Enhancement of coercivity in cobalt nanowire arrays electrodeposited from aqueous solutions containing citric acid

Co nanowire arrays were prepared by pulsed-potential electrodeposition into nanopores of anodized aluminum oxide membranes (AAOMs) using an aqueous bath including citric acid. For comparison, Co nanowire arrays also synthesized by constant-potential electrodeposition. Based on the electron microscope images, the diameter and aspect ratio of electrodeposited nanowires were ca. 25 nm and ca. 1400, respectively. In addition, many crystal defects faults were introduced into the Co nanowires electrodeposited by pulsed-potential electrolysis. According to the XRD patterns, the crystallite size of Co nanowires electrodeposited from aqueous bath including citric acid decreased compared to that synthesized using a bath including boric acid. The formation of Co²⁺-citrate complexes suppressed the electrodeposition, and H₂ gas generation seems to inhibit the movement and arrangement of Co atoms on the electrode surface (i.e., crystal growth process). The axial direction of nanowires, which were prepared using a bath including citric acid, corresponded to the c-axis of the HCP-Co crystal. In this work, the squareness and coercive force of Co nanowire arrays fabricated by pulsed-potential electrodeposition reached up to 0.93 and 2.68 kOe, respectively. It seems that magnetization reversal was suppressed because the magnetic domain structure within the crystal grains changed from multiple domains to a single domain. In addition, the crystal defects can cause domain-wall-pinning effect and enhance the hard magnetic characteristics of electrodeposited Co nanowire arrays. Furthermore, the synergism of crystallomagnetic and shape anisotropies improved the vertical magnetization characteristics.