Effect of Lubricant Thickness on the Nanoscale Heat Transfer at the Head-Disk Interface

Abstract

I. INTRODUCTIONIn the head-disk interface (HDI) of heat-assisted magnetic recording (HAMR), a laser is employed over the read/write area to locally heat the recording media to its Curie temperature to facilitate data writing. During this process, the lubricant on the disk undergoes harsh cyclic thermal processes so that the lubricant is partially depleted by the laser [1]. Thus, the lubricant thickness is locally reduced during the writing, but afterwards it recovers gradually due to lubricant reflow [2]. In this study, the effect of the lubricant thickness on the nanoscale heat transfer across the HDI is studied using a thermal fly-height control (TFC) heater and an embedded contact sensor (ECS). The dR/dP, the rate of change in the ECS resistance with respect to the TFC power, was measured to indicate the HDI heat transfer coefficient [3]. It is demonstrated that the ECS is a good thermal sensor for the lubricant thickness through the contact cooling effect.II. EXPERIMENT SETUPExperiments were performed on a component-level spin-stand stage. When flying over the HAMR disks with the lubricant thickness from 8.0 Å to 12.5 Å, the TFC heater was energized until head-disk touchdown occurred. During the touchdown process, the ECS, biased with a constant DC current, was used to monitor the head temperature. An acoustic emission (AE) sensor was used to detect the head-disk contact. The ECS resistance versus the TFC power relation (R-P) was measured. Then the R-P curve’s derivative, dR/dP, was obtained to indicate the head-disk nanoscale heat transfer.III. RESULTS AND DISCUSSIONFig. 1 shows the results of dR/dP versus backoff TFC power using different ECS bias currents, where zero backoff power is the TFC power at the head-disk contact onset detected by the AE sensor. The figure presents the contact cooling effect around +1 mW backoff TFC power, which appears as the local minimum. It is seen from the figure that the cooling effect is stronger with a higher ECS bias current, since the hotter ECS undergoes a larger temperature drop when touching the rotating disk surface. Thus, the ECS biased with a higher current delivers better sensitivity for the experiments.Fig. 2 shows the dR/dP curves for the disks with the lubricant thickness varying from 8.0 Å to 12.5 Å. As the lubricant thickness increases, the dR/dP local minimum rises and hence the contact cooling effect weakens. Therefore, the lubricant behaves as a thermal barrier to the head-disk heat transfer and its effect can be quantified by the ECS.IV. CONCLUSIONSIn this digest, touchdown experiments are introduced to quantify the effect of the lubricant on the nanoscale heat transfer at the head-disk interface. The experimental results show that a higher ECS bias current ensures a better sensitivity for the experiments and that the lubricant layer weakens the head-disk heat transfer. Taking advantage of this effect, the ECS can be further used to measure the local lubricant thickness depletion and reflow during HAMR operations. **