We report the results from a study of lanthanum doping in the Sr 2 FeMoO 6 half-metallic double perovskites. The thermoelectric power systematically increases with electron doping, and the temperature dependence is remarkably similar to that reported in the superconducting cuprates. This suggests a common mechanism and we interpret the data in terms of a model applied to the superconducting cuprates. There is a systematic decrease in the saturation magnetization with La doping, which is predominately due to an increase in the Fe-Mo antisite disorder. Even though there is a large decrease in the saturation magnetization, surprisingly we find that the low-temperature magnetization can be described within the three-dimensional Heisenberg model. The saturation magnetization is shown to extrapolate to zero for ϳ44% antisite disorder, which is close to the maximum of 50% ͑i.e., completely random Mo-Fe site occupancy͒.

We report the results from magnetic and transport measurements on SrFeO x where we focus on oxygen contents about the Sr 4 Fe 4 O 11 phase ͑x = 2.75͒ that antiferromagnetically orders at ϳ230 K. We find an exchange-bias-like effect in Sr 4 Fe 4 O 11 that commences near the Fe͑2͒ moment Néel temperature ͑ϳ230 K͒. While there is a weak spin-glass component, possibly arising from spin-frustrated moments on the Fe͑1͒ sites, the spin-glass temperature is significantly less than 230 K, and hence the exchange-bias-like effect is unlikely to be due to a spin glass. The exchange-bias-like effect clearly arises from an exchange interaction term that includes the Fe͑2͒ spins but its exact nature is not clear and awaits a detailed theoretical study. Roomtemperature thermopower measurements show a large change in sign for oxygen contents on either side of the Sr 4 Fe 4 O 11 phase, which we show can be qualitatively interpreted by assuming that the Fermi level is within a narrow band. High-field resistivity measurements show a large magnetoresistance that commences near the Sr 4 Fe 4 O 11 Néel temperature.

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