Realization Of Sr₄Fe₆O₁₃ Epitaxial Film And Its Growth Phase Diagram

With the size of semiconductor devices approaching the physical limit,it is also very important to improve the performance of the device itself while further miniaturizing the integrated circuit.Optimizing the functional materials used in these devices is an important step.Perovskite-like oxides have attracted much attention due to their great flexibility in structure and composition regulation.As a kind of perovskitelike material,Sr4Fe6O13 is famous for its excellent ion-electron mixed conductivity and has become one of the research hotspots.In addition to its broad application prospects in the field of electrical transport,it has also shown great potential in electromagnetic regulators,spintronic devices,oxygen separation filters,and gas detectors,etc.However,most of the current researches focus on polycrystals and lacks high-quality single crystals to explore their intrinsic properties,such as ferromagnetism and unique mixed conductivity.At the same time,due to the close Gibbs free energy of strontium iron oxide with different stoichiometric ratios,it is difficult to achieve high-quality single crystal preparation by traditional methods.Pulsed laser deposition technology is a good alternative for the preparation of high-quality single crystals.The single crystal epitaxial film deposited by this technology will have a certain orientation due to the effect of the substrate,and there will be substrate induced stress in it.Therefore,it is of great significance to find suitable growth conditions to grow high-quality Sr4Fe6O13 epitaxial films and then investigate their properties.This can not only enrich the research system of perovskite-like oxides,but also effectively provide high-quality single crystal samples for the research of Sr4Fe6O13,thus promoting the research on the intrinsic properties and related applications of this kind of materials.This thesis is devoted to the preparation of high-quality Sr4Fe6O13 epitaxial films,and the growth conditions of epitaxial films in strontium ferrite system have been systematically studied.In this paper,the specific research content can be divided into two parts.The first part is to compare the phase and quality of each film under different growth conditions,determine the best growth conditions of Sr4Fe6O13 epitaxial films,and carry out the structural characterization of the films,including X-ray diffraction,high-angle annular dark field scanning transmission electron microscopy and so on,as well as the magnetic properties of the film.Under the optimal growth condition,the high-quality epitaxial films with no impurity,no obvious line defects and clear Von Laue oscillations in X-ray diffraction can be prepared.In the second part,the experimental growth phase diagram of strontium ferrite epitaxial film is summarized by analyzing the phase of the epitaxial film deposited under different growth conditions.The calculated results of energy based on density functional theory are introduced into thermodynamic analysis,and the calculated growth phase diagram is compared with the experimental results.The paper is divided into the following chapters:In the first chapter,based on perovskite-like materials(including Sr4Fe6O13),the crystal structure and common preparation methods of them are briefly introduced.Then,the magnetic properties,electrical properties,and other properties of Sr4Fe6O13 are reviewed.Its rich application prospects are also given.Finally,the research status of Sr4Fe6O13 and its meaningful scientific issues in this field are summarized,following the research contents of this paper.The second chapter of this paper mainly introduces the material preparation methods involved in the experiment process of this paper,and focuses on the pulsed laser deposition system.On the other hand,this chapter also gives a detailed introduction to the material characterization technology used during the whole test process,including the film sample quality and structure characterization technology(such as X-ray diffraction technology,X-ray reciprocal space imaging,and high-angle annular dark-field scanning electron microscope),the element analysis methods of the films(energy spectrometer and X-ray photoelectron spectroscopy),and the performance measurements of the film.In the third chapter,the most suitable growth conditions for high-quality Sr4Fe6O13 epitaxial films are discussed and determined by adjusting the growth condition parameters during the films’ growth.A series of structural measurement results provide strong macroscopic and microscopic evidence for the successful growth of the epitaxial film.Subsequently,the average valence state of Fe and oxygen vacancy content in the film is determined.At the end of this chapter,the test of film properties is described.By comparing the hysteresis loop of Sr4Fe6O13 epitaxial film and SrTiO3:Nb(0 0 1)substrate,it is found that the film’s magnetism changes,which may root in the tetragonal structure with higher symmetry of the grown film than that of the ceramics.The above research can provide scientific reference for other researchers to grow Sr4Fe6O13 epitaxial films,and its good single crystal interface can be used to study the origin of many physical phenomena.In the fourth chapter,the corresponding phases of the films under different growth conditions are summarized,and then the corresponding phase diagrams of the films are drawn.In this chapter,by analyzing the phase of films under different growth conditions,the experimental growth phase diagram is summarized to reflect the coupling relationship between different growth conditions.The high quality Sr4Fe6O13 epitaxial films are more inclined to grow in a slight oxygen environment.Then,the stable material energy calculated based on the density functional theory is introduced into the Gibbs free energy change of the formation of Sr4Fe6O13 and SrFeO2.5,getting the relationship between the temperature,oxygen pressure and ΔG.During the process of the film growth,the phase competition between SrFeO2.5 and Sr4Fe6O13 is revealed.This method can throw new light on the epitaxial growth of oxides with complex structures and contribute to the development of research on the growth of oxides with complex structures.The last chapter summarizes the full text and puts forward some prospects according to the obtained results.