Abstract
The crystal growth process and ferromagnetic properties of electrodeposited cobalt nanowires were investigated by controlling the bath temperature and cathodic overpotential. The cathodic overpotential during electrodeposition of cobalt nanowire arrays, ΔEcath, was theoretically estimated by the difference between the cathode potential, Ecath, and the equilibrium potential, Eeq, calculated by the Nernst equation. On the other hand, the activation overpotential, ΔEact, was experimentally determined by the Arrhenius plot on the growth rate of cobalt nanowire arrays, Rg, versus (vs.) reciprocal temperature, 1/T. The ferromagnetic cobalt nanowire arrays with a diameter of circa (ca.) 25 nm had the preferred crystal orientation of (100) and the aspect ratio reached up to ca. 1800. The average crystal grain size, Ds, of (100) peaks was estimated by X-ray diffraction patterns and was increased by decreasing the cathodic overpotential for cobalt electrodeposition by shifting the cathode potential in the noble direction. Axial magnetization performance was observed in the cobalt nanowire arrays. With increasing Ds, coercivity of the film increased and reached up to ca. 1.88 kOe.
Highlights
The standard electrode potential of metallic cobalt (Co/Co2+ −0.28 V vs. normal hydrogen electrode (NHE)) is nobler than that of metallic iron (Fe/Fe2+ −0.44 V vs. NHE)
Pirota et al reported that Co nanowire arrays, which were electrodeposited into two-step anodized anodized aluminum oxide (AAO) nanochannels, exhibited a coercivity, Hc, of up to ca. 1.0 kOe and squareness, Mr /Ms, of ca. 0.38 [17]
In our previous work we demonstrated that Co nanowire arrays that were electrodeposited into one-step anodized AAO nanochannels, showed a coercivity, Hc of up to ca. 2.4 kOe and squareness, Mr /Ms of ca. 1.0 [16]
Summary
The standard electrode potential of metallic cobalt (Co/Co2+ −0.28 V vs. normal hydrogen electrode (NHE)) is nobler than that of metallic iron (Fe/Fe2+ −0.44 V vs. NHE). Cobalt-based magnetic materials, compared with iron-based, have excellent corrosion resistance when a magnet is used under a corrosive environment [1,2]. Cobalt alloy nanowires with a large aspect ratio show excellent magnetic performance, including large coercivity, large squareness and high Curie temperature [3,4,5]. Several fabrication processes of metallic alloy nanowire arrays have so far been reported by researchers. The template-based approach, using a thin film with numerous nanochannels such as ion-track etched polycarbonate templates and anodized aluminum oxide (AAO) templates [6,7,8,9,10,11,12], is a highly recommended process. The AAO nanochannels with numerous through-holes can realize straight nanowires with large aspect ratios
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