Fabiracation Of Novel Magnetic Single-crystalline Oxide Thin-films And Exploration Of The Mechanism Of Their Multi-degree-of-freedom Coupling

Perovskite oxides have been at the frontier of materials science and condensed matter physics researches.In the middle of last century,materials such as BaTiO3 and PbTiO3 attracted considerable interests in the scientific community.Later in 1980s,layer structured copper oxide compounds became the star-materials owing to their high temperature superconductivity.Rapid developments of quantum-spintronic devices and information storage have attracted more and more attentions to the magnetic oxides,especially perovskite or layered-perovskite materials.Due to strong correlations,perovskite or layered-perovskite oxides exhibit rich physical properties and excellent application potentials.Explorations of high-quality synthesis of single-crystalline thin films stand as not only the foundation of finding new physics and new material properties,but also the premise of prototype devices and commercial products.Understanding the coupling between multi degrees of freedoms,including lattice,charge,spin and orbital,is a necessary step to explore the underlying mechanism.Besides,it is an indispensable part before the real applications of magnetic oxides.Owing to the structural differences,the research contents are composed of two parts.One is the simple perovskites and the other is the more complex layer structured perovskites.The first part(in chapter 3)focuses on the study of multi-degree-of-freedom coupling in precisely synthesized high-quality thin films of perovskite structures and the manipulation of their magnetic state.Based on the first part,the second part(in chapter 4)is mainly about the investigation of magnetic and multiferroic properties of complex layer-structured perovskite oxides with large periods.Great efforts have been made to the synthesis of high-quality films,the exploration of their physical properties and understanding of the physical mechanism in the changing-period and changing doping-concentration samples.The main contents of chapter 1 include the characteristics of perovskite oxides,the magnetic interactions,and the recent developments in strained LaCoO3 films and multiferroic Aurivillius oxides.Chapter 2 introduces the experimental methods employed in the study,including the thin film growth,characterizations of surface morphology,electric and magnetic characterizations,the electronic structure,and the nano-device fabrication.Chapter 3 describes the research we conducted on strained LaCoO3 films with simple perovskite structure,including the investigation of the electro-magnetic performances and tuning of the multi-degrees-of-freedom coupled magnetism.Using TiO2 terminated SrTiO3 substrates and in-situ reflective high energy electron diffraction(RHEED),we fabricated different series of films in the pulsed laser deposition system.Meanwhile,the effects of strain and defects(charge doping)on the ferromagnetism were systematically studied.We eventually found an approach to fabricate the LaCoO3 films as an intrinsic and high temperature perovskite ferromagnetic insulator.It stands as a great progress in the development of ferromagnetic-insulator based spintronic devices.Moreover,in chapter 3,we carefully explored the thickness dependences of the lattice structure,surface morphology,magnetic domain structure,magnetic dynamics,electric conductivity and optical absorptions.We were able to manipulate the magnetic domain structure and magnetic anisotropy by varying the film thickness.In this part of work,we also found interesting LaAlO3 thickness effects on the LaCoO3 magnetic behavior in the LaCoO3/LaAlO3//SrTiO3 bilayer samples.With the half-order peaks measurements,scanning transmission electron micrograph and soft X ray absorption results,we have obtained better understanding of multi-degree-of-freedom coupling between strain,charge,and oxygen octahedral rotation near the interface.In Chapter 4,we found an effective route to synthesize and character the complex layer-structured perovskite oxides.Firstly,in order to realize the periodic strcutere tuning effect on the physical properties,we attempted to synthesize unreported Bi11(Fe5CoTi3)10/9O33 films with super large period of 10 and explored their new magnetic and ferroelectric properties.We found that the film with a self-modulated structure exhibited ferromagnetism and ferroelectricity at room temperature.Secondly,to realize tuning of the physical properties by element doping,we used a two-target deposition method to obtain doped films with systematically varied doping concentrations.Doping concentration was found to influence both the lattice structure and the electronic structure.Under-doped films with the targeted structure exhibit enhanced ferromagnetism and ferroelectricityIn Chapter 5,we briefly summarize the entire thesis and the key points of the innovation.Meanwhile,some future research plans are also proposed.