  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.
