| Anodization of aluminum in acid solutions can generate unique morphologies of the resulting amorphous alumina films, consisting of long and columnar nanopores with approximately hexagonal ordering (alumite). Alumite has attracted broad attention, both on a fundamental and applied level, for more than half a century. In particular, the possibility of synthesizing nano-structures for magnetic recording technology makes this classic self-assembly process even more attractive. Previous studies have shown that the pore size and pore distance could be easily tuned by controlling anodization conditions. Recently, excellent hexagonal ordering of these nanopores has been achieved by multi-step anodization. Although the maximum ordered area could be increased to 100 μm 2 by further extending the anodization time and using a multi-step anodization, the defects and grain structure in the Al metal surface hamper the achievement of longer-range order.; Alumite structures are an almost ideal template for the synthesis of magnetic nanoparticle arrays, which have possible applications in advanced magnetic recording technology and as model systems for the study of well-defined and uniform magnetic nanoparticles. Electroplating can allow a systematic study of how best to make such arrays controlled. Various voltage and current waveforms were employed to control the nucleation, growth, and consequently the magnetic properties of the magnetic metal array in ordered alumite. At first, Fe nanowires were deposited in disordered pores, then Co nanowires were deposited in ordered pores by AC sinusoidal waveform electrodeposition. The average length and diameter of these nanowires can be controlled, but a wide distribution of nanowire lengths is observed. Successively, particle uniformity was achieved by a novel electrodeposition scheme, utilizing pulse-reverse voltage waveforms (pulse-reverse electrodeposition) to control nucleation and growth of the particles. The resulting nanoparticles are polycrystalline, and the grains are randomly oriented.; The magnetic properties of the resulting magnetic metal arrays are dominated by particle shape and by interparticle magnetostatic interactions. A very clear transition of the anisotropy, from perpendicular to in-plane, is observed at an aspect ratio a of about two. The arrays exhibit good thermal stability, demonstrating a great potential of these structures as future recording media in a patterned scheme. Since other metals and alloys can be easily fabricated by utilizing the same technique, the pulse-reverse electrodeposition shows great promise for a reliable synthesis of uniform nanostructures of many metals, with potential applications in the fabrication of devices not only for magnetics, but also for optical and opto-electronic nanodevices. |
