Field-induced transformation between triangular and square skyrmion crystals in a tetragonal polar magnet

Magnetic skyrmions with topologically nontrivial spin textures form a variety of periodic structures depending on microscopic interactions and lattice symmetry. We theoretically investigate a transformation between triangular and square skyrmion crystals against an external magnetic field in a polar tetragonal magnet. By performing the simulated annealing for a classical spin model, we show that the competition of the Dzyaloshinskii-Moriya interaction at multiple wave vectors is a key ingredient in inducing the structural transition in terms of the skyrmion crystals. The present results indicate the importance of magnetic frustration in momentum space as the origin of exotic topological phase transitions.