Higher-order modulations in the skyrmion lattice phase of Cu2 OSeO3

Using small-angle neutron scattering, we have investigated higher-order peaks in the skyrmion-lattice phase of Cu2OSeO3 in which two different skyrmion lattices, SkX1 and SkX2, are known to form. For each skyrmion-lattice phase, we observed two sets of symmetrically inequivalent peaks at the higher-order-reflection positions with the indices (110) and (200). Under the condition where the SkX1 and SkX2 coexist, we confirmed the absence of the scattering at Q positions combining reflections from the two phases, indicating a significantly weak double-scattering component. Detailed analysis of the peak profile as well as the temperature and magnetic-field dependence of the peak intensity also supports the intrinsic higher-order modulation rather than the parasitic double scattering. The two higher-order modulations show contrasting magnetic-field dependence; the former (110) increases as the field is increased, whereas the latter (200) decreases. This indicates that, in Cu2OSeO3, skyrmions are weakly distorted, and the distortion is field dependent in a way that the dominant higher-order modulation switches from (110) to (200) under the field. Monte Carlo simulations under sweeping external magnetic field qualitatively reproduce the observed magnetic-field dependence and suggests that the higher-order modulations correspond to the superlattices of weak swirlings appearing in the middle of the original triangular-latticed skyrmions.