| In this dissertation, we fabricated various magnetic nanostructures including free-standing nanowire, tube, ring and core-shell cable arrays on silicon substrate. For these materials, induction-coupled plasma spectrometer (ICP) was used to investigated their compositions, transmission electron microscope (TEM), scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM) were used to characterize their morphologies, x-ray diffraction (XRD) was used to analyze their structures, and vibrating sample magnetometer (VSM) and superconducting quantum interference device magnetometer (SQUID) were used to measure their magnetic properties. Main work includes two parts below:1. preparation and magnetic properties of free-standing CoPt alloy and CoPt/FeCo core-shell nanocable arraysFree-standing and AAO-embedded Co-Pt alloy nanowire arrays fabricated on silicon substrates were comparatively studied. The as-deposited nanowires were magnetically soft and anisotropic. After annealing in hydrogen atmosphere, magnetic hardening with coercivity up to6000Oe was achieved for both free-standing and AAO-embedded Co43Pt57nanowires. The existence of AAO template has been found no obvious effect on phase transformation from fcc to fct structure. However, comparing to the free-standing annealed Co43Pt57nanowires, an anisotropic magnetic behavior was observed for AAO-embedded annealed wires. It is assumed that the expansion of<100> and<010> axis of the crystal during phase transformation was restricted by the surrounding AAO template and thus lead to an anisotropic distribution of c-axis of the crystals.Large area free-standing CoPt/FeCo core-shell composite nanocbale arrays were fabricated on silicon substrate using free-standing CoPt nanowires as template. The thickness of the FeCo shell can be modulated by varying the plating time. With the deposition of FeCo shell, obvious "slender waist" was observed in the hysteresis loop, and the coercivity decreases sharply with the increasing thickness of FeCo shell, which could be attributed to the fact that the soft FeCo shell is partly or even completely decoupled from the hard CoPt core. The magnetization reversal processes of the cables with different shell thicknesses were also investigated by micromagnetic simulation.2. Large area Co nanoring arrays fabricated on silicon substrate by anodic aluminum oxide template-assisted electrodepositionA simple approach based on anodic aluminum oxide (AAO) template-assisted electrodeposition was developed to fabricate large-area Co nanoring arrays on silicon substrate. The ring diameter and interring distance can be modulated by varying the anodization parameters. Magnetic measurements and micro-magnetic simulations reveal that the onion to vortex (O-V) transition is strongly diameter dependent. Upon increasing the outer diameter from100nm to300nm, the O-V switching field gradually changes sign from positive to negative. This is also proved by in situ observations of the magnetic states of the rings under different external fields using magnetic force microscope.3. Fabrication and magnetic properties of free-standing Ni nanotube arrays with controllable wall thicknessFree-standing Ni nanotubes were fabricated on silicon substrate using anodic aluminum oxide (AAO)/polypyrrole composite template. The diameter, length and wall thickness of the nanotubes can be precisely and independently controlled. Magnetic measurements show that the magnetic anisotropic properties are strongly dependent on the wall thickness of nanotubes. Theoretical analysis and micromagnetic simulation were performed to explain the wall thickness-dependent anisotropic behavior. 4. Fabrication and magnetic properties of Co/polypyrrole composite nanowire arraysCo/polypyrrole composite nanowire arrays were fabricated by AC electrodeposition using anodic aluminum oxide templates. Transmission electron microscopy studies show that each nanowire is consisted of many Co nanofibers with diameters less than10nm. These nanofibers are separated by the polypyrrole matrix, the possible growth mechanism of Co/PPy composite nanowires is illustrated. It is considered that during the positive half-period of the AC voltage, PPy is polymerized into the AAO pore by the electrochemical oxidation of Py monomer with LiClO4·3H2O as oxidant.9Then during the negative half-period, Co is deposited from Co2+. Therefore, the depositions of PPy and Co are two independent and alternate processes instead of a co-deposition process. However, because the frequency of the AC voltage is50Hz, the single electrodeposition (electropolymerized) pulse is thus only0.01second. It is so short that the Co (PPy) can not fully fill the bottoms of the AAO pores. Both the Co and PPy seeds are formed at the bottom of the AAO template during the first AC voltage period. Subsequently, they grow upward from their own seeds and lead to the composite structure. Magnetic measurments reveal that Co/PPy composite nanowires show much better room temperature permanent magnetic performance than pure Co nanowires. The squareness and coercivity of Co/PPy composite nanowires are0.99and1700Oe, much larger than that of the pure Co wire. Theoretical analysis reveals that the competition between shape anisotropy, magnetocrystalline anisotropy and magnetostatic interaction between wires are responsible for the excellent permanent magnetic performance. The polypyrrole content dependent room temperature (300K) and low temperature (5K) magnetic properties were also investigated. |
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