Direct observation of sub-terahertz magnon resonances in a topological kagome ferromagnet

Magnons (the quanta of spin waves) in ferromagnets are typically resonant in the gigahertz range. Here we report the direct observation of two sub-terahertz magnon resonances (0.61 and 0.45 THz at 4 K) in a Weyl semimetal kagome ferromagnet Co₃Sn₂S₂ using time-resolved Kerr rotation spectroscopy. These frequencies are the highest among all the reported magnetic resonances in ferromagnets due to its giant magnetocrystalline anisotropy. While the stronger high frequency mode can be modeled with conventional ferromagnetic resonance, remarkably, the much weaker low frequency mode may originate from the antiferromagnetic coupling exhibiting in the kagome lattice. These findings not only contribute to a deeper understanding of the magnetic and topological properties of this material but also pave the way for the design of novel terahertz spintronic devices.