Recording Head Design

Abstract

Magnetic recording is literally as old as the hills. Recently a satellite, flying high over t he polar regions of Mars, detected st ripes of magnetisation spaced at more th an 50 miles. This is proof that Mars had a significant magnetic field and a liquid core in the distant past. In order to detect finer structures, the height of the orbit would have to be reduced. At fly heights, h, greater than one third of the plus to minus spacing of the stripes, S _b , the field intensity from a periodic source falls as: B α e ^−πh/Sb This is known as the Wallace spacing loss formula. At this time the fly height in disk drives is about 25 nm. At this height the spacing loss factor is about 1/5 for a 50 nm bit space. This is approaching the minimum that a modern partial response maximum likelihood (PRML) channel detector can cope with, for a reasonable minimum signal to noise (SNR) requirement. The fly height in a disk drive is the key variable from which all th e others scale. Recently density has doubled each year. This has been achieved by scaling all of the salient system dimensions, including tolerances, down by 30% per year.