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.

The formation work of a two-dimensional hcp-Co (metallic cobalt crystal with hexagonal close packed structure) nucleus, Whkl, was calculated by Pangarov’s theory. W002 was estimated to be smaller than W100 in a cathode potential range nobler than the transition potential, Etra (ca. −0.77 V vs. Ag/AgCl). To confirm the above estimation, ferromagnetic nanocomposite thick films, which contained (002) textured hcp-Co nanocrystal arrays, were synthesized by potentiostatic electrochemical reduction of Co2+ ions in anodized aluminum oxide (AAO) nanochannel films with ca. 45 µm thickness. The aspect ratio of hcp-Co nanocrystals with a diameter of ca. 25 nm reached up to ca. 1800. Our experimental results revealed that the texture coefficient, TC002, increased when decreasing the overpotential for hcp-Co electrodeposition by shifting the cathode potential nobler than Etra. In a similar way, TC002 increased sharply by decreasing the growth rate of the hcp-Co nanocrystals so that it was smaller than the transition growth rate, Rtra (ca. 600 nm s−1). The perpendicular magnetization performance was observed in AAO nanocomposite films containing hcp-Co nanocrystal arrays. With increasing TC002, the coercivity of the nanocomposite film increased and reached up to 1.66 kOe, with a squareness of ca. 0.9 at room temperature.

The time-dependence of electrochemical reduction current, which was observed during the one-dimensional (1-D) crystal growth of ferromagnetic cobalt nanowire arrays, was analyzed by Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory. Textured hcp-Co nanowire arrays were synthesized by potentio-static electrochemical reduction of Co 2+ ions in anodized aluminum oxide (AAO) nanochannel films. Crystal growth geometry factor n in the JMAK equation was determined to be ca. 1. Hence, the electrochemical crystal growth process of a numerical nanowires array can be explained by 1-D geometry. The crystal nucleation frequency factor, k in JMAK equation was estimated to be the range between 10 −4 and 10 −3 . Our experimental results revealed that the crystal nucleation site density N d increased up to 2.7 × 10 −8 nm −3 when increasing the overpotential for cobalt electrodeposition by shifting the cathode potential down to −0.85 V vs. Ag/AgCl. The (002) crystal orientation of hcp-Co nanowire arrays was, remarkably, observed by decreasing N d . Spontaneous magnetization behavior was observed in the axial direction of nanowires. By decreasing the overpotential for cobalt electrodeposition, the coercivity of the nanocomposite film increased and reached up to 1.88 kOe, with a squareness of ca. 0.9 at room temperature. could synthesize the Co nanowires with a diameter of ca. 15 nm by using a solvothermal chemical process. In their report, the coercivity of 10.6 kOe was achieved at room temperature [5]. However, the average length of Co nanowires was only ca. 200 nm. Hence, there are still several issues to overcome in addressing the gap, which is required for the permanent magnetic applications with anisotropic performance. In the template-assisted method, ferromagnetic metal is electrodeposited into the template with numerous nanochannels with large aspect ratio (cylinder-like nanopores). This method is a very attractive approach to synthesizing ferromagnetic metal nanowires because it can be carried out using simple experimental devices and its cost is low [6]. Typical template materials include ion track-etched polycarbonate films [7-10] and anodic aluminum oxide (AAO) films [11]. In particular, pore diameter and thickness of AAO templates can be adjusted by changing experimental conditions such as anodic oxidation voltage. As a result, the diameter (d), length (L) and aspect ratio (L/d) of nanowires, which are electrodeposited into nanochannels, can be adjusted. Therefore, many researchers have synthesized ferromagnetic metal nanowire arrays by using the AAO templates [6,[12][13][14][15][16]. In addition, hexagonal close-packed cobalt (hcp-Co) has large magneto-crystalline anisotropy along the c-axis [17,18]. Hence, the magnetic anisotropy of the cobalt nanowire arrays could be enhanced, when the long axis of nanowires corresponds to the c-axis by adjusting the cobalt crystal orientation.Research to date has focused on the crystal structure and the magnetic properties of ferromagnetic cobalt nanowire arrays, which were synthesized ...

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