Non-Griffiths-like clustered phase above the Curie temperature of the doped perovskite cobaltite La1-x Srx Co O3

The existence of preformed clusters above the Curie temperature of the doped perovskite manganites is well established and, in many cases, conforms to the expectations for a Griffiths phase. We show here that the canonical perovskite cobaltite (La1-x Srx Co O3) also exhibits a clustered state above the Curie point in the ferromagnetic phase. The formation of magnetic clusters at a well-defined temperature (T*) is revealed in the small-angle neutron scattering and dc susceptibility. Remarkably, the characteristics of this clustered state appear quite unlike those of a Griffiths phase; the deviation from Curie-Weiss behavior is opposite to expectations and is field independent, while T* does not correspond to the undiluted Curie temperature. These results demonstrate that, although the Griffiths model may apply to many systems with quenched disorder, it is not universally applicable to randomly doped transition metal oxides.