| In recent years, one-dimensional (1D) magnetic nanomaterials, such as nanotubes, nanobelts, nanowires and nanofibers, have attracted an intensive attention because of their distinctive physical and chemical properties from their bulk and nanoparticle counterpartes due to their large specific surface area, high aspect ratio and unique shape anisotropy, leading potential applications in high-density magntic recording, magnetic sensors, micro-nano functional devices, magnetic composites, spintronic devices, electromagnetic wave absorbing materials, calalyst, biomedicine, etc. Among the vaious methods for preparation of 1D micro-nanostructured materials, the electrospinning due to simple, low cost and high yield is proved to be a versatile and the most popular technique utilized in fabrication of functional micro/nanofibers with both solid and hollow interiors that are exceptionally long in length, uniform in diameter, and diversified in composition. Spinel ferrites are a kind of widely used soft magnetic materials in both industry and daily life, and their performances are found to be directly related to their stoichiometric composition, microstructure and morphology. Up to now, the researches on spinel ferrite 1D micro-nanostructures have been mainly focused on some simple spinel ferrites and a few for the complex spinel ferrites with excellent performances and wide applications. Therefore, there is important scientific interests and practical applications to develop 1D micro-nanostructured materials of the complex spinel ferrites and relative composites.In this dissertation, we combined electrospinning technique with sol-gel and heat treatment processes and carried out abundant research work on the controlled fabrication, structural adjustment and magnetic property characterization of the 1D complex spinel ferrite micro/nanofibers and relative composite materials. The main works and innovative results are as follows:1. Fabrication, structure and magnetic properties of the complex spinel ferrite micro/nanofibers. (1) We successfully prepared Mn0.5Zn0.5Fe204, Ni1-xZnxFe2O4(x= 0.0-0.8), Co0.5Ni0.5Fe2O4, Co1-xZnxFe2O4 (x= 0.0～0.5) and Li0.5-0.5xZnxFe2.5-0.5x04 (x= 0.0～0.8) binary spinel ferrite micro/nanofibers as well as Ni0.5-xCuxZn0.5Fe2O4 (x=0.0-0.5) ternary spinel ferrite micro/nanofibers with a narrow diameter distribution and uniform cross-section by calciantion of the electrospun PVP/inorganic component composite micro/nanofibers. (2) These micro/nanofiber samples were characterized using TG-DTA, FT-IR, XRD, SEM, (HR)TEM, SAED, EDS and VSM techniques. The effects of calcination temperature and chemical composition on the structures, micromorphologies and macro-magnetic properties of samples were systematically investigated and the reasons were analyzed and discussed. (3) We revealed and preliminarily understood the relationship between structure and performance of the investigated complex spinel ferrite in the one-dimensional or quasi one-dimensional case. It finds that the variation of the lattice constant with Zn content shows a nonmonotonic behavior and deviates from the Vegard's law for the Co1-xZnxFe2O4 nanofibers, but for Ni1-xZnxO4 and Li0.5-0.5xZnxFe2.5-0.5xO4 micro/nanofibers, this variation relationship basically complies with the Vegard's law. The single-domain critical sizes for Ni0.5Zn0.5Fe204, Co0.5Zn0.5Fe2O4, Co0.5Ni0.5Fe204, Lio.35Zno.3Fe2.35O4 and Nio.3Cu0.2Zn0.5Fe2O4 micro/nanofibers are estimated to be about 33,39,30,35 and 53 nm, respectively. Futhermore, the dependent relationships of the coercivity (Hc) on the average crystalline size (D) in a D range below the critical size are obtained for Coo.5Zno.5Fe204, Co0.5Ni0.5Fe2O4 and Ni0.3Cu0.2Zn0.5Fe204 micro/nanofibers. It is showed that Hc of these fiber samples follows a D power law with an exponent of 0.65, 0.70 and 0.71, respectively, which verifies the predicted results (i.e. Hc～D2/3) for 1D nanosystems based on the random anisotropy model. (4) In addition, we investigated the magnetic anisotropy and difference in magnetic behaviors between the fiber and the corresponding powder samples for the prepared Ni0.5Zn0.5Fe204 and Ni0.3Cuo.2Zno.5 Fe2O4 micro/nanofibers and reasonably explained the reason for the observed phenomena.2. Fabrication, structure and magnetic properties of the complex spinel ferrite/non-magnetic oxide composite micro/nanofibers. Based on the successful preparation of NiZn ferrite micro/nanofibers, we fabricated the non-magnetic Nio.5Zno.5Fe204/Si02 or Al2O3 micro/nanofibers by electrospinning technique combined with sol-gel process. The effects of SiO2 or Al2O3 additives on the phase, microstructure and morphology as well as macro-magnetic properties were studied in detail, and the related physical mechanisms were also discussed. The results indicate that the addition of amorphous non-magnetic oxides is an effective way to control and tune the microstructure and magnetic performances of spinel ferrite micro/nanofibers.3. Fabrication, structure and magnetic properties of the spinel ferrite/magnetic alloy composite micro/nanofibers. We successfully synthesized the Fe-Ni alloy/nickel ferrite composite micro/nanofibers with an average diameter of 170 nm by electrospinning and subsequent partial reduction process control technology for the first time. The crystal structure, phase composition, micromorpholgy and macro-magnetic properties of these micro/nanofibers were studied, in comparison with the pristine nickel ferrite micro/nanofibers before reduction. It finds that the synthesized composite micro/nanofibers consist of Fe-Ni alloy with the face centered cubic (FCC) and body centered cubic (BCC) mixed structures and nickel ferrite with the spinel structure. The two phases of the Fe-Ni alloy and nickel ferrite are well magnetic exchange-coupled, and consequently the prepared composite micro/nanofibers exhibit enhanced magnetic properties compared to the pristine nickel ferrite micro-nanofibers. The saturation magnetization and coercivity are increased from 49.5 emu/g and 17.4 kA/m before reduction to 103.9 emu/g and 36.6 kA/m after reduction, respectively. This new synthetic route can be conveniently expanded to prepare other types of magnetic metal or alloy/spinel ferrite composite micro/nanofibers as well.
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