METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 38A, APRIL 2007––717 Templated Assembly of Magnetic Cobalt Nanowire Arrays A.K. SRIVASTAVA, R.S. SINGH, K.E. SAMPSON, V.P. SINGH, and R.V. RAMANUJAN A template-assisted assembly technique combined with a chemical synthesis approach has been used to produce high density magnetic cobalt nanowire arrays. Cobalt nanowires were formed using chemical synthesis techniques, by borohydride reduction of cobalt salt in the pores of anodic alumi- num oxide (AAO) templates; the samples were annealed in order to achieve nanowires with hcp crystal structure. The morphology and the crystal structure were controlled by the template geometry and annealing conditions, respectively. Magnetic property measurements showed the influence of morphology and crystal structure on the magnetic properties of the arrays. I. INTRODUCTION THE development of ferromagnetic nanowire arrays [1–10] has revealed various unusual properties relevant to applica- tions in high density data storage devices; e.g., their aniso- tropic behavior is of current interest. [1,2,4–8] The development of such magnetic nanowires is also useful in bioengineering applications such as cell separation where they exhibit dis- tinct advantages compared to coated magnetic particles. [9] T emplate-assisted assembly is a well-known route to fab- ricate ordered nanostructures of different sizes and shapes. [1,2,4–12] Various efforts to fabricate one-dimensional, two-dimensional (2-D), and three-dimensional (3-D) nano- structures have been made using conventional lithogra- phy, [12] nanosphere lithography, [10,11] di-block copolymers template, [13] and anodic aluminum oxide (AAO) tem- plates. [1] Magnetic-field-assisted assembly [14,15] has also been used to form magnetic nanowires, but arranging them in long-range periodic order is still a challenge. In previous reports, magnetic nanowires have been fabricated by elec- trodeposition [1,2,4–8] of the material in an AAO template. The high anisotropy hcp phase of cobalt is of particular interest for data storage applications and cobalt nanowire array with high coercivity value is a promising candidate for high density data storage devices. Various nanostruc- tures of cobalt have been reported, e.g., nanobowls, [10,11] nanowires, [1,2,4–8] and nanobelts. [16] The borohydride reduc- tion of metal salts has been shown to produce cobalt and cobalt ferrite nanobowl arrays using 3-D polystyrene col- loidal templates, while a surfactant-assisted hydrothermal reduction process has been used for the synthesis of cobalt nanobelts. Recently, infiltration of cobalt ferrite nanopar- ticles in the pores of AAO templates for the synthesis of nanowires has been demonstrated. [17] In this report, we present a chemical synthesis approach to fabricate Co nanowires into pores of the AAO templates by borohydride reduction of cobalt salt. Cobalt nanopar- ticles were also synthesized without using AAO template. The vacuum-assisted infiltration method was employed to infiltrate the precursors into the pores of the AAO template. Cobalt has a wire morphology because the reduction reac- tion occurs in the pores. The Co nanowires were fabricated in the pores of AAO template with pore sizes of 35 of 200 nm, respectively. The wire length and average wire-wire separation for the 35-nm pore diameter template are 30 mm and 117 nm, respectively. The corresponding values for the 200-nm pore diameter template are 65 mm and 450 nm, respectively. This synthesis was followed by annealing at 350 °C in air. In comparison to physical methods such as sputtering or pulsed laser deposition (PLD), chemical syn- thesis with vacuum infiltration has the advantage that high aspect ratio nanowires with better signal-to-noise ratio can be produced. This is because the ‘‘shadowing effect,’’ due to the geometry of the long pores, limits the aspect ratio obtained by sputtering or PLD but does not play a role in chemical synthesis techniques. The chemical technique can also be extended to the formation of superparamagnetic cobalt nanowires using templates with pore diameter of less than 7 nm. Transmission electron microscopy (TEM), vibrat- ing sample magnetometry (VSM), scanning electron micros- copy (SEM), atomic force microscopy (AFM), and X-ray diffraction (XRD) techniques were used to investigate these cobalt nanowire arrays. The nanowires show enhanced coer- civity compared to cobalt nanoparticles. The role of mor- phology and crystal structure on magnetic properties of the array is also presented. II. EXPERIMENTAL A. Synthesis of Nanoparticles and Nanowires The cobalt nanoparticles were synthesized by mixing aqueous solutions of cobalt chloride and sodium borohy- dride. Just after mixing, cobalt chloride solution turns from pink to black, as a result of the reduction of cobalt salt. The sample was then annealed for 350 °C for 3 hours in air in order to produce cobalt with desired hcp crystal structure. For the synthesis of cobalt nanowires, AAO templates with a mean diameter of 200 nm (Whatman International Limited, Maidstone, England) were used, while AAO templates with a mean diameter of 35 nm were prepared by anodization of aluminum film. The AAO templates were fabricated by anodizing an Al foil in 0.3M oxalic acid at 40 V for 25 minutes; phosphoric acid was used for pore opening. [18] A.K. SRIVASTAVA, Graduate Student, and R.V. RAMANUJAN, Asso- ciate Professor, are with the School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798. Contact e-mail: ramanujan@ntu.edu.sg R.S. SINGH, Senior Research Scientist, K.E. SAMPSON, Research Associate, and V.P. SINGH, Professor and Department Chair, are with the Center for Nanoscale Science & Engineer- ing, Department of Electrical and Computer Engineering, University of Kentucky, Lexington, KY 40506-0046. This article is based on a presentation made in the symposium entitled ‘‘Phase Transformations in Magnetic Materials,’’ which occurred during the TMS Annual Meeting, March 12–16, 2006, in San Antonio, Texas, under the auspices of the Joint TMS-MPMD and ASMI-MSCTS Phase Transformations Committee. DOI: 10.1007/s11661-007-9096-7 © The Minerals, Metals & Materials Society and ASM International 2007