Study Of Spin Transport In Tunnel Junctions With Multiferroic Barriers

The purpose of this work is to study the spin transport properties, including spin filtering (SF), tunneling magnetoresistance (TMR) and tunneling electroresistance (TER) effects, in the tunnel junctions with multiferroic barriers. The mechanisms responsible for the effects of multiferroism, including ferromagnetism and ferroelectricity, and converse piezoelectricity in the barrier on the spin transport properties have been analyzed in detail. Several theoretical models of tunnel junctions have been set up to study the spin transport properties under the influence of multiferroism and converse piezoelectricity in the barrier. The mechanisms and methods to enhance, control and adjust these effects have also been investigated.Our main work and results are listed bellow:1. Multiple switching of spin polarization injected into a semiconductor by a multiferroic tunnel junctionA method for switching between multiple spin polarization of the electric current injected into a semiconductor is proposed, based on injecting spins from a diluted magnetic semiconductor through a multiferroic tunnel barrier. We show that it is important to combine a diluted magnetic semiconductor electrode with a multiferroic barrier to realize the coexistence of two spin filtering mechanisms and multiple switching of spin polarization on a wide range. The reversal of either electric polarization or magnetization in a multiferroic barrier results in a sizable change in the spin polarization of the injected current both in magnitude and in sign, thereby providing a four-state electrical control of spin polarization. The electroresistance and electromagnetoresistance effects can be also realized in this structure. Our investigations may stimulate experimental studies of the multiferroic tunnel junctions and offer a new route towards spin injection into the semiconductors.2. Tunnel junctions with a ferroelectric-ferromagnetic composite barrier A theoretical model for a tunnel junction with a ferroelectric-ferromagnetic (insulator) composite barrier separating two metallic electrodes is proposed. By using free electron direct quantum tunneling method and transfer matrix formalism, taking into account screening of polarization charges in metallic electrodes and dielectric response in ferromagnetic barrier, we investigate the SF, TMR and TER effects in the junction. It is shown that the large SF effect, hence TMR and TER effects, can be achieved. Eight resistive states in the junction can also be realized by the reversal of electric polarization in the ferroelectric layer and magnetization either in the ferromagnetic layer or in the electrodes.3. Converse piezoelectric effect on the electron tunneling across the multiferroic junctionsConverse piezoelectric effect on the electron tunneling across the tunnel junctions with a single-phase or ferroelectric-ferromagnetic two-phase composite multiferroic barrier is investigated theoretically. It is found that the SF, TMR and TER are enhanced or reduced due to the presence of the strain caused by the converse piezoelectricity in the barrier when the electric polarization is oriented antiparallel or parallel to the applied field. The TMR and TER can even increase with the increasing applied voltage when the converse piezoelectric effect is very strong in the barrier, which is totally different from the voltage dependence in the junction with the barrier with nonpiezoelectricity. The investigations offer a new route towards controllable spin transport.