In the weak ferromagnetic ${\mathrm{MnCO}}_3$ system, a low-frequency collective spin excitation (magnon) is the hybridized oscillation of nuclear and electron spins coupled through the hyperfine interaction. By using a split-ring resonator, we performed transmission spectroscopy measurements of the ${\mathrm{MnCO}}_3$ system and observed avoided crossing between the hybridized nuclear magnon mode and the resonator mode in the NMR-frequency range. The splitting strength is quite large due to the large spin density of ${}^{55}\mathrm{Mn}$, and the cooperativity value $C=0.2$ (the magnon-photon coupling parameter) is close to the conditions of strong coupling. The results reveal a new class of spin systems, in which the coupling between nuclear spins and photons is mediated by electron spins via the hyperfine interaction.

• Received 25 December 2014

DOI:https://doi.org/10.1103/PhysRevLett.114.226402