Areal density limits for perpendicular magnetic recording

It is becoming clear that conventional longitudinal magnetic recording is approaching the superparamagnetic limit and that further density increases must be achieved using an alternate approach. Perpendicular recording with a soft underlayer provides one such approach. Relative to longitudinal recording, it is commonly claimed to possess three major advantages, each exhibiting an apparent flaw that we argue can be solved: 1. The record field is approximately twice that obtainable in longitudinal recording owing to the image field generated by the soft underlayer. However, the coercivity of a perfectly oriented perpendicular grain is also nearly twice that of the unoriented longitudinal grain with equal anisotropy field. Solutions to this problem include a head field that generates some longitudinal components or an incompletely aligned perpendicular media. 2. The demagnetization fields of perpendicularly oriented media favor thicker films that can better resist thermal fluctuations. However, at high densities, the enhanced distance between head and soft underlayer will weaken the head fields and gradients. The solution to this problem rests in the observation that even the recording layer has non-zero susceptibility. 3. Magnetostatic fields do not destabilize transitions in perpendicular recording, unlike longitudinal recording. Unfortunately, they do destabilize the middle of the bit. However, this problem can be overcome by introducing a uniform exchange coupling within the media.