A Novel Surface-confined Spiral State With The Double Period In The Cubic Chiral Helimagnet Cu₂OSeO₃

The chiral magnetoelectric insulator Cu₂OSeO₃ hosts a rich and anisotropic magnetic phase diagram that includes helical, conical, field-polarized, tilted conical, and skyrmion lattice phases. Using resonant elastic x-ray scattering (REXS), we uncover a new spiral state confined to the surface of Cu₂OSeO₃. This surface-confined spiral state (SSS) displays a real-space pitch of ~120 nm, which remarkably is twice the length of the incommensurate structures observed to-date in Cu₂OSeO₃. The SSS phase emerges at temperatures below 30~K when the magnetic field is applied near the 110 crystallographic axis. Its surface localization is demonstrated through a combination of REXS in reflection and transmission geometries, with complementary small-angle neutron scattering measurements suggesting its absence from the bulk. We attribute the stabilization of the SSS to competing anisotropic interactions at the crystal surface. The discovery of a robust, surface-confined spiral paves the way for engineering energy-efficient, nanoscale spin-texture platforms for next-generation devices.