Abstrad -The superparamagnetic effect dictates a lower limit to the size of magnetic grains in the recording medium. At very high areal densities each bit of information will thus be recorded on relatively few magnetic grains. Since the grains have random positions and sizes, there will be large statistical fluctuations or noise on the recovered signal. Modelling the media grains as random Voronoi regions, we simulate the process of writing, reading, detection and error-correction. Although the signal-tonoise ratio is very poor, we find that it possible in the simulation to recover data reliably with as few as four grains per bit.

Low media noise and high thermal stability are required to achieve high density magnetic recording. Reduction of magnetic grain size is required in order to minimize transition jitter. However, very small grains become unstable and spontaneously reverse their magnetization due to thermal fluctuations. Future high density (>20 Gbit/in 2 ) recording systems are likely to operate in a media noise dominated environment and could exhibit a certain amount of signal decay. Recording performance degradation caused by media noise and thermal decay is evaluated. Density-dependent amplitude decay is caused by the presence of demagnetization fields and results in increasing level of nonlinear distortions and overwrite degradation. Magnetization decay is accompanied by changes in media noise in an unstable media as determined by the relative contributions of the transition and particulate noises. Experimental measurements suggest, that channel bit error rate degradation in an unstable medium is determined by signal and noise evolution, as well as by pattern-dependent distortions.

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It is demonstrated that the nonlinear transition shift (NLTS) increases with time in a thermally unstable medium. The measurements were performed using an experimental medium with low magnetic moment. The increase of NLTS during thermal decay can be explained by the density dependence of the decay rates. This density dependence of thermal decay rates is caused by demagnetization field from adjacent transitions. A random magnetization pattern demonstrates different amplitude decay at high and low densities, and an increased level of NLTS. Error rates of PRML channel were estimated before and after the thermal decay.