Hybridized magnon modes in the quenched skyrmion crystal

Magnetic skyrmions have attracted attention as particlelike swirling spin textures with nontrivial topology, and their self-assembled periodic order i.e., the skyrmion crystal (SkX) is anticipated to host unique magnonic properties. In this paper, we investigate magnetic resonance in the quenched SkX state, which is obtained by the rapid cooling of the high-temperature equilibrium SkX phase in the chiral magnetic insulator ${\mathrm{Cu}}_{2}\mathrm{O}\mathrm{Se}{\mathrm{O}}_{3}$. At low temperatures, sextupole and octupole excitation modes of skyrmions are identified, which are usually inactive for oscillating magnetic fields ${B}^{\ensuremath{\nu}}$ with GHz-range frequency $\ensuremath{\nu}$ but turn out to be detectable through the hybridization with the ${B}^{\ensuremath{\nu}}$-active counterclockwise and breathing modes, respectively. The observed magnetic excitation spectra are well reproduced by theoretical calculations, which demonstrates that the effective magnetic anisotropy enhanced at low temperatures is the key for the observed hybridization between the ${B}^{\ensuremath{\nu}}$-active and ${B}^{\ensuremath{\nu}}$-inactive modes.