The Limits of the Wallace Approximation for PMR Recording at High Areal Density

This paper discusses the validity of the Triple Harmonic method and the Wallace readback model for the determination of head-disk spacing variations in today´s perpendicular recording (PMR) systems at >; 500 Gb/in² areal density. For the recording system investigated here, and in the limit of high frequency, approximately beyond the frequency corresponding to the 2 T (0011) pattern, this method fails, as head and electronic noise start to interfere. In the low-frequency range of PMR systems, the Wallace equation does not apply any longer. Accurate magnetic spacing variations can be obtained by using the FFT of the intensity ratio of the readback signals, and fitting with the appropriate algebraic expression. Using this method, a repeatability of less than 0.1 nm was obtained, by using a random data pattern of only four 512 bytes data sector long.