Conductivity in a disordered medium and carrier localization in weakly doped lanthanum manganites

The dc and 9.2-GHz electrical resistivities and magnetoresistance observed in La1−xAxMnO3 crystals (A=Sr, Ce, x≤0.1) in the temperature interval 77–300 K are accounted for by the contributions due to carriers, both nonlocalized and localized in the valence-band tail and near the Fermi level. The localized-state tail extends to a depth of 0.15–0.25 eV inside the band gap, and the hopping activation energy varies from 0.06 to 0.15 eV, depending on the sample composition. Within the temperature region where magnetic ordering sets in, the variations of the electrical resistivity and magnetoresistance with temperature and magnetic field are caused by variations in the carrier mobility and concentration.