Magnon polaritons in a van der Waals ferromagnet coupled to a superconducting resonator

TL;DR

This paper demonstrates magnon-photon hybridization in thin van der Waals ferromagnetic flakes, showing potential for integrating magnetic excitations with superconducting circuits at the monolayer scale.

Contribution

It reports the first observation of magnon polaritons in exfoliated van der Waals ferromagnets with ultra-thin layers, using a low-impedance superconducting resonator.

Findings

Reproducible avoided crossings in six devices confirm magnon-photon coupling.

Coupling strength scales with the square root of the magnetic layer thickness.

Extrapolation suggests hybridization is achievable in monolayer Cr$_2$Ge$_2$Te$_6$.

Abstract

Achieving magnon-photon hybridization in the microwave regime is essential for integrating magnetic excitations with superconducting circuits. While this has been extensively demonstrated in bulk magnetic systems, realizing it in two-dimensional van der Waals materials remains challenging due to their reduced magnetic volume and increased dissipation. Here, magnon-photon hybridization is observed in exfoliated flakes of the van der Waals ferromagnet Cr$_2$Ge$_2$Te$_6$, with thicknesses down to 30 nm. The resulting magnon polaritons-hybrid excitations of cavity photons and magnons-are evidenced by reproducible avoided crossings across six devices, enabled by a low-impedance superconducting resonator design. The coupling strength follows the expected square-root dependence on thickness, and extrapolation of this scaling indicates that hybridization in the monolayer limit is within reach.