Formation of square skyrmion lattice in a centrosymmetric magnet without geometrical frustration

Magnetic skyrmion, a topologically stable swirling spin texture, has attracted attention as a particle-like object potentially suitable for high-density information bit. The previous observation of skyrmions has mostly focused on noncentrosymmetric systems with Dzyaloshinskii-Moriya interaction, while the further search of novel mechanisms to realize smaller skyrmion is highly demanded. Here, we report the observation of square skyrmion lattice for a centrosymmetric non-frustrated tetragonal magnet GdRu₂Si₂ by performing resonant x-ray scattering and Lorentz TEM experiments. The possible origin of observed skyrmion formation has been discussed in terms of four-spin interaction mediated by itinerant electrons. The present results demonstrate that skyrmions can be stabilized even without geometrical frustration and inversion symmetry breaking, and suggest that rare-earth intermetallics with highly-symmetric crystal lattice may be ubiquitously platforms to explore nanometric skyrmions of exotic origins.