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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 ͑La 1−x Sr x CoO 3 ͒ 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.

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We have found ferromagnetism in epitaxially grown superlattices of CaRuO(3)/CaMnO(3) that arises in one unit cell at the interface. Scanning transmission electron microscopy and electron energy loss spectroscopy indicate that the difference in magnitude of the Mn valence states between the center of the CaMnO(3) layer and the interface region is consistent with double exchange interaction among the Mn ions at the interface. Polarized neutron reflectivity and the CaMnO(3) thickness dependence of the exchange bias field together indicate that the interfacial ferromagnetism is only limited to one unit cell of CaMnO(3) at each interface. The interfacial moment alternates between the 1 μ(B)/interface Mn ion for even CaMnO(3) layers and the 0.5 μ(B)/interface Mn ion for odd CaMnO(3) layers. This modulation, combined with the exchange bias, suggests the presence of a modulating interlayer coupling between neighboring ferromagnetic interfaces via the antiferromagnetic CaMnO(3) layers.

X rays produced during electron-beam deposition of metallic electrodes drastically change the performance of organic spintronic devices. The x rays generate traps with an activation energy of ≈0.5  eV in a commonly used organic. These traps lead to a dramatic decrease in spin-diffusion length in organic spin valves. In organic magnetoresistive (OMAR) devices, however, the traps strongly enhance magnetoresistance. OMAR is an intrinsic magnetotransport phenomenon and does not rely on spin injection. We discuss our observations in the framework of currently existing theories.