Effect of growth rate on the crystal orientation and magnetization performance of cobalt nanocrystal arrays electrodeposited from aqueous solution

Abstract

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.