Interfacial electronic states and magnetoresistance in tunnel junctions

The relation between the tunnel magnetoresistance (TMR) and the spin polarization (P) of ferromagnetic tunnel junctions (FTJ) is investigated, focusing on the effect of electronic states at interfaces of FTJ. We calculate the electronic states of transition-metal (TM)/Al2O3 interfaces, and show that the presence of imperfectly oxidized Al or O ions at the interface gives rise to positive P values due to spin-dependent s–d or p–d mixing at the interface. Model calculations of the tunnel conductance and TMR reveal that the presence of a few imperfectly oxidized ions at the interfaces govern the tunnel conductance, and the P of these ions is correlated with the TMR. The presence of imperfectly oxidized Al or O ions at the TM/Al2O3 interfaces can well account for the TMR observed in TM/Al2O3/TM and TM/Al2O3/La–SrMnO3 junctions.