Magnetic Recording at High Densities on Protein-Derived CoPt Nanoparticles
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MAGNETIC RECORDING AT HIGH DENSITIES ON PROTEIN-DERIVED CoPt NANOPARTICLES Jay Hoinville, Angus Bewick, David Gleeson, Richard Jones, Oksana Kasyutich, Eric Mayes, Artur Nartowski, Barnaby Warne, Jason Wiggins, and Kim Wong Nanomagnetics Ltd., 108 Longmead Road, Emerald Park East, Emersons Green, Bristol BS16 7FG, United Kingdom ABSTRACT This paper reports on the progress in developing self organized nanoparticulate arrays for magnetic recording at densities beyond 30 Gbit/cm2 (200 Gbit/in2), and in particular describes the beneficial use of biological templates in developing such arrays. Chemically synthesized, high magnetocrystalline anisotropy magnetic nanoparticles have demonstrated extremely narrow size distributions that are critical in reducing media noise. Due to their monodispersity, they also exhibit emergent self-patterning that could potentially support bit-per-particle densities up to 2-8 Tbit/cm2 (10-50 Tbit/in2). High anisotropy L10 CoPt precursor grains are prepared within apoferritin from aqueous reactants, with synthesis conditions controlling grain size, structure and composition. Smooth films on glass disk substrates are produced by either spin- or dip-coating from aqueous dispersions of the precursor material. Films are typically annealed at 590 °C for 60 minutes with a 19 kPa (190 mBar) partial pressure of H2 to form the L10 phase. We report on recently produced films that have demonstrated moderate-density recording using a contact drag tester with commercial, 0.35 µm-wide magnetoresistive (MR) heads. The highest areal density currently achieved for these nanoparticulate films is 0.93 Gbit/cm2 (6.0 Gbit/in2). INTRODUCTION Magnetic recording media composed of monodisperse high anisotropy nanoparticles are emerging as likely candidates to extend areal densities beyond 200 Gbit/in2. One contender, chemically synthesized equiatomically alloyed L10 phase CoPt exhibits extremely narrow volume distributions. These CoPt nanoparticles are thermally stable down to a grain size of 8 nm in diameter. This is primarily due to its large anisotropy, Ku, equal to 5x107 erg/cm3 1. Synthetic techniques have been used to prepare various types of nanoparticles2, but have not shown sufficient control in monodispersity. Aqueous synthesis has provided greater size control3, but the production of monodisperse precursors for the L10 phase of CoPt has only been demonstrated using the biological template ferritin4. One of the great advantages of using ferritin is that it limits the maximum size of a grain to the internal size of the ferritin protein. Ideally when the CoPt precursor is fully formed within the ferritin cavity, there is no grain size distribution in films made from the synthesized material. Another advantage of ferritin is its spherical shape, which encourages the formation of self-organized films. Ferritin’s 2 nm-thick protein coating discourages sintering of grains at the high annealing temperatures required for the transformation to the L10 phase. On sufficiently smooth substrates ferritin has pr
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