Details of Research Outputs

TitleTOPOLOGICAL SPIN TEXTURES IN FERROMAGNETS AND FRUSTRATED MAGNETS
Author (Name in English or Pinyin)
XIA, Jing
Author (Name in Chinese)夏静
Degree TypeDoctor of Philosophy
Supervisor(s) (Name in English or Pinyin)ZHOU, Yan
Date Issued2020
Degree GrantorThe Chinese University of Hong Kong
Place of ConferralShenzhen
Degree DisciplineComputer and Information Engineering
languageEnglish
Abstract

Topologically non-trivial spin textures, such as skyrmions and bimerons, are particle-like objects carrying integer topological charges and can be used as non-volatile information carriers in future electronic and spintronic applications. In this thesis, Ibtheoretically and computationally study the physical properties of different topological spin textures in conventional ferromagnetic and frustrated ferromagnetic materials. In particular, I focus on the static and dynamic properties of magnetic skyrmions and magnetic bimerons. This thesis consists of two main parts. In the first part, I study the current-driven dynamics of skyrmions in ferromagnetic thin-film and nano disk systems. I first study the isolated skyrmion and skyrmion cluster in ferromagnetic nano disks with uniform interface-induced Dzyaloshinskii-Moriya interactions (DMI). The spin-polarized current with a spatially varied polarization, e.g., a vortex-like polarization, is adopted to drive the dynamics of skyrmions. The skyrmions driven by the spin-polarized current with a vortex-like polarization can reach either a dynamic steady state or a static state, which depend on the given profile of the spatially varied polarization. For the dynamic steady state, the dkyrmion moves along a circular trajectory, while for the static state, the skyrmion does not move. Therefore, using the spin-polarized current with a vortex-like polarization. I demonstrate that it is possible to rotate, assemble, compress, and delete skyrmions in nano disks in a quantitative manner. Then, I study the current-driven dynamics of an isolated skyrmion in ferromagnetic thin films with anisotropic DMI, where the skyrmion has an elliptical shape instead of a perfect circular shape. The skyrmion Hall effect of the elliptical skyrmion driven by either the spin-orbit or the spin-transfer torques is investigated. It is found that the skyrmion Hall angle can be controlled by tuning the profile of the elliptical skyrmion. For arbitrary spin current polarization or spin-orbit torque mechanism, I suggest that it is always possible to find the best current direction to minimize the skyrmion Hall angle when the skyrmion is of an elliptical shape. In the second part of this thesis, I investigate the current-driven dynamics of magnetic skyrmions and bimerons in frustrated ferromagnetic ultra-thin-film systems, including monolayers and bilayers. The frustration is caused by competing exchange interactions. I first study the frustrated skyrmions in a ferromagnetic monolayer system and an antiferromagnetically exchange-coupled bilayer system, where the biayer skyrmion consists of two monolayer skyrmions with opposite topological charges. It is found that the frustrated monolayer skyrmion could move in a straight or circular trajectory, depending on the type of driving current. And, the in-plane current-driven frustrated monolayer skyrmion could move in a straight or circular trajectory, depending on the type of driving current. And, the in-plane current-driven