In the weak ferromagnetic MnCO$_3$ system, a low-frequency collective spinexcitation (magnon) is the hybridized oscillation of nuclear and electron spinscoupled through the hyperfine interaction. By using a split-ring resonator, weperformed transmission spectroscopy measurements of MnCO$_3$ system andobserved, for the first time, avoiding crossing between the hybridizednuclear-electron magnon mode and the resonator mode in the NMR-frequency range.The splitting strength is quite large due to the large spin density of$^{55}$Mn, and the cooperativity value $C=0.2$ (magnon-photon couplingparameter) is close to the conditions of strong coupling. The results reveal anew class of spin systems, in which the coupling between nuclear spins andphotons is mediated by electron spins via the hyperfine interaction, and inwhich the similar normal-mode splitting of the hybridized nuclear magnon modeand the resonator mode can be observed.
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