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Unusual Exchange Bias Associated with Phase Separation in Perovskite Cobaltites

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.