frustrated bilayer skyrmion moves in a straight trajectory, while the out-of -plane current-driven frustrated bilayer skyrmion moves in a circular trajectory. It is also found that the in-plane current-driven mobility of a frustrated bilayer skyrmion is much better than the monolayer one at a large ratio of β/α, where α and β denote the damping parameter and non-adiabatic spin-transfer torque strength, respectively. Besides, the out-of-plane current-driven mobility of a frustrated bilayer skyrmion is much better than the monolayer one when α is small. Then, I study the magnetic bimerons in a frustrated ferromagnetic monolayer system. Magnetic bimeron is a topological counterpart of skymions in in-plane magnets, which can also be used as spintronic information carrier. I study the static properties of frustrated bimenrons with different topological structures. I also report the dynamics of frustrated bimerons driven by the spin-orbit torques, including both the damping-like and field-like torques. It is found that the isolated bimeron can be driven into linear or elliptical motion, when the spin-polarization direction is perpendicular to the easy axis. The isolated bimeron can also be driven into rotation by the damping-like torque when the spin polarization direction is parallel to the easy axis. And, the rotation frequency is proportional to the driving current density. In addition, I numerically demonstrate the possibility of creating a bimeron state with different topological charges. Finally, at the end of this thesis, I provide a prospective outlook on future research of topological spin textures, such as the topological spin textures in antiferromagnets and ferromagnets. 拓扑非平庸的磁自旋结构,例如斯格明子和双半子,具有整数拓扑数和准粒子特征,有潜力作为未来电子和自旋电子学应用中的非易失性信息载体。本文从理论解析和数值模拟两个方面对普通铁磁系统和磁阻挫系统中不同类型的拓扑磁结构进行了详细的研究,尤其是磁斯格明子和磁双半子的静力学和动力学性质。本文主要由两个部分组成。第一部分研究了斯格明子在铁磁薄膜和铁磁纳米碟盘中由电流引起的动力学。首先,研究了在具有各向同性界面型DMI的铁磁纳米碟盘中的单个斯格明子和斯格明子团簇。采用具有空间分布极化的自旋极化电流,例如类涡旋态极化分布,对斯格明子进行调控。结果表明,斯格明子在具有类涡旋态极化的自旋极化电流驱动下,根据自旋极化电流的不同极化方向分布,可达到动态或静态的稳定。在动态的稳定中,斯格明子做圆周运动,而在静态的稳定中,斯格明子保持静止不动。因此,利用具有类涡旋态极化的自旋极化电流,可以定量地旋转,组装,压缩和消除纳米碟盘中的斯格明子。本文第一部分还研究了由各向异性DMI导致的椭圆形斯格明子的动力学,包括了自旋轨道转矩和自旋转矩引起的斯格明子霍尔效应。结果表明,通过调节椭圆形斯格明子的形状可以调控斯格明子霍尔角。此外,当斯格明子形状为椭圆时,对于任意自旋极化方向或自旋转矩机制,总是可以找到一个最佳的电流方向,已得到最小化的斯格明子霍尔角。本文第二部分研究了磁阻挫薄膜系统中的斯格明子和双半子动力学。在本文的模型中,阻挫效应是由若干相互竞争的交换耦合作用产生。首先,研究了铁磁单层膜系统和反铁磁交换耦合双层膜系统中的阻挫斯格明子的动力学,其中双层斯格明子是由两个具有相反拓扑数的单层斯格明子构成。结果表明,在磁阻挫单层膜系统中,根据不同类型的驱动电流,斯格明子可沿直线或圆形轨迹运动。在磁阻挫双层膜系统中,由面向电流驱动的双层斯格明子沿直线运动,由面向(垂直)电流驱动的双层斯格明子则做圆周运动。此外,当β/α大时,由面向电流驱动的双层斯格明子的运动速度比单层斯格明子的速度更高,其中,α和β分别表示系统的阻尼系数和非绝热自旋转矩强度。当α较小时,由面外电流驱动的双层斯格明子的运动速度比单层斯格明子的速度更快。本文第二部分还研究了磁阻挫单层膜系统中的双半子。双半子是具有面内磁晶各向异性的磁体中斯格明子的对应态,也可作为非易失性信息载体。本文研究了具有不同拓扑结构的双半子的静力学性质,还研究了双半子在类阻尼自旋转矩和类场自旋转矩驱动下的动力学。结果表明,当驱动自旋电流的极化方向与磁体易轴垂直时,双半子沿直线或椭圆轨迹运动。当驱动自旋电流的极化方向与磁体易轴平行时,双半子发生周期性的旋转,其旋转的频率与驱动电流密度成正比。此外,利用数值模拟,还演示了通过具有不同拓扑数的双半子之间的碰撞和合并来产生具有更高或更低拓扑数的双半子。本文的最后展望了未来拓扑磁结构领域的研究方向,例如反铁磁和亚铁磁系统中的拓扑磁结构。

LibraryUniversity Library
Location Theses & Dissertations Collection
Call NumberM.Phil. X53 2020
Document TypeThesis
Identifierhttps://irepository.cuhk.edu.cn/handle/3EPUXD0A/2726
LinksPRIMO
CollectionSchool of Science and Engineering
Recommended Citation
GB/T 7714
XIA, Jing. TOPOLOGICAL SPIN TEXTURES IN FERROMAGNETS AND FRUSTRATED MAGNETS[D]. The Chinese University of Hong Kong, Shenzhen,2020.
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夏静.pdf(10622KB)Thesis-- No AccessCC BY-NC-SA
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