Electronic Structure,Magnetic And Transport Properties In Multiferroic BiFeO₃ Heterostructures

BiFeO₃ is a single-phase room-temperature multiferroic material with the ferroelectric and antiferromagnetic properties,which has potential applications in spintronic devices.Multiferroic BiFeO₃ becomes one of the hottest materals in the field of condensed matter physics and materials science.In this dissertation,the electronic structure,magnetic and transport properties in multiferroic BiFeO₃heterostructures are investigated by density functional theory and nonequilibrium Green's function formalism.The novel physical phenomena in multifferoic BiFeO₃heterostructures are predicted,where the mechanisms are clarified.All the results provide the theoretical basis for the practical applications of multiferroic BiFeO₃heterostructures in spintronic devices.The main results of this dissertation are as follows:(1)In the tetragonal BiFeO₃/Mott insulator LaTiO₃ heterostructures,the metal-insulator transition in LaTiO₃ can be induced by BiFeO₃.The two-dimensional electron gas appears at the interface of BiFeO₃/LaTiO₃ heterostructures.These results provide theoretical foundations for the multiferroic-modulated electronic structure in Mott insulator.(2)In the tetragonal BiFeO₃/BiCoO₃ heterostructures,the ferroelectric and metallic properties simultaneously appear,which enriches the physical properties in complex oxide interfaces.(3)In the rhombohedral BiFeO₃/BiIrO₃ superlattices,the valley polarization in BiIrO₃ can be induced by varying the relative direction of the ferroelectric polarizations in rhombohedral BiFeO₃ and BiIrO₃.Moreover,the spin polarization in valley state can be tailored by the orientation of Fe magnetic moment in antiferromagnetic BiFeO₃.These results offer theoretical basis for designing the oxides valleytronic devices.(4)In the tetragonal BiFeO₃/Fe₄N heterostructures,the perpendicular magnetic anisotropy(PMA)and high spin polarization appear in Fe₄N,which can be ascribed to the tetragonal distortion and interfacial couplings.When the in-plane biaxial strains were applied,the induced PMA in Fe₄N can be preserved as the orbital oscillation appears.The PMA in each Fe₄N layer can be effectively modulated by external electric field,which can be attributed to the broken spin screening effect in BiFeO₃/Fe₄N heterostructures.Particularly,in Fe₄N/BiFeO₃/Fe₄N tunnel junctions,the negative tunnel magnetoresistance is observed,which can be reversed by the applied bias voltage.These results provide the theoretical basis for magnetic storages devices by using BiFeO₃/Fe₄N heterostructures.(5)In the tetragonal BiFeO₃/La_2/3Sr_1/3MnO₃ heterostructures with MnO₂ termination,the net Fe magnetic moment appears in the first interfacial layer of BiFeO₃,which can be tuned by the external electric field.The strong interfacial magnetoelectric coupling appears in the La O-terminated BiFeO₃/La_2/3Sr_1/3MnO₃ heterostructures.These results provide much useful information to deeply understand the magnetoelectric coupling in multiferroic heterostructures.(6)In the La_2/3Sr_1/3MnO₃/BiFeO₃/Fe₄N multiferroic tunnel junctions,four resistance states can be induced by the direction of ferroelectric polarization in barrier and the alignment of the magnetization in two ferromagnetic electrodes.Meanwhile,when the linearly polarized light was applied,the spin-polarized photocurrent can be observed,resulting in the multiple resistance states in the junctions.So,the coupling between the spin,ferroelectric and optical vectors is predicted in multiferroic tunnel junctions,which provides the theoretical basis for designing the multi-fields modulated novel spintronic devices.