Vortex dynamics and GHz-range washboard frequencies in NbN/ferromagnet hybrids without vortex–magnon coupling

Vortex dynamics and high vortex velocities in NbN/ferromagnet hybrids set the conditions for detecting vortex–magnon interactions. We studied 10-nm NbN films capped with Pt, Co, and (Pt/Co) multilayers, introducing AlN spacers to mitigate proximity effects. We find that vortex–defect interactions govern the dynamics: stronger pinning enhances the critical current density but suppresses the attainable velocity, whereas reduced pinning enables high-velocity flux flow followed by Larkin–Ovchinnikov instabilities, with vortex motion exceeding 10 km s −1 at low fields and remaining in the ≈ 1–2 km s −1 range up to 2 T, with the upper bound attained in the weak-pinning Pt/Co-based stacks and the lower bound in the more disordered Co-capped configuration. The corresponding washboard frequencies span ≈ 27–41 GHz, within the previously reported ferromagnetic spin-wave range, yet we do not observe dc-transport anomalies that could be attributed to vortex–magnon coupling within our experimental sensitivity . These results underscore the need for structural and dynamical optimization and provide a framework for designing superconductor/ferromagnet hybrids aimed at probing vortex–magnon interactions. • Ultrafast vortex flow in NbN hybrids: v > 10 km s −1 , washboard frequencies up to ∼40 GHz. • Weaker pinning enables faster flux-flow in superconducting / ferromagnetic hybrids. • Vortex–magnon fingerprints may be pinning-sensitive, masked by disorder/domains.