We have measured the dc and ac electrical and magnetic properties in various magnetic fields of the recently reported superconducting ferromagnet RuSr 2 GdCu 2 O 8 . Our reversible magnetization measurements demonstrate the absence of a bulk Meissner state in the compound below the superconducting transition temperature. Several scenarios that might account for the absence of a bulk Meissner state, including the possible presence of a spongelike non-uniform superconducting or a crypto-superconducting structure in the chemically uniform Ru-1212, have been proposed and discussed.

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Highly conductive perovskite La0.5Sr0.5CoO3 thin films were grown on (012) LaAlO3 by pulsed-laser deposition at different substrate temperatures. Magnetoresistance (MR) measurements show only a negative effect for the films grown at a temperature of 800 °C. However, a positive MR effect was found for the films grown at lower temperatures of 400 and 500 °C. The sign of the magnetoresistance changed from positive to negative near 150 K under an applied magnetic field of 5.0 T, and the largest positive MR of 2.6% was obtained at a temperature of 4.2 K for the film grown at 500 °C. Low-temperature magnetic hysteresis of the films was also studied, and it is suggested that the scattering of the carrier at the domain boundary may lead to the positive-MR effect.

We have measured the oxygen isotope effect on the metal-insulator transition and the electrical properties of La 0.65 Ca 0.35 MnO 3 under hydrostatic pressure up to 1.7 GPa. The conductivity activation energy E A in the insulating phase is a linear function of T c and coincides for both isotopes 16 O and 18 O if compared at different pressures but for the same T c . The pressure and isotope effects can be qualitatively understood in terms of the double exchange model and the adiabatic hopping of Jahn-Teller polarons. Higher order corrections to the Holstein model in the intermediate coupling regime explain the unusually large isotope effect on the relative pressure coefficient of T c .

Magnetization, resistivity, thermal expansion, thermopower, and neutron powder diffraction data are presented for the perovskite manganite Pr 0.7 Ba 0.3 MnO 3 . A distinct ferromagnetic insulating state, similar to that observed in the La 0.88 Sr 0.12 MnO 3 system, is observed in the temperature range between the paramagneticferromagnetic transition at T FM Ϸ180 K and the broad metal-insulator transition near T p ϭ120 K. The large A-site cation-size mismatch appears to suppress the ferromagnetic double-exchange interactions relative to the competing ferromagnetic superexchange interactions, thereby giving rise to the ferromagnetic insulating state.