| Perovskite-type transition metal oxides have exhibited novel physical properties such as electricity,magnetism and photoelectric conversion due to the correlation coupling between their spins,orbitals,charges and lattices,which has attracted extensive attention in the past decades.Recently,due to the design requirements of multifunctional integration and miniaturization of integrated circuits and electronic devices,as well as the current traditional semiconductor electronic devices affected by Moore’s law and quantum interference effect,perovskite oxide epitaxial films and heterojunction gradually come into the attention of researchers as ideal new electronic functional materials.In order to effectively control the structure and properties and even induce the development of new material properties,it has become the frontier direction of multifunctional magnetoelectric materials to deeply understand the relationship between the novel physical properties and interface structure of perovskite oxide heterojunction.Considerable previous research of perovskite oxide heterojunctions has focused on regulating oxygen octahedral structure coupling at the interface by changing the relative proportion or thickness of different oxide layers,and the multi-periodic alternating growth of different oxide layers can highlight the interface interaction.However,the specific growth order of the different oxide layers may lead to the reconstruction of the electrons and atoms at the interface,and the oxide layer may be stressed from both the lower and upper films at the same time,resulting in weakened stress effects from the substrate.The stress effect from the substrate cannot be ignored in the study of perovskite oxide epitaxial bilayers/trilayers,because the upper layer will be directly subjected to the stress of the bottom layer during the growth process.In addition,in addition to the extensional stress and octahedral coupling effect,various exchange coupling effects at the interface,including interface charge transfer,orbital reconstruction,structural asymmetry and ion diffusion,will reconstruct the oxide interface in the perovskite oxide bilayers/trilayers,thus causing it to exhibit novel physical properties.On the other hand,a large number of novel physical properties with application prospects have been found in high-quality complex oxide multilayers prepared by pulsed laser deposition(PLD)or the like.However,it is difficult to fabricate wafer-level complex oxide multilayer films by the PLD method,due to the limitations of film preparation cost and test cycle.The polymer assisted deposition(PAD)method has the advantages of easy operation,low cost,high quality,and easy doping,which is suitable for large-scale industrial production.The highly oriented perovskite oxide bilayers/trilayers can be obtained by PAD method,which film quality can be very close to that prepared by PLD method through the exploration and optimization of various preparation conditions.As a result,study on the films prepared by PAD method is of great significance for the physical mechanism research and practical industrial application of perovskite oxide multilayer films.In this thesis,a simple polymer-assisted deposition(PAD)method was used to epitaxially grow the perovskite oxide bilayers/trilayers,and the interfacial oxygen octahedral structure coupling and corresponding electromagnetic properties are effectively tuned by means of epitaxial stress cooperation of substrate orientation,oxygen content,ion doping,and exchange layer sequence,and the mechanism of action inherent to the novel physical phenomena is explored.The main research content and results are as follows:In Chapter 1,the crystal structure,electronic structure,magnetic structure and magnetoelectric interaction are introduced for perovskite oxides.The physical properties and research progress of several common perovskite oxides are presented.The behaviors of the epitaxial heterojunction interface and oxygen octahedra of perovskite oxides are discussed.And the popular research directions and current problems of perovskite oxides are summarized,highlighting the important significance of the study.In Chapter 2,SrRuO3/NdNiO3(SRO/NNO)bilayers with different orientations were epitaxially grown by the PAD method,and the physical properties of the bilayers were systematically investigated by interfacial effects and stress modulation.The 3D island-grown SRO layer in the bilayer helps to reduce the lattice relaxation and stress effects,so that the transport properties of the bilayer are determined by the lower NNO layer;while the stress-induced structural adjustment of the 2D lamellar-grown NNO layer directly affects the interfacial structure.It is suggested that the competition and combination of the stress effect on the NNO layer and the interfacial effect in SRO/NNO bilayers(including the interfacial orbital hybridization/reconstruction and stronger interlayer exchange coupling),dominate the interfacial properties with substrate-orientation dependence.In Chapter 3,LaCoO3/LaMnO3/LaCoO3 trilayers were epitaxially grown in Ar,air and O2 heat treating atmospheres by the PAD method,respectively,and the effects of oxygen content on the structure and magnetism of the trelayer were studied in detail.It was found that the magnetic properties of the trilayers were influenced by stress effects and exchange coupling.In addition to two ferromagnetic transitions corresponding to the LaCoO3 and LaMnO3 layers,an anomalous ferromagnetic transition with the contribution of interfacial charge transfer is exhibited at 216 K.All three ferromagnetic transition temperatures are regulated by the oxygen content.Both interfacial coupling and diffusion-induced interlayer coupling are present in the three films and are regulated by oxygen content-dependent charge transfer.The cooperative function of the orbital/charge structure and the interlayer coupling dominates the magnetic properties of LLL trilayer under the tensile stress.In Chapter 4,NdNi1-xCoxO3/NdMnO3(x=0,0.05,0.1)bilayers were grown by PAD method,and the effects of Co doping and interface effect on the electromagnetism of the bilayers were studied.Under the effect of stress and interfacial coupling,the metal-insulator transition temperature of the bilayer is higher than that of the NdNi1-xCoxO3 single-layer film with the same amount of Co doping.In the bilayers,in addition to the ferromagnetic transition at low temperature from the contribution of NdMnO3 single-layer film,an anomalous ferromagnetic transition at high temperature with the contribution of interfacial charge transfer is also exhibited,and the ferromagnetic transition temperature at low(high)temperature shows a regular increasing(decreasing)trend with the increase of Co doping.The study of the electromagnetic properties,which are regulated solely by ion doping within the heterojunction layer,highlights the close relationship between interfacial coupling and heterojunction.In Chapter 5,NdNiO3-La0.7Ca0.3MnO3(NNO-LCMO)bilayers with different sequences were grown by PAD method,and the effects of growth sequences on the electromagnetism properties of the bilayers were systematically studied.Compared with the Curie temperature Tc of the LCMO single-layer,the Tc of the NNO/LCMO bilayer increases,and the electric transport shows the thermal hysteresis behavior similar to that of the NNO single-layer film,and the abnormal metallic behavior in the low temperature region.However,the Tc of LCMO/NNO bilayers decreases,showing an abnormal ferromagnetic transition due to interfacial charge transfer at high temperature,and the electrical transport shows similar colossal magnetoresistance effect behavior of LCMO single-layer films.It is shown that the magnetic and transport properties of the bilayers are affected by the interlayer exchange coupling and the interface structure difference,which reveals the corresponding relationship between the interlayer coupling and the interface structure.In Chapter 6,the effects of the interfacial oxygen octahedral structure coupling and corresponding electromagnetic properties regulation of perovskite oxides by means of epitaxial stress cooperation,such as substrate orientation,oxygen content,ion doping and exchange sequence are summarized,and the innovation of this paper is further condensed,which highlight the interface effect under epitaxial stress and the relationship between heterojunction structure and physical properties.In addition,several key directions of future research are also prospected,which will broaden the path for studying the effective regulation of heterostructure and physical properties,and indicate the potential application value of perovskite oxide bilayer/trilayer devices in multifunctional magneto-electric coupled electronic devices. |
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