Effect of the calcination temperature on the magnetic and transport properties of rhombohedral LaMnO3+δ compounds

Rhombohedral LaMnO3+δ powders were successfully synthesized by annealing gel precursors obtained by a polymerized complex method. The average grain size increases with increasing calcination temperature. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves indicate that LaMnO3+δ compounds obtained at 700 and 800 °C are ferromagnetic, while both ferromagnetic and antiferromagnetic components are present in those calcined at 900 and 1000 °C. This is because the concentration of Mn4+ ions drops with increasing calcination temperature. The temperature dependence of the resistivity of LaMnO3+δ compounds exhibits an insulator–metal transition at about 90 K and also a low-temperature upturn with a resistivity minimum at about 40 K. Upon increasing particle size, the zero-field resistivity of the LaMnO3+δ powders increases due to suppression of the conduction by the insulating antiferromagnetic component. The negative magnetoresistance of a sample pellet prepared at 700 °C ranges from −82% to −88% in the temperature range of 40–100 K due to suppression of the spin-dependent scattering by an applied field.