UC Berkeley

The hard drive industry widely views heat assisted magnetic recording (HAMR) as the technology to achieve 4 Tb/in^{2 and greater storage densities and recapture the aggressive storage density growth rates of years past so that hard disk drives are able to meet the world's exploding data storage demand. While traditional magnetic media is thermally unstable at room temperature for the small bit sizes needed for high density recording, the high coercivity HAMR magnetic media can safely store digital data at very small bit sizes of (25 nm)2. In order to write data, a near-field optical system confines electromagnetic energy below the diffraction limit to locally heat the HAMR recording bit to 400-500°C within a few nanoseconds. This adds new thermal complications to the already difficult mechanical and tribological design challenges for the head-disk interface (HDI) region. The reliability of hard drive read-write performance depends on the ability of the recording head slider, which contains the read and write elements, to stably fly in close proximity (< 5 nm) to the spinning recording disk. HAMR technology introduces heat-dissipating components and rapid thermal fluctuations to the HDI system not seen in traditional hard drives. Numerical simulations provide insightful information into the performance of HDI components that are difficult or impossible to attain experimentally.}

This dissertation focuses on numerically simulating the mechanics of two components of the HDI under HAMR conditions: (1) the air bearing---pressurized airflow dragged in between the rapidly spinning disk and the slider---that supports the flying slider to maintain a < 5 nm minimum spacing above the disk and (2) the 1&ndash2-nm-thick polymer lubricant that coats the disk to protect it against intermittent contact with the slider. Both are modeled using lubrication theory that is modified for gas rarefaction or thin-film polymer effects in order to provide useful system-level predictions.

In this work, the fully generalized molecular gas lubrication equation that allows for non-isothermal conditions is the basis for a simulation tool used to numerically study the effects of heat dissipation by inefficient near-field optics system components on the air bearing performance. The iterative HAMR static solver solves the coupled problem of air bearing pressure generation and slider thermal deformation, linked by the heat transfer coefficient and pressure profile at the slider's air bearing surface (ABS). Static simulations are conducted for a simple HAMR slider in which the heat dissipating components are a thermal flying height control (TFC) heater, the near-field transducer (NFT), and laser diode. The NFT induces an additional 1&ndash2 nm of localized protrusion compared to traditional TFC sliders, and it has the highest temperature of 175&ndash300°C for the conditions tested. The waveguide dissipates heat away from the NFT and lowers the ABS maximum temperature, leading to a smoother NFT protrusion. Thermal creep, a rarefied gas flow driven by temperature gradients on the boundary, causes additional flying height drop of 0.05&ndash0.15 nm for sliders with minimum flying heights below 2 nm. The efficiency of the read/write transducer and NFT are extremely sensitive to flying height, so even these differences of 0.1 nm will be significant in 4 Tb/in^{2 HAMR systems in which the minimum flying height will only be 1 nm.}

The lubricant covering the disk in a HAMR drive must be able to withstand the writing process. As a first step in modeling a robust lubricant, a simulation tool is developed that incorporates previously proposed film thickness variations of viscosity and an additional component of disjoining pressure due to functional end-groups. Here the simulation tool is applied to a conventional perfluoropolyether lubricant, Zdol 2000, for which there exists experimental data. Simulations at small length and time scales that are unobservable with current experimental capabilities are performed. For films thicker than 1 nm, the inclusion of polar disjoining pressure suppresses the lubricant thickness change due to evaporation and thermocapillary shear stress compared with cases without this component. Thin-film viscosity is an important property to consider for thinner lubricants. The smaller spot lubricant profiles have side ridges due to thermocapillary shear stress while the larger spot profiles show no side ridges, only a trough due to evaporation. The lubricant depletion zone width and depth increase with increasing thermal spot maximum temperature.

The lubricant must also sufficiently recover the lubricant depletion and accumulation zones so as to allow for stable flying heights and reliable read/write performance. Simulation results indicate that lubricant deformation caused by small thermal spots of 20-nm full-width half maximum (FWHM) recover on the order of 100&ndash1000 times faster than larger 1-&mum FWHM spots. However, the lubricant is unable to recover from sufficiently high writing temperatures. An optimal thickness at which HAMR writing deformation recovers fastest is apparent for sub-100-nm FWHM thermal spots. Simulations show that simple scaling of experimental observations using optical laser spots of diameters close to 1 &mum to predict lubricant phenomena induced by thermal spots close to 20-nm FWHM may not be valid. Researchers should be aware of the possibility of different lubricant behavior at small scales when designing and developing the HAMR head-disk interface.