Multiferroic And Two-dimensional Magnetism Study Of Perovskite-type Oxide Quantum Materials

Perovskite oxides and their layered siblings are important functional materials,and they have attracted tremendous interests in recent years due to their fertile physical properties.The perovskite and layered perovskite magnetic oxides play a crucial role in the applications of multi-state storage,sensors and spintronic devices.Multiferroics and two-dimensional magnets are two kinds of novel magnetic quantum materials.Multiferroics refer to materials that are ferrolectric and magnetic(ferromagnetic or antiferromagnetic)simultaneously.The applications of multiferroics require the strong coupling between ferroelectricity and magnetism at room temperature.Whereas,due to the incompatibility between ferroelectricity and magnetism,it is difficult for the coexistence of ferroelectricity and magnetism in single-phase materials above room temperature.BiFeO3 is the most widely investigated multiferroic material in experiments.It exhibits a coexistence of ferroelectricity and anti-ferromagnetism above room temperature.Up to now,multiferroics with the coexistence of ferroelectricity and ferromagnetism in single-phase materials above room temperature have rarely been realized experimentally.Aurivillius phase oxides have a unique layer structure which is comprised of fluorite-like(Bi2O2)+2 and perovskite-type layers.Inserting several ferromagnetic perovskite layers into the ferroelectric matrix may lead to the coexistence of ferroelectricity and ferromagnetism in single-phase materials above room temperature.Different from ferromagnetism in three-dimensional cases,the long-range order in two-dimensional systems can be suppressed by thermal fluctuations and spin excitations.There is no ferromagnetic ordering in isotropic two-dimensional Heisenberg models at finite temperatures.While the ferromagnetic ordering in two-dimensional systems can be stabilized by strong magnetic anisotropies.Thus,the long-range magnetic ordering can exist in two-dimensional Ising and XY models.Because of the significant values of two-dimensional magnetism in both theoretical and application aspects,there have been much progress made in developing two-dimensional ferromagnetic materials.Whereas,until now,there are very limited progress in oxides,considering that realizing two-dimensional ferromagnetism in oxides will greatly facilitate the integration with modern electronic technologies based on semiconductor industries.In this thesis,we mainly focus on two parts,i.e.,the studies of magnetism in Aurivillius phase oxides and two-dimensional ferromagnetism in SrRuO3/SrTiO3 superlattices.We modulate the magnetic properties of Aurivillius phase oxides and SrRuO3/SrTiO3 superlattices by changing the dopants and lattice periods.And we performed comprehensive investigations on the electronic structures of the films using soft X-ray absorption spectroscopy,X-ray emission spectroscopy,and resonant inelastic X-ray scattering.We explore the underlying linkage between the ferromagnetism and electronic structures,to further understand the mechanism of ferromagnetism in these films.The main contents of the thesis include:The first chapter is the general overview of the thesis,we mainly introduce:(1)The concept,research background and mechanism of multiferroic materials.(2)The lattice structure of Aurivillius phase oxides and their current situation,we mainly introduce the studies on ceramics and epitaxial films.(3)The basic knowledge,research background and current situation of two-dimensional magnetic materials.The second chapter is a brief introduction of the fabrication and characterization of epitaxial thin films.We first introduce the pulsed laser deposition method and the reflective high energy electron diffraction systems.We use X-ray diffraction and atomic force microscope to characterize the crystalline structures and surface morphologies of the films,respectively.And we use physical property measurement system and superconducting quantum interference device to measure the electric and magnetic properties of the films.Finally,we introduce the principle and technology of X-ray absorption spectroscopy,X-ray emission spectroscopy,and resonant inelastic X-ray scattering,and their applications in detecting the electronic structures of materials.In chapter three,we use pulsed laser deposition to grow Bi6Fe2Ti3O18,Bi6FeCoTi3O18,and LaBi5FeCoTi3O18 epitaxial thin films on(LaAlO3)0.3(Sr2AlTaO3)0.7 substrates.We improve the magnetism of Bi6FeCoTi3O18 by the doping of Co and La,and investigate their electronic structures using X-ray absorption spectroscopy,X-ray emission spectroscopy,and resonant inelastic X-ray scattering.The X-ray absorption spectroscopy measurements demonstrate that the doping of Co and La reduces the hybridization between Ti/Fe/Co and O 2p orbitals in Bi6FeCoTi3O18 and LaBi5FeCoTi3O18 films,and the doping of La can reduce the concentration of oxygen vacancies.The experimental and simulated resonant inelastic X-ray scattering studies demonstrate that the doping of Co and La can modulate the crystal-field parameters of FeO6 and CoO6 octahedra.The X-ray absorption spectroscopy and resonant inelastic X-ray scattering measurements suggests that the evolution of electronic structures results from the lattice distortions in the films.The larger lattice distortion will enhance the canting of Fe and Co ions,which offer experimental evidences for the intrinsic ferromagnetism in Aurivillius phase oxides.In chapter four,we use pulsed laser deposition to prepare atomic-layer SrRuO3/(SrTiO3)N(N = 1-5)superlattices on SrTiO3 substrates.We modulate the magnetic performance of the superlattices by varying the thickness of SrTiO3 layer.With increasing the SrTiO3 layer number,the magnetic transition temperature of the superlattices decreases.Our mgnetotransport measurements demonstrate that the insertion of SrTiO3 layer induce a gradual transition from two-dimensional Ising ferromagnetism to three-dimensional non-Ising ferromagnetism in the superlattices.The two-dimensional Ising ferromagnetism depends both on the temperature and SrTiO3 layer number.The X-ray absorption spectroscopy measurement reveals that,owing to the lager electronegativity of Ru atoms than Ti atoms,the electrons of apical oxygen atoms at the interfaces between SrRuO3 and SrTiO3 will be polarized to Ru atoms,resulting in the orbital anisotropies between Ru/Ti dxy and dyz,zx orbitals.With the increasing of SrTiO3 layer number,the anisotropy between Ti 3dxy and 3dyz,zx orbitals increase while the anisotropy between Ru 4dxy and 4dyz,zx orbitals decreases.This suggests that the transition from two-dimensional Ising ferromagnetism to three-dimensional non-Ising ferromagnetism in SrRuO3/SrTiO3 superlattices can be attributed to the orbital reconstructions of inerfacial Ru and Ti ions.In chapter five,we summarize the thesis and give an overlook of future studies on Aurivillius phase oxides and SrRuO3/SrTiO3 superlattices.