Muon-spin spectroscopy studies of materials hosting topological magnetic states

Abstract

This thesis applies muon-spin spectroscopy (muSR) to materials hosting topological magnetic states. Each study focuses on a different material family that hosts either a skyrmion lattice (SkL), or chiral soliton lattice (CSL) state. We study the static and/or dynamic magnetism of each material, distinguishing the behaviour of the topological magnetic states from those of the materials.

Longitudinal field (LF) muSR measurements of Cu2OSeO3, CoxZnyMn20-x-y, and GaV4S8-ySey reveal that the SkL phase exhibits emergent dynamic behaviour at megahertz frequencies. These dynamics in CoxZnyMn20-x-y and GaV4S8-ySey are well described by gigahertz excitations that reduce in frequency near the critical temperature. We find that as crystallographic site disorder increases, so too does the complexity of the dynamic response on the muon timescale, suggesting that site disorder plays a role in stabilising complex excitations. The temperature-evolution of the magnetic ground state of GaV4S8-ySey is revealed through zero-field muSR measurements, whilst LF measurements show that the SkL exists under low-levels of substitution at both ends of the series. muSR measurements of polycrystalline Zn-substituted Cu2OSeO3 show that the reported splitting of the SkL phase in the B-T phase diagrams of these materials occurs due to different levels of Zn-substitution in different grains of the sample.

Finally, in Cr1/3MS2 (M=Nb or Ta) the presence of a gap-like feature (width in range 40-100 meV) in the density of states is confirmed by magnetometry measurements. Dynamic spin
fluctuations driven by excitations across this gap are seen over a wide range of frequencies (0.1 Hz to MHz) with AC susceptibility and muSR measurements. Effects due to the CSL in Cr1/3NbS2, as detected with muSR, dominate over the gap-driven magnetism when the CSL is stabilised as the majority phase.