Zoology of multiple-Q spin textures in a centrosymmetric tetragonal magnet with itinerant electrons

Magnetic skyrmion is a topologically stable particle-like swirling spin texture potentially suitable for high-density information bit, which was first observed in noncentrosymmetric magnets with Dzyaloshinskii-Moriya interaction. Recently, nanometric skyrmion has also been discovered in centrosymmetric rare-earth compounds, and the identification of their skyrmion formation mechanism and further search of nontrivial spin textures are highly demanded. Here, we have exhaustively studied magnetic structures in a prototypical skyrmion-hosting centrosymmetric tetragonal magnet GdRu2Si2, by performing the resonant X-ray scattering experiments. We identified a rich variety of double-Q magnetic structures, including the antiferroic order of meron(half-skyrmion)/anti-meronlike textures with fractional local topological charges. The observed intricate magnetic phase diagram has been successfully reproduced by the theoretical framework considering the four-spin interaction mediated by itinerant electrons and magnetic anisotropy. The present results will contribute to the better understanding of the novel skyrmion formation mechanism in this centrosymmetric rare-earth compound, and suggest that itinerant electrons can ubiquitously host a variety of unique multiple-Q spin orders in a simple crystal lattice system.