Research Topics
Our activities evolve around understanding the transport and response properties of solids which root in spin-orbit interaction and non-collinear magnetism, with particular focus on effects of disorder and non-trivial topology of electronic states in various spaces. The concrete directions of our research lie in ab-initio theory of:
- spin, anomalous and quantum anomalous Hall effects in metals and insulators
- the effect of spin-orbit torque in ferromagnets and antiferromagnets
- spin accumulation and spin pumping
- topological insulators and metals
- transport properties of skyrmions and chiral magnets
- Dzyaloshinskii-Moriya interaction and orbital magnetism
- topological characterization of solids and magnetization dynamics
- ballistic transport in nano-scale magnets
- thermal transport phenomena
- interaction of matter with laser pulses and optical excitation of magnets
- orbital magnetism of metals and insulators
Research Highlights
Chiral Hall Effect in Noncollinear Magnets from a Cyclic Cohomology Approach
Fabian R Lux, Frank Freimuth, Stefan Blügel, Yuriy Mokrousov
Physical Review Letters 124, 096602 (2020)
We demonstrate the emergence of an anomalous Hall effect in chiral magnetic textures which is neither proportional to the net magnetization nor to the well-known emergent magnetic field that is responsible for the topological Hall effect. Instead, it appears already at linear order in the gradients of the magnetization texture and exists for one-dimensional magnetic textures such as domain walls and spin spirals. It receives a natural interpretation in the language of Alain Connes’ noncommutative geometry. We show that this chiral Hall effect resembles the familiar topological Hall effect in essential properties while its phenomenology is distinctly different. Our findings make the reinterpretation of experimental data necessary, and offer an exciting twist in engineering the electrical transport through magnetic skyrmions.
Physical Review Letters 125, 177201 (2020)
Current-induced spin-orbit torques (SOTs) allow for the efficient electrical manipulation of magnetism in spintronic devices. Engineering the SOT efficiency is a key goal that is pursued by maximizing the active interfacial spin accumulation or modulating the nonequilibrium spin density that builds up through the spin Hall and inverse spin galvanic effects. Regardless of the origin, the fundamental requirement for the generation of the current-induced torques is a net spin accumulation. We report on the large enhancement of the SOT efficiency in thulium iron garnet (TmIG)/Pt by capping with a CuOxlayer. Considering the weak spin-orbit coupling (SOC) of CuOx, these surprising findings likely result from an orbital current generated at the interface between CuOx and Pt, which is injected into the Pt layer and converted into a spin current by strong SOC. The converted spin current decays across the Pt layer and exerts a “nonlocal” torque on TmIG. This additional torque leads to a maximum colossal enhancement of the SOT efficiency of a factor 16 for 1.5 nm of Pt at room temperature, thus opening a path to increase torques while at the same time offering insights into the underlying physics of orbital transport, which has so far been elusive.
Theory of current-induced angular momentum transfer dynamics in spin-orbit coupled systems
Dongwook Go, Frank Freimuth, Jan-Philipp Hanke, Fei Xue, Olena Gomonay, Kyung-Jin Lee, Stefan Blügel, Paul M Haney, Hyun-Woo Lee, Yuriy Mokrousov
Physical Review Research 2, 033401 (2020)
Motivated by the importance of understanding various competing mechanisms to the current-induced spin-orbit torque on magnetization in complex magnets, we develop a theory of current-induced spin-orbital coupled dynamics in magnetic heterostructures. The theory describes angular momentum transfer between different degrees of freedom in solids, e.g., the electron orbital and spin, the crystal lattice, and the magnetic order parameter. Based on the continuity equations for the spin and orbital angular momenta, we derive equations of motion that relate spin and orbital current fluxes and torques describing the transfer of angular momentum between different degrees of freedom, achieved in a steady state under an applied external electric field. We then propose a classification scheme for the mechanisms of the current-induced torque in magnetic bilayers.
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.