UC Berkeley

Ever since the first hard disk drive (HDD) was invented in 1956, it has played a dominant role in global data storage needs. Conventional HDDs, however, have reached the theoretical density limit, called the superparamagnetic limit. Heat-assisted magnetic recording (HAMR) is a novel HDD variant that has been proven to break this theoretical limit. In this technology, a tiny laser is embedded in the read-write head that heats a nanoscale spot in the disk to several hundred Kelvin. The elevated temperature enables writing on highly coercive disks that resist the superparamagnetic effect. The physics underlying HAMR has proven to work; however, a commercial implementation has been met with numerous engineering challenges. Due to its complex nature, the head-disk interface (HDI) remains susceptible to premature failure. One key challenge is called smear, which is a contamination buildup on the head. This smear ultimately causes the head to crash on the disk, severely limiting the lifetime of HAMR drives. Therefore, mitigating smear is indispensable in the quest for reliable hard drives. This dissertation uses a range of numerical techniques to study the transport of smear nanoparticles in the head-disk interface.

We begin with a study of the HDI using classical continuum analysis. The temperature field and thermal protrusion on the head and disk surfaces are calculated by employing the transient heat conduction equation and a thermo-mechanical model of a slider. These calculations revealed the presence of a thermal spot in both the head and the disk surface, with the disk exhibiting significantly higher temperatures. We then explored the effect of the thermal fly-height control (TFC) power and the disk rotation speed on the head temperature. Additionally, a laser-induced protrusion was observed and studied on the head surface.

Next, we introduced the concept of smear as nanoparticles by calculating the optical and air-bearing related force. Using the Rayleigh approximation, the optical force was calculated on a spherical and ellipsoidal nanoparticle. The resulting force field revealed the presence of an optical trap just under the near-field transducer (NFT). A comparison with appropriate drag and thermophoresis force showed that optical forces could be significant for nanoparticles with large volumes, such as flat ellipsoids. Further, metals of all sizes are sensitive to this force due to their permittivity satisfying the Frohlich condition. Also, dielectric particles are found to congregate near large metallic contaminants due to the formation of a secondary surface plasmon on its surface. We then quantified the air-bearing related forces considering the Chapman-Enskog velocity distribution of the air-bearing. The resulting equation revealed three forces: drag, thermophoresis, and lift. Of these, lift forces were found to be negligible. Then, a sensitivity analysis over different parameters revealed the conditions where each force dominates. We found that smaller nanoparticles in light gases experience higher thermophoresis force, whereas heavier nanoparticles in heavy gases experience higher drag. These results can help control the growth of smear in the head-disk interface.

We also devised a novel hybrid simulation strategy to model the head surface smear growth. The technique accelerates a molecular dynamics simulation by calculating the force field derived from prior calculations. This simulation strategy successfully replicated the streak-like features on the head found in experiments. Two types of streaks were observed: a thick streak that occurred due to direct disk-to-head transport and a thin streak due to the oscillating motion of the nanoparticles in the air-bearing. The hybrid simulation was demonstrated to be an effective tool for simulating smear over long timescales. Further, we used the direct simulation Monte Carlo (DSMC) method to study the air bearing under nanoscale spacings. The DSMC method incorporated the consistent Boltzmann algorithm (CBA) to account for dense gas behavior. It revealed the presence of a vertical drift in the air bearing due to the temperature difference between the head and the disk. We observed significant water vapor levels due to the high saturation pressure of the head disk interface. We also observed the particles' near ballistic trajectory under ultra-low spacing. This prompted the examination of material transport in ultra-low flying conditions where the air-bearing molecules are sparse. The imbalance in the head and disk temperatures resulted in an imbalance in the van der Waals forces, which transported smear nanoparticles to the head. This effect was found to be significant for clearance less than 2 nm.

Finally, we conclude by summarizing this dissertation's novel results, commenting on the nature of the smear formation, and proposing mechanisms that may help mitigate it.