Xiaodong Che

General analysis of noise in recorded transitions in thin film recording media

General noise voltage relations based on magnetization correlations in thin film recording media have been developed. Transition position jitter or amplitude fluctuations are a consequence rather than an input to the model. The effect of nonstationarity due to recorded data patterns is included by a simplified first-order model of the down track variation of the correlation function. Estimates of position jitter and amplitude fluctuations are given. An example of peak detection is used to illustrate the determination of error rates. >

Studies of nonlinear bit shift and partial erasure using pseudo-random sequences

A pseudo-random sequence method to identify nonlinear effects has been utilized at high recording densities. This method is extended to distinguish two different nonlinear effects: nonlinear bit shift and partial erasure. Following the nonlinear bit shift echo derivation in the pseudo-random sequence method, it is shown that partial erasure also produces an echo that is only 1/2 bit cell apart from the echo for nonlinear bit-shift. A numerical simulation of writing 31 bit long pseudo-random sequences is utilized to study this phenomenon. The magnetization instead of the waveform is analyzed to obtain higher spatial numerical resolution. The two nonlinear effects are identified from their echoes and their response to the writing precompensation is studied. It is found that partial erasure is not reduced efficiently by precompensation. >

The relationship of medium noise to system error rate in a PRML channel

High density disk drive systems utilize fine grain polycrystalline thin films as storage medium. These films exhibit noise that is pattern dependent and has appreciable magnitude near the center of each recorded transition. A model for error rates in a PRML channel is presented to explore how variations in medium noise parameters effect system performance. The channel is assumed to be characterized by both pattern dependent nonstationary medium noise and white Gaussian electronic and/or head noise. Results are presented in terms of both error rate and sensitivity of error rate to changes in medium parameters. >

Dynamics of nonlinearities in high density recording

A micromagnetic model is used to study nonlinearities in high-density recording. Head imaging is taken into account approximately to model near contact recording. The simulations are able to show detailed steps of a dibit transition formation and illustrate the effect of the image field. Nonlinear amplitude reduction as a result of zigzag transition percolation is calculated at different recording densities and flying heights. The numerical results agree very well with experiment. Effects of medium exchange coupling and medium orientation on the amplitude loss are also investigated. >

Magnetization correlations in thin film media in the presence of a recorded transition

Magnetization correlations in thin film media in the presence of a written transition have been studied utilizing numerical micromagnetic simulation. Polycrystalline films are modeled by a hexagonal array of identical grains each with random anisotropy axis orientation. A transition is written in the medium and the positional variance, the stationary cross track correlation, and the nonstationary down track correlation functions are determined by suitable averaging over statistical ensembles.

PRML performance estimation considering medium noise down track correlations

Application of partial-response maximum-likelihood (PRML) sequence detection in magnetic recording has been demonstrated with substantial increase in the linear density. This method achieves optimal performance when the noise is white and Gaussian. In a recording channel dominated by medium noise, the noise is correlated due to inter-granular interactions. The characteristics of the noise strongly affect the PRML performance. The signal dependent medium noise is characterized approximately by the cross track average magnetization in the presence of transitions. Error rates for detecting a single bit, a dibit, and a tribit with medium noise correlations included are calculated. It is found that errors in detecting a dibit or a tribit are mainly induced by the magnetic noise addition of adjacent transitions. >

Noise analysis and image focusing for magnetic force microscopy

Magnetic force microscopy (MFM) has been widely utilized to reveal magnetization distributions by sensing the external magnetic‐field distribution very close to the sample. The resolution of MFM is determined by the size of the sensor tip and by the spacing between the tip and the measured sample. A technique is developed to analyze the noise and linearity of a MFM image, and consequently to improve the spatial resolution by reducing the spacing loss. As a demonstration, a series of MFM images of a single permalloy particle is obtained and numerically analyzed. The spacing loss is reduced and a much higher resolution image is obtained.

Non Linearities And Noise In Perpendicular Dibit Recording

Nonlinearities and medium noise in perpendicular recording are studied by simulating the dibit recording process. Strong nonlinear bit shift is observed: the second transition shifts away from the first transition. Transition noise on the second transition decreases with a reduction of the bit separation due to a shortening of the second transition. To a very good approximation, even when the bit separation is very small, medium fluctuations are determined only by the cross track average magnetization. The cross track correlation of nonexchange coupled granular media with sufficiently low magnetization is roughly one grain diameter. >

Study of magnetic recording transitions at very low flying heights

In this article, a micromagnetic model is utilized to study dibits recorded under an almost contact condition. Transitions, which are sharply written at contact, suffer irreversible demagnetization while they leave the recording head. Dibit stability under a small perturbing field is examined. A small dc field and a thin‐film edge field are utilized for the perturbing field. It is found that, in contact recording, Mrδ/Hc should be sufficiently small for magnetic pattern stability.

Analysis of correlation of magnetization and noise after ac erasing

Normally, theories of magnetic noise after ac erasing take into account residual correlations among magnetic particles either through an ad hoc assumption or by numerical simulation of the erasing process [IEEE Trans. Magn. MAG 23, 1767 (1987); IEEE Trans. on Magn. MAG 20, 412 (1984); IEEE Trans. Magn. MAG 19, 899 (1983).] In this paper we present a theory for a particulate medium based on a pseudo‐equilibrium calculation of the correlation, with the ac field acting as the heat bath. The results are compared with experiment.