  It is shown that the coupling between magnetic layers separated by
non-magnetic metallic superlattices can decay exponentially as a function of
the spacer thickness $N$, as opposed to the usual $N^{-2}$ decay. This effect
is due to the lack of constructive contributions to the coupling from extended
states across the spacer. The exponential behavior is obtained by properly
choosing the distinct metals and the superlattice unit cell composition.
