Zero damping effect of magnetic bilayer in microwave resonant cavity

Experimental and theoretical studies have shown that a single magnon mode and cavity photon can be coupled coherently and dissipatively, with the interference between two types of coupling creating zero damping effect. In magnetic bilayers or multilayers, there exists more than one magnon mode which can be directly coupled by interface exchange interaction. In this work, a single-magnon mode is extended to a two-magnon mode and the effect of the two-magnon mode on zero damping condition is investigated. Using eigenfrequency analysis and microwave transmission spectra, the analytical expressions of the zero damping condition and the frequency detuning can be derived. By comparing analytical results with numerical results, the dependence of zero damping condition on system parameters can be obtained. In the absence of direct interface exchange magnon-magnon coupling, the zero damping condition occurs for dissipative coupling or hybrid coupling. As the coupling strength increases, the distance between two zero damping points increases. For hybrid coupling, the two zero damping points turn no longer symmetric, which is different from the case of pure coupling. Moreover, the effect of interface exchange magnon-magnon interaction on zero damping condition is studied. The interface exchange coupling results in the splitting of microwave transmission spectra, but the zero damping condition occurs only in the low-frequency mode. As the interface exchange coupling strength increases, the frequency at which the zero damping condition happens will shift toward lower frequency. Due to extremely narrow line-width of microwave transmission dip under the zero damping condition, the result in this work is expected to be useful for designing the magnon-based quantum sensing devices.