Mechanisms of hopping conductivity in weakly doped La1−xBaxMnO3

The resistivity, ?, of ceramic La1?xBaxMnO3 with x = 0.02?0.10 corresponding to the concentrations of holes c?0.15?0.17 displays an activated behaviour both above and below the paramagnetic to ferromagnetic transition temperature TC = 175?209?K, obtained from measurements of the magnetization. Above T~310?390?K ?(T,x) is determined by nearest-neighbour hopping of small polarons with activation energy Ea = 0.20?0.22?eV. Below the onset temperature Tv = 250?280?K, depending on x, a Shklovskii?Efros-like variable-range hopping conductivity mechanism, governed by a soft temperature independent Coulomb gap, ??0.44?0.46?eV, and a rigid gap, ?(T), is found. For the range T~50?120?K, ?(T) is connected to the formation of small lattice polarons in conditions of strong electron?phonon interaction and lattice disorder. The rigid gap obeys a law ?(T)~T1/2 within two temperature intervals above and below TC, exhibits an inflection at TC and reaches at Tv a value of ?v?0.14?0.18?eV. Such behaviour suggests a spin dependent contribution to ?(T). The localization radius of the charge carriers, a, has different constant values within the temperature intervals where ?(T)~T1/2. With further decrease of T, a increases according to the law expected for small lattice polarons.