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.
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