The current revolution in magnetic recording technology is leading to remarkable increase in areal density. Innovations such as MR heads and PRML channels have prompted this sudden upturn in the growth rate for areal density. Products at 1 Gbit/sq.in., packing an incredible 10 GBytes into a single 3.5" disk-drive, seem to be just around the corner. Central to this revolution is our improved understanding of the read/write process and the ability to apply powerful readback detection techniques. The fascinating and challenging properties of the recording channel at high densities and data-rates will be described.

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The performance of decision feedback equalization (DFE) with a maximum-likelihood Abstract (ML) detector in high-density magnetic recording channels is estimated analytically, in terms of the length of the feedback-filter and the depth of the ML detector. This estimation takes into account the performance degradation due to error propagation in the feedback-loop and the ML-detector. In this analysis, it is shown quantitatively that the signal-to-noise-ratio (SNR) performance in high-density recording channels can be improved by combining ML detector and feed-back filter. In particular, it is found that near-optimum performance is achieved by increasing the total of the feedback-filter length and ML-detector depth to 6 bits within the information density range from 0 to 3.

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Nonlinear phenomena in perpendicular magnetic recording employing a single pole head were investigated. First, a measurement on linear superposition in time domain indicated that some amount of nonlinearity existed. The nonlinearity was concluded that it caused by transition shift, not by waveform distortion. By interpreting the results we could prove the transition shift strongly related with head field gradient and interference field from recorded magnetization. Dependence on head parameters was therefore examined through following experiments. Based on these results, a single pole head with which transition shift can be reduced was proposed.

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The readout characteristics of the partial response (PR) channel of an magneto-optical disk system is presented. The PR class 11 or its modification PR(1,u,1) channel, is found to be favorable in detecting data from MO channel in which the minimum mark size is about 30% of the diameter of readout optical spot. The dual-level independent edge-detection (DLE) method is newly developed for reproducing method instead of a simple threshold-detection method, with considering fluctuations of media and head. The DLE method has a 2.5-dB advantage in SNR compared with the simple threshold-detection. Assuming a 20-dB SNR, the window margin with 10^4 samples is more than fifty percent of the window.

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To make higher density on optical disk, a new recording method-two dimensional recording- which can increase the track-density rather than the linear density was proposed. The performance of this method on increasing density is estimated by the simulation and confirmed by measuring the read-out signal. Consequently, it is clarified that this method can attain more than twice the areal density independently and realize the density of 2.2 Gbit/in^2 which is 8 times as large as that of first generation using 680 nm laser diode.

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Ultrathin Co/Pd multilayered films were prepared by rf magnetron sputtering and the effects of deposition conditions such as sputtering gas pressure and palladium underlayer thickness on the perpendicular magnetic characteristics of the multilayer were investigated. It was found that predominant mode of magnetic reversal changed from domain wall motion to magnetic moment rotation as sputtering gas pressure increased. Palladium underlayer also influenced the magnetization reversal features, and contribution of magnetic rotation to the magnetic reversal increased as palladium underlayer thickness increased. It turned out that there existed an optimum pressure for maximum perpendicular anisotropy energy, and the pressure was lower for the film with thinner cobalt layer.

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Exchange-coupled NdTbFeCo/TbFeCo double layered structure is one of the promising candidates for magneto-optic recording media in short wavelengths. Kerr llipticity should be considered with Kerr rotation angle to evaluate Figure of Merit in short wavelength, due to phase difference. In this research, Carrier to Noise ratio is measured with phase compensator to eliminate the effect of the phase difference in the short wavelengths.

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Artificially modulated magnetic Co/Pd multilayer is one of the promising materials for magneto-optic recording in short wavelength laser beam. But due to the fact that the Co/Pd multilayer is in a non-equilibrium state in terms of free energy and the marneto-optic recording is a kind of thermal recording which is conducted around Curie temperature (^TC), the assessment of stability in the Co/Pd multilayer is crucially interested in both structural and magnetic properties. As the parameter of the assessment of stability, changes in particle size and perpendicular magnetic anisotropy of the Co/Pd multilayers as the structural and magnetic properties, respectively are discussed below and above ^TC. The temperature dependence of the both properties in the Co/Pd multilayers shows a great deviation below and above ^TC. We attribute the deviation in the properties to the role of magnetic exchange energy below ^TC in the Co/Pd multilayers.

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