Effect of Functional End-Groups on the Lubricant Recovery After Heat-Assisted Magnetic Recording (HAMR) Writing

In the developing heat-assisted magnetic recording (HAMR) technology, a laser heats up the magnetic media to the Curie temperature of a few hundred degrees Celsius for a short time of the order of a few nanoseconds. Accordingly, the thin-film lubricant coating on the disk experiences effects such as thermo-capillary shear stress, evaporation, viscosity drop, and eventually, lubricant depletion [1, 2]. Our previous work [3] studied these effects on the lubricant depletion for various lubricants and a prescribed Gaussian temperature distribution with a peak temperature of 350°C and a Full-Width Half Maximum (FWHM) of L_s = 20nm, close to the target laser spot size for HAMR. In order to maintain a reliable head-disk interaction, the lubricant needs to recover to the initial uniform profile. A previous work by Dahl and Bogy [4] studied the recovery process after HAMR writing for Z-dol 2000 using lubrication theory. In this paper, we focus on the effect of the lubricant functional end-groups on the recovery process, using the same method. Z-dol, Z-tetraol, and ZTMD lubricant families have the same polymer backbone but different numbers of hydroxyl end-groups [5]. This difference modifies two key parameters, both affecting the recovery time significantly. First, it changes the bonding ratio and the disjoining pressure properties of the lubricant. Figure 1 shows the disjoining pressure derivative, including both the polar and dispersive components, as a function of film thickness for Z-dol2000, Z-tetraol2200, and ZTMD2200 based on experiments [1, 6, 7]. Second, a major increase in lubricant viscosity occurs as the number of hydroxyl end-groups increases from two (Z-dol) to four (Z-tetraol) to eight (ZTMD) [3]. Fig. 2 shows the viscosity μ(h) as a function of the local lubricant thickness for all three different families of lubricants at the room temperature.