Thermal switching has been recognized as a possible limit to archival storage in advanced high density recording media . Thinner films with more isolated grains suffer from large magnetic viscosities -1Q%/decade. Previous work has shown that the observed remanent coercivity H,,(s) in high viscosity tapes can be predicted from Sharrock's Law down to 109s. Here we report high speed switching results on thin film media which show that the remanent coercivity at 10% is increased -40% relative to the VSM value in these films. A quantitative relationship between the viscosity, irreversible susceptibility, and the increase in HCR(s) is demonstrated.
The continued increase in areal densities in magnetic recording makes it crucial to understand magnetization reversal in nanoparticles. We present finite-temperature micromagnetic simulations of hysteresis in Fe nanopillars with the long axis tilted at angles from 0 degrees to 90 degrees to the applied sinusoidal field. The field period is 15 ns, and the particle size is 9 × 9 × 150 nm. The system is discretized into a rectangular pillar of 7 × 7 × 101 spins each with uniform magnetization. At low angles, reversal begins at the endcaps and proceeds toward the center of the particle. At ninety degrees, reversal proceeds along the entire length of the particle (save at the ends). The switching field was observed to increase over the entire range of angles, consistent with recent experimental observations. A second, lower-resolution micromagnetic simulation with 1 × 1 × 17 spins, does not agree with experiment, but shows behavior very similar to that of the Stoner-Wohlfarth model of coherent rotation.
It has been established previously that thermal switching in high density magnetic media causes a significant time dependence of the coercivity at least for times > 10 −8 s. Here, the classical Arrhenius-Neel model assuming coherent rotation is applied to systems with distributions in volume, anisotropy and orientation and the numerical results for the time dependent remanent coercivity ( ), the viscosity , and the irreversible susceptibility, compared to new experimental data on CrO 2 particulate tapes with varying orientation. The results for an anisotropy distribution are significantly different from a volume distribution and are in better agreement with the data. All distributions produce Sharrock type behavior for ( ), regardless of the distribution width. The model predicts that increasing the orientation distribution width will have very little effect on the fractional time dependence of , in agreement with experiment which showed that and ( ) were essentially independent of orientation. Finally, the direct connection between and ( ) is confirmed theoretically and experimentally.Index Terms-Dynamic coercivity, high speed switching, magnetic viscosity, thermal switching.
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