Vortex Crystals with Chiral Stripes in Itinerant Magnets

We study noncoplanar magnetic ordering in frustrated itinerant magnets. For a family of Kondo square lattice models with classical local moments, we find that a double-Q noncoplanar vortex crystal has lower energy than the single-Q helical order expected from the Ruderman-Kittel-Kasuya-Yosida interaction when the lattice symmetry dictates four global maxima in the bare magnetic susceptibility. By expanding in the small Kondo exchange and the degree of noncoplanarity, we demonstrate that this noncoplanar state arises from a Fermi surface instability occurring in independent sections connected by two ordering wave vectors.