Synthesis And Magnetic Field-Induced Assembly Of Nanoscale Magnetic Materials

Driven by the promising applications in high-density magnetic data storage media, ferrofluids, and biomedicine, an important research effort has been directed towards the study on the synthesis of nanoscale magnetic materials. Moreover, ordered functional structures of magnetic nanomaterials have some brand-new properties resulted from the spatial orientation and arrangement of the building blocks and can be viewed as the foundation for the formation of novel miniaturized nanodevices. So research on the assembly of nanoscale magnetic materials is very important from the point of view of science and applications.The objective of this dissertation is to explore new avenue for the solution-based synthesis of nanoscale magnetic materials, investigate the relationship between their magnetic parameters and their structure (size and/or shape), and develop some simple and effective technologies for assembling the magnetic block buildings into highly ordered functional structures. The main parts of the results are summarized below:1. Single-crystalline, square-like Bi₂Fe₄O₉ nanosheets were synthesized via a mild hydrothermal process using ferric nitrate, bismuth nitrate and sodium hydroxide as starting materials. The edge length and the thickness of a representative square-like nanosheet were 100-150 nm and 17.5 nm, respectively. The squareness and the size (edge length and thickness) of nanosheets could be adjusted to a certain extent through varying the concentration of metal ions and OH^- and adding the surfactant CTAB. Possible mechanisms for the formation of square-like nanosheets have been tentatively proposed on the basis of experimental data. Electron spin resonance (ESR) measurement suggested that the small size of Bi₂Fe₄O₉ nanosheets could effectively lower the Néel temperature (T_N) about 20 K and the novel sheet-like structure with thin thickness would induce the antisymmetric superexchange interaction between Fe to appear the weak ferromagnetic moment below T_N. These results would help to the more understand of the abnormal magnetic properties of other antiferromagnetic ternary transition metal oxide nanomaterials.2. Fe₃O₄/C and Fe₃O₄/α-Fe/C nanocomposites were synthesized through the formation of polymer gel with highly dispersed magnetic iron oxide nanoparticles in hydrothermal system, followed by carbonizing the gel at different temperatures under N₂ atmosphere, respectively. The diameter and the size distribution of spherical Fe₃O₄ nanoparticles could be controlled by varying the temperature and the concentration of oxidizing agent in the hydrothermal process. After the PVP gels was carbonized at 400℃, these spherical Fe₃O₄ nanoparticles were homogeneously embedded in amorphous carbon matrix. Increasing the annealing temperature to higher temperature (1000℃) could result in the partial reduction of iron oxide to a-Fe through nanoscale carbothermic reaction. Magnetic measurement indicated that the Fe₃O₄/C nanocomposites was superparamagnetic, but the Fe₃O₄/α-Fe/C nanocomposites showed obvious ferromagnetic. Due to the imbalance of the electromagnetic match, the as-prepared carbon-based magnetic nanocomposites showed the bad microwave adsorption in the 0.5-16 GHz. These results could be in favor of the investigation of other magnetic nanocomposites with excellent microwave adsorption in the range of GHz.3. A unique one-dimensional (1D) legumelike structure of cobalt nanoparticles was prepared by a simple magnetic field-induced assembly approach under the assistance of polyvinylpyrrolidone (PVP). In each "legume", cobalt nanoparticles were regularly aligned along the lines of magnetic force in a row with visible spacing between adjacent nanoparticles, which were permanently conserved through the interpenetration and bridging between PVP molecules adsorbed on the surface of Co nanoparticles even after the removal of external magnetic field. Varying the concentration of PVP and changing the strength of applied magnetic field could adjust to a certain extent the shape and size of one-dimensional legumelike structures. Magnetic measurement showed that the legumelike assemblies of cobalt nanoparticles could be ferromagnetically coupled and hence behaved as magnetic nanowires rather than as individual particles. This novel legumelike structure would provide a new model system for the study of magnetization and transport properties of one-dimensional ordered magnetic nanostructures.4. Single-crystalline magnetite nanoparticles with a relatively narrow size distribution were solvothermally synthesized in a water-alcohol mixed solvent solution using ferrocene ((C₅H₅)₂Fe), polyvinylpyrrolidone (PVP) and hydrogen peroxide (H₂O₂) as starting materials. These magnetite nanoparticles with average size of 48 nm exhibited almost standard cubic-like shape and were coated with a thick PVP layer (～8 nm) to form magnetite-PVP core-shell structure. Driven by strong magnetic dipolar attractions, magnetite nanocubes could be assembled into flux-clusure rings around 200-500 nm in diameter, which consisted of several to dozens of nanocubes. The rings had one-particle annular thickness and individual nanocubes were spaced fully together. It was found that only nanocubes, the average size of which was close to 50 nm, could be assembled into rings, while slight smaller particles were aligned in dipolar chains, suggesting that ring self-assembly could be produced by the degradation of dipolar chains that were metastable structure with respect to rings, and only larger nanocubes with strong magnetic dipoles could overcome the potential barrier for the transformation from chains to rings.