Magneto-conductive mechanisms in the La-site doped double-layered La1.4Ca1.6Mn2O7 manganites
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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 R-h(T) and mean hopping energy E-h(T) increase. This may explain the resistivity increase and the crossover to the ES-VRH model. Values of R-h(T) vary between 1.811 and 1.030 nm, which allow us to suggest 3D-VRH as the best model fitting current results. E-h(T) values range from 0.1157 to 0.2635 eV. The DOS increases as increasing magnetic field while R-h(T) and E-h(T) decrease, which is in agreement with the observed decrease of resistivity. (C) 2016 Elsevier B.V. All rights reserved.