  A skyrmion crystal (SkX) expressed as a multiple number of spiral modulations
manifests itself not only in its peculiar magnetic texture but also in
nontrivial transport properties originating from an emergent magnetic field. We
here report our numerical results for multiple SkXs in a centrosymmetric
tetragonal crystal system. By performing simulated annealing for an effective
spin model for itinerant magnets, we find that three types of the SkXs with the
skyrmion numbers of one and two, which are characterized by different
superpositions of helices, are stabilized in the ground state, and are
transformed by an external magnetic field. The essence of the emergent multiple
SkXs is lied in itinerant frustration where exchange interactions are competed
in momentum space due to the nature of itinerant electrons.
