Cobalt Nanowire Arrays Grown on Vicinal Sapphire Templates by DC Magnetron Sputtering

A modified magnetron sputtering setup was used to fabricate planar nanowire (NW) arrays of Co. The structural characterization of these polycrystalline Co NW arrays confirms the coexistence of both hcp and fcc phases, in which their relative fractions depend on the Co deposition time as well as the presence (or absence) of a bcc nanowire seed layer, as shown for tungsten case. 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 the fcc Co phase. While different contributions to the magnetic anisotropies are considered, we find that the magnetic properties are dominated by shape effects with an easy axis oriented along the average direction of the wire axis. Thicker NW arrays also have a significant contribution from the magnetocrystalline anisotropy. From the behavior of the temperature dependent magnetization, we find that for thicker nanowires the magnetization reversal is governed by a curling reversal mode, whereas thinner nanowires switch their magnetization via coherent rotation.