Neutron Studies of Magnetic Recording Media
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Neutron Studies of Magnetic Recording Media S.L. Lee1, T. Thomson2, F.Y. Ogrin3, C. Oates1,6, M. Wismayer1, C. Dewhurst4, R. Cubitt4 , S. Harkness5 1 School of Physics and Astronomy, University of St. Andrews, Fife, KY6 9SS, UK 2 Hitachi San Jose Research Centre, 650 Harry Road, San Jose, CA 95120 USA 3 School of Physics, University of Exeter, Exeter EX4 4QL, U.K. 4 Institut Laue-Langevin, B.P. 156, 38042 Grenoble Cedex 9, France 5 Seagate Technologies, 47010 Kato Road, Fremont, CA 94538 USA. 6 AGH School of Science and Technology, al Mickiewicza, 30-059, Krakow Poland. ABSTRACT Small-angle neutron studies have been performed on samples of continuous inhomogeneous magnetic recording media. This has allowed the local magnetic structure to be probed at a subnanometre resolution, revealing some interesting information about the size and shape of the magnetic grains and their relation to the physical grains. INTRODUCTION There have been relatively few neutron studies of the local magnetic structure of continuous magnetic recording media [1,2]. This paucity is in part due to the difficulties of performing scattering experiments on the tiny volumes of magnetic material available in real systems. Experiments have therefore sometimes focused on thick films of related alloys, that lack the dimensions, underlayers and processing history that give thin film recording media their very particular structure and properties. We have performed a number of studies on longitudinal thin film media intended for use in hard disk drives. These experiments are extremely challenging to perform, due to the small magnetic volume of the active layer (ca. 20 nm) and the large amount of background scattering from the substrate and underlayers. Nonetheless, for those systems where the distribution of grain sizes is relatively narrow, a surprising amount of information can be extracted concerning the local magnetic structure. In this paper we focus on results from one particular material, a CoCrPtB alloy with a grain diameter of around 11nm. EXPERIMENTAL DETAILS The samples were sputtered onto Al substrates, beginning with a thick layer of NiP (ca. 1000 nm), followed by a series of non-magnetic seed layers of typical thickness of 10-20 nm, and finally the CoCrPtB layer of thickness 20nm, this layer being capped with 7 nm of carbon. Due to the large amount of metallurgical scattering from microscopic structure at small angles, much of the Al substrate was removed by careful abrasion to reduce this contribution. In order to increase the volume of magnetic material available for scattering, the sample comprised around thirty polished coupons of diameter 18 mm. The small-angle neutron (SANS) experiments were carried out using the D11 spectrometer at the ILL, Grenoble. The neutrons have both a nuclear and a magnetic scattering cross-section that are roughly comparable in magnitude. However, due to the tiny magnetic volume of these samples, the
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scattering from the structural disorder dominates the raw scattering data, to the extent t
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