  Magnetic tunnel junctions (MTJs), composed of two ferromagnetic electrodes
separated by a thin insulating barrier layer, are currently used in spintronic
devices, such as magnetic sensors and magnetic random access memories.
Recently, driven by demonstrations of ferroelectricity at the nanoscale,
thin-film ferroelectric barriers were proposed to extend the functionality of
MTJs. Due to the sensitivity of conductance to the magnetization alignment of
the electrodes (tunnelling magnetoresistance) and the polarization orientation
in the ferroelectric barrier (tunnelling electroresistance), these multiferroic
tunnel junctions (MFTJs) may serve as four-state resistance devices. Based on
first-principles calculations we demonstrate four resistance states in
SrRuO3/BaTiO3/SrRuO3 MFTJs with asymmetric interfaces. We find that the
resistance of such a MFTJ is significantly changed when the electric
polarization of the barrier is reversed and/or when the magnetizations of the
electrodes are switched from parallel to antiparallel. These results reveal the
exciting prospects of MFTJs for application as multifunctional spintronic
devices.
