Megahertz Dynamics In Skyrmion Systems Probed With Muon-Spin Relaxation

We present longitudinal-field muon-spin relaxation (LF mu SR) measurements on two systems that stabilize a skyrmion lattice (SkL): Cu2OSeO3, and CoxZnyMn20-x-y for (x, y) = (10, 10), (8, 9), and (8, 8). We find that the SkL phase of Cu2OSeO3 exhibits emergent dynamic behavior at megahertz frequencies, likely due to collective excitations, allowing the SkL to be identified from the mu SR response. From measurements following different cooling protocols and calculations of the muon stopping site, we suggest that the metastable SkL is not the majority phase throughout the bulk of this material at the fields and temperatures where it is often observed. The dynamics of bulk Co8Zn9Mn3 are well described by similar or equal to 2 GHz excitations that reduce in frequency near the critical temperature, while in Co8Zn8Mn4 we observe similar behavior over a wide range of temperatures, implying that dynamics of this kind persist beyond the SkL phase.