Megahertz dynamics in skyrmion systems probed with muon-spin relaxation

Abstract

We present longitudinal-field muon-spin relaxation (LF μSR) 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 μSR 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 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.

Spin order and fluctuations in the EuAl4 and EuGa4 topological antiferromagnets: A μSR study

“…In the EuAl 4 and EuGa 4 case, where there is no skyrmion phase in zero field, the relaxation rates diverge at T N , followed by a significant drop at T < T N due to the slowing down of spin fluctuations, a typical feature of magnetically ordered materials. A similar behavior is observed in Co 10 Zn 10 [40], a parent compound of the Co-Mn-Zn alloys, which lacks any skyrmion phases. According to Hall-resistivity measurements, the skyrmion phase may exist in a field range ∼1-2.5 T in EuAl 4 and ∼4-7 T in EuGa 4 [6,7].…”

“…Since most of the skyrmion phases appear in a field range not easily accessible by standard μSR instruments, up to now, only a handful of results have been reported where LF-μSR could be used to study skyrmion-hosting compounds. These include GaV 4 (S,Se) 8 [39], Cu 2 OSeO 3 [40], and the Co-Zn-Mn alloy [40,41], whose skyrmion phases are stabilized by a relatively small field (< 0.1 T). However, for many newly discovered skyrmion systems, i.e., GdRu 2 Si 2 and Gd 3 Ru 4 Al 12 (as well as for EuAl 4 and EuGa 4 studied here) [6,7,29,30], the critical field required for stabilizing the skyrmion phase is above 1 T. In their AFM state, EuAl 4 and EuGa 4 exhibit comparable spin fluctuations to other well-studied skyrmion compounds.…”

Coupling microwave photons to topological spin textures in Cu2OSeO3

“…Hence, we see a decrease in the coupling strength between photons and skyrmion resonance. This is further supported by the temperature dependence of the intensity of small-angle neutron scattering (SANS) and the relaxation rate of muon-spin relaxation measurements in Cu 2 OSeO 3 [32][33][34], where the intensity and relaxation rate, respectively, have a maximum around 57 K and a drop was observed at different temperatures in the skyrmion phase.…”

Spin dynamics in bulk MnNiGa and Mn1.4Pt0.9Pd0.1Sn investigated by muon spin relaxation

“…It is plausible that near the highertemperature transition we are probing dynamics arising from characteristic excitations of the antiskyrmion state at this lower field. However, this would require that the antiskyrmion state is characterized by a lower relaxation rate than the neighbouring magnetic states, in contrast with published data on Bloch and Neel skyrmions, where an increase in this quantity is seen [22][23][24][25]. and 100 K for both fields, which roughly correspond with kinks in the baseline asymmetry, and a rise in the relaxation rate at the lowest measured temperatures which is correlated with a further drop in the relaxing asymmetry.…”

“…2(b)], which shows a sharp peak 15 K lower in temperature than the peak in the µSR relaxation for the 0.01 T dataset. AC susceptibility probes a substantially lower frequency range than µSR, and therefore we might expect the influence of fluctuations with a single characteristic , where ν0 is the zero-temperature frequency, ζ = 0.7048 is the correlationlength critical exponent, and z = 2.035 is the dynamical critical exponent [25]. (b) Relaxation rate (x-axis) as a function of frequency (y-axis) following the Redfield formula, for the case of constant ∆, for two different values of the applied field (ω0 = γµBL).…”