MBE-grown self-assembled iron nanowires

In recent years, magnetic nanowires (NWs) have been extensively studied because of their novel physical properties and their potential applications in a number of magnetic nanotechnologies such as high-density magnetic recording media, magnetic field sensors, magnetic nanoprobes for spin-polarized microscopy and cell manipulation in biomedical technology. The unique and tunable magnetic properties of magnetic NWs arise from the shape anisotropy and the low dimension. We have recently found that two types of self-assembled iron NWs can be formed on a ZnS thin film surface by the molecular beam epitaxy (MBE) technique. The type-A Fe NWs orient along the ZnS [011] direction with irregular shape, while the type-B Fe NWs orient along either closely to the ZnS [001] or [010] direction with seemingly straight shape. Detailed high-resolution transmission electron microscopy and selected area electron diffraction characterizations revealed that both types of Fe NWs were bcc single-crystalline with their elongated axis along the Fe <100> direction family. In this thesis study, we focus on the studies of the dependence of the shape, alignment orientation and aspect ratio of these NWs on the Fe coverage, growth temperature, as well as the orientation and miscut angle of the substrates. Phenomenological models are introduced to address these dependences. The analysis carried out on cross-sectional TEM images of both types of Fe NWs leads us to believe that surface energy minimization drives the formation of the observed specific three dimensional shapes of both types of Fe NWs. The observation that the number density of the resulting Fe NWs regardless their types is higher as the growth temperature decreases is well explained by a surface diffusion model of Fe adatoms. It was also observed that a lower growth temperature favors the formation of type-A Fe NWs while a higher growth temperature favors the formation of type-B Fe NWs, which is in good agreement with an existing model regarding the kinetics and energetics of metal adatoms versus temperature. We have addressed how the Fe coverage could affect the width of type-B Fe NWs and the narrowest width achieved in this study is about 30 nm. The role of a pseudo-morphic ZnS surface and the uniqueness of the (100) orientation in the formation of Fe NWs were also discussed through detailed studies on the effects of substrate or buffer materials, quality of the buffer layer, orientations and miscut angle of the substrates. The realization of highly aligned magnetic NWs lying on a non-magnetic and highly resistive surface, as achieved in this study, will certainly shed some light on both fundamental research and practical applications related to one-dimensional (1-D) spintronics