  A magnetic susceptibility which decreases with decreasing temperature is
observed in all $CuO_2$ based superconductors with less than optimal doping. We
propose that in $La_{2-x} Sr_x CuO_4$ this is due to antiferromagnetic ordering
which is prevented by the low spatial dimensionality while in $YBa_2 Cu_3
O_{6.6}$ it is due to the interplay between antiferromagnetic fluctuations
within a plane and singlet pairing of electrons between nearest neighbor
planes.
