Observation of dark magnons in a van der Waals antiferromagnet/ferromagnet interface

Magnons, the quanta of spin waves, offer a promising platform for low-power information processing owing to their charge-free propagation. Dark magnon modes, which are intrinsically decoupled from electromagnetic fields, exhibit high robustness against environmental perturbations and enable stealth-like propagation, making them highly attractive for magnonic applications. However, the excitation and detection of dark magnons in antiferromagnets remain highly challenging. Here, we report the direct observation of a magnon dark mode in a van der Waals antiferromagnet CrSBr interfaced with a ferromagnetic insulator YIG. In CrSBr, the canted (spin-flop) phase is nearly bypassed and the spin-flip phase has two magnon branches, one of which, the dark mode, has nearly zero net dynamic magnetization. We demonstrate that the interfacial exchange interaction enables the experimental detection of the magnon dark mode, while its presence is confirmed by the observation of coherent magnon-magnon coupling between YIG and CrSBr magnons. Micromagnetic simulations and a theoretical model reveal that the dark mode in CrSBr at the heterostructure interface hybridizes with YIG magnons via interfacial exchange interaction. Our results provide a new route for detecting dark magnon modes in antiferromagnets and open up opportunities for stealth-like magnon transport and hybrid magnonic devices based on van der Waals heterostructures. Dark magnons refer to magnons that are decoupled from electromagnetic fields, making them robust against external perturbations. This feature also makes dark magnons challenging to detect, but here, Yu, Wang, Hu, He, and coauthors succeed in directly observing the dark magnon mode in the van der Waals antiferromagnet CrSBr, by interfacing a thin flake of CrSBr with ferrimagnetic YIG.