Nanofios crescidos por magnetron sputtering com feixe de vapor em incidência rasante: mecanismo de crescimento das nanoestruturas e características magnéticas.

Abstract

In this thesis, an experimental setup was developed for production of W, Co and W/Co nanowires (NWs) on sapphire vicinal surfaces, using sputtering equipment with magnetrons positioned in a confocal geometry, in which the vapor beam reaches the substrate in a grazing incidence. The suitable conditions to obtain a nanoscale corrugated surface with a high-order correlation were determined concerning the annealing procedure (1300 °C for 5 h) of the sapphire substrates. Depending on the entrance angle β of the tungsten vapor beam, different nanoparticles and/or nanowires were systematically produced under controlled conditions. It was possible to establish the condition that preferentially allows the growth of nanowires, that is, it was demonstrated that nanowires can be produced when β ≈ Ω (miscut angle of the vicinal substrate). Co nanowires with different dimensions (thickness and width) were deposited directly on the sapphire vicinal surface or on W nanowires (with seed layer function) producing W/Co bilayers. Co nanowires are polycrystalline and are composed of the hcp and fcc phases, whose relative fractions depend on the time of Co deposition and the presence (or absence) of the W-bcc seed layer. A model to explain how the Co nanowires grow is proposed: hcp Co phase is firstly stabilized on the vicinal sapphire substrate (or W seed layer), but as the NW thickness increases, a structural relaxation leads to fcc Co phase. Though different contributions of anisotropies are considered, the magnetic properties are dominated by shape effects with an easy axis oriented along the average direction of the wire axis. Thicker nanowire arrays also have a significant contribution from the magnetocrystalline anisotropy. From the behavior of the temperature dependent magnetization, we find that for nanowires with larger sectional area the magnetization reversal is governed by a curling reversal mode, whereas nanowires with smaller sectional area switch their magnetization via coherent rotation. The effective volumes ( ) of the magnetic domains involved in the magnetization reversal were estimated at ~103 nm3 , which occur in smaller volumes than those calculated for nanowire dimensions (~105 nm3 ).