Noise in Disk Data-Recording Media

Measurements were made of recording medium noise in erased disks using an in-contact magnetoresistive element and an inductive head supported on an air bearing slider. Four types of coatings on aluminum disks were examined: thin, transition-metal alloy film, CrO,, FeCo particle, a n d y-Fe,O:,. Results obtained by means of three measurement techniques are in qualitative agreement and indicate that: (1) dc-erased noi\e of alloy film disks is 14 to 20 dB lower than that of particulate disks measured; (2) dc-erased noise of particulate disks measured is 6 to 16 dB above their bulk-erased noise: (3) although dc noise of particulate disks increases with write current, dc noise of alloy film disks is independent of write current: (4) the shapes of the noise spectra are similar in dc-erased particulate y-Fe,O, disks and FeCo particle coated disks: and (5) significant modulation noise is detected on particulate disks but not on alloy film disks. The observed dc-erased noise spectrum is compared with the model for small particle noise and is then used to estimate the size of particle agglomerates or voids. Introduction Several theoretical and experimental investigations of noise in magnetic tape systems have been published [ 1-61, but until recently there was little interest in measuring the corresponding noise in disk surface media because it does not limit performance of present disk data storage systems. Disk noise measurements are of theoretical interest because of the relatively well-known head-medium spacing and the expected absence of noise caused by surface irregularities, tape flutter, and inverse magnetostrictive effects associated with head-tape contact. Although most tape noise studies have emphasized the noise of ac-erased media, the emphasis here is on the dc-erased, or uniformly magnetized, state which resembles the saturated digital state. The dc-erase noise provides information about the homogeneity of the magnetic medium, and its measurement can shed light on the suit-ability of the medium for use at high storage densities. The difference between the observed noise power spectrum and that which would be caused by small particles is interpreted as the noise spectrum of the inhomogenei-ties. Measurements of particulate coatings, both here and in the literature, indicate higher noise levels for dc erase than for ac erase. For comparison, measurements are presented here of a thin transition-metal alloy film medium, for which the reverse is true.