Magnetotransport in granular LaMnO_3+δ manganite with nano-sized particles

Transport and magnetic properties of compacted LaMnO_3+δ manganite nanoparticles of an average size of 18 nm have been investigated in the temperature range 5-300 K. The nanoparticles exhibit a paramagnetic-to- ferromagnetic (FM) transition at the Curie temperature T_C ∼ 246 K. However, the spontaneous magnetization disappears at a higher temperature of about 270 K. It was found that at low temperatures the FM core occupies about 50% of the particle volume. The temperature dependence of the resistivity shows a metal-insulator transition and a low-temperature upturn below the resistivity minimum at T ∼ 50 K. The transport at low temperatures is controlled by the charging energy and spin-dependent tunnelling through grain boundaries. It has been found that the charging energy decreases monotonically with increasing magnetic field. The low temperature I-V characteristics are well described by an indirect tunnelling model while at higher temperatures both direct and resonant tunnelling dominates. The experimental features are discussed in the framework of a granular ferromagnet model.