Interface Effects Of BiFeO₃-Based Multilayer Films Synthesized By Laser Molecular Beam Epitaxy

The interplay between lattice,orbital,charge,and spin degrees of freedom at artificial heterointerfaces of complex oxides could lead to fascinating physics and functionalities,such as two-dimensional electron gas,superconductivity,magnetoelectric coupling,and so forth.Thus,the study of epitaxial oxide heterostructures are crucial for the development of next-generation stable,sustainable,and low-power electronic and magnetic devices,which have been attracting great research interest.BiFeO₃（BFO）,which exhibits strong ferroelectric polarization and G-type antiferromagnetism at room temperature,is a magnetoelectric multiferroic material that is currently the most widely studied.In this dissertation,we construct BFO-based oxide multilayer films by means of laser molecular beam epitaxy system,investigate the influence of interface effects on BFO ferroelectric polarization and transport and magnetic properties of SrRuO₃（SRO）,and obtain several results that have major practical significance.These results possess potential applications for the development of nonvolatile logic and memory devices with high density.The main research contents of this dissertation are listed as follows:BFO/SrTiO₃（STO）/BFO multilayer films with atomically sharp interfaces were fabricated on STO（001）substrates by using laser molecular beam epitaxy system.We achieved interface control of ferroelectric polarization of BFO bilayer via precisely creating two different kinds of termination layers on the surface of STO substrates and middle STO thin films.Specifically,BFO layer at the bottom was directly grown on TiO₂-terminated STO substrate,whereas BFO layer at the top was grown on the surface of SrO-terminated STO thin film.Thus,the atomic configurations of interface at the bottom and top are-TiO₂-Bi O-and-SrO-FeO₂-respectively.On the other hand,we fabricated another sample in which the atomic configurations of interface at the bottom and top are-SrO-FeO₂-and-TiO₂-Bi O-respectively.Atomically resolved energy-dispersive X-ray spectroscopy maps confirmed that the stacking sequence of interfaces in the multilayer film are good agreement with our design.High-resolution high-angle annular dark field scanning transmission electron microscopy（HAADF-STEM）images indicate that the as-grown out-of-plane polarization direction of the BFO layer grown on TiO₂-terminated interface is upward,whereas the BFO layer grown on SrO-terminated interface is downward.Therefore,the polarization state of ferroelectric bilayer in the BFO/STO/BFO multilayer films can be controlled through precise design of termination layer at the interface on an atomic scale.In addition,we grown multilayer films with a stacking order of BFO₁₂/(SRO_m/BFO₁₂)_n on TiO₂-terminated STO（001）substrates,investigated the influence of ferroelectric layers on lattice structure and transport and magnetic properties of SRO layers,and observed ferroelectric proximity effect and topological Hall effect related to the interfaces.High-resolution HAADF-STEM images reveal that a displacement between the B-site Ru and the mass center of the four nearest A-site cations appears in the ultra-thin SRO regions near the BFO/SRO interfaces.This demonstrates that ferroelectric-driven ionic displacements in BFO can penetrate into SRO,attesting the presence of ferroelectric proximity effect.Hall measurements reveal that a topological Hall effect appears in the m=9 multilayer film,arising from the broken inversion symmetry of the SRO structure near the BFO/SRO interface.Furthermore,magnetic results indicate that the interfacial SRO exhibit smaller coercive field compared to that of the middle SRO,which is related to Ru O₆ octahedral tilting induced by the ferroelectric proximity effect.Therefore,ferroelectric proximity effect induces the lattice distortion of the SRO near the BFO/SRO interfaces,which has an effect on transport and magnetic properties of SRO in the multilayer films.