Helicity-Dependent Ellipticity and Interstitial Vortices in Confined Anisotropic Skyrmions

Skyrmions carry robust topology, yet their geometry is highly tunable and can shape interactions, dynamics and functionality. However, how geometric confinement couples to helicity and reshapes the effective energy landscape in intrinsically anisotropic skyrmions remains unexplored. Here we demonstrate that skyrmion ellipticity and its magnetic-field evolution are strongly helicity dependent in 500-nm-wide FeNiPdP nanostripes with anisotropic Dzyaloshinskii-Moriya interaction. Confinement compresses clockwise (CW)-helicity skyrmions toward near-circular profiles, whereas counterclockwise (CCW)-helicity skyrmions elongate along the nanostripe axis, breaking the reciprocal ellipticity relation expected in unconfined geometries. Increasing the perpendicular field further reveals a monotonic circularization of CW skyrmions but a clear nonmonotonic response for CCW skyrmions. Energy analysis shows that confinement reshapes the field-dependent energy landscape and shifts the optimal skyrmion geometry, giving rise to this nonlinear response. The nanostripe constraint also introduces a surface-localized interstitial vortex texture in anisotropic skyrmions that can evolve into a vortex pair, consistent with our micromagnetic simulations. Our results establish helicity-resolved confinement as a route to engineer anisotropic skyrmion morphology and emergent surface textures, enabling helicity-programmable shape and interaction control for multistate skyrmion-based devices.