Spin-polarized tunneling and polaronic transport properties of polycrystalline (Sm1−y Gd y )0.55Sr0.45MnO3 (y = 0.5 and 0.7) compounds

The detail investigations on the magneto-transport properties of the polycrystalline (Sm0.3Gd0.7)0.55Sr0.45MnO3(SGSMO-1) and (Sm0.5Gd0.5)0.55Sr0.45MnO3(SGSMO-2) compounds, having a glassy-like and ferromagnetic ground states respectively have been carried out in details. Due to the existence of two different magnetic ground states in the above mentioned systems, the magneto-transport properties are markedly differed from each other, specially at the low temperature region. The highly semi-conducting nature of the SGSMO-1 compound is suppressed with the application of magnetic field, whereas the SGSMO-2 compound exhibits a metal-insulator transition in its pristine state. The high-temperature semiconducting state of both the systems can be well-explained with the polaronic transport mechanisms via small-polaron hopping and variable-range-hopping models. The low-temperature metallic states for both the systems are explored by considering the various contributions arise from the grain boundary effect, electron-electron, electron-phonon, electron-magnon etc scattering processes. The spin-polarized tunneling transport mechanism at the grain boundaries plays a crucial role in the enhancement of low-field magnetoresistance in the studied systems.