Department of Physics, University of New Hampshire, USA

Abstract:

Chiral magnets are a series of magnets with broken inversion symmetry. A new type of spin interaction therein, the Dzyaloshinskii-Moriya interaction, stimulates the formation of many novel topological spin textures. One typical example is the emergence of magnetic skyrmion, whose nontrivial topology enables unique dynamical property and thermal stability and gives out promise on future spintronic devices.  A key transport signature of the skyrmion is the topological Hall effect, that is, the electron moves in sideway under the effect of real space Berry phase induced by spin chirality. In the first part of the talk, I will argue that the presence of topological Hall in many experiments is not enough to identify skyrmions. It could originated from atomic scale chirality from thermal fluctuations.

In the second part of the talk, I will generalize the skyrmion texture from two dimensions (2D) to three dimensions (3D), and discuss two relevant 3D spin textures in chiral magnets. One is the target skyrmion we recently observed, both theoretically and experimentally, in ultra-small nanodisks of chiral magnets. A target skyrmion consists of concentric helical rings and can be stabilized in the absence of external magnetic field. Switch between two types of target skyrmions can be enabled by a field loop. The other texture to be discussed is the magnetic hopfion. We propose the presence of zero-field hopfion in synthetic chiral magnetic multilayers. The transition from hopfion to the ground state, a monopole-antimonopole pair, can be fully understood as the topological transition between torus and sphere. These works could stimulate the development of 3D spintronics.

报告人简介：

臧佳栋，2007年获复旦大学学士学位，2012年获复旦大学理论物理博士学位，毕业后到美国霍普金斯大学做博士后研究，并于2015年起在美国新罕布什尔大学物理系和材料系担任助理教授。长期从事凝聚态理论研究，重点研究低维磁性材料和拓扑物态，主持美国能源部和科学基金委多项课题。发表论文50余篇，引用2600余次，其中包括Nature Materials, Science Advances, Nature Communications, Phys. Rev. Lett.等多篇有影响力文章。受邀为Science, Nature, Physical Reviews等国际期刊的论文审稿人。