  Topological swirling spin textures, such as skyrmions and merons, have
recently attracted much attention as a unique building block for high-density
magnetic information devices. The controlled transformation among different
types of such quasi-particles is an important challenge, while it was
previously achieved only in a few non-centrosymmetric systems characterized by
Dzyaloshinskii-Moriya interaction. Here, we report an experimental discovery of
multi-step topological transitions among a variety of meron and skyrmion
crystal states in a centrosymmetric magnet GdRu$_2$Ge$_2$. By performing the
detailed magnetic structure analysis based on resonant X-ray and neutron
scattering experiments as well as electron transport measurements, we have
found that this compound hosts periodic lattice of elliptic skyrmions,
meron/anti-meron pairs, and circular skyrmions as a function of external
magnetic field. The diameter of these objects is as small as 2.7 nm, which is
almost two orders of magnitude smaller than typical non-centrosymmetric
magnets. Such an intricate manner of topological magnetic transitions are well
reproduced by a theoretical model considering the competition between RKKY
interactions at inequivalent wave vectors. The present findings demonstrate
that even a simple centrosymmetric magnet with competing interactions can be a
promising material platform to realize a richer variety of nanometric magnetic
quasi-particles with distinctive symmetry and topology, whose stability may be
tunable by various external stimuli.
