In the past years, the advent of geometrical, or Berry phases, in condensed matter physics caused a mini-revolution in the field of transport and dynamical properties of electronic and spin systems exposed to external magnetic and electric fields. Topological concepts caused a paradigm shift in modern electronics since they manifest themselves in a manifold of novel observed and predicted effects and properties of real materials, such as dissipationless spin currents or the appearance of topologically non-trivial states of matter.
Our group is dedicated to exploring the appearance of and ways of utilizing the geometrical concepts and phenomena related to geometrical phases in the solid state for use in future nanoelectronics.
As a group, we focus on novel response and transport effects in complex magnetic systems ranging from interfaces of transition metals to skyrmions. We make extensive use of the predictive power of density functional theory as our main tool for investigating topological phases in real materials, thereby bridging the gap between experimental advances and progress in theoretical understanding of topological effects in realistic materials.
As a result, we dedicate a large part of our activities to developing first-principles methodologies for addressing the electron and spin properties which are rooted in the topological nature of electrons in solids.
Latest Research Highlights
Mixed topological semimetals driven by orbital complexity in two-dimensional ferromagnets
C. Niu, J.-P. Hanke, P. M. Buhl, H. Zhang, L. Plucinski, D. Wortmann, S. Blügel, G. Buhlmayer, and Y. Mokrousov
The concepts of Weyl fermions and topological semimetals emerging in three-dimensional momentum space are extensively explored owing to the vast variety of exotic properties that they give rise to. On the other hand, very little is known about semimetallic states emerging in two-dimensional magnetic materials, which present the foundation for both present and future information technology. Here, we demonstrate that including the magnetization direction into the topological analysis allows for a natural classification of topological semi- metallic states that manifest in two-dimensional ferromagnets as a result of the interplay between spin-orbit and exchange interactions. We explore the emergence and stability of such mixed topological semimetals in realistic materials, and point out the perspectives of mixed topological states for current-induced orbital magnetism and current-induced domain wall motion. Our findings pave the way to understanding, engineering and utilizing topological semimetallic states in two-dimensional spin-orbit ferromagnets.
Long-range chiral exchange interaction in synthetic antiferromagnets
D.-S. Han, K. Lee, J.-P. Hanke, Y. Mokrousov, K.-W. Kim, W. Yoo, Y. L. W. van Hees, T.-W. Kim, R. Lavrijsen, C.-Y. You, H. J. M. Swagten, M.-H. Jung and M. Kläui,
Nature Materials (2019) https://doi.org/10.1038/s41563-019-0370-z
The exchange interaction governs static and dynamic magnetism. This fundamental interaction comes in two flavours—symmetric and antisymmetric. The symmetric interaction leads to ferro- and antiferromagnetism, and the antisymmetric interaction has attracted significant interest owing to its major role in promoting topologically non-trivial spin textures that promise fast, energy-efficient devices. So far, the antisymmetric exchange interaction has been found to be rather short ranged and limited to a single magnetic layer. Here we report a long-range antisymmetric interlayer exchange interaction in perpendicularly magnetized synthetic antiferromagnets with parallel and antiparallel magnetization alignments. Asymmetric hysteresis loops under an in-plane field reveal a unidirectional and chiral nature of this interaction, which results in canted magnetic structures. We explain our results by considering spin–orbit coupling combined with reduced symmetry in multilayers. Our discovery of a long-range chiral interaction provides an additional handle to engineer magnetic structures and could enable three-dimensional topological structures.