Terabit Density Cobalt Nanowire Arrays With Tunable Magnetic Properties
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Terabit Density Cobalt Nanowire Arrays With Tunable Magnetic Properties Andrei Ursache1, Mustafa Bal1, James T. Goldbach2, Robert L. Sandstrom3, C. T. Black3, Thomas P. Russell2, and Mark T. Tuominen1 1 Department of Physics, University of Massachusetts, Amherst, MA 2 Polymer Science and Engineering Department, University of Massachusetts, Amherst, MA 3 IBM T. J. Watson Research Center, Yorktown Heights, NY ABSTRACT Nanoporous templates made from diblock copolymer films are used for electrochemical fabrication of hexagonal arrays of vertical cobalt magnetic nanowires at terabit/in2 density. The nanowire diameter and areal density are determined by the copolymer molecular weight, whereas the nanowire length and internal crystal morphology are controlled through the dc electrodeposition growth process. The array magnetic properties can be modified substantially by electrodeposition pH conditions while keeping wire size and interwire distance constant. Optimum pH control results in preferential growth of c-axis oriented crystallites with large perpendicular coercivity. The appearance of exchange bias behavior, exhibited at low temperatures, is also investigated. INTRODUCTION One possibility for future magnetic recording media consists of ultrahigh density arrays of nanoscale single-domain magnetic elements with well-controlled magnetic behavior. Ideally, the internal crystal morphology of the array elements should be controlled independently of element dimensions and array geometric factors. Recent experiments demonstrate that a nanoporous template derived from a self-assembled diblock copolymer film can be used as an electrodeposition template to produce hexagonal arrays of nanoscale cobalt magnetic cylinders with element density in excess of 1012 /in2 [1]. These vertically aligned nanowires are a candidate system for perpendicular magnetic data storage media. The thrust of the present work is to systematically tune the magnetic properties of such arrays by modification of the nanowire internal structure through changes in the electrodeposition bath pH. In general, electrodeposition conditions can influence crystalline phase [2], crystallite orientation [3], crystallite size, and incorporation of cobalt oxide or cobalt hydroxide into the nanowires. Changes to nanowire crystalline structure result in dramatically different array magnetic properties. Specifically, the largest perpendicular coercivity is expected when the magnetocrystalline anisotropy axis lies parallel to the shape anisotropy axis. In the present context, this corresponds to the case where the nanowires are composed of c-axis oriented hcp cobalt crystallites with the orientation axis aligned with the nanowire axis. SAMPLE FABRICATION Nanoporous polymer films were used as templates for the electrochemical synthesis of cobalt nanowires. The nanoporous templates were produced by a method presented in [1], and briefly described as follows. A silicon wafer (thickness 0.1 mm) is used as a rigid substrate, on which additional chromium and gold layers are therm
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