Controlled large-scale synthesis and magnetic properties of single-crystal cobalt nanorods

We report detailed synthesis studies of large-scale fabrication of single-crystalline cobalt nanorods using thermally evaporated gold as a conducting electrode on a nanoporous anodized aluminum oxide (AAO) template. By varying the time (i.e., adjusting the current density), it is possible to control the length of the nanorods and these rods grow in highly anisotropic face-centered (L10) phase. The large-scale isolation of these nanorods was achieved using a mild HF solution to dissolve the AAO membranes, leaving behind large scale ensemble of the isolated Co nanorods. Scanning electron microscopy and transmission electron microscopy examinations show clear evidence for metallic Co nanorods (about 100 nm in diameter) formed inside an array of AAO nanopores. We observe more uniform length of nanorods when an external magnetic field is applied during synthesis. A preferred perpendicular (shape) anisotropy is observed in these nanorod arrays. Magnetic properties indicate both coercivity and thermal activation volume increase with increasing nanorod length.