  Using Tyablikov's decoupling approximation, we calculate the initial
suppression rate of the Neel temperature, $R_{IS}=-lim_{x-> 0} T^{-1}_{N}
dT_{N}/dx$, in a quasi two-dimensional diluted Heisenberg antiferromagnet with
nonmagnetic impurities of concentration $x$. In order to explain an
experimental fact that $R^{(Zn)}_{IS}=3.4$ of the Zn-substitution is different
from $R^{(Mg)}_{IS}=3.0$ of the Mg-substitution, we propose a model in which
impurity substitution reduces the intra-plane exchange couplings surrounding
impurities, as well as dilution of spin systems. The decrease of 12% in
exchange coupling constants by Zn substitution and decrease of 6% by Mg
substitution explain those two experimental results, when an appropriate value
of the interplane coupling is used.
