Orthorhombic Structure And Ferromagnetic Properties Of Freestanding Manganese Oxide Films

Perovskite oxides have been a major research in materials or physics field,because of their exotic and diverse physical properties,such as metal-insulator transition,phase separation,colossal magnetoresistance,spin-orbit coupling,and so on.Due to symmetry breaking and dimensional effect,as well as the demand for miniaturized electronic devices,perovskite oxide films have attracted increasing attentions.And manganese oxide film is one of the most widely studied material in magnetic perovskite oxide film,the study of its structure and physical properties is conducive to deeply understanding the coupling between multiple degrees of freedom in perovskite oxide,even exploring more novel physical properties and broadening its application.Different from the traditional epitaxial manganese oxide films as well as heterojunctions and superlattices,the method of water-soluble sacrificial layers provides a prerequisite for the fabrication of freestanding manganese oxide films.The research of multifunctional freestanding films provides a potential development direction for the maturation of integrated semiconductor devices and the rise of flexible electronic devices.Understanding the structure and magnetic properties of freestanding ferromagnetic oxide films is beneficial to broaden the research and application of freestanding films and promote oxide films toward deviceization.In this thesis,the first part introduces the structural and magnetic characteristics of perovskite oxides,as well as the current fabrication and research progress of freestanding oxide films.Then,it describes the fabrication of epitaxial thin films and the transfer process of freestanding thin films,as well as some material characterization methods of structural,magnetic and spectral measurement.Finally,the studies about the orthorhombic structure and ferromagnetism of freestanding manganese oxide films,including magnetic anisotropy,strain-coupled magnetism and magnetic proximity effect.The details are as follows:1.Photoinduced evolution of orthorhombic structure and magnetism.The application of ultrafast electron diffraction,which is strongly dependent on sample preparation,brings a technological innovation to uncover the photoinduced changes in the orthorhombic structure and magnetic properties of perovskite manganese oxides.Freestanding manganese oxide films placed on hollow copper grids exhibit features distinct from thermal steady state upon photoexcitation,including nearly conserved Bragg intensity,suppressed lanthanum and oxygen ion shifts,and long-range lattice orthogonality evolution.After 10 ps of photoexcitation,the material system entered a quasi-equilibrium state.In this state,an enhanced ferromagnetic order and an insulator-to-metal transition are predicted using first-principles calculations.This study demonstrates new opportunities in light-tuning the structure and properties of colossal magnetoresistive oxides,and provides new ideas for material selection and design of ultrafast switchable quantum devices.2.Modulation of magnetic anisotropy and multidirectional soft ferromagnetism.A series of freestanding manganese oxide films of different thicknesses were transferred onto a silicon substrate.Different from epitaxial films and thicker freestanding films,the 4 nm freestanding LaMnO3 ultrathin films exhibited soft ferromagnetism along both in-plane and out-of-plane directions.Meanwhile,it is accompanied by a ferromagnetic transition temperature up to 200 K and an increase in saturation magnetization.Energy loss spectra and theoretical calculations reveal the layered model of manganese valence state:the hydrogen-doped surface and interface are rich in Mn2+,while the interior of the film is Mn3+,and the uneven distribution of manganese valence states leads to the convergence of in-plane and out-of-plane magnetism.The discovery of isotropic soft ferromagnetism in freestanding singlecrystal films will facilitate the development of flexible magnetic devices that need to be regulated or detected in multiple directions,and can also simplify the fabrication of devices without being limited by magnetic anisotropy.3.Strain-coupled ferromagnetic superelastic oxide films.Different from the external bending or tensile strain,ferromagnetic oxide films that spontaneously form bending,folding or even multilayer rolls rarely exist.A series of freestanding manganese oxide films with different thicknesses were transferred to hollow copper grids,which still maintained stable single crystallinity and ferromagnetism.The state of spontaneous formation of films with different thicknesses is different,and the bending strain of the folded film with a thickness of 40 nm is the largest,which is 4%.Meanwhile,the saturation magnetization and remanent magnetization increase with the degree of bending.By fitting the coupling coefficient,it is proved that the magnetization and bending strain are coupled.The spontaneous formation of superelastic ferromagnetic oxide films with large strain is beneficial to promote the application of flexible functional materials and provide a development direction for the expansion of self-assembled flexible electronic devices in the magnetic field.4.Magnetic proximity effect in novel van der Waals heterostructures.The novel heterojunctions composed of complexly coupled transition metal oxides with multiple degrees of freedom and two-dimensional materials with valley spin characteristics can serve as a potential platform to study magnetic exchange interactions.The freestanding oxide films provide new possibilities to study the magnetic proximity effect of van der Waals heterojunctions on silicon-based substrates.Photo luminescence spectra with different circular polarization indicate that the interface of the novel van der Waals heterojunction has a magnetic proximity effect,and the temperature dependence of valley depolarization is similar to the magnetic transition of freestanding manganese oxide films.The novel van der Waals heterojunctions formed from transition metal chalcogenides and transition metal oxides demonstrate that magnetic thin films can provide magnetic field exchange via remanence without requiring any magnetic field for magnetization.