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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 aluminum 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 has revealed various unusual properties relevant to applications in high density data storage devices; e.g., their anisotropic 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 distinct advantages compared to coated magnetic particles.[9] Template-assisted assembly is a well-known route to fabricate 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) nanostructures have been made using conventional lithography,[12] nanosphere lithography,[10,11] di-block copolymers template,[13] and anodic aluminum oxide (AAO) templates.[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 electrodeposition[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 nanostructures of cobalt have been reported, e.g., nanobowls,[10,11] nanowires,[1,2,4–8] and nanobelts.[16] The borohydride reduction of metal salts has been shown to produce cobalt and cobalt ferrite nanobowl arrays using 3-D polystyrene colloidal templates, while a surfactant-assisted hydrothermal reduction process has been used for the synthesis of cobalt nanobelts. Recently, infiltration of cobalt ferrite nanoparticles 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 [1–10]

A.K. SRIVASTAVA, Graduate Student, and R.V. RAMANUJAN, Associate Professor, are with the School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798. Contact e-mail: [email protected] R.S. SINGH, Senior Research Scientist, K.E. SAMPSON, Research Associate, and V.P. SINGH, Professor a