Materials Science Research Lecture
Abstract: Magnetic materials like Fe, Ni, and Co are among the oldest and best studied systems in materials science. Yet, the delicate balance of correlated quantum interactions and long-range classical fields give rise to a richness that we are just starting to reveal. Here, I will discuss two emerging topics that are of particular interest: chiral soliton dynamics and non-equilibrium physics. The key to explore these new territories of materials physics is direct time-resolved imaging, which can be realized via time-resolved x-ray holography [1] at synchrotrons and free electron lasers with unprecedented resolution and depth of information.
In magnetism, strong chiral interactions can lead to the formation of localized chiral solitons with non-trivial topology, so-called skyrmions. Skyrmions can move like particles but their dynamics exhibit fascinating topological signatures, such as gyration [1], inertia [1], the skyrmion Hall effect [2], and topological damping [3]. The properties of skyrmions are intricately linked to the underlying material. For example, room-temperature skyrmions are abundant in ultrathin ferromagnetic films sandwiched between dissimilar materials with high spin-orbit interactions [1-4]. In these materials, skyrmion size and stability are determined by long-range stray fields while quantum interactions at the interfaces lead to efficient and deterministic skyrmion motion and skyrmion nucleation by electrical currents [2,4]. I will discuss these physics based on quantitative theoretical modeling [3] and show that there is a new class of materials, chiral ferri- and antiferromagnets, with a small phase pocket for purely quantum-stabilized skyrmions. These quantum-stabilized skyrmions are insensitive to classical fields, their size can be smaller than 10 nm at room temperature, and their dynamics is ultrafast and without topological signatures [3]. I will show how high resolution x-ray holography led to the discovery of these skyrmions in the material predicted by the model [5]. As an outlook, I will present first results of skyrmion dynamics in the far-from-equilibrium state after optical excitation with extremely bright femtosecond laser pulses and discuss the perspective to image these and other ultrafast processes at new free electron laser sources.
[1] Büttner et al., Nat. Phys. 11, 225 (2015).
[2] Litzius et al., Nat. Phys. 13, 170 (2017).
[3] Büttner et al., Sci. Rep. 8, 4464 (2018).
[4] Büttner et al., Nat. Nanotech. 12, 1040 (2017).
[5] Caretta, Mann, Büttner et al., Nat. Nanotech. 13, 1154 (2018).
More About the Speaker: Felix Büttner is a postdoctoral researcher at the Department of Materials Science and Engineering at MIT, where he specializes in chiral and topological magnetic materials (experiment and theory) as well as high resolution time-resolved x-ray imaging at synchrotrons. He received his MS from the University of Goettingen and his PhD from the University of Mainz, both in Germany. He has won the Best Young Researcher Award of the IEEE Italy and IEEE Magnetics Society, the Postdoctoral Fellowship of the German Research Foundation, and the NSLS-II Director's Postdoc Fellowship.