Fabrication And Magnetic Study Of One-dimensional Nickel Nanostructures With Different Diameter Porous Anodic Aluminum

One-dimensional (1D) nanostructures have attractedconsiderable interest due to their unique magnetic, electronic,mechanical and optical properties and their potential applicationsin fields of information storage, magnetoresistive nanosensors,drug delivery, magnetic separation, racetrack memory andmicrowave devices, etc. Many strategies have been developed tosynthesis these nanostructures such as chemical vapor deposition,molecular beam epitaxy, and thermal decomposition of precursors,hydrothermal synthesis, spontaneous coalescence of nanoparticlesand template assisted electrodeposition, and so on. Among thesemethods, porous anodic aluminum oxide (PAA) template-basedelectrodeposition has facilitated the researchers due to itssimplicity, cost effectiveness, and precise control over length,diameter, interwire distance and composition of nanostructures.Here, various PAA templates were fabricated in differentelectrolytes by two-step anodization process. Then, different1Dnickel nanostructures were obtained via DC electrodeposition usingthe PAA templates. The morphology, composition, crystalstructures and magnetic properties of the products werecharacterized by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), transmission electronmicroscopy (TEM), X-ray diffraction (XRD) and vibrating samplemagnetometry (VSM). The main work is as follows:(1) Various PAA templates were fabricated in sulfuric acid, oxalicacid, citric and citric/oxalic acid mixture by two-step anodizationprocess respectively. Ni nanochains composed of a chain ofinterconnected nanoparticles rather than uniform nanowires werefabricated via DC electrodeposition in PAA templates, which werefirst reported to our knowledge. Ni nanowires and nanotubes werealso obtained using PAA templates with different diameters.(2) The magnetic properties of Ni nanochains, Ni nanowires andNi nanotubes were compared. The magnetic measurement resultsindicate that the magnetic properties are strongly dependent on themorphology and size of Ni nanostructures. Ni nanochains have noobvious easy axis direction and present a very small coercivity. Theeasy axis of Ni nanowires is oriented parallel to the wire axis, andthe effective magnetic anisotropy is mainly from shape anisotropy.For Ni nanotubes, easy axis is perpendicular to the tube axis whichis dominated by the magnetic dipole interaction. The angulardependence of the coercivity indicates that the weakly couplednanoparticles within the Ni nanochains reverse their magnetizationnearly independently, however for Ni nanowires, magnetic reversalprocess is dominated by localized quasi-coherent nucleation mode,and Ni nanotubes shows a complicated reversal behavior.(3) The dependence of magnetic properties on outer diameterand wall thickness of Ni nanotubes were investigated. The resultsshow that the large wall thickness and small outer diameter make the easy magnetization axis much easier to be saturated when thefield is perpendicular to the tube axis.