Influence of Zeeman splitting and thermally excited polaron states on magneto-electrical and magneto-thermal 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 magneto-resistive manganites is investigated through a thorough experimental study of the magnetic (AC susceptibility and DC magnetization) and transport (resistivity and thermal conductivity) properties. Measurements are reported in the case of well characterized polycrystalline La_{0.8}Sr_{0.2}MnO_3 samples. The experimental results suggest rather strong field-induced polarization effects in our material, clearly indicating the presence of ordered FM 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=0.5nm and m_p=3.2m_e for estimates of the localization length (size of the spin polaron) and effective polaron mass, respectively.