Stability and Metastability of Skyrmions in Thin Lamellae of Cu$_2$OSeO$_3$

TL;DR

This study uses small angle X-ray scattering to investigate skyrmion phases in thin Cu$_2$OSeO$_3$ lamellae, revealing an expanded equilibrium skyrmion region, anisotropic distortions, and stable metastable skyrmions unaffected by cooling rate.

Contribution

It demonstrates that thin lamellae of Cu$_2$OSeO$_3$ exhibit an expanded skyrmion phase and stable metastable skyrmions, highlighting the effects of reduced thickness and crystalline anisotropy.

Findings

Expanded skyrmion phase between 30-50 K in lamellae

Elliptical distortion of skyrmions in [110] sample

Stable metastable skyrmions unaffected by cooling rate

Abstract

We report small angle X-ray scattering (SAXS) measurements of the skyrmion lattice in two 200~nm thick Cu$_2$OSeO$_3$ lamellae aligned with the applied magnetic field parallel to the out of plane [110] or [100] crystallographic directions. Our measurements show that the equilibrium skyrmion phase in both samples is expanded significantly compared to bulk crystals, existing between approximately 30 and 50~K over a wide region of magnetic field. This skyrmion state is elliptically distorted at low fields for the [110] sample, and symmetric for the [100] sample, possibly due to crystalline anisotropy becoming more important at this sample thickness than it is in bulk samples. Furthermore, we find that a metastable skyrmion state can be observed at low temperature by field cooling through the equilibrium skyrmion pocket in both samples. In contrast to the behavior in bulk samples, the volume fraction of metastable skyrmions does not significantly depend on cooling rate. We show that a possible explanation for this is the change in the lowest temperature of the skyrmion state in this lamellae compared to bulk, without requiring different energetics of the skyrmion state.