主讲人：严鹏 教授（电子科技大学）

题目：When spintronics meets topology

时间：

地点：理工楼1226报告厅

邀请人：贾成龙

报告摘要：

In the first part of my talk, we show the emergence of second-order topological insulator in the dynamics of spin texture based metamaterials. Quantized Chern number and ZN Berry phase are proposed to completely characterize the nontrivial topology. By studying the collective gyration of magnetic vortices in a breathing honeycomb lattice, we derive the full phase diagram and show that the topological “zero-energy” corner mode is protected by a generalized chiral symmetry in the sexpartite lattice, leading to particular robustness against disorder and defects. Interestingly, we observe corner states at either obtuse-angled or acute-angled corners, depending on whether the lattice boundary has an armchair or zigzag shape. Full micromagnetic simulations confirm the theoretical predictions with good agreement. This finding opens up a promising route for realizing higher-order symmetry-protected corner states in magnetic systems and for finally achieving topological spintronic memory, imaging, and computing.

Wave fields with spiral phase dislocations carrying orbital angular momentum (OAM) have been realized in many branches of physics, such as for photons, sound waves, electron beams, and neutrons. However, the OAM states of magnons (spin waves), the building block of modern magnetism, and particularly their implications have yet to be addressed. In the second part of my talk, we theoretically investigate the twisted spin-wave generation and propagation in magnetic nanocylinders. The OAM nature of magnons is uncovered by showing that the spin-wave eigenmode is also the eigenstate of the OAM operator in the confined geometry. Inspired by optical tweezers, we predict an exotic “magnetic tweezer” effect by showing skyrmion gyrations under twisted magnons in exchange coupled nanocylinder|nanodisk heterostructure, as a practical demonstration of magnonic OAM to manipulate topological spin defects. Our study paves the way for the emerging magnetic manipulations by harnessing the OAM degree of freedom of magnons.

Three-magnon effects have been known to be important for nonlinear processes in magnetic thin films, since they can give rise to very different output waves. Conventional three-magnon processes are triggered by the weak nonlocal magnetic dipole-dipole interaction in uniform magnetic thin films. Exchange coupling and magnetic anisotropy (including both the magnetocrystalline anisotropy and the shape anisotropy due to the local part of the dipolar interaction), on the other hand, are often much stronger than the nonlocal dipole-dipole interaction in ferromagnet. In homogeneous ferromagnets without external magnetic fields, the lowest-order nonlinear process by these two interactions is the four-magnon scattering. In the third part of my talk, we show that magnetic textures can significantly enhance nonlinearities in magnetic systems, such as the three-magnon splitting and confluence processes. Based on these findings, we propose to eavesdrop on spin waves inside the domain-wall nanochannel, and to nonlocally measure the DMI parameter in a narrow ferromagnetic strip or nanowire, via three-magnon processes. The nonlinear effect on the topological phases in magnetic domain wall racetrack is also analyzed.

An optical frequency comb consists of a set of discrete and equally spaced frequencies and has found wide applications in the synthesis over a broad range of spectral frequencies of electromagnetic waves and precise optical frequency metrology. Despite the analogies between magnons and photons in many aspects, the analogue of an optical frequency comb in magnonic systems has not been reported. Here, we theoretically study the magnon-skyrmion interaction and find that a magnonic frequency comb (MFC) can be generated above a threshold driving amplitude, where the nonlinear scattering process involving three magnons prevails. The mode-spacing of the MFC is equal to the breathing-mode frequency of skyrmion and is thus tunable by either electric or magnetic means. The theoretical prediction is verified by micromagnetic simulations and the essential physics can be generalized to a large class of magnetic solitons. Our findings open a new pathway to observe frequency comb structures in magnonic devices that may inspire the study of fundamental nonlinear physics in spintronic platforms in the future.

个人简介：

严鹏，教授，博士生导师。2006年本科毕业于中国科技大学近代物理系，2011年于香港科技大学获得物理学博士学位。2011-2015年期间，先后在荷兰代尔夫特理工大学，德国美因兹大学从事博士后研究工作。2015年底回国组建电子科技大学自旋电子学理论研究所，2016年入选国家特聘青年人才计划，主要研究领域为自旋电子学，拓扑磁动力学，非平衡态热力学与统计，以及生物磁性与磁导航，多次应邀参加重要国际学术会议并作邀请报告，如2021年美国物理学会三月会议，2020年国际磁学会议，2019年国际光学工程学会第十二届自旋电子学会议等。以第一作者或通讯作者完成的主要学术成果包括：(1) 预言了磁振子传递自旋转移力矩的新机制(PRL 2011)，最近被两篇实验工作所证实(Science 2019)；(2) 揭示了原子尺度磁结构对自旋流和热流的调控机制(PRL 2013)；(3) 提出了基于宇称-时间反演对称破缺的铁磁-反铁磁相变新原理(PRL 2018)；(4) 提出了涡旋态自旋波轨道角动量转移的新机制，发现了一种全新的磁性镊子效应(PRL 2020)；(5) 首次在磁孤子晶格中实现了受广义手征对称性保护的拓扑零能模，并开展了拓扑电路实验检验该理论，系列研究成果发表在Physical Review, Nano Letters等著名期刊; (6) 受邀撰写专著《Chirality, Magnetism and Magnetoelectricity》其中章节“Topological Dynamics of Spin Texture Based Metamaterials”；(7) 受邀为物理学顶尖综述期刊Physics Reports撰写长篇综述《Topological insulators and semimetals in classical magnetic systems》。