Small-polaron hopping conduction in bilayer manganite La_1.2Sr_1.8Mn₂O₇

We report anisotropic resistivity measurements on a La_1.2Sr_1.8Mn₂O₇ single crystal over a temperature T range from 2 to 400 K and in magnetic-fields H up to 14 T. For T ≥ 218 K, the temperature dependence of the zero-field in-plane resistivity ρ_ab(T) obeys the adiabatic small polaron hopping mechanism, while the out-of-plane resistivity ρ_c(T) can be ascribed by an Arrhenius law with the same activation energy. Considering the magnetic character of the polarons and the close correlation between resistivity and magnetization, we developed a model which allows the determination of ρ_ab,c(H,T). The excellent agreement of the calculations with the measurements indicates that small polarons play an essential role in the electrical transport properties in the paramagnetic phase of bilayer manganites.