Magnetoelectric Transport Properties In Multiferroic Tunnel Junctions: Orbital Effect

The requirements of miniaturization,integration and low power consumption for electronic devices increase over time.Hence,we need to fabricate possible devices that specifically exploit spin properties instead of or in addition to charge degrees of freedom.Devices based on giant magnetoresistance(GMR)and tunnel magnetoresistance(TMR)effects are already in use in industry as a read head successfully.However,it becomes more and more unreliable to write a data by an extra magnetic field with the decrease of the size of device.In order to solve this dilemma,we urgently need to develop a non-magnetic technology.Multiferroics,in which ferroelectricity and ferromagnetism can coexist and be coupled to each other,makes it possible to control the magnetization of materials non-magnetically.On the other hand,the asymmetrical spatial distribution of spin occurring at the surface/interface results in additional topological spin orbit coupling(SOC)which together with Rashba SOC opens a new way to manipulate the spin by controlling the orbital angular momentum.In this dissertation we have made a series of investigations on the influence of spin orbit couplings(SOCs)whose strength can be controlled by electric field on the tunnel magnetoelectric properties across multiferroic tunnel junctions and related physical phenomena like Spin Hall Effect,Anomalous Hall Effect,and Spin relaxation so on.These studies will provide necessary theoretical support and practical guidance for micro-nano spintronic devices in the future.In chapter one we review the development of multiferroics theoretically and experimentally and show the advantage of studying the composite structures.In chapter two we study the influence of spin orbit coupling(SOC)on tunnel anisotropic magnetoresistance effect in multiferroic tunnel junctions numerically and phenomenologically.The spin-dependent interaction results in an oscillatory tunneling anisotropic magnetoresistance(TAMR)with C2 vsymmetry.A large modulation of TAMR is realized by an external electric field due to the gate-controllable Rashba spin-orbit interaction and the magnetoelectric coupling.TAMRmax? 1% and it is about one order in magnitude larger than the value in the case without the Rashba-type SOC.Such an enhancement of TAMR is highly desirable for practical future applications and adds yet one more advantage of oxide electronics.In chapter three we investigate the Seebeck and spin Seebeck effect in multiferroic tunnel junctions.As a result of the existence of SOC,the thermoelectric coefficients are anisotropic and dependent on the magnetization orientation.On the other hand,the figure of merit of the tunnel junctions is large,which is important to the production of high-efficiency thermoelectric devices.Considering that there will exist the magnetoelectric coupling when ferroelectrics are adjancent to the ferromagnets.we study the influence of magnetoelectric effect on the magnetic relaxation in chapter four.The Gillbert damping shows the C2 vsymmetry in the presence of the spin orbit coupling.Furthermore,when the polarization is inversed,the magnitude of Gillbert damping will change accordingly.In chapter five we ignore the scattering of impurities and study the Tunnel Spin Hall Effect and Anomalous Hall effect caused by the spin orbit interaction.The last chapter gives the conclusion and outlook of this dissertation.