Low-Dimensional Magnetic Ferrites:Probing Of Ion Occupancies,Tailoring Of Magnetization And Their Practical Applications

Low-dimensional magnetic ferrites have been widely used in modern civil and military industries due to their novel physical properties,distinguished magnetic properties and outstanding chemical stability.The microscopic crystal structure of ferrites,that is,the occupation and distribution of ions,determines their microscopic magnetic structure and macroscopic magnetic properties.The observation and analysis of ions occupancies and distribution,the relationship between microstructure and microscopic magnetic structure,and the effect of magnetic anisotropy on magnetization distribution at the atomic scale are not only helpful to deeply understand the origins of ferrite magnetism,but also provide a guidance for the tailoring of ferrite performance and even the design and applications of magnetic multifunctional devices.This thesis systematically studied the microstructure（orthogonal perovskite structure,garnet structure and spinel structure）of low-dimensional magnetic ferrite nanofibers with rich structures and excellent performance by using spherical aberration-corrected scanning transmission electron microscopy（Cs-STEM）.The macroscopic magnetic properties and distribution of microscopic magnetic structures were obtained by using macroscopic physical property measurement systems and advanced electron holography.By linking the microstructures,magnetic moment distribution and magnetic properties,the magnetism origin of the ferrites and the effect of magnetic anisotropy on the magnetization distribution are revealed at the atomic scale.Based on the obtained results,the magnetic properties of the ferrites were then precisely regulated.Taking Co_0.5Ni_0.5Fe₂O₄ as an example,the application prospect of one-dimensional multi-shelled transition metal oxide nanofibers in the field of lithium ion energy storage was explored.The main research contents are summarized as follows:1.Atomic observation of cation distribution in rare-earth orthoferrites YFe O₃ and its magnetic property.YFe O₃ single-particle-chain nanofibers were prepared by an electrospinning technique.It showed that the YFe O₃nanofibers were stacked with single particles each by each along the length direction.The morphology,chemical composition and crystal structures were characterized by low magnification HAADF-STEM,EDS-mapping and XRD.It was found that the YFe O₃ nanofibers were uniform in size and had a typical orthogonal perovskite structure with a space group of Pnma（62）.The accurate occupation and distribution of cations were further studied by Cs-STEM.The results showed that each particle stacked into the nanofibers is a single crystal structure,and their crystal orientations are random.Their magnetic property measured by VSM revealed a 26,772 Oe coercivity（H_c）and weak ferromagnetic phase at zero field.2.Ion occupying analysis of Y₃Fe₅O₁₂nanofibers and its shape anisotropy effect on micromagnetic structure.The Y₃Fe₅O₁₂ single-particle-chain nanofibers with garnet structure were preparedby electrospinning method too.The occupied sites and orderly distribution of cations in the Y₃Fe₅O₁₂ single-particle-chain nanofibers were investigated at the atomic scale by Cs-STEM and EDS-mapping.The spontaneous magnetization and external-field-magnetized magnetization of a single nanofiber were captured by electron holography.The magnetization distribution of the single nanofiber was distributed along the axis and was independent of crystal orientation.However,in the process of applying a vertical magnetic field,the easy magnetization direction[111]was preferentially saturated.By combining the microstructure and magnetization state,the experimental result showed that the magnetization distribution of Y₃Fe₅O₁₂ single-particle-chain nanofibers is dominated by shape anisotropy,whilst,its the magnetization anisotropy has a weak effect.3.Atomic-scale imaging of dopant sites in Ni-doped ideal normal spinel ZnFe₂O₄nanofibre and its correlated magnetism origin.Direct identification of the cationic occupation and distribution in Ni-doped normal spinel ZnFe₂O₄ is not only helpful to understand its physical mechanism of tunable magnetic properties,but also provides a theoretical basis for the synthse of specific functional ferrites.In this chapter,ZnFe₂O₄ and Ni-doped ZnFe₂O₄ single-particle-chain nanofibers were prepared by electrospinning technology.Then,an ordering distribution of cations in Ni-doped normal spinel ZnFe₂O₄ was directly observed.The data evidently clarified that divalent Zn²⁺cations occupied all tetrahedral sites and trivalent Fe³⁺cations occupied all octahedral sites in the ZnFe₂O₄ single-particle-chain nanofibres.When Ni were doped,Ni²⁺preferentially occupied the octahedral sites,and the excess Fe³⁺were squeezed to the tetrahedral sites.The redistribution of cations can cause changes in magnetic properties.The doping of ZnFe₂O₄ by Ni can improve the magnetic properties by analyzing the data of PPMS and electron holography.Electron holography and micromagnetic simulations further disclose that shape anisotropy and dipolar anisotropy dominated the magnetic moment distributions of the Ni-doped ZnFe₂O₄.4.The design and synthesis of spinel one-dimensional multi-shelled nanostructures for energy storage applications.Rational design,synthesis and massive production of one-dimensional（1D）spinel composite oxides with multi-shelled nanostructures are critical to the realization of highly efficient energy conversion and storage.In this chapter,we designed a simple and general electronspinning method to fabricate 1D transition metal oxides with complex architectures.It was found that the concentration of precursor polymers PAN could control the structures of the products at a reasonable heating rate,including hollow nanofibers,wire-in-tube nanofibers and tube-in-tube nanofibers.This technique was extended to various inorganic multi-element oxides and binary-metal oxides,such as:Co_0.5Ni_0.5Fe₂O₄,Co Fe₂O₄ and ZnFe₂O₄.More twin boundaries（TBs）were found within the Co_0.5Ni_0.5Fe₂O₄tube-in-tube nanofibers.Benefiting from the porous multi-shelled nanostructures and twin boundary defects,the tube-in-tube hollow nanostructures possess superior electrochemical performances with high energy and stability in lithium-ion storage.