  We consider s=1/2 spins in the presence of a constant magnetic field in
z-direction and an AC magnetic field in the x-z plane. A nonzero DC
magnetization component in y direction is a result of broken symmetries. A
pairwise interaction between two spins is shown to resonantly increase the
induced magnetization by one order of magnitude. We discuss the mechanism of
this enhancement, which is due to additional avoided crossings in the level
structure of the system.
