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.

Noise in disk data-recording media

In the early days of electronic data processing, new application requirements arose that could not be met adequately by batch processing, a mode dictated by the sequential nature of card and tape input-output equipment. Users needed a device to store and directly access relatively large amounts of data on-line. The first such products, IBM's RAMAC disk file and Sperry Rand's Randex drum, partially answered these needs. However, their use could be justified only for storage of highly active data due to their limited capacity and high cost compared to off-line tape storage.

Magnetic disks for bulk storage: past and future

A linear, statistical model is described which predicts the power spectrum of measured noise in bulk-demagnetized (i.e. AC-erased) thin-film magnetic recording media. It is shown that the noise is the result of magnetic flux which is ascribed to erasure-induced transitions along the track length in the medium. The noise power spectrum for a rigid disk medium is shown to correspond to the power spectrum of Poisson-distributed induced transitions along the track length, while noise along the track width is sufficiently described in terms of a uniform, average magnetization with small variance. Experimental data from two thin-film disks are used with the model to estimate the Poisson parameter for each disk. It is demonstrated that AC-erased noise from particulate media can be considered as a limiting case of the Poisson model. >

/pdf/model-for-demagnetization-induced-noise-in-thin-film-rggi9r7chp.pdf

Model for demagnetization-induced noise in thin-film magnetic recording media

The leading magnetic material for use in data recording tapes is the metal particulate (MP) composition. The trend to smaller particle size necessarily entails an increase in thermally induced time effects. An Arrhenius-Ne/spl acute/el switching model is used, together with the dependence of the experimental coercivity on field duration, to determine an effective switching volume. The model takes account of the highly imperfect magnetic orientation present in actual recording media. Results for coated media made with current advanced particles having various degrees of orientation (squareness) show that the calculated switching volume is not sensitive to orientation above a squareness value of about 0.6 (essentially random in plane). This switching volume correlates well with particle volume, as determined by transmission electron microscopy (TEM). These results confirm the practical relevance of the model, both for realistic tapes and for disks that have random grain orientation, and are evidence that the model adequately takes account of preferred-axis misalignment.

Magnetic time effects in small metallic particles for recording: orientation dependence

/pdf/detection-of-digital-information-from-erased-magnetic-discs-35vgr5i3fn.pdf

Detection of Digital Information From Erased Magnetic Discs

http://ieeexplore.ieee.org/iel5/20/22905/01066571.pdf

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Papers

Maximum Signal-to-Noise Ratio of a Tape Recorder