Direct Atom-resolved Imaging Of Ion Occupations In Low-dimensional Ferrites And Correlated Magnetic Properties

The determination of the atomic structure of magnetic ferrite materials is critical for an understanding of their magnetic properties.A comprehensive study of the correlation between atomic structure and magnetic properties is essential and significant for not only guiding the optimization of material properties,but also facilitating the designs of desired electronic devices.In this thesis,we systematically studied three representative types of ferrites,including spinel ferrite,magnetoplumbite ferrite and garnet ferrite,by using aberration-corrected scanning transmission electron microscopy?STEM?analysis combined with the traditional characterization methods,theoretical analysis simulation and the first-principles calculation.Great efforts have been devoted to deeply explain the correlation between atomic structure and magnetic properties.And the effects of element doping on the atomic structure of ferrites have been further investigated,which should be significant for not only tailoring magnetic properties,but also preparing high performance ferrites.The main contents are summarized as follows:1.Preparation and characterization of single-particle-chain CoFe₂O₄nanofibres and its magnetization reversal mechanism.The correlation between microstructure and magnetic properties and the dynamic magnetization behavior were systematically studied in CoFe₂O₄ ferrites.Single-particle-chain cobalt CoFe₂O₄ ferrites were synthesized by electrospinning.The morphology,chemical composition and interface between the two particles were systematically characterized by scanning electron microscope and transmission electron microscopy.The results show that the nanofibres have a continuous structure and relatively uniform distribution.Lattice-resolution HRTEM images of the interface of two neighbored crystallites on a single CoFe₂O₄ nanofiber demonstrate that individual CoFe₂O₄ nanoparticles stack along the nanofiber axis and each particle has a random orientation.Then we used the off-axis electronic holography to perform direct observations of the micromagnetic structure of individual CoFe₂O₄ nanofibres.Micromagnetic simulation performed by OOMMF software was carried out to obtain magnetization distribution of a single nanofiber for a comparison with the experimental results,which discovers that the magnetization reversal process of individual CoFe₂O₄ nanofibres should be governed by the coherent rotation of the moments.2.Direct observation of cation distributions of ideal inverse spinel CoFe₂O₄nanofibres and correlated magnetic properties.The correlation between atomic structure and magnetic properties of ideal inverse spinel CoFe₂O₄ nanofibres was investigated.We provide a first observation of the distribution of cations in an ideal inverse spinel structure of CoFe₂O₄ nanofibres by using aberration-corrected transmission electron microscopy?TEM?,which were further verified by QSTEM simulations.The results show that half of trivalent Fe³⁺cations occupy all tetrahedral A sites and half of the Fe³⁺cations,and all the divalent M²⁺cations occupy the octahedral B sites.The ordering of Co²⁺and Fe³⁺at the octahedral sites imaged along either[001],[011]or[-112]orientation was identified as 1:1,in accordance with an ideal inverse spinel structure.Simultaneously,theoretical HAADF-STEM images of normal spinel structure of CFO nanofibres were also simulated,which give a good fitting of converse contrast of cation occupations in comparison with those of inverse spinel CFO nanofibres.The saturation magnetisation calculated based on the crystal structure as determined from the TEM image agrees well with that measured experimentally on the spinel CoFe₂O₄nanofibres,further confirming results from TEM.3.Direct observation of cation distributions of Zn-doped normal spinel MgFe₂O₄ ferrite and its magnetic properties.The atomic structures of normal spinel MgFe₂O₄ ferrite and the effects of Zn²⁺ions on the microstructure and magnetic properties were studied.The accurate occupations of cations in MgFe₂O₄ ferrite was studied by aberration-corrected transmission electron microscopy.The results suggest that the MgFe₂O₄ nanofibres in this work should have an ideal normal spinel structure.The experimental STEM images under three orientations are the same as the three schematic oriented crystal unit-cell models.We further studied the preferred sites of Zn²⁺ions in the Zn-doped MgFe₂O₄ ferrite nanofibres.Our measurements reveal that the doped Zn cations prefer to occupy partial Fe³⁺at the octahedral B sites.Combined with magnetic measurements,it can be found that the saturation magnetization of the MFO nanofibres increases after Zn-doping,and the coercive force and the Curie temperature decrease to different extents.4.Direct imaging of dopant sites in rare-earth-element-doped permanent magnet and correlated magnetism origin.The atomic structures of M-type strontium hexaferrite with magnetoplumbite structure and the effects of the doped La cations on the microstructure and magnetic properties were systematically investigated.Three relevant atomic resolution HAADF-STEM images of M-type strontium hexaferrite projected from[001],[110]and[210]three orientations were first imaged by aberration-corrected transmission electron microscopy?TEM?and ABF technique was employed to view the image of O atoms.Theoretical STEM images of the SFO with ideal magnetoplumbite structure were also simulated by using QSTEM,which agree well with the experimental ones.We then present a direct observation of the preferred atomic sites of La atoms in La-doped M-type SrFe₁₂O₁₉ hexaferrite.Our data solidly clarify that only the Sr²⁺cations are replaced by La³⁺cations and the La-doping causes the changes of valence states of iron cations located at 4f₁ and 2a crystallographic sites.First-principle calculations further unveil that after La-doping the changes of the spin states of Fe³⁺cations located at 4f₁ tetrahedral sites result in the magnetization enhancement and those of 2a octahedral sites contribute electrical neutrality,well matching with the experimental atomic-column resolution EELS and magnetic measurements.5.Synthesis and cation distributions of garnet structural Tm₃Fe₅O₁₂ ferrites.Garnet structural Tm₃Fe₅O₁₂ epitaxial film grown on[111]-oriented SGGG?substituted GGG?substrates have been successfully prepared by Pulsed Laser Deposition?PLD?method and the chemical composition and interface were systematically characterized.Subsequently,we use aberration-corrected transmission electron microscopy to obtain the atomic-level STEM images taken along the[0-11],[012],[11-2]and[1-22]axes.The corresponding theoretical unit-cell models were made respectively.The analytical results display that the experimental STEM atomic occupations are completely consistent with the theoretical site occupations of the cations in the garnet structural TIG ferrites,which plays a guiding role to further study the correlation between the crystal structure and the magnetic properties and invent new spintronic devices.