Modulation Of Structures And Physical Properties In Magnetic Perovskite Oxide Thin Films By Interface Design

Perovskite oxides cover a series of important physical phenomena such as ferromagnetism,antiferromagnetism,ferroelectricity,ferroelasticity,magnetoelectric coupling,superconductivity,and two-dimensional electron gas.The d-orbital electrons and its strong correlation effects of transition metal elements in perovskite oxides determine the strong coupling between lattices,charges,orbitals and spins,so that regulating one degree of freedom can effectively induce the response of other degrees of freedom,and obtain multifunctional devices with potential applications.Doping cations or applying physical pressure are usually utilized to change the charge,spin,and structure in bulk perovskite oxides,achieving manipulation and exploration of functions.Preparing perovskite oxides into thin films and forming heterostructures provides more effective strategies to modulate the structure and physical properties of oxides.In oxide heterostructures,heterointerfaces formed by materials with different structures,chemical compositions,or physical properties may produce structural symmetries and physical properties that are completely different from the bulk oxides,thus stabilizing novel metastable phases.Interface effects such as interfacial structural coupling,charge transfer,interfacial magnetic coupling,and inversion symmetry breaking have been extensively explored.However,limited by the interfacial quality and the complex coupling effect at the interface,the interfacial modulation mechanisms obtained by each research group are different,which is largely due to the competition of various interactions at the interface and the resulting metastable states with close energy.Therefore,discovering new interface effects and interface-induced novel phases,and understanding the modulation mechanism of interface effects on physical properties are very important issues in the research of condensed matter physics.This dissertation focuses on the following three interfacial modulation strategies:(1)The structural coupling,especially the interfacial oxygen octahedral coupling at the atomic level interface,is utilized to control the structure and physical properties of oxide thin films.This strategy requires high interfacial quality of epitaxial thin films,and this coupling effect can only effectively propagate at atomically sharp interfaces.In this thesis,the high-quality SrRuO3(SRO)/LaCoO3(LCO)heterostructures are taken as the research objects.By utilizing the oxygen octahedral rotation phase mismatch between SRO and LCO,the SRO films are stabilized at different symmetries and the lattice modulation of the magnetic anisotropy of SRO are realized.(2)Another extreme case is the modulation of film properties by the large structural mismatch at the interface.For example,when perovskite oxides are grown on non-perovskite substrates,the interfacial structural mismatch can seriously change the oxide structures(resulting in polycrystalline or noncrystalline oxide films)and properties,which is increasingly demanded for practical applications.In this thesis,the large structural mismatch at the interface of La2/3Sr1/3MnO3(LSMO)/MgAl2O4(MAO)is utilized to induce the disordered amorphous phase of LSMO films and achieve a ferromagnetic insulating state close to room temperature.(3)Interfacial magnetic coupling,which requires the interface quality being between the above two cases,is an important method to manipulate the spin structure and is the magnetic exchange interaction created by different magnetic oxides at the interface.Unlike the interfacial structural coupling,it can occur even at interfaces without chemical bonding.In this thesis,taking advantage of the large difference in magnetic anisotropy between SRO and LCO,a non-collinear spin structure is created in the SRO layer of the SRO/LCO superlattice,and interesting magnetic transport phenomena are induced.The dissertation is divided into the following chapters for discussion:In the first chapter,the structure and distortion of perovskite oxides,and the utilization of interfacial oxygen octahedral coupling to control the structure and magnetic anisotropy of oxide films were introduced.Then,the changes in the structure and properties of thin films caused by the interfacial structural mismatch were introduced when perovskite oxide thin films are grown on non-perovskite substrates.In the last part of this chapter,starting from the physical basis of chiral spin structures,the chiral spin structures induced by interfacial magnetic interactions in perovskite oxide films(especially SRO films)and their characterization methods were introduced.Finally,the idea of the topic selection of this dissertation was expounded.The second chapter mainly introduces the preparation and characterization methods of perovskite oxide thin films involved in this thesis.The oxide epitaxial films were prepared by laser molecular beam epitaxy.The crystal structure of the films was mainly characterized by X-ray diffraction and asymmetric diffraction reciprocal space mapping.The flatness of interface of atomic level quality was determined by X-ray reflection,and the oxygen octahedra at the interface were observed by transmission electron microscopy.And the electrical transport and magnetic characterization of thin films were introduced.In Chapter 3,the metastable phases and lattice-dependent magnetic anisotropy of SRO thin films by interfacial oxygen octahedral coupling were realized.The SRO films of different thicknesses were prepared by laser molecular beam epitaxy on SrTiO3 substrates with an LCO buffer layer.X-ray asymmetric diffraction reciprocal space mappings and atomic imaging were utilized to analyze the structural symmetries of SRO films.The results show that SRO films were stabilized in the distortedrhombohedral and tetragonal phases in turn as the films thickness increases.The magnetic and transport measurements demonstrate that the SRO films exhibited strong lattice-dependent magnetic anisotropies.This work provides systematic experimental evidence for utilizing oxygen octahedral rotation phase mismatch to induce the novel phases and properties of perovskite oxides,and confirms its feasibility.In Chapter 4,the non-crystalline phases and ferromagnetic insulating state of LSMO films by the interfacial structural mismatch were studied.Using X-ray diffraction,reflection and Rutherford backscattering,the LSMO films were confirmed to be amorphous and disordered and possess consistent stoichiometry as the target.Electrical transport and magnetic measurements show that the amorphous LSMO films possess a ferromagnetic insulating state close to room temperature over a wide thickness range.The fitting results of the transport curves demonstrates that the insulating state originates from the disorder and defects of the films.This work provides a new idea for applying perovskite oxides to spintronics research.In Chapter 5,the non-collinear spin structure of SRO films was successfully realized by means of magnetic exchange at the interface between a perpendicular anisotropic ferromagnetic metal SRO and an in-plane anisotropic ferromagnetic insulator LCO.The SRO and LCO thin films were constructed as superlattices,and the spin structure within the SRO layers was explored through anomalous Hall effect and magnetic field-angle-dependent magnetotransport measurements.The anomalous Hall effect of SRO can be explained by the "two-channel model",which confirms the existence of the non-collinear spin structure.Magnetic field-angle-dependent magnetotransport measurements confirm that the non-collinear spin structure originates from the interfacial magnetic interaction and the intrinsic perpendicular anisotropy of the SRO.This work provides a new way to induce nontrivial spin structures.In Chapter 6,the brief summary of the research work in this thesis is presented,and the future research prospects are proposed focusing on the limitations of the research work in this thesis.