Effect of sintering temperature on electrical transport properties of La_0.67Ca_0.33B>MnO₃

Abstract

A systematic investigation of lanthanum-based manganite, La_0.67Ca_0.33MnO₃, has been undertaken with a view to understand the influence of varying crystallite size, in the nanoscale, on various physical properties. The materials were prepared by the sol-gel route by sintering at four different temperatures starting from 800 to 1100 ° C, with an interval of 100 °C. After the usual characterization of these materials structurally by XRD, their metal-insulator transition (T_P) as well as magnetic transition (T_C) temperatures were determined. Surprisingly these materials are found to exhibit two different types of behaviors, viz, while T_C is found to decrease from 253 to 219 K, T_P is increasing from 145 to 195 K with increasing sintering temperature. A systematic study of electrical conductivity of all four materials was undertaken not only as a function of temperature (80-300 K), but also as a function of magnetic field up to 7 T mainly to understand the detailed conduction mechanism in these materials. On analyzing the data by using several theoretical models, it has been concluded that the metallic (ferromagnetic) part of the resistivity (ρ) (below T_P) fits well with the equation ρ(T)=ρ₀+ρ_2.5T^2.5, indicating the importance of grain/domain boundary effects (ρ₀) and electron-magnon scattering processes (~T^2.5). On the other hand, in the high temperature (T>T_P) paramagnetic insulating regime, the adiabatic small polaron and VRH models fit well in different temperature regions, thereby indicating that polaron hopping might be responsible for the conduction mechanism.