  We have investigated the $Ln^{3+}$-substitution ($Ln$ = lanthanoid) effect of
a quasi two-dimensional ferromagnet Sr$_3$Fe$_{2-x}$Co$_x$O$_{7-\delta}$ ($x$$
= $0.5). %$Ln^{3+}$-substitution creates antiferromagnetic insulating clusters
around $Ln^{3+}$. With increasing $Ln^{3+}$-concentration, the ferromagnetism
is gradually suppressed and the resistivity is increasing, which are ascribed
to an increase in antiferromagnetic (AFM) clusters created by
$Ln^{3+}$-substitution. In
Sr$_{2.7}$Gd$_{0.3}$Fe$_{1.5}$Co$_{0.5}$O$_{7-\delta}$, the magnetoresistance
(MR) is enhanced by about 20 % compared with that of
Sr$_3$Fe$_{1.5}$Co$_{0.5}$O$_{7-\delta}$. Coexistence of ferromagnetic (FM) and
AFM phases is essential for the enhancement of the<EMAIL_ADDRESS>Applied magnetic
fields align the FM clusters in the same direction, resulting in a reduction in
the resistivity. A metamagnetic transition observed in the $Ln^{3+}$-doped
samples also contributes to the enhancement of the MR.
