Topological mode switch by exceptional point encircling in a cavity magnonic system

Cavity magnonic systems have attracted wide interest for their potential in bridging quantum electrodynamics and magnetism, while non-Hermitian physics has similarly garnered increasing attention. Here, we propose and experimentally investigate a tunable cavity magnonic system comprising a microwave cavity mode coupled to the Kittel mode in an yttrium iron garnet (YIG) ferromagnetic sphere. By controlling the damping and adjusting the cavity photon-magnon coupling strength, we realize and characterize exceptional points (EPs) in this system. We observe a transition in the transmission spectra from anticrossing to crossing as the coupling strength is tuned across the EP threshold, confirming the EP's presence. Furthermore, by carefully encircling the EP in a parameter space, we demonstrate dynamic topological mode switching, wherein the system's eigenvalues interchange upon completing a loop around the EP. Our results highlight the flexibility of a cavity magnonic platform for exploring non-Hermitian phenomena and pave the way for EP-based device applications in quantum information processing and topological photonics.