ARTICLES Growth of Single-Crystalline Ni and Co Nanowires via Electrochemical Deposition and Their Magnetic Properties Hui Pan, ² Binghai Liu, ‡ Jiabao Yi, ‡ Cheekok Poh, ² Sanhua Lim, ² Jun Ding, ‡ Yuanping Feng, ² C. H. A. Huan, ² and Jianyi Lin* ,²,§ Departments of Physics and Material Science, National UniVersity of Singapore, 2 Science DriVe 3, Singapore 117542, and Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, Singapore 627833 ReceiVed: October 21, 2004; In Final Form: December 23, 2004 Single-crystalline Ni nanowires have been successfully fabricated with anodic aluminum oxide as template by electrodeposition. Structural characterization (X-ray diffraction, XRD, and high-resolution transmission electron microscopy, HRTEM) shows that the single-crystalline Ni nanowire has a preferred orientation along the [220] direction. The effects of electrochemical deposition conditions on the structure of Ni nanowires are systematically studied to investigate the growth mechanism. Possible reasons for the growth of the single- crystalline Ni nanowires were discussed on the basis of electrochemistry and thermodynamics. These single- crystalline Ni nanowires have exhibited excellent magnetic properties (large anisotropy, large coercivity, and high remanence). By a similar process, single-crystalline Co nanowires with hexagonal close-packed (hcp) structure were achieved, also having large anisotropy, large coercivity (1.8 kOe), and high remanence ratio (80.8%). Introduction Nanostructured ferromagnetic materials are of great interest in theoretical physics, solid-state science, and practical tech- nological applications, such as ultra-high-density magnetic recording and spintronics. 1-6 Methods used to produce the metallic nanowires include lithographic patterning, 7 which is comparatively cumbersome, expensive, and not suitable for large-scale production, and “template synthesis”, 1,2,8 which involves electrochemically depositing metal into nanopores in a template. A commonly used template is anodic aluminum oxide (AAO). 9 To date, most metallic nanowires have been produced on AAO templates, including low melting point metals, such as Au, Ag, and Zn nanowires, 10-13 and high melting point metals, such as Fe, Co, and Ni nanowires. 8 Successful growth of single-crystal nanowires of low melting point metals have been reported, 13 but growth of single-crystal nanowires of high-melting-point metals was claimed to be very difficult if not impossible. 13 Single-crystal Fe or Ni nanowires have been reported in the literature. 8,14 But the single-crystalline size was rather small, around 40 nm along the wire axis, and there was no discussion on the mechanism of single-crystal growth and the effect of single crystallinity on their magnetic properties. In this work we demonstrated the successful AAO template growth of single-crystalline Ni and Co nanowires and the excellent magnetic properties of these nanowires. Experimental Section The AAO template was prepared following the two-step anodization procedure. 9 Briefly, the pure Al foil was first annealed and electropolished in a mixture of perchloric acid and ethanol. The anodization was performed at 40 V in an oxalic acid solution of 3 wt % at room temperature. After removal of the alumina in a mixture of phosphoric acid and chromic acid, the ordered pore arrangement was achieved during the second anodization with the same conditions as in the first anodization. The remaining Al was removed in CuCl 2 solution. The oxide layer at the bottoms of the pores was removed in acid. To facilitate electrodeposition, a Pt layer was sputtered to the back of the AAO as the electrode. A Ni sulfate electrolyte (240 g of NiSO 4 ‚7H 2 O/45 g of NiCl 2 ‚6H 2 O/40 g of H 3 BO 3 ‚2H 2 O) with a pH value of 2.5 was used. DC electrodeposition was performed at various applied voltages ranging from 0.4 to 4.0 V and temperatures ranging from 25 to 60 °C to investigate their effects on the structures and magnetic properties of Ni nanowires. Five Ni samples were prepared under different electrodeposition conditions. They are sample 1 [an applied voltage of 0.4 V and room temperature (RT)], sample 2 (1.0 V, RT), sample 3 (4.0 V, RT), sample 4 (1.0 V, 40 °C), and sample 5 (1.0 V, 60 °C). To keep the length of nanowires equal in all the samples, long deposition time was applied at low electrodeposition voltages, and the AAO used were prepared under the same anodizing conditions so that they have the same pore structures. The morphology of the deposited Ni nanowires was observed by scanning electron microscopy (SEM, JEOL JSM-6700F). The structure of the nanowires was characterized by high-resolution transmission electron microscopy (HRTEM) and X-ray diffrac- tion (XRD) (Brucker AXS D8). The magnetic properties of * Corresponding author: e-mail Lin_jianyi@ices.a-star.edu.ag or phylinjy@nus.edu.sg. ² Department of Physics, National University of Singapore. ‡ Department of Material Science, National University of Singapore. § Institute of Chemical and Engineering Sciences. 3094 J. Phys. Chem. B 2005, 109, 3094-3098 10.1021/jp0451997 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/08/2005