Growth and Characterization of Self-Assembled Ferromagnetic Iron Nanowires
抄録
In recent years perpendicular ferromagnetic nanowires have attracted considerable interest due to their potential use in many areas of advanced nanotechnology, specifically high-density recording media. An ideal recording medium in this regard consists of a densely organized assembly of nanometer-scale ferromagnets with high magnetization and suitable coercivity.
In this dissertation a novel and simple approach to create self-assembled nanowires of &#945;-Fe through the decomposition of a suitably chosen perovskite is reported. We illustrate the principle behind this approach using the reaction 2La<sub>0.5</sub>Sr<sub>0.5</sub>FeO<sub>3</sub> &#x2192; LaSrFeO<sub>4</sub> + Fe + O<sub>2</sub> that occurs during the deposition of La<sub>0.5</sub>Sr<sub>0.5</sub>FeO<sub>3</sub> under reducing conditions. This leads to the spontaneous formation of an array of single crystalline &#945;-Fe nanowires embedded in LaSrFeO<sub>4</sub> matrix, which grow perpendicular to the substrate.
The embedded iron nanowires are an illustration of three-dimensional heteroepitaxy. The lateral diameter and spacing of the nanowires are strongly dependent on growth temperature. By reducing the temperature of deposition, the size and spacing between the iron nanowires decrease. The changes in the diameter of the nanowires follow an Arrhenius behavior and suggest that the growth of the nanowires is kinetically controlled by diffusion. In addition, their in-plane shape evolves from circular to octahedral and square shape with [110] facets dominating as the growth temperature increases and the elastic energy dominates over the surface energy.
The iron nanowires exhibit square shaped out-of-plane hysteresis loops with a relatively large uniaxial anisotropy with the easy access normal to the film plane. The calculated anisotropy field indicates that the anisotropy observed is due to the cylindrical shape of the nanowires and shape anisotropy.
The nanowires are also used as the nucleation sites for growth of vertically aligned multi-walled carbon nanotubes for field emission applications. The diameter and shape of the carbon nanotubes are controlled by the diameter and length of the nanowires.
It is believed that this approach to self-assembly of the ferromagnetic nanowires is generic and can be expected to yield epitaxial nanocomposites from other complex oxides as well. This opens a new and exciting path for processing of a broad range of nanomaterials through self-assembly.