Modification of the magnetic properties of longitudinal thin-film media by ion-beam irradiation
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Modification of the magnetic properties of longitudinal thin-film media by ion-beam irradiation Jason D. Wright1 and Kannan M. Krishnan Materials Sciences Division, Lawrence Berkeley National Laboratory Berkeley, CA 94720 1 Department of Materials Science, University of California, Berkeley Berkeley, CA 94720 ABSTRACT The modification of conventional longitudinal recording media by ion-beam irradiation is of both scientific and technological interest. In particular, patterning by irradiation through a stencil mask at the 50 nm length scale may fulfill the promise of a commercially viable patterned media architecture. In this context, the magnetic properties and microstructural evolution of high-coercivity longitudinal thin film media were investigated after ion-beam irradiation. TRIM simulations were used to calculate the depth profiles and damage distributions as a function of energy and dose for carbon, nitrogen, and chromium ions and three different media (C, Cr, no capping layer). Corresponding implantations were carried out and hysteresis curves were measured using a vibrating sample magnetometer (VSM). Using chromium ion implantation at 20 keV, both remanence and coercivity were reduced to zero, i.e., rendering the ferromagnetic thin film paramagnetic, at doses as low as 1×1016 cm-2. For C+ implantation at 20 keV, remanence and coercivity were also reduced to varying extent up to doses of 5×1016 cm-2 after which further irradiation had no effect. Slight decreases in remanence and coercivity were observed for 20 keV N2+ irradiation. XRD measurements indicate that the hexagonal cobalt alloy phase remains intact after irradiation. The physical and magnetic domain structures at the surface were assessed by atomic force and magnetic force microscopy. Combined with the development of a suitable stencil mask, such chromium ion implantation can be used to develop a viable patterned media with nanoscale dimensions, consisting of locally defined ferromagnetic and paramagnetic regions. This work is in progress. INTRODUCTION Improvements in the ability to carefully control the magnetic properties of thin films have made significant contributions to the recent 60% growth rate in the areal bit density of magnetic recording media. Historically, these improvements have been associated with reductions in grain size and advances in grain isolation (made necessary by statistical noise constraints requiring a certain number of grains per bit). The superparamagnetic effect, a grain size limit that renders magnetic moments unstable with respect to thermal fluctuations, represents a fundamental roadblock to the current regime of media development. Since the magnetic properties of interest in such recording systems occur in the near-surface region, ion beam processing may be an appropriate means of asserting yet further control over the important physical, chemical and magnetic microstructures of these films. Baglin et al.[1] have previously reported the loss of uniaxial anisotropy and T3.6.1
an increase in coercivity in Permalloy
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