界电流密度时斯格明子才能够在纳米条带中连续移动.临界电流密度随应变振幅的增加而增加,随应变周期的增加而减小.铁磁斯格明子在周期性应变的调制下会产生周期性运动,轨迹为波浪式,其横向速度受到边界的影响,而纵向速度则与应变梯度成正比.反铁磁斯格明子在周期性应变调控下运动方向不变,但其移动速度则剧烈变化.     

Quantum Hall skyrmions with higher topological charge

We have investigated quantum Hall skyrmions with more than one unit of topological, and hence electric, charge. Using a combination of analytic and numerical methods we find the counterintuitive result that when the Zeeman energy is tuned to values much smaller than the interaction energy ( ), the creation energy of a charge two skyrmion becomes less than twice the creation energy of a charge one skyrmion, i.e. skyrmions bind in pairs. The doubly charged skyrmions are stable to further accretion of charge and exhibit a larger spin per unit charge than charge one skyrmions which would, in principle, signal this pairing. 1997 Elsevier Science B.V. All rights reserved.     

Brownian motion of massive skyrmions in magnetic thin films

We report on the thermal effects on the motion of current-driven massive magnetic skyrmions. The reduced equation for the motion of skyrmion has the form of a stochastic generalized Thiele’s equation. We propose an ansatz for the magnetization texture of a non-rigid single skyrmion that depends linearly with the velocity. By using this ansatz it is found that the skyrmion mass tensor is closely related to intrinsic skyrmion parameters, such as Gilbert damping, skyrmion-charge and dissipative force. We have found an exact expression for the average drift velocity as well as the mean-square velocity of the skyrmion. The longitudinal and transverse mobility of skyrmions for small spin-velocity of electrons is also determined and found to be independent of the skyrmion mass.