Multi-step topological phase transitions among meron/antimeron and skyrmion crystals in a centrosymmetric magnet*

Topological swirling spin textures, such as skyrmions and merons, have attracted much attention as a candidate for potential high-density information bits.

Previously, skyrmions were reported in noncentrosymmetric compounds with Dzyaloshinskii-Moriya interaction. On the other hand, nanometric skyrmions have recently been reported in centrosymmetric compounds with alternative formation mechanisms. So far, most of the reported materials host only a lattice state of circular skyrmions in equilibrium, while other types of solitonic spin textures have rarely been reported. 

In this research, we exhaustively studied magnetic structures in a centrosymmetric magnet GdRu₂Ge₂ by neutron and resonant X-ray scattering experiments. We identified the multi-step topological transitions among elliptic skyrmion, meron/anti-meron pair, and circular skyrmion crystals, where the diameter of each quasi-particles is as small as 2.7 nm. Our detailed theoretical analysis successfully revealed that such an intricate manner of topological transitions is well reproduced by considering the competition of RKKY interactions at different wave vectors. The present result will pave a new way for developing a centrosymmetric simple lattice system with itinerant electrons as a unique platform to realize versatile nanometric topological textures.