Micromagnetic Study of Composite Media for High Density Heat Assisted Magnetic Recording

Abstract

The hard disk drive industry is a market with around 25 billion revenue each year. The annual average areal density growth rate is about 40% before 2012. With cloud computing and storage technology emerge, hard disk drives with high area density and low price are required. However, the areal density of current perpendicular magnetic recording technology tends to saturate at 1 Tb per square inch. Therefore, new technologies like Heat Assisted Magnetic Recording (HAMR) are needed. On the other hand, the Solid State Drive (SSD) has developed quickly as another candidate for high density and high speed information media which makes this situation urgent. In this thesis, micromagnetic simulations of HAMR media are conducted based on the renormalized Landau-Lifshitz-Gilbert (LLG) method. L10 FePt is one promising recording media candidate for HAMR. The transition noise and transition jitter are calculated through magnetic recording simulation accelerated by GPU parallel computing. Thermal fluctuations and Curie temperature variance are verified to be two import noise sources for FePt recording media besides the grain size distribution and anisotropy variance. A more easily implemented method called thermal switching probability distribution (SPD) is proposed. It can provide two important factors for evaluating the recording performance: 𝜎𝑆𝑃𝐷 and write temperature. Under a certain fabrication technology (certain average grain size), the transition jitter can be estimated by 𝜎𝑆𝑃𝐷 . Furthermore, the grain volume dependence of 𝜎𝑆𝑃𝐷 and write temperature are investigated. The dependence follows 1 ⁄ 𝑉 and 1 ⁄ √𝑉 power law respectively. This knowledge greatly helps the noise analysis and new media design. To mitigate the noise from thermal fluctuations and Curie temperature variance, a new composite media design based on a bilayer structure with two different Curie temperatures is proposed. The substantial anisotropy of the write layer differentiates this design with respect to previous work. This ensures that the composite structure has small transition noise and high areal density. The user density can reach 3.4 Tb per square inch under traditional recording and 4.7 Tb per square inch with shingled magnetic recording (SMR) technology. The interlayer exchange coupling effects are found to affect the energy barrier during the dynamic recording process. Both the thermal effects and write temperature can be tuned by optimizing the interlayer exchange coupling effects. Further research verified that this is due to the linear temperature dependence of energy barrier at temperatures close to Curie temperature. More research need to be done to offer a good explanation of the high areal density achieved by PMR-ECC-like structure for HAMR. Possible directions include the effective field temperature gradient and switching speed during the recording process as the temperature changes.