Importance of double-exchange interaction in low-doped ferromagnetic insulator

Transport and magnetic properties of ferromagnetic (FM) insulating La0.875Ca0.125MnO3 (LCMO) manganite have been reported. Temperature variation of field cooled (FC) magnetization shows paramagnetic to ferromagnetic (PM–FM) transition at 168 K ( T C ) while ac susceptibility exhibits an additional cusp like downfall around 115 K ( T f ), which shifts with frequency indicating a frustrated magnetic state. Resistivity shows insulating behaviour with a distinct inflection near T C . The Shklovskii–Efros (S–E) type variable range hopping mechanism is seen to govern the conduction within the temperature range 156–282 K. Thermoelectric power (TEP) exhibits a peak at ∼ T C and the field dependant resistivity shows different curvatures above and below T C . Such behaviours suggest that the PM–FM transition is mediated by double exchange (DE) mechanism similar to that observed in FM metallic manganites. Unexpectedly, magnetoresistance data analysis shows that DE mechanism persists in this sample even down to 50 K. Thus the DE mechanism plays an important role even in case of ferromagnetic insulating manganite La0.875Ca0.125MnO3, governing the magnetic and electrical properties.