反铁磁畴壁和斯格明子的动力学行为研究

Spintronics is a field to manipulate electron spin as well as electron charge and it is promising to innovate the information and computing technology. The traditional spintronics focuses on the manipulation of ferromagnetic nanostructures such as domain walls, skyrmions and spin waves. Recently, antiferromagnet (AFM) spintronics has attracted a lot of attention due to the superb stability and terahertz dynamics of an AFM. Various interesting aspects of AFM have been investigated such as the AFM domain wall dynamics, ultrafast control of AFM spin wave, electric manipulation of an AFM and current driven motion of skyrmions. However, the understanding of the physics in these fields is far from satisfactory. For example, the damping of an AFM determines the domain wall velocity and the switching time of Neel order of an AFM, but its value is never determined by either experiments or theory. The exact form of damping-like torque in the dynamic equation is artificially added instead of rigorous theoretic treatment. For another example, skymirion is promising to storage information due to its smaller size and straight motion along electric current. But the stability of AFM skyrmions, the spin wave excitation around a skyrmion and the current/temperature gradient driven skyrmion dynamics are not clear. ..Based on our research experience on ferromagnetic domain wall pinning and depinning, the influence of nonlocal damping on domain wall dynamics and skyrmion manipulation using current pulses, the current project will focus on two aspects of AFM spintronics. Firstly, we investigate the equations that govern the AFM dynamics and the resultant influence on the AFM domain wall motion. Special attention will focus on the form of damping-like torque and the new torque generated by spin pumping effect in a non-collinear structure. Secondly, we investigate the stability of AFM skyrmions against temperature, external fields and frustration and try to find an optimized window that may guide the experiments to find the skyrmions. Also we investigate the dynamics of skyrmions driven by electric current, temperature gradient and spin wave especially the effect of defects and dislocations on skyrmion dynamics. Our research should be important to understand the dynamic behaviors of AFM domain walls and skyrmions and to the experimental realization and engineering fabrication of AFM-based devices.

自旋电子学是研究如何在固体系统里调控电子自旋的新兴学科，它对革新现在的信息存储方式和提升信息读取速度有重要意义。传统的自旋电子学关注的是纳米尺度的铁磁结构，例如畴壁，斯格明子，自旋波等。近年来，因为反铁磁更强的稳定性和太赫兹尺度的动力学行为，反铁磁自旋电子学吸引了广泛的研究兴趣。其中，反铁磁的能量耗散机制和耗散系数大小、自旋泵浦效应对畴壁速度的影响、斯格明子的稳定性和动力学行为等问题的研究还不够深入。本课题首先分析反铁磁的能量耗散机制和由此产生的耗散力矩对动力学方程的修正，特别是自旋泵浦效应对耗散力矩的影响。其次，研究反铁磁斯格明子的稳定性受温度和阻挫的影响，并进一步探究斯格明子的低激发态的性质和在外加电流驱动下的动力学行为。本课题为研究反铁磁动力学提供理论基础、为实验中观测到斯格明子提供新思路、也为反铁磁自旋电子学器件的制备提供参考。

自旋电子学是在调控电子电荷自由度的基础上，进一步调控电子自旋的新兴学科，它在低能耗、高密度和快速信息处理领域有重要应用价值。传统的自旋电子学关注铁磁系统里自旋的输运行为，近来，由于反铁磁系统的太赫兹动力学尺度、较强的稳定性和电学操控的便利性，吸引了诸多研究兴趣。然而，人们对反铁磁动力学的基本描述，特别是能量耗散通道、磁结构（如磁畴、磁畴壁、斯格明子等）的动力学行为以及其与新兴量子信息科学的融合依然没有清楚的理解。本项目首先探明了反铁磁系统的自旋耗散通道，推导出了自旋泵浦效应对动力学方程的修正，并从第一性原理角度证实了我们理论对系统耗散系数的预测。在此基础上，我们阐明了驱动反铁磁畴壁运动的物理机制和实现超快畴壁运动的可能性，其中对畴壁运动速度的预测最近被实验证实。其次，我们探究了磁性斯格明子的稳定性和钉扎效应，并提出了利用参数泵浦和磁镊效应来精准调控斯格明子的运动。这种调控方式不需要电荷流作为媒介，因而极大降低了热损耗。最后，我们从量子尺度重新审视了反铁磁自旋电子学，发现了反铁磁体内磁振子间的量子纠缠效应，进而查明了磁振子与谐振腔里的光子实现能级耦合和信息交互的渠道。我们这些结果不仅加深了人们对于反铁磁动力学的认识，同时为设计基于磁畴壁和斯格明子的高性能自旋电子器件提供了理论依据。进一步地，反铁磁里量子效应的探究，延拓了传统自旋电子学的研究范畴，同时提供了一个自旋电子学和量子信息科学交叉研究的思路。