  We report the observation of unusual exchange bias phenomena in the doped
perovskite cobaltites La_{1-x}Sr_{x}CoO_{3} (x = 0.15, 0.18, and 0.30) in which
a spontaneous phase separation into ferromagnetic clusters embedded in a
non-ferromagnetic matrix occurs. When the La_{1-x}Sr_{x}CoO_{3} samples are
cooled in a static magnetic field through a freezing temperature, the
magnetization hysteresis loops exhibit exchange bias, i.e., the loops shift to
the negative field and the magnetization becomes asymmetric. Moreover, exchange
bias disappears when the measuring magnetic field is high enough. These results
suggest that the intrinsic phase inhomogeniety in a spontaneously
phase-separated system may induce an interfacial exchange anisotropy after a
field cooling. The diminution of exchange bias in high magnetic fields is
ascribed to the propagation of ferromagnetic regions with increasing magnetic
field.
