  We report the effect of Al substitution on the Mn site of the bilayered half
doped manganite La{_1}Sr{_2}Mn{_2}O{_7}. This substitution dilutes the
magnetically active Mn-O-Mn network without introducing an appreciable
distortion in the lattice and ionic considerations leads to a predominant
reduction of Mn{^{4+}} with increasing Al. The rate of fall of the long range
antiferromagnetic transition temperature as a function of substitution is seen
to match well with established quasi 2D Heisenberg systems indicating that the
nature of magnetic interactions in this quasi 2-D system is of the short range
Heisenberg type. The magnetic contribution of the specific heat estimated using
Fisher's relation could be fitted with a function incorporating the presence of
a gapped Fermi surface appropriate for this type of systems. The resistivity
was seen to increase as a function of substitution due to the weakening of the
double exchange within the ferromagnetic layers of these A type of
antiferromagnets and justifiably, in the paramagnetic region the data could be
fitted to Mott's equation for the variable range hopping of polarons in 2
dimensions.
