Low Dimensional Magnetic Nanostructures:Controlled Synthesis, Nanoscale Characterization And Their Applications

Low dimensional magnetic nanostructure with well-controlled shape and size are recently one of the hottest research topics in material science and condensed matter physics, which is not only because of their unique and novel physical properties from fundamental point of view but also the future industrial demands to systematically reduce the size of spin and tunneling devices, racetrack memory, continuously increase the density of magnetic recording. In this thesis, several low dimensional magnetic nanostructures have been synthesized by electrospinning and liquid phase reduction method. Their structures and magnetic properties were comprehensively studied. Besides, as an example, the prepared magnetic transition metal oxide (Co3O4) has been explored to apply into high-performance electrochemical energy storage devices. The main results are listed as follows:(1) BaFe12O19 single-particle-chain nanofibers have been successfully prepared by an electrospinning method. It is found that individual BaFe12O19 nanofibers consist of single-nanoparticles which are found to stack along the nanofiber axis. The crystal structure of the BaFe12O19 single-particle-chain nanofibers is proved to be M-type hexagonal. The single crystallites on each BaFe12O19 single-particle-chain nanofibers have random orientations. A formation mechanism is proposed based on thermo gravimetry/differential thermal analysis, X-ray diffraction and transmission electron microscope. The magnetic measurement of the BaFe12O19 single-particle-chain nanofibers reveals that the coercivity reaches a maximum of 5943 Oe and the saturated magnetization is 71.5 emu/g at room temperature. Theoretical analysis at the micromagnetism level is adapted to describe the magnetic behavior of the BaFe12O19 single-particle-chain nanofibers.(2) NiFe2O4 multi-particles-chain nanofibres and NiFe2O4 nanotubes have been successfully fabricated using electrospinning followed by calcination. It is found that individual NiFe2O4 nanofibres and nanotubes are consisted of multi- nanocrystallites stacking along the nanofibre axis. The crystal structure of individual these NiFe2O4 nanomaterials proved to be polycrystalline with a face centered cubic (fcc) structure. To fully understand the magnetization process of NiFe2O4 nanofibres and nanotubes, we have developed models named "chain of sheets" and "chain-of-rings" from the theory of micromagnetism, respectively. The high frequency magnetic properties of NiFe2O4 nanotubes were also fully investigated.(3) CoFe2O4 nanotubes have been directly fabricated by single-capillary spinneret electrospinning. The morphology and structure characterizations show that individual CoFe2O4 nanotubes are made of CoFe2O4 nanocrystals stacking along the nanotubes with no preferred growth directions and these individual nanocrystals are single crystal with a cubic spinel structure. Each nanocrystal was shown to be a single magnetic domain. The magnetic measurements show that the coercivity (Hc) of the CoFe2O4 nanotubes decreases from 10400 Oe at 5 K to 300 Oe at 360 K. The CoFe2O4 nanotubes have a spin reorientation (SR) at 5 K, which is different from CoFe2O4 nanorods and nanoparticles.(4) A novel architecture, conductive metal/transition oxide (Co@Co3O4) core-shell three-dimensional nanonetwork (3DN), has been constrcuted by surface oxidating Co 3DN in situ, for high-performance electrochemical capacitors. It is found that the Co@Co3O4 core-shell 3DN consists of petal-like nanosheets with thickness of<10 nm interconnected forming a 3D porous nanostructure, which preserves the original morphology of Co 3DN well. In the application of electrochemical capacitors, the electrodes exhibit a high specific capacitance of 1049 F g-1 at scan rate of 2 mV s-1 with capacitance retention of ～52.05% (546 F g-1 at scan rate of 100 mV s-1) and relative high areal mass density of 850 F g-1 at areal mass of 3.52 mg cm-2. It is believed that the good electrochemical behaviors mainly originate from its extremely high specific surface area and underneath core-Co "conductive network". And the underneath core-Co "conductive network" enables an ultrafast electron transport.