  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.
