  Motivated by the discovery of spiral and modulated collinear phases in
several magnetic materials, we investigate the magnetic properties of
Heisenberg spin S=1/2 antiferromagnets in 2 and 3 dimensions, with frustration
arising from 2nd-neighbor couplings in one axial direction (the ANNNH model).
Our results clearly demonstrate the presence of an incommensurate spiral phase
at T=0 in 2 dimensions, extending to finite temperatures in 3 dimensions. The
crossover between N\'eel and spiral order occurs at a value of the frustration
parameter considerably above the classical value 0.25, a sign of substantial
quantum fluctuations. We also investigate a possible modulated collinear phase
with a wavelength of 4 lattice spacings, and find that it has substantially
higher energy and hence is not realized in the model.
