Spin-dependent transport in a multiferroic tunnel junction: Theory for Co/PbTiO3/Co

Spin-dependent electronic transport through multiferroic Co/PbTiO3/Co tunnel junctions is studied theoretically in view of the recent observation of an inverse TMR in Co/PbTiO3/LaSrMnO3 heterostructures. Conductances calculated within the Landauer-Buttiker formalism yield a four-conductance state characterized by sizable positive tunnel magnetoresistances (TMR) and tunnel electroresistances (TER). The conductances depend crucially on the details of the electronic structure at the interfaces. In particular, the spin polarization of the tunneling electronic states is affected by the hybridization of orbitals and the associated charge transfer at both interfaces. Digital doping of the PbTiO3 barrier with Zr impurities at the TiO2/Co-2 interface removes the excessive metalization of the barrier and significantly enhances the TMR but is not sufficient to switch the TMR's sign. Our results indicate that the origin of the TMR inversion might be attributed to the magnetoelectrically active LaSrMnO3/PbZr0.2Ti0.8O3 interface.