Inhomogeneous ferromagnetic insulating state and isotope effect in Pr1−x CaxMnO3

The magnetic and transport properties of the Pr1−xCaxMnO3 ceramic samples were studied with the main emphasis on the ferromagnetic insulating (FI) state. A non-monotonic dependence of the Curie temperature TC on x with the maximum corresponding to x=0.25 was observed. A pronounced effect of 16O→18O isotope substitution on TC was found. The samples exhibited a noticeable drop in the activation energy for resistivity at TC and negative magnetoresistance. A special attention was paid to possible manifestations of inhomogeneity in the FI state.