Voltage and temperature dependence of the grain boundary tunneling magnetoresistance in manganites

We have performed a systematic analysis of time voltage and temperature dep endence of the tunneling magnetoresistance (TMR) of grain boundaries (GB) i n the manganites. We find a strong decrease of the TMR with increasing volt age and temperature. The decrease of the TMR with increasing voltage scales with an increase of time inelastic tunneling current due to multi-step ine lastic tunneling via localized defect, states in the tunneling barrier. Thi s behavior can be described, within a three-current model for magnetic tunn el junctions that extends thc two-current Julliere model by adding an inela stic, spin-independent tunneling contribution. Our analysis gives strong ev idence that the observed drastic decrease of the GB-TMR in manganites is ca used by an imperfect tunneling barrier.