  Magnetic skyrmions are topologically stable spin swirls with particle-like
character and potentially suitable for the design of high-density information
bits. While most known skyrmion systems arise in noncentrosymmetric systems
with Dzyaloshinskii-Moriya interaction, also centrosymmetric magnets with a
triangular lattice can give rise to skyrmion formation, with
geometrically-frustrated lattice being considered essential in this case. Until
today, it remains an open question if skyrmions can also exist in the absence
of both geometrically-frustrated lattice and inversion symmetry breaking. Here,
we discover a square skyrmion lattice state with 1.9 nm diameter skyrmions in
the centrosymmetric tetragonal magnet GdRu2Si2 without geometrically-frustrated
lattice by means of resonant X-ray scattering and Lorentz transmission electron
microscopy experiments. A plausible origin of the observed skyrmion formation
is four-spin interactions mediated by itinerant electrons in the presence of
easy-axis anisotropy. Our results suggest that rare-earth intermetallics with
highly-symmetric crystal lattices may ubiquitously host nanometric skyrmions of
exotic origins.
