Magnetic field of a ring head on a double layer medium

A simple theoretical model is developed for a Karlquist-type recording head interacting with a two layer recording medium in which the sublayer has a high but finite permeability. During readback the vertical H fields far from the gap are shown to decay exponentially with a characteristic length r given by
r = t \ast h \ast u
where t is the sublayer thickness, h is the spacing between the head and the sublayer, and u is the relative permeability of the sublayer. The long tails of this field account for observed readback wave forms. The model also predicts that the spatial extent of sublayer saturation during the write process will be twenty to one hundred gap lengths, justifying the assumption that during write one may ignore the sublayer. The write prediction was tested experimentally by making a standstill recording with a Winchester-type head on a double layer medium and measuring the fringing field above the medium with a high-resolution Hall probe. The data were compared to the predictions of an iterative simulation in which the sublayer was either present or absent during write. A significantly better fit was found when the simulated medium contained no sublayer during the write process.