Jakub Zázvorka1, Raphael Gruber2, Takaaki Dohi2, Nico Kerber2, Peter Virnau2, Mathias Kläui2
1 Institute of Physics, Faculty of Mathematics and Physics, Charles University,
Prague, Czech Republic
2 Institute of Physics, Johannes-Gutenberg University Mainz, Mainz, Germany
Ferromagnetic skyrmions, whirls of magnetization with topological charge, are efficiently displaced using spin-transfer torques and spin-orbit torques1. Thermally induced skyrmion motion was by theory and has initially been deemed negligible compared to the current induced motion. Recently, we have uncovered thermal diffusive skyrmion dynamics in a multilayer material with engineered ultra-low pinning2. At low skyrmion densities, the motion of skyrmions observed was following the diffusive motion assuming rigid particles with no correlation, having the mean-squared displacement linearly proportional to the time (Fig. 1a). Investigating the temperature dependence, we observed that skyrmions move in a non-flat energy landscape, revealing an exponential dependence of the diffusion coefficient on temperature. At higher skyrmion densities, skyrmion lattices emerge in the material (Fig. 1b). While from previous measurements it
is established that the skyrmions tend to move and diffuse throughout the material, the ordering of the lattice is highly dependent on the skyrmion-skyrmion interactions. The structure of the skyrmion lattice is investigated in dependence on temperature and skyrmion radius variation by external magnetic field using the pair-correlation function and the angular ordering parameter3. Deviations from a hexagonal order are observed with formation of defects which can propagate throughout the system. Observing the dynamics of the lattice and its evolution with time and external parameters, we study the lattice formation process and the structural phases of the two-dimensional system.
References
1 Litzius, K. et al. Skyrmion Hall effect revealed by direct time-resolved X-ray microscopy.
Nature Physics 13, 170-175, doi:10.1038/nphys4000 (2016).
2 Zazvorka, J. et al. Thermal skyrmion diffusion used in a reshuffler device. Nat. Nanotechnol 14, 658-661, doi:10.1038/s41565-019-0436-8 (2019).
3 Zázvorka, J. et al. Skyrmion Lattice Phases in Thin Film Multilayer. Advanced Functional Materials 30, doi:10.1002/adfm.202004037 (2020).