Cavity-magnon polaritons are hybrid excitations from the interaction between cavity photons and magnons, the quanta of collective spin oscillations. Along with the tunability of the magnon-photon coupling strength, fast information transfer and conversion speed are desired in hybrid devices. This can be achieved utilizing the propagating nature of spin waves with nonzero momentum for their ultrafast time dynamics and reduced ohmic dissipation. Antiferromagnets are particularly interesting as hosts for magnons since stray-field interactions are minimized and they support multiple modes with distinctive magnetic-field behavior across the phase diagram. Chromium trichloride (${\mathrm{CrCl}}_3$) is a van der Waals layered antiferromagnet having a strong easy-plane anisotropy and a weak in-plane easy-axis anisotropy. Despite some magnetic resonance studies, the impact of magnetic reorientation of spins in ${\mathrm{CrCl}}_3$ on the cavity-magnon-polariton interaction strength as a function of magnetic field remains largely unexplored. In this study, we investigate the coupling between magnons in ${\mathrm{CrCl}}_3$ and photons in a coplanar waveguide resonator as a function of magnetic field. In particular, we find that the magnon-photon coupling strength varies nonmonotonically and distinctly with the magnetic field for the acoustic and the optical magnons, which can be utilized to tune the magnon-photon coupling strength using an external magnetic field as a knob. We find the signature of spin-flop transition in the two harmonics of the cavity due to a stronger dispersive coupling between optical magnons and cavity photons at lower fields. Additionally, we find standing modes formed by spin waves with nonzero momentum associated with the two hybrid magnons when the external field is applied at an angle with the crystal plane. These modes do not undergo substantial coupling with the cavity mode unlike the antiferromagnetic modes and can be used as low-loss propagation channels in hybrid devices.

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• Magnetic anisotropy
• Magnetization dynamics
• Magnons
• Polaritons
• Spin dynamics
• Superconducting RF