  The presence of macroscopic phase separation between the superconducting and
magnetic phases in \cfcaf is demonstrated by muon spin rotation (muSR)
measurements conducted across their phase boundaries (x=0.05-0.15). The
magnetic phase tends to retain the high transition temperature (T_m > T_c),
while Co-doping induces strong randomness. The volumetric fraction of
superconducting phase is nearly proportional to the Co content $x$ with
constant superfluid density. These observations suggest the formation of
superconducting "islands" (or domains) associated with Co ions in the
Fe$_2$As$_2$ layers, indicating a very short coherence length.
