Particle Size Dependent Magnetoresistance and Magnetothermoelectric Power Of La_0.5Pb_0.5MnO₃ Showing Metal-Insulator Transition

Abstract

Particle size dependent transport properties (resistivity and thermopower) of La_0.5Pb_0.5MnO₃ has been investigated both in presence and in absence of magnetic field B=0.0-1.5T (maximum). All the samples show metal-insulator transition (MIT) with a peak at the MIT temperature (T_p). Magnetic field decreases the resistivity with an increase in the peak temperature T_p. Particle size, conductivity and T_p of the sample increase with increasing annealing time. High temperature semiconducting (insulating) part of the resistivity curve is divided into two distinct regimes. Resistivity data for T>qϘ/2, can be well fitted with the nearest neighbor small polaron hopping (SPH) model. Polaron hopping energy (WH) decreases with increase of particle size. The lower temperature part (T_p>T>qϘ/2) of the semiconducting (insulating) regime is found to follow variable range hopping (VRH) model. With the increase of particle size, the temperature range of validity of the VRH mechanism decreases. The low temperature metallic regime (for T<T_p) of the resistivity (both in absence and in presence of field) data fit well with ρ = ρ0 +ρ_2.5 T^2.5 and transport mechanism in this region is mainly dominated by magnon-carrier scattering (~T_2.5). Particle size has, however, comparatively little effect on Seebeck coefficient (S). In all the samples with different particle sizes, S changes sign below T_p. In contrast to magnetoresistance, application of magnetic field increases S at low temperature (T<T_p) for these samples. Similar to the resistivity results, thermopower data in the metallic phase (both for B=0.0 and 1.5T) can also be analyzed by considering magnon-scattering term along with an additional spin-wave fluctuation term (~T⁴).