Non-adiabatic small polaron hopping and the effect of spin-polarized tunneling in electron-doped Ca0.85Nd0.15MnO3

The electron-doped manganite Ca0.85Nd0.15MnO3 has been investigated through magnetic and electrical transport studies. It shows insulating behavior down to the lowest temperature (5 K) measurement even on application of high magnetic field (up to 15 T) indicating a robust insulating behavior. Small polaron hopping and variable range hopping conduction have been observed at high- and low-temperature region, respectively. High-T conductivity of the sample follows non-adiabatic hopping conduction mechanism. Magnetic field-dependent resistivity has been thoroughly examined in terms of a phenomenological model taking into account the spin-polarized tunneling (SPT) of the conduction electrons. In order to explore the nature of the temperature-dependent magnetoresistance (MR), we have separated the contribution related to SPT and spin fluctuation basing on the model. Scaled isothermal MR data collapse into two curves separating the charge order temperature (T (CO)), which indicates the existence of general formalism of MR and also suggests temperature-independent behavior of the reduced MR.