Effect of annealing on the electrical and magnetic properties of electrodeposited Ni and permalloy nanowires

The influence of annealing on the microstructure and the electrical and magnetic properties of cylindrical nickel-based nanowires has been investigated. Nanowires of nickel of ~275 nm diameter and of permalloy (Py) of ~70 nm diameter were fabricated by electrochemical deposition into nanoporous templates of polycarbonate and anodic alumina, respectively. Characterization was carried out on as-grown and up to 650 °C heat-treated nanowires. Transmission electron microscopy imaging and diffraction of the nanowires showed a temperature-correlated grain growth of an initially nanocrystalline structure (untreated) with <8 nm (Ni) and <20 nm (Py) grains towards coarser poly-crystallinity after heat treatment with grains up to ~160 nm (Ni) and ~70 nm (Py), the latter being limited by the nanowire width. The electrical conductivity of individual as-grown and 650 °C annealed Ni nanowires was measured in-situ by scanning electron microscopy. At low current densities, the conductivity of annealed nanowires was estimated to have doubled over as-grown nanowires. We attribute this increase to the observed grain growth. The annealed nanowire was subsequently subjected to increasing current densities. Above 120 kA.mm−2 the nanowire resistance started to rise. At 450 kA.mm−2, the nanowire melted and current flow ceased. Magnetometry of as-grown and annealed nanowire arrays showed them to display quasi-thin film magnetic properties. Coercivity and saturation field were inversely correlated in annealed wires and a 25% tunability in these properties was achieved at just 200 °C.