Discovery of terahertz-frequency orbitally coupled magnons in a kagome ferromagnet

In ferromagnetic materials, magnons—quanta of spin waves—typically resonate in the gigahertz range. Beyond conventional magnons, while theoretical studies have predicted magnons associated with orbital magnetic moments, their direct observation has remained challenging. Here, we present the discovery of two distinct terahertz orbitally coupled magnon resonances in the topological kagome ferromagnet Co₃Sn₂S₂. Using time-resolved Kerr rotation spectroscopy, we pinpoint two magnon resonances at 0.61 and 0.49 terahertz at 6 kelvin, surpassing all previously reported magnon resonances in ferromagnets due to strong magnetocrystalline anisotropy. These dual modes originate from the strong coupling of localized spin and orbital magnetic moments. These findings unveil an unconventional category of magnons in a ferromagnet stemming from orbital magnetic moments and position Co₃Sn₂S₂ as a promising candidate for high-speed terahertz spintronic applications.