Self-induced Floquet magnon states

Katrin Schultheiss, Helmholtz-Zentrum Dresden–Rossendorf

Friday 3pm ET, June 7th, 2024 

Magnetic vortices (Fig. 1a) are prominent examples for topology in magnetism. They exhibit an intricate magnon spectrum and show an eigen-resonance of the vortex texture itself, the gyration of the vortex core. While there have been studies about magnon-assisted reversal of the core polarity [1], the impact of the vortex core gyration on the magnon spectrum has not been addressed so far. 

The fundamental modes of both excitations are clearly separated in their resonance frequencies. While the core typically gyrates at a few hundred MHz (Figs. 1b,d), the magnon modes typically have frequencies in the lower GHz range (Figs. 1c,d). This separation allows for experiments studying the temporal evolution of the magnon spectrum when the gyration of the vortex core is driven externally. Under the influence of such a periodic driving field, Floquet magnon states emerge due to a temporal periodicity imposed on the system’s equilibrium state (Fig. 1e), much like the formation of Bloch states in the periodic potential of a crystal lattice. While Bloch states are shifted in momentum space, Floquet states are shifted in energy by multiples of the drive frequency, facilitating the design of novel properties and functionalities in condensed matter systems. 

This talk delivers experimental results and numerical simulations on how the regular magnon modes in a magnetic vortex transform into Floquet bands, when the vortex core gyration is driven simultaneously. The observed magnon Floquet states are both distinct from the well-known regular magnon modes as well as from the vortex gyration and represent unique excitations providing new opportunities to study and control nonlinear magnon dynamics. 

This work has received funding from the EU Research and Innovation Programme Horizon Europe under grant agreement no. 101070290 (NIMFEIA). 

[1] M. Kammerer, et al. Nature Comm. 2, 279 (2011)