Normal-Mode Splitting in the Coupled System of Hybridized Nuclear Magnons and Microwave Photons

In the weak ferromagnetic MnCO₃ 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 MnCO₃ 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 ⁵⁵Mn, 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.