Magneto-conductive mechanisms in the La-site doped double-layered La1.4Ca1.6Mn2O7 manganites

Magneto-conductive mechanisms and properties of La1.2Re0.2Ca1.6Mn2O7 (Re=La, Pr, Y, Gd and Eu) have been investigated. In the low temperature range, combining residual resistivity, weak localization effect, electron–electron and electron–phonon interactions in a model, fit well the resistivity curves of undoped, Pr-doped and Y-doped samples. The Gd-doped and Eu-doped ones require the introduction of the small polaron metallic conduction. In the high temperature range, 3D-Mott's variable range hopping (3D-VRH) is the best model to fit resistivity of the undoped, Pr-doped and Y-doped samples, while Effros-Shklovskii model (ES-VRH) is the best one for Gd-doped and Eu-doped samples. In the entire temperature range, the percolation model fit well the resistivity. Using 3D-VRH model, the density of state (DOS) decreases with doping by Gd and Eu, whereas mean hopping distance Rh(T) and mean hopping energy Eh(T) increase. This may explain the resistivity increase and the crossover to the ES-VRH model. Values of Rh(T) vary between 1.811 and 1.030 nm, which allow us to suggest 3D-VRH as the best model fitting current results. Eh(T) values range from 0.1157 to 0.2635 eV. The DOS increases as increasing magnetic field while Rh(T) and Eh(T) decrease, which is in agreement with the observed decrease of resistivity.