Megahertz dynamics in skyrmion systems probed with muon-spin relaxation

TL;DR

This study uses muon-spin relaxation to investigate the dynamic behavior of skyrmion lattices in specific materials, revealing megahertz to gigahertz excitations and their temperature dependence, thus providing insights into skyrmion dynamics and phases.

Contribution

It demonstrates the use of LF μSR to probe skyrmion dynamics at MHz frequencies and explores the metastability and bulk behavior of skyrmion phases in selected materials.

Findings

SkL phase in Cu2OSeO3 shows emergent MHz dynamics.

Metastable SkL is not the dominant phase in Cu2OSeO3.

Co8Zn9Mn3 exhibits GHz excitations that decrease near the critical temperature.

Abstract

We present longitudinal-field muon-spin relaxation (LF $\mu$SR) measurements on two systems that stabilize a skyrmion lattice (SkL): Cu$_2$OSeO$_3$, and Co$_x$Zn$_y$Mn$_{20-x-y}$ for $(x,y)~=~(10,10)$, $(8,9)$ and $(8,8)$. We find that the SkL phase of Cu$_2$OSeO$_3$ 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 Co$_8$Zn$_9$Mn$_3$ are well described by $\simeq~2$ GHz excitations that reduce in frequency near the critical temperature, while in Co$_8$Zn$_8$Mn$_4$ we observe similar behavior over a wide range of temperatures, implying that dynamics of this kind persist beyond the SkL phase.