Perpendicular Recording Media
Perpendicular recording has been frequently proposed as a technique for surpassing the limits placed on longitudinal recording by thermal fluctuations. Basically, superparamagnetism will be a problem when the product of the anisotropy K and the granular v
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Perpendicular Recording Media
Randall Victora
Perpendicular recording has been frequently proposed as a technique for surpassing the limits placed on longitudinal recording by thermal fluctuations. Basically, superparamagnetism will be a problem when the product of the anisotropy K and the granular volume V is no longer sufficiently greater than the product of the Boltzman constant kB and the temperature T. At this point thermal fluctuations will erase stored data, the granular volume can no longer be decreased, and the rapid density growth of the recording industry will end. Perpendicular recording is thought to offer several advantages relative to longitudinal recording. Several of these advantages stem from the nature of the destabilizing field generated by neighboring bits. In longitudinal recording, a bit boundary forms a charged domain wall. This stored charge generates strong destabilizing fields that work to widen the transition and switch the magnetization of grains into unintended directions. The strength of these fields increases with media thickness. As density is increased, the number of charged domain walls and the requirement for sharp transitions increase, thus aggravating the problem. In perpendicular recording, a bit boundary places plus and negative magnetostatic charges of grains on opposite sides of the boundary in close alignment. This is magnetostatically favorable. In other words, magnetostatics favors transitions and prefers high density. Furthermore, thicker media exhibit ever more favorable magnetostatics. Thus, the granular volume can be increased without affecting the areal density. These two effects account for most of the perceived benefit of single layer perpendicular media. However, it is generally argued that addition of a magnetically soft underlayer generates the largest benefits for perpenicular recording. This soft underlayer is placed between the recording layer and the substrate and serves to image the magnetic recording head. This effectively doubles the field generated, thus allowing the coercivity and anisotropy field to be doubled. This, of course, stabilizes the grain. There are also several secondary advantages to perpendicular recording. Longitudinal recording for disk drives employs 2-d isotropic media meaning that the anisotropy axes of individual grains are not aligned. This produces noise that would be eliminated by the highly aligned perpendicular materials. This also implies that track edges are sharper in perpendicular recording. This is a consequence of the range of coercivities found in the disoriented longitudinal grains that may be written by the stray side fields generated by
M.L. Plumer et al. (eds.), The Physics of Ultra-High-Density Magnetic Recording © Springer-Verlag Berlin Heidelberg 2001
8 Perpendicular Recording Media
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the recording head. In perpendicular recording, the coercivities will be more uniform owing to the better orientation and the track edge will be sharper. The promise of perpendicular recording can only be met if the media me
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