Effects of low frequency near metal-insulator transition temperatures on polycrystalline La0.65Ca0.35Mn1-xFe1-yO3 (where y=0.05-0.10) ceramic oxides

The ac electrical properties of 5-10% Fe doped polycrystalline sample have been investigated by complex impedance analysis over the frequency and temperature ranges of 1-100 kHz and 77-300 K, respectively. The average normalized change (Delta Z'/Delta f)/Z(0) has been deduced for these Fe doped CMR samples which shows an increasing trend with iron doping. The most pronounced effect of frequencies is at T-c, with the increase of Fe doping it is observed that not only T-c is lowered Substantially but also the height of the peaks of real part of impedance (Z') is increased which in turn decreases considerably with the increase of the ac field. An equivalent circuit model, R-g(RgbCgb), i.e. a resistor-capacitor network, has been proposed to explain the impedance results at different temperatures. The plot between tau and 1/T gives a straight line from where relaxation time (TO) has been deduced. The correlated barrier hopping (CBH) model has been employed and the binding energy of the defect states is estimated to be between 0.39 and 0.25 eV while the minimum hoping distance varies within the range of 2.93-5.21 angstrom for these 5-10% Fe doped LCM samples. (c) 2005 Elsevier Ltd. All rights reserved.