Insulator-metal like transition in air-synthesized Mn⁴⁺-rich La1-xBaxMnO₃: grain boundary phase effect

The air-synthesized La 1- x Ba x MnO 3 system was revisited for 0.4⩽ x ⩽0.9. We show that this system exhibits an insulator-metal like transition, T IM (175-230 K), far below the Curie temperature ( T C ≈335 K). In contrast to previous studies, we demonstrate that this system is multiphase, i.e. involving a hexagonal phase and a ferromagnetic cubic perovskite. We suggest that the T IM transition at low temperature is due to the presence of a secondary perovskite at the grain boundary. In less oxidizing conditions, the synthesis of cubic perovskites for x ≈0.50, with T IM ranging from 170 to 240 K, supports this interpretation. Tunnelling magnetoresistance up to 22% at 10 K under 0.2 T is also observed.