  We systematically studied the magnetic and transport properties for the
polycrystalline samples of Fe-doped perovskite cobaltites:
Pr$_{1-y}$Ca$_{y}$Co$_{1-x}$Fe$_x$O$_3$ ($y$=0.3, $x$=0-0.15; $y$=0.45,
$x$=0-0.3) and Gd$_{0.55}$Sr$_{0.45}$Co$_{1-x}$Fe$_x$O$_{3}$ ($x$=0-0.3). Fe
doping leads to an enhancement of the ferromagnetism in the systems of
Pr$_{1-y}$Ca$_{y}$Co$_{1-x}$Fe$_x$O$_3$, while the ferromagnetism is suppressed
with further increasing Fe content and spin-glass behavior is observed at high
doping level of Fe. In contrast, the ferromagnetism is suppressed in the system
Gd$_{0.55}$Sr$_{0.45}$Co$_{1-x}$Fe$_x$O$_{3}$ as long as Fe is doped, and no
spin-glass behavior is observed in the sample with Fe doping up to 0.3. The
competition between ferromagnetic interactions through
Fe$^{3+}$-O-(LS)Co$^{4+}$ and antiferromagnetic interactions through
Fe$^{3+}$-O-Fe$^{3+}$ and Fe$^{3+}$-O-(IS)Co$^{3+}$ is considered to be
responsible for the behavior observed above. The average radius of the ions on
A sites plays the key role in determining what type of interactions Fe doping
mainly introduces.
