  While both La$_{0.7}$Sr$_{0.3}$MnO$_3$ and La$_{0.7}$Sr$_{0.3}$CoO$_3$ are
ferromagnets with metallic conductivity below $T_\mathrm{c}$ (360K and 220K,
respectively), partial substitutions of Co by Mn in
La$_{0.7}$Sr$_{0.3}$Co$_{1-y}$Mn$_y$O$_3$ ($y < 0.1$) drastically suppress the
ferromagnetic long-range order pre-established by Co-O-Co double exchange and
tune the conductivity towards insulating behavior. Since Mn-O-Mn
double-exchange interactions are avoidable at low Mn-substitution levels, the
deterioration of ferromagnetism and conductivity thus provides evidence for no
Mn-O-Co double exchange (but antiferromagnetic superexchange) in the present
system. At $y = 0.1$, the ferromagnetism is no longer observed; the system
becomes an insulating spin-glass with $T_\mathrm{g}\approx 62$K as estimated
from the ac-susceptibility data using the conventional critical slowing-down
scaling law. With further substitution ($y\geq 0.3$), the ferromagnetism is
recovered ($T_\mathrm{c} = 165$K for $y = 0.3$ and 200K for $y = 0.5$) while
the resistivity continues increasing and exhibits insulating behavior. These
results indicate that the substitution is not simply a mixture of
La$_{0.7}$Sr$_{0.3}$MnO$_3$ and La$_{0.7}$Sr$_{0.3}$CoO$_3$, but produces a
more complicated scenario of spin states, interactions and disorder.
