Intrinsic inhomogeneities of low-doped lanthanum manganites in the paramagnetic temperature range

The nature of the electrical resistivity for low-doped lanthanum manganites is elucidated. The electrical resistivity is described by the Efros-Shklovskii law (lnρ √ (T0/T)−1/2, where T0 √ 1/Rls) in the temperature range from T* ≈ 300 K ≈ TC (TC is the Curie temperature for conducting manganites) to their TC and is explained by the tunneling of carriers between localized states. The magnetoresistance is explained by a change in the size of localized states Rls in a magnetic field. The patterns of change in Rls with temperature and magnetic field strength determined from magnetotransport properties are satisfactorily described in the model of phase separation into small-radius metallic droplets in a paramagnetic matrix. The sizes Rls and their temperature dependence have been estimated through magnetic measurements. The results confirm the existence of a Griffith phase. The intrinsic inhomogeneities produced by thermodynamic phase separation determine the electrical resistivity and magnetoresistance of lanthanum manganites.