Content area
Abstract
To increase the areal density of the hard disk drive, the average grain size and the film thickness of the recording media must be reduced. However, the reduction decreases signal-to-noise ratio (SNR) and induces thermal instability to the written information. In this dissertation, micromagnetic simulations have been performed to study effects of intergranular magnetostatic and exchange interactions on SNR and thermal stability in longitudinal and perpendicular media. The principles and algorithms of simulating a recording system, generating a non-uniform size distribution, and analyzing a thermal decay process are discussed.
For longitudinal media, the intergranular exchange coupling increases both the cross-track correlation length and the transition parameter, leading to a substantially large transition noise power. Without an exchange, the pure magnetostatic interactions reduce the transition noise power significantly. However, with decreasing the transition noise power, the contribution from the DC particulate noise is not negligible. Formulas to calculate the transition noise power, the DC noise power, the total noise power, and the corresponding signal-to-noise ratios have been derived. In addition, effects of a non-uniform grain size distribution and a c-axis orientation on recording performances have been examined.
Dynamic remanent hysteresis loops have been simulated and the thermal energy barrier distributions have been calculated. Compared to the physical volume distribution width, the intergranular exchange reduces the thermal energy barrier width. Intergranular magnetostatic interactions exhibit a very small effect. However, for the dibit recordings of advanced isotropic longitudinal media, due to the reduced demagnetization field, the intergranular exchange induces a larger thermal decay and a smaller SNR. For slightly highly oriented longitudinal media, with increasing exchange, though thermal stability is improved, SNR is still smaller. Thus, for longitudinal media, the intergranular exchange interaction must be reduced to achieve a higher SNR.
For perpendicular media, the intergranular exchange coupling increases the cross-track correlation length but decreases the transition parameter. Simulation results have demonstrated that to increase SNR and to enhance thermal stability simultaneously, medium with reduced magnetostatic interactions is required and a weak exchange should be included.
