Influence of Zeeman splitting and thermally excited polaron states on magnetoelectrical and magnetothermal properties of magnetoresistive polycrystalline manganite La 0.8 Sr 0.2 MnO 3

Abstract

Some possible connection between spin and charge degrees of freedom in magnetoresistive manganites is investigated through a thorough experimental study of the magnetic [alternating current susceptibility and direct current (dc) magnetization] and transport (resistivity and thermal conductivity) properties. Measurements are reported in the case of well characterized polycrystalline La0.8Sr0.2MnO3 samples. The experimental results suggest rather strong field-induced polarization effects in our material, clearly indicating the presence of ordered ferromagnetic regions inside the semiconducting phase. Using an analytical expression which fits the spontaneous dc magnetization, the temperature and magnetic field dependences of both electrical resistivity and thermal conductivity data are found to be well reproduced through a universal scenario based on two mechanisms: (i) a magnetization dependent spin polaron hopping influenced by a Zeeman splitting effect and (ii) properly defined thermally excited polaron states which have to be taken into account in order to correctly describe the behavior of the less conducting region. Using the experimentally found values of the magnetic and electron localization temperatures, we obtain L=0.5 nm and mp=3.2me for estimates of the localization length (size of the spin polaron) and effective polaron mass, respectively.