Studies On Interfacial States And Straintronics Of Perovskite Oxide Heterostructures

Perovskite oxide heterostructures have mesmerized the scientific community in the last decade due to the possibility of creating tunable novel multifunctionalities,which are possible owing to the strong interaction among charge,spin,orbital,and structural degrees of freedom.Artificial interfacial modifications,which include defects,formal polarization,structural symmetry breaking,and interlayer interaction,have led to novel properties in various complex oxide heterostructures.These emergent phenomena not only serve as a platform for investigating strong electronic correlations in low-dimensional systems but also provide potentials for exploring next-generation electronic devices with high functionality.Based on these advantages,this thesis using Density Functional Theory,the Nonequilibrium Green's Function method,and experimental research with partners,by the substrate material Sr Ir O₃ as the starting point of research,for the construction of the perovskite oxide heterojunction,electronics,properties and its stress on application of new information storage device is studied.In the first chapter,we review the research progress of perovskite-type oxide heterojunction in recent years.This mainly includes two parts,one is interface effect,the other is emergence effect.Then we focus on the research progress of perovskite oxide heterojunction in stress electronics,the basic principle of stress electronics and device applications are introduced in detail.Finally,a brief introduction is given to the research objectives and main research contents of this paper.In the second chapter,we systematically introduce the theoretical research methods used in this paper.We first take the development of density functional theory as the main line and introduce the various approximate methods and theoretical methods that people put forward to solve the multi-body problem in history.Then,we introduce the theoretical basis of the non-equilibrium Green's function and its application in the calculation of conductance.Finally,we introduce the theoretical calculation toolkit used in this paper.In the third chapter,the magnetic and dynamical properties of tetragonal Sr Ir O₃under the influence of epitaxial strain have been studied using DFT calculations.Two magnetic states,i.e.,strong ferromagnetic and weak ferromagnetic states,are found with the application of different epitaxial strains.The Stoner model can provide a reasonable explanation of these phenomena.In addition,the calculations reveal that tetragonal Sr Ir O₃ can be a kind of polar metal under both compressive and tensile strains.This may stimulate further investigation in the properties of oxide superlattices consisting of Sr Ir O₃ films.After studying the unusual magnetic transitions of Sr Ir O₃under strain,we used tetragonal Sr Ir O₃ as the base material of the heterojunction to splice Ba Ti O₃ to construct supercells,thereby studying the stress electronics of the perovskite oxide heterojunction nature.We use first-principles calculations to explore the magnetoelectric coupling effect of Sr Ir O₃/Ba Ti O₃ heterojunction and its stress electronics properties.The Sr Ir O₃/Ba Ti O₃ heterojunction shows obvious changes in ferroelectric polarization and magnetic moment under different stresses.We combined density functional theory and non-equilibrium Green's function method to study the electrical conductivity and giant control resistance ratio of Sr Ir O₃/Ba Ti O₃ multi-iron tunnel junctions under different stresses.Through this research,we have designed a new type of multi-state memory device based on stress electronics that can be controlled by an external electric field.Our research may lay a certain theoretical basis for future research on heterojunctions based on tetragonal Sr Ir O₃,and stimulate more attention for the application of stress electronics in polymorphic storage in the future.In the fourth chapter,the four stacking configurations of Zn O/BTO/Zn O supercell with two layers of Zn O and three layers of(111)BTO have been studied using DFT calculations.The Mechanism of polarization direction in(111)BTO influenced by the interfaces are investigated.Combining density functional theory calculations with nonequilibrium Green's function formalism,two giant TER effects of 581%and 112%are observed in our newly designed SRO/Zn O/BTO/Zn O/SRO and SRO/Zn O/BTO/SRO FTJ heterostructure.The thickness of Zn O/BTO in this heterostructure is less than 4 nm.The proposed strategy in our study is applicable to design higher performance FTJs.We hope this work will stimulate the experimental endeavors of fabricating FTJs with a giant TER effect to accelerate their commercial applications into ultralow-power,high-speed,and nonvolatile nanoscale memory devices.In the fifth chapter,we fabricated high quality 7 u.c.BTO film on Nb-doped single crystalline Sr Ti O₃(NSTO)to study its polar order and polarization sitching.While ordinary PFM characterization suggests linear piezoelectricity as well as apparent polarization reversal under electric poling,careful examination reveals that there is absence of ferroelectric switching under the practically applied electric bias,where the apparent contrast is resulted from charging injection,which is unstable and can be eliminated by grounding.The existence of weak polar order consistent with linear piezoelectricity is confirmed at atomic scale by high resolution integrated differential phase contrast(IDPC)of transmission electron microscopy as well as at macroscopic by the optic second harmonic generation(SHG),while the lack of polarization reversal under the voltage applied is supported by density functional theory calculation showing the persistence of dead layer on the surface.Nevertheless,resisting switching with electric conduction different by two orders of magnitude is observed,demonstrating that polarization reversal is not necessary for resisting switching in ferroelectric heterostructure.In the sixth chapter,we have fabricated SRO/STO superlattices with sharp interfaces on STO.The structural properties have been established by XRD and STEM.By investigating the temperature dependence of resistance,MR and magnetic field dependence of MR,we have revealed that the transport and magnetic properties can be tuned by varying the thickness between SRO and STO layers.In addition,the appearance of atypical humps in the magnetic field dependence of the Hall resistance was confirmed to be resulted from several contribution including the one from ferromagnetism and others from a change of the Berry curvature rather than THE.Thus,the AHE with atypical humps can be decoupled into several AHE components.The results indicate the presence of two AHE components as well as the Berry curvature can be engineered by STO layers between SRO in the form of superlattices.The last chapter is the summary and prospects of the whole works of this paper.We have made a systematic summary and arrangement of the research conclusions obtained in this paper.Through the summary of the current work,we look forward to the new problems to be explored in the future and the potential research direction.