Enhanced Magnon-Photon Coupling at the Angular Momentum Compensation Point of Ferrimagnets.

We theoretically show that the coupling between magnons in an antiferromagnetically coupled ferrimagnet and microwave photons in a cavity is largely enhanced at the angular momentum compensation point (T_{A}) when T_{A} is distinct from the magnetization compensation point. The origin of the enhanced magnon-photon coupling at T_{A} is identified as the antiferromagnetic spin dynamics combined with a finite magnetization. Moreover, we show that strong magnon-photon coupling can be achieved at high excitation frequency in a ferrimagnet, which is challenging to achieve for a ferromagnet due to low magnon frequency and for an antiferromagnet due to weak magnon-photon coupling. Our results will invigorate research on magnon-photon coupling by proposing ferrimagnets as a versatile platform that offers advantages of both ferromagnets and antiferromagnets.