Magnetotransport in granular LaMnO_3+δmanganite with nano-sized particles

Abstract

Transport and magnetic properties of compacted LaMnO3+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 TC ∼ 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.