  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 whenever 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, and it is generic for a wide range of
electron filling fractions whenever the two ordering wave vectors connect
independent sections of the Fermi surface.
