Thermal Transport, Thermal Protrusion, and Thermally-Induced Material Transfer in the Head-Disk Interface of Heat-Assisted Magnetic Recording

As data generated worldwide are growing explosively, it is crucial to increase the areal density of traditional storage devices to satisfy the requirements. Conventional hard disk drive (HDD) technology, perpendicular magnetic recording (PMR), has reached the superparamagnetic limit of ~ 1 Tb/in2. To realize the areal density over 1 Tb/in2, the size of the media bits must be further decreased to tens of nanometers, which requires high coercivity magnetic media. The high coercivity can avoid superparamagnetism and thus store data safely at the small bit size under room temperature, but it makes data writing challenging. To assist the writing process, energy is input to the media to lower its coercivity temporarily. Current technologies such as heat-assisted magnetic recording (HAMR) and microwave-assisted magnetic recording (MAMR) utilize two different methods to lower the coercivity. HAMR integrates a laser to locally heat the media to its Curie temperature (400–500 °C), while MAMR uses a spin torque oscillator to induce ferromagnetic resonance in the media grains./nIn the HAMR head-disk interface (HDI), a recording head flies over a rotating disk with a relative velocity of 5–40 m/s and an initial spacing of 10–15 nm controlled by an air bearing. Then, the spacing is reduced by energizing a joule heater inside the head. The heater generates a protrusion on the head surface to lower the initial spacing to 1–2 nm so that data reading/writing can be performed using the read/write transducers in the head. The head is also integrated with a laser diode, a waveguide (WG) and a near-field transducer (NFT) for laser delivery. The laser beam is launched from the recording head and is focused on the recording disk to locally heat the disk (400–500 °C), which is even hotter than the head temperature (150–250 °C). Therefore, the head-disk interface of HAMR is a system that combines nanoscale spacing (< 15 nm), high temperatures (head ∼ 150–250 °C, disk ∼ 400–500 °C), steep thermal gradient (∼ 10 K/nm), and a high-speed sliding condition (5–40 m/s). Furthermore, the introduction of the laser affects thermal transport and thermal protrusion, and causes thermally-induced material transfer in the interface, which needs to be investigated both for fundamental understanding and for practical applications such as HAMR and other microelectronics devices./nTo study the thermal transport across a closing gap between the head and the disk, we conducted static touchdown experiments using a custom-made setup where the disk is not rotating to exclude the air cooling effect. The head temperature rise was measured as a function of the heater power under various conditions such as different substrate materials, relative humidity and laser on/off. An enhanced thermal transport due to phonon heat conduction is observed for the gap < ∼ 2 nm. The thermal transport across the gap becomes stronger when a better thermal conductor is used as the substrate or when the humidity is higher than 75%. With the presence of the laser, the head undergoes a joule heat dissipation inside the head and a back-heating from the hot spot on the substrate./nIn the HAMR operations, the laser delivery involves energy loss, which leads to a localized angstrom-level laser-induced protrusion (LIP) and a fly height change (FHC). They need to be considered and compensated in the spacing control. Flying touchdown experiments were performed to evaluate their overall effect on the spacing change, then they were separated using their different time constants in microseconds and milliseconds. In addition, HAMR operations may utilize two heaters in the head. It is demonstrated that the head protrusion shape can be modulated by use of the dual heaters, and that the touchdown area can be controlled precisely./nDuring the laser exposure under HAMR operations, material transfer also happens due to the high level of thermal transport. The temperature of the hot spot on the disk is much higher than the lubricant evaporation temperature, so the lubricant is evaporated from the disk and then condenses on the head surface. The material accumulation on the head surface, also known as smear, is a challenging reliability issue for HAMR. We experimentally investigate the smear formation mechanism and propose two smear mitigation strategies. The results show that the smear forms when the lubricant evaporation occurs for a certain time, and that the smear can be mitigated by a mechanical burnishing approach or a thermal approach./nNext, we report a thermal mapping technique using a phase change material Ge2Sb2Te5. Ge2Sb2Te5 undergoes a crystalline transition at 149 °C with changes in its density and optical reflectivity. By use of these changes, we can map surface temperatures from nanoscale to microscale with minimal calibration, which is demonstrated using a recording head./nFinally, we propose a near-field thermal transport based scheme for lubricant thickness measurement. The thermal effect of the lubricant is investigated when the head approaches the disk in the flying touchdown experiments, which is then used to determine the lubricant thickness. Most previous lubricant measurements require an ex-situ tool such as optical surface analyzer (OSA), but the proposed scheme is an in-situ method with a sub-angstrom resolution and a faster response time. Using the scheme, we performed in-situ measurements of the lubricant depletion and reflow dynamics under HAMR operations.

Advisor: Bogy, David B.