Physics of perpendicular magnetic recording: Playback

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The physics of the playback process in perpendicular recording is explored. It is shown that due to the existence of effectively two layers of the “magnetic charges” in a perpendicular medium, the stray field sensed by a reader rolls off with the areal density essentially differently than it does in longitudinal recording. Comparing three recording modes, longitudinal recording and perpendicular recording without and with a soft underlayer (SUL), at equivalent conditions, with a 10 nm thick recording layer at a bit aspect ratio of 1:1, the density roll-off is the fastest for the longitudinal mode, while it is the slowest for the perpendicular mode without a SUL. It is shown that although at relatively low linear and track densities the use of a SUL increases the playback signal, the signal does not depend on the use of a SUL at high densities. It is shown that for both perpendicular modes, although at sufficiently low track densities (below ∼50 ktpi) the signal disappears at relatively low linear densities, there is a significant nonzero signal even at zero linear density if the track density is sufficiently high (above ∼300 ktpi). Using the magnetic image model, it is shown that, fundamentally, the use of a SUL could not improve the resolution of a recording system. As an example, the Reciprocity Principle is utilized to analyze and compare four different reader designs.