Microwave Spectroscopy of the Low-Temperature Skyrmion State in Cu$_{2}$OSeO$_{3}$

In the cubic chiral magnet Cu$_{2}$OSeO$_{3}$ a low-temperature skyrmion state (LTS) and a concomitant tilted conical state are observed for magnetic fields parallel to <100>. In this work, we report on the dynamic resonances of these novel magnetic states. After promoting the nucleation of the LTS by means of field cycling, we apply broadband microwave spectroscopy in two experimental geometries that provide either predominantly in-plane or out-of-plane excitation. By comparing the results to linear spin-wave theory, we clearly identify resonant modes associated with the tilted conical state, the gyrational and breathing modes associated with the LTS, as well as the hybridization of the breathing mode with a dark octupole gyration mode mediated by the magnetocrystalline anisotropies. Most intriguingly, our findings suggest that under decreasing fields the hexagonal skyrmion lattice becomes unstable with respect to an oblique deformation, reflected in the formation of elongated skyrmions.