Underlayer Noise in Rigid-Disk Perpendicular Recording

Abstract

In vertical recording schemes that employ a soft magnetic underlayer, high amplitude noise spikes can be induced during readback. We have focussed in our study on thick (nominal 2 um), high permeability (nominal 2000) NiFe which appears to be necessary from modelling considerations to establish a reasonable head efficiency for a pole-keeper structure. We report here data showing in the rigid-disk environment the noise is dispersed low enough in the spectrum to suggest modest high pass filtering as a solution. We also report hard layer erasure associated with large spikes. From some simple calculations of the flux coupling the head from these spikes, a model of noisespike-induced, hard-layer erasure is proposed. By demonstrating that the noise scales in both amplitude and time with media velocity we have ruled out Barkhausen jumps as the primary mechanism of flux change.