  A large magnetoresistance (MR) effect was observed in the layered compounds
NdCoAsO and SmCoAsO, in which ferromagnetically ordered itinerant-electrons of
Co are sandwiched by localized 4$f$-electrons of $Ln^{3+}$, below
ferromagnetic-antiferromagnetic transition (FAFT) temperature $T_\mathrm{N}$ as
observed in other FAFT compounds. In SmCoAsO, the large MR effect is also
observed up to the Curie temperature $T_\mathrm{C}$, and it is found to be
originating in the presence of another antiferromagnetic phase in the
low-magnetic field region of the ferromagnetic phase.
