Large Magnetoresistance in Fe3O4/4,4′-Bipyridine/Fe3O4 Organic Magnetic Tunnel Junctions

Organic magnetic tunnel junctions (OMTJs) are promising systems thanks to their chemically tunable electronic property, long spin lifetime, and easy functionalizations. Here, the spin-dependent electronic transport properties in Fe3O4/4,4′-bipyridine/Fe3O4 OMTJs are investigated by first-principles quantum transport calculations. Since the transport properties of junctions are sensitive to device details, two types of terminations of Fe3O4 electrodes are considered. The device with tetrahedral Fe termination shows anomalous negative tunnel magnetoresistance (TMR), that is, which has a higher and lower junction resistance in the parallel and antiparallel magnetization configurations, respectively. When the contact termination is octahedral Fe, a large positive TMR of 180% appears. The difference in TMR sign of two OMTJs originates from the electrons transmission mediated by frontier molecular levels coupled differently to Fe d states. Furthermore, TMR can be effectively controlled by applied electrical bias by changing states of octahedral Fe involved in transport, which can reach 22000% at 0.1 V. Moreover, a perfect spin-filter effect is demonstrated irrespective of the contact geometry. The results contribute to a fundamental understanding of spin-dependent transport properties in OMTJs.