  From a Monte Carlo study of the ferromagnetic Kondo lattice model for doped
manganites, including the antiferromagnetic superexchange interaction
($J_{AF}$), we found that the ferromagnetic ordering was suppressed as $J_{AF}$
increased. The ferromagnetic transition temperature $T_c$, as obtained from a
mean field fit to the calculated susceptibilities, was found to decrease
monotonically with increasing $J_{AF}$. Further, the suppression in $T_c$
scales with the bandwidth narrowing induced by the antiferromagnetic
frustration originating from $J_{AF}$. From these results, we propose that the
change in the superexchange interaction strength between the $t_{2g}$ electrons
of the Mn ions is one of the mechanisms responsible for the suppression in
$T_c$ observed in manganites of the type
(La$_{0.7-y}$Pr$_{y}$)Ca$_{0.3}$MnO$_3$.
