First-principles Study Of Metallic Mechanisms Of BaTiO₃ Barrier And Its Multiferroic Tunnel Junctions

Multiferroic tunnel junctions?MFTJs?is usually composed of two magnetic metal electrodes sandwiching a thin ferroelectric?FE?film.By applying voltage pulse to change the polarization state of ferroelectric barrier,MFTJs can be switched between low resistance state and high resistance state,which is the tunneling electroresistance?TER?effect.By changing the magnetization arrangement direction of two ferromagnetic electrodes,MFTJs can also be switched between low resistance state and high resistance state,which is the tunneling magnetoresistance?TMR?effect.Multiferroic memory is one of the competitive next generation non-volatile memory,and MFTJs can be used as the core storage unit of multiferroic memory.The works of this dissertation focus on the first-principles calculation of BaTiO₃-based MFTJs,which can be divided into the following parts:?1?We report a first-principles study of the ferroelectricity and spin-dependenttransport properties through Co/BTO/CoO/CoMFTJs.How to design MFTJs is avery worthy research topic in order to read data conveniently and obtain high TMRand high TER at the same time.High TMR is usually obtained by the magneticelectrodes with half-metallic property and a barrier with spin-filtering effect,whilehigh TER is usually obtained by an asymmetric tunnel junction.In theory,highTMR can be obtained by using Co as electrode which is half-metal for?₁ and?₅band,and using BaTiO₃ as barrier which has slower attenuation for?₁ and?₅ band.A larger TER can be obtained by inserting a CoO layer into one interface of Co andBTO to produce asymmetry.We find the coexistence of large TMR ratio and largeTER ratio in the Co/BTO/CoO/CoMFTJs.The large TMR effect originates fromthe spin-filter tunneling through the BTO barrier,while the TER effect is due to themodulation of orbitals overlap by polarization reversal.?2?Research on reversible metal ferroelectric barrier of MFTJs.Coexistence ofmetallicity and ferroelectricity has become an interesting but not yet appliedphysical effect.It is both because free electron screening due to metallicity preventsthe polarization to be switched by an electric field,diminishing the value offerroelectricity,and because metallicity is usually achieved by doping,which leadsto disorder and is often detrimental to other electronic properties.Here,we predictvia first-principles calculation a switchable metallic ferroelectric barrier in?Co₂?₉-TiO₂-?BaO-TiO₂?₅-CoO-CoMFTJs without doping.The metallic ferroelectricity iscaused by an electrode proximity effect that is common to ionic ferroelectricmaterials and shifts the Fermi energy as a function of the termination layer at theinterfaces.This effect is accentuated by the large polarization of CoO layerrelatively to that of BTO,leading to a larger electrostatic potential drop on theinterface containing CoO thus further pulling the conduction band bottom of theentire BTO region below the Fermi energy.Increasing the polarization of BTOrelative to that of CoO,e.g.,by applying strain,or reducing the polarization of CoOregion can remove the metallicity,allowing the polarization to be switchedelectrically.When the polarization is reversed,the Fermi level is also close to theconduction band due to the electrode proximity effect,but it remains in the bandgap almost everywhere in the BTO barrier,maintaining an insulating state.Thismetallic-insulating BTO difference between two polarization configurations isshown to appear in?Co₂?₉-TiO₂-?BaO-TiO₂?_N-CoO-Co system,where N=4,5,6,7,8,9,and leads to a sizable TER.?3?On the basis of work?2?,we calculate the Co-?TiO₂-BaO?₅-CoMFTJs withoutdoping by first-principles calculation.When the polarization orientation points fromthe BaO-Co₂ interface to Co₂-TiO₂ interface,the entire BTO barrier region becomesmetallic.This metallic ferroelectric BTO originates from the large polarization ofthe interfacial BaO-Co₂region relative to BTO,resulting in a very large electrostaticpotential drop in BaO-Co₂region and pulling the BTO conduction band below theFermi level.Increasing the polarization of BTO relative to that of BaO-Co₂,e.g.,byapplying strain,or reducing the polarization of BaO-Co₂ region can remove themetallicity,allowing the polarization to be switched electrically.When thepolarization is reversed,the BTO in Co-?TiO₂-BaO?₅-Co system is a normalinsulating state.This metallic-insulating BTO difference between two polarizationconfigurations is of great significance for the application of metal ferroelectricphenomena to information storage.?4?Reversible electric field switching of spin polarization in Co/BTO interface isstudied.The spin polarizability of Ti atoms at the Co/BTO interface can becontrolled between positive and negative values by the polarization reversal of BTO.The changes of Ti atoms at the interface mainly come from the orbital hybridizationof Ti-3d_xz 3d_yz,Co1-3d_xz?Co1 and Ti atoms are in the same xz plane?and Co2-3d_yz?Co2 and Ti atoms are in the same yz plane?orbitals.This phenomenon is verysensitive to electrode material,ferroelectric barrier material and interface terminalstructure.The symmetrical Co/BTO/Co transport results show that the resistance ofthe tunnel junction is smaller when the two Co electrodes are arranged inantiparallel configuration than when they are arranged in parallel configuration.thatis,TMR is negative.It is suggested that the spin polarizability of the Ti atom at theinterface plays an important role in the transport.The phenomenon of spinpolarizability switched between positive and negative values controlled by BTOpolarization can be used to change the sign and amplitude of TMR effect by electricfield in MFTJs.This method is expected to be used in the non-volatile informationstorage and processing of electric field control in the future.