Researchers from Helmholtz-Zentrum Dresden-Rossendorf, CNRS AND Radboud University recently demonstrated that tiny magnetic vortices can host self-induced Floquet states driven purely by internal magnon dynamics, without the need for high-power laser fields. By periodically modulating the vortex core with low-power microwave excitation, they engineer Floquet bands in the magnon spectrum and observe clear frequency-comb signatures in nanometer-scale magnetic disks.
Floquet engineering uses a periodic drive to create effective Hamiltonians and band structures that do not exist in equilibrium, enabling exotic states and modified spin interactions. In this work, the periodic drive is not an external optical field but arises from internal modes of a magnetic vortex in an ultrathin disk, where the magnetization curls in-plane and forms a nanoscale vortex core with out-of-plane orientation. When microwave magnons are driven strongly enough, nonlinear coupling transfers energy into a circular gyration of the vortex core, which then acts as a time-periodic perturbation that renormalizes the magnon band structure.
Magnetic vortices form in ultrathin nickel-iron disks whose lateral size is scaled from micrometers down to a few hundred nanometers. Within these disks, collective spin-wave modes (magnons) propagate as phase-coherent oscillations, much like a coordinated “wave” moving through a stadium crowd. "These magnons can transmit information through a magnet without the need for charge transport," explains project leader Dr. Helmut Schultheiß, underscoring their relevance for energy-efficient spintronics and neuromorphic computing.
In especially small disks, the team observed that instead of a single resonance peak, the microwave spectrum develops a series of evenly spaced lines, forming a frequency comb. This comb arises because pumped azimuthal magnon modes nonlinearly excite vortex-core gyration; the gyrating core periodically modulates the magnetic configuration and generates Floquet sidebands of the original magnon mode. By tuning the microwave drive and thus the core-motion amplitude, the researchers can systematically reshape the magnon spectrum via this self-induced Floquet mechanism.
A key result is that these Floquet states and frequency combs appear at microwave powers in the microwatt range, far below the energies used in laser-driven Floquet experiments. Combined with the fact that magnons carry information without charge currents, this points to ultralow-power operation for future spintronic and neuromorphic devices. The team envisions that such magnonic frequency combs could act as a "universal adapter," synchronizing otherwise incompatible frequency ranges and helping interconnect electronics, spintronics, and quantum information technologies.