Growth And Analysis Of Bi₆Fe_2-XCo_XTi₃O₁₈ Epitaxial Thin Films

Single-phase multiferroic materials have drawn significant attentions because of the potential applications in multi-state memory, spintronic device, sensor, etc. However, due to the incompatibility of electronic structures with ferroelectricity and ferromagnetism, single-phase multiferroic materials are very rare in nature. Aurivillius-type layered bismuth oxides (Bi4Bin.3Fen-3Ti3O3n+3) have been studied as potential room-temperature multiferroic materials. Many previous studies have enhanced the porperities of ferroelectricity and magnetism by doping modification, and remarkably, by doping Co ions, this material exists coexistence of ferromagnetism and ferroelectricity at or above room temperatures. However, almost all previous studies have mainly focused on polycrystalline ceramics or film samples. The investigation on epitaxial thin films with multiferroic properities is much fewer. Thus it is highly challenging to probe the intrinsic physical properties of the Aurivillius oxides in the absence of a clean structure. Because single-crystalline materials, compared with the polycrystalline material, may generate superior performance and novel physical phenomena, it is imperative to fabricate high quality single-crystalline epitaxial films.This thesis includes six chapters:the first chapter introduces the related background knowledge. The second chapter describes the deposition and characterization methods of the thin film. The third chapter describes the growth and characterization of the Bi6FeCoTisO18 epitaxial thin films. The fourth chapter describes the effects of metallic buffer layer on the growth temperature of the thin films. The fifth chapter describes the interface effect in multiferroic films on different bottom layers. The sixth chapter describes the conclusion of the thesis study and prospects.In chapter one, the related background knowledge and the current research highlight are introduced. First of all, the concept, the reaserach history and the development of the multiferroic materials are introduced. Magnetoelectric coupling effect, incompatibility between ferroelectricity and ferromagnetism, mechanisms of magnetoelectric coupling are reviewed. Secondly, the history and recent progresses in the multiferroic areas of the Aurivillius-type layered bismuth oxides are described. Thirdly, the interfacial screening effect and electrostatic boundary conditions in heterostructures of metal oxides are described. Fourthly, the great challenges in fabricating Aurivillius epitaxy thin films are explained. Finally, the research outline and the significance of this thesis are explained.In chapter two, the deposition and characterization methods of epitaxial thin films are introduced. First, the fabrication processes of the Bi6FeCoTi3O18 (BFCTO) target are introduced. Secondly, the technique of the laser molecular beam epitiaxy (L-MBE) is introduced, including the methods and theory of the reflection high-energy electron diffraction (RHEED). L-MBE has a great advantage in preparation of complex oxide films. Then, we introduce the characterization methods of the structures of the film, such as Atomic Force Microsope (AFM), X-ray Diffraction (XRD), Scanning Transmission Electron Microscope (STEM). Finally, we introduce the characterization of properties, such as Physical Property Measurement System (PPMS) and Superconducting Quantum Interference Device (SQUID).In chapter three, we successfully fabricate high-quality BFCTO epitaxial thin films on (LaA103)o.3(Sr2AlTa06)o.7 (LSAT) substrate by L-MBE. Then, we systemically study the effects of growth temperature and oxygen pressure on the BFCTO film. We found that the range of growth window is very small. When the substrate temperature is high or oxygen pressure is low, the layer period n decrease, and the secondary spinel phase become observable. In contrast, when the substrate temperature is low or oxygen pressure is high, the BFCTO thin films exhibit rougher surfaces and lower single-crystalline qualities. Furthermore, we characterize the properties of the ferroelectricity and ferromagnetism of the films, and the results indicate coexistence of ferromagnetism and ferroelectricity of the BFCTO films at or above room temperatures.In chapter four, we choose two different underneath layers, LAST and LaNiO3 (LNO) buffer layer, to grow epitaxial BFCTO thin films. We find the great differences of optimal deposition temperature for growth on conductive layers and on insulating bottom layers. Subsequently, we grow many films around the optimal conditions, which confim that the large growth temperature disparities are real. Then, we design the experiments to confirm that the optimal growth temperature is corrrelated with the conductance of the bottom layer. Finally, we give preliminary discussions on the reason of the difference of the growth temperatures.In chapter five, the effects of the interface on the quality of thin films are discussed. BFCTO films have been grown on two different types of underneath layers: insulating LSAT substrates and conducting LNO buffer layers. It is found that the conductive LNO buffer layer can assist the growth of BFCTO thin films, resulting to significantly enhanced epitaxial quality. Then, we find the difference of the atomic interface between the two types of samples using STEM. It is found that the interface between BFCTO and LSAT is very rough, while the interface between BFCTO and LNO is uniform and sharp. The sharp interface in the latter can be explained by the screening effect of the conductive LNO. Finally, we show the results of the ferromagnetic and ferroelectric measurements of the high quality BFCTO films grown on the LNO buffer layer.In chapter six, we conclude the whole thesis study and look forward to the future study.