| In the design and exploration of quantum functional materials,perovskite and layered perovskite-like oxides have attracted many attentions from researchers owing to their rich structures and new physics.The coupling between parametric orderings(such as spin,orbital,lattice and charge)in materials can induce numerous novel physical functional properties,including multiferroics,transparent conductor,topological insulator,high-temperature superconductor and colossal magnetoresistance,etc.,which are of great significance in developing information storage,semiconducting,and spin electronic devices.Nevertheless,the flexibility of multi degree of freedoms also signifies the complexity and challenge in perovskite oxide researches.A better understanding of the intrinsic physical mechanisms in materials is urgently needed,which will provide more possibilities for the development and application of perovskite-like oxide multifunctional devices.Based on the preparation and physical properties analysis of perovskite-like oxide epitaxial thin films,the research contents of this thesis can be divided into two parts according to the material types:simple perovskite and complex bismuth layer-structured perovskite oxide.Starting with the simple perovskite structure materials,we selected barium stannate as study subject,which has great potential in transparent conducting oxide,and discussed the influence of oxygen vacancy on the structure and electrical transport properties of material.On this basis,we integrated barium stannate with ferroelectric single crystal via interface engineering to prepare perovskite oxide heterostructures,and further investigated the in situ control of electrical properties by multiple order parameter coupling at the heterointerfaces.Finally,based on the previous experiences accumulated in the preparation and research of simple perovskite thin films,we constructed the complex bismuth layer-structured perovskite natural superlattice for realization of magnetoelectric coupling in single-phase materials.Chapter 1 gives an introdution of the research background.Firstly,we briefly introduce the basic characteristics of perovskite-like oxide structures and physical properties,as well as the modulation effects of lattice defects and heterointerfaces on perovskites.After that,the recent developments in BaSnO3-based transparent conducting oxide and multiferroic bismuth layered perovskite-like oxides were reviewed.Chapter 2 introduces the important preparation and characterization methods of thin film materials.The epitaxial single-crystalline thin films were prepared by pulsed laser deposition in this thesis.In order to obtain the expected film samples,X-ray diffraction(XRD),atomic force microscope(AFM)and scanning transmission electron microscope(STEM)were used to confirm the structures and compositions of the films.The characterization techniques of magnetic,ferroelectric and electrical transport properties,which are needed in further physical property testing and mechanism analysis,are also described in detail.In Chapter 3,we discussed the influence of oxygen vacancy on the electrical transport properties of La0.04Ba0.96SnO3 thin films and its internal physical mechanisms.Normally,oxygen vacancies are implicitly assumed as isolated point defects and dope the oxygen-deficient BaSnO3-1 with electron carriers.Whereas,contrary to the typical behaviors of reported BaSnO3-?,the oxygen-deficient La0.04Ba0.96SnO3 films in our work exhibit an apparent metal-to-insulator transition combined with the lower carrier concentrations and mobility with increased oxygen vacancies.Combined with the first-principles calculations,we attribute these contrary phenomena to the effect of extra introduced oxygen vacancies in the system,and propose a theoretical explanation for the strong electron localization caused by defect-defect interactions between the VO and LaBa.Based on the La3+ions impurity doping,in-gap states are introduced into BaSnO3 with extra oxygen vacancies originated from lower growth oxygen pressures.Therefore,free electrons mainly originated from the La3+ donors are localized around the nearby O and Sn atoms in the supercell,while the in-gap states are mainly composed of the combination of 0 2p and hybridized Sn 5s5p with some extents of Sn 4d orbitals.In Chapter 4,we fabricated epitaxial heterostructures consisting of La0.04Ba0.96SnO3 thin films on(001)0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 ferroelectric single crystal substrate via interface engineering.By applying ferroelectric gating,the LBSO channels could be in situ modulated under both the interfacial charge and strain effect and exhibit distinct electrical characteristics with different thicknesses.The model consists of depletion layer based interfacial charge effect and relaxation based strain effect have been quantitatively established and well applied to simulate the observed resistance change under ferroelectric gating.The lattice strain modulation remains stable throughout the thicker films,while increases a bit in thinner films which could be relevant to the changed initial strain or defect states near the film-substrate interfaces.Meanwhile,the interfacial charge effect dominates the electroresistance near the interface due to the short charge screening length,and enhanced significantly with the decrease of film thickness.As a result,the resistance change versus electric field evolves from buttefly-like shape towards square-like shape when decreasing the film thickness,due to the joint effect of both strain and interfacial polarization screening charges.In Chapter 5,we grew the high-quality single-crystalline(001)-oriented SrBi5Fe0.5Co0.5Ti4O18(SBFCT)thin films.XRD,AFM and STEM measurement results indicate that the SBFCT films exhibit a high epitaxial quality with smooth surface morphology and a perovskite-like period of n=5.The magnetic measurements of the thin films show the weak ferromagnetic behaviors and M-H hysteresis loop at room temperature.In the piezoresponse force microscopy characterizations,obvious ferroelectric domain structure with dominant c-axis spontaneous polarization direction are observed with perpendicular bias,suggesting the presence of ferroelectricity at room temperature.More importantly,using scanning probe microscope to study electrical switching of magnetization in SBFCT thin films,we visually present the phenomenon of electrically controlled magnetic polarization,which is a substantial evidence for room temperature magnetoelectric coupling in SBFCT.It is attractive for practical application toward data storage with electrically written and magnetically read.Finally,in Chapter 6,we briefly summarize the thesis and propose some perspectives of future work. |
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