Higher-order modulations in the skyrmion lattice phase of Cu2OSeO3

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 skyrmionlattice phase, we observed two sets of symmetrically inequivalent peaks at the higher-order-reflection positions with the indices (1 1 0) and (2 0 0). 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 magneticfield 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 (1 1 0) increases as the field is increased, whereas the latter (2 0 0) 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 (1 1 0) to (2 0 0) 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 triangularlatticed skyrmions.