Magnetization, resistivity, thermal expansion, thermopower, and neutron powder diffraction data are presented for the perovskite manganite ${\mathrm{Pr}}_{0.7}{\mathrm{Ba}}_{0.3}{\mathrm{MnO}}_3.$ A distinct ferromagnetic insulating state, similar to that observed in the ${\mathrm{La}}_{0.88}{\mathrm{Sr}}_{0.12}{\mathrm{MnO}}_3$ system, is observed in the temperature range between the paramagnetic-ferromagnetic transition at $T_{\mathrm{FM}}\approx 180\hspace{0.5em}\mathrm{K}$ and the broad metal-insulator transition near $T_p=120\mathrm{}\hspace{0.5em}\mathrm{K}.$ 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.

• Received 5 November 2001

DOI:https://doi.org/10.1103/PhysRevB.65.214423