Stability and metastability of skyrmions in thin lamellae ofCu2OSeO3

Abstract

We report small angle X-ray scattering (SAXS) measurements of the skyrmion lattice in two 200 nm thick Cu2OSeO3 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. arXiv:1909.07855v1 [cond-mat.str-el]

Real-Space Experimental Observation of Magnetic Surface Spirals in FeGe

“…Figure 4(a) shows the field dependence of the periodicity of both states. The solid red (purple) lines show the well documented theoretical periods for the helicoid (conical) state [33,34], while the blue line shows the theoretical surface spiral period from Rybakov et al [25]. This data shows that the field dependence of the two states is significantly different, and that the longer wavelength state matches the predictions of Rybakov et al for the surface spiral.…”

“…1(e,h) mark the observed q-value of the zero-field helical state. Magnetic helices are known to evolve with increasing magnetic field, when pinning due to uniaxial or shape anisotropies resists transition to a conical or field- polarized state [33,34], forming a distorted helicoid with an increased period. In out-of-plane magnetic fields we observe a longer period modulated state, however it does not match the expected distortion of helices in an applied field, and instead shows significant similarities to the surface spiral state predicted by Rybakov et al [25].…”

Megahertz dynamics in skyrmion systems probed with muon-spin relaxation

“…As variation in the amplitude of the fluctuating field /γ μ is likely to follow the magnetization, the variation in λ likely H (mT) (1) Above T c , the relaxation rate λ is well described by power-law behavior [39] typical of critical fluctuations in a three-dimensional (3D) Heisenberg magnet [40][41][42][43] with a fluctuation time 1/ν ∝ |T − T c | −w with w = 0.709, typical for a 3D Heisenberg magnet. Below T c the same critical parameters do not account for λ, which should show a sharp rise very close to T c [ Fig.…”

Spontaneous creation and annihilation dynamics of magnetic skyrmions at elevated temperature

“…The magnetic skyrmion is a topological nontrivial magnetization configuration, shown to be found in magnetic materials which exhibit antisymmetric exchange, also known as the Dzyaloshinskii-Moriya interaction (DMI) [1][2][3][4][5]. The DMI relies on spin-orbit coupling, which usually is provided by the heavy metal layer in the system [6].…”