Dean Palmer

Identification of nonlinear write effects using pseudorandom sequences

Nonlinear distortion causes significant degradation at high linear recording densities. Nonlinear distortion cannot generally be corrected by equalization and is thus particularly important to correctly identify and quantify. A technique is described which is easily applied, clearly separates linear from nonlinear effects, can be related to the underlying physical processes, and provides a good basis for modeling the channel. The technique involves the recording of a binary pseudorandom sequence and the capture and processing of the playback waveform. The technique is used to identify four distinct nonlinear effects and to examine their behavior as a function of flying height and record current.

Characterization of the read/write process for magnetic recording

The extracted dipulse technique, or pseudorandom sequence method, provides a means for the characterization and measurement of a variety of linear and nonlinear recording effects. In the past the technique has been used to identify and quantify several transition shift phenomena. The technique is extended to characterize readback distortions and media noise. The technique can be applied to any binary or even to multilevel channels; a ternary example is included.

Overwrite in thin media measured by the method of pseudorandom sequences

The method of pseudorandom sequence channel identification is applied to the measurement of bit shift due to overwrite in magnetic recording. The experimental results are compared to results of other overwrite measurement techniques. The proposed method has the advantage of simultaneously quantifying the bit shift due to overwrite and that due to other nonlinear effects. A simple magnetic model is proposed to explain the main features of the overwrite phenomenon. The bit shift is attributed to the interaction of the magnetic field produced by the head and the demagnetizing fields in the media produced during the write process. >

Texture-induced modulation noise and its impact on magnetic recording performance

The mechanical texturing of recording media which is necessary for good stiction characteristics can have an undesirable side effect on magnetic performance. A simple technique is described to measure the modulation noise caused by the texture in the substrate. Measurements made on CoPtCr media show a strong connection between the degree of modulation and the error rate. >

Write bubble measurement under realistic recording conditions

A simple method to characterize the write process of a recording head by measuring the write bubble is described. The advantage of the technique is that the measurements are made under normal recording conditions. Changes in the write bubble can be observed in response to changes in key recording parameters, such as write current, medium velocity, and data rate.

An experimental study of the effect of thermal decay on noise and nonlinear distortions in perpendicular media

The time dependences of both noise and nonlinear distortions were studied using pseudorandom sequences. Transition noise and dc remanence noise were separated from the total media noise by curve fitting the transition noise with the most significant eigenmodes based on the Karhunen-Loeve expansion. The results show that the transition noise power and the signal decrease and the dc remanence noise power increases with time, resulting in a degradation of the total signal to noise ratio. Using the extracted dipulse technique, it was found that the main peak narrowed and a positive echo at 25-b position increased with time, which indicates that the decay of low-frequency components is faster than that of high-frequency components in perpendicular media.

Recording properties of multilayered thin film media

The recording properties of layered film media, including the bit error rate, have been measured for comparison with the results found on single layer film media. This investigation centered on the ability of magnetostatic coupling between layers to affect the magnetic performance in ways other than noise reduction. Multilayered magnetic films were fabricated with interlayer thickness from 25 to 330 AA and with one to six magnetic layers using a CoPtCr alloy on a Cr underlayer. In most tests the single layer media perform better than the multilayer media, but the differences are small and do not appear to influence the error rate significantly. The largest distinguishing feature. is the improvement in the signal-to-noise ratio which comes from two thin layers that are coupled magnetostatically. This leads to an improvement of up to four orders of magnitude in the bit error rate for the bilayer media. Increasing the number of layers gives additional improvement. >

Determination of grain size distributions in magnetic recording media by grazing incidence X-ray diffraction

The grain size distribution of magnetic recording media has been measured with in-plane grazing incidence X-ray diffraction (GIXRD), without using synchrotron radiation, via the Bertaut-Warren-Averbach (BWA) technique. Lognormal-like grain size distributions are observed in all recording media. The obtained distributions are consistent with the TEM observations. In addition, the relevant measurement issues are also discussed.