Experimental Study of Nanoscale Head-Disk Heat Transfer In Heat-Assisted Magnetic Recording

To study the nanoscale heat transfer and laser-related protrusions in heat-assisted magnetic recording (HAMR), we performed static touchdown experiments between HAMR waveguide heads and non-rotating media such as a silicon wafer and a recording disk with an AlMg substrate. During the static touchdown, the laser element is energized with DC current and the embedded contact sensor (ECS) is used to monitor the head temperature. The experimental results show that the thermal fly-height control (TFC) touchdown power decreases with increasing laser current. Meanwhile, the head temperature increases due to the laser heating. From this the ECS resistance rise induced by the laser is extracted. The results show that the silicon wafer dissipates heat effectively under the laser exposure, while the AlMg-substrate disk undergoes a higher temperature rise, which in turn heats the head.

Ranking of Thermal Asperity (TA) Interaction During Seeking Under Various Fly-Height Conditions

HDD heads have various interaction mechanisms with thermal asperities (TAs), and protection mechanisms need to be put in place to ensure the head-disk interaction (HDI) resulting from them is eliminated or minimized to the highest extent possible. It is straightforward to not allow the head sit-on-track on cylinders that have such TAs on them, and the same principle can be extended to so-called high TAs (HTAs), whose height is more than the fly height of the head, so heads do not inadvertently interact with the TA even when motion is triggered on another head, since the entire head stack moves together. Similar TA interactions also occur when the head seeks across the tracks. Typical short seeks have thermal fly-height control (TFC) turned on while it is turned on during long seeks, which is greater than a few hundred tracks. Heads can also interact with TAs during retract and arrival of the head during such long seeks. Finally, background media scan (BGMS), which is an industry standard, when the drive enters an idle state. Interaction with HTAs can also occur when the drive enters such a state. Typical seek avoidance attempts to eliminate TA interaction during seeking, however it is not straightforward to determine which of the seek mechanism: TFC On during short seeks, retract/arrival during long seeks, HTA interaction during long seeks with TFC off, or idle TA interaction causes the greatest HDI. Through theoretical analysis and experimental corroboration, this paper intends to rank the various modes of TA interaction, so by developing features for eliminating or minimizing them in that order could help bring the maximum benefit for achieving minimum lifetime reduction of the head due to such interaction.

A Method to Reduce Head-Disk Interface (HDI) Degradation Risk During Thermal Asperity (TA) Mapping in a Hard Disk Drive

One of the issues in thermal asperity (TA) detection using an embedded contact sensor (ECS) is the degradation caused to the read/write elements of the head while interacting with the TA. We propose a method to reduce such head-disk interaction (HDI) during TA detection and classification by flying higher at low thermal fly-height control (TFC) power, which minimizes the interaction of the TA with the head. The key idea is to scan the head at higher fly height, but with higher ECS bias voltage. Initial experiments have shown that the TA count follows a negative cubic relationship with the backoff at various bias levels, and that it follows a square relationship with bias at various backoff levels. Using a sample set, the calibration curves i.e. the golden relationship between these parameters can be established. Using these, one can start the TA detection at the highest backoff and high ECS bias, and start to estimate the nominal TA count. By mapping out these TAs and ensuring the head does not fly over them again to prevent HDI, the fly height can then be lowered, and the rest of the TA cluster can be scanned. Following this method iteratively, the entire TA cluster can be mapped out with minimal interaction with the head. Although this method entails an increase in the test time to detect and map all TAs, compared to detecting them with TFC being on, this can help improve the reliability of the drive by protecting the sensitive read/write elements especially for energy assisted recording from HDI.

A Method for Characterizing the Head-Disk Interface Dynamics Near the Touchdown Conditions Using the Magnetic Fly-Height

Recent growth in the cloud storage industry has created a massive demand for higher capacity hard disk drives (HDD). A sub-nanometer head media spacing (HMS) remains the most critical pre-requisite to achieve the areal density needed to deliver the next generation of HDD products. Designing a robust head-disk interface (HDI) with small physical clearance requires the understanding of slider dynamics, especially when the head flies in proximity to the disk surface. In this paper, we describe a method using the magnetic read-back signal to characterize the head fly-height modulations as it undergoes a transition from a free-flying state to soft contact with the disk surface. A technique based on the magnetic fly-height sensitivity is introduced for the identification of the transition plane that corresponds to the onset of the touchdown process. Additionally, the proposed magnetic spacing based meteorology is used to study the effect of the air bearing stiffness on the magnitude of the slider vibrations induced by intermittent head-disk interactions. The information about the minimum spacing while maintaining the stable flying conditions can help in reducing the head-disk interaction risk that can enable a low clearance interface.

Numerical and Experimental Investigation of Nanoscale Heat Transfer Between a Flying Head Over a Rotating Disk

With the minimum fly height less than 10 nm in contemporary hard-disk drives, understanding nanoscale heat transfer at the head-disk interface (HDI) is crucial for developing reliable head and media designs. While flying at near-contact, the fly height and spacing dependent nanoscale heat transfer are significantly affected by interfacial forces in the HDI (such as adhesion force, contact force etc.). Moreover, with the emergence of technologies such as Heat-Assisted Magnetic Recording and Microwave-Assisted Magnetic Recording, head failure due to overheating has become an increasing concern. In this study, we present a numerical model to simulate the head temperature profile and the head-disk spacing for a flying head over a spinning disk and compare our results with touchdown experiments performed with a magnetic recording head flying over a rotating Al-Mg disk. In order to accurately predict the fly height and heat transfer at near-contact, we incorporate asperity based adhesion and contact models, air & phonon conduction heat transfer, friction heating and the effect of disk temperature rise in our model. Our results show that the incorporation of adhesion force between the head and the disk causes a reduction in the fly height, leading to a smaller touchdown power than the simulation without adhesion force.

Numerical and Experimental Investigation of Nanoscale Heat Transfer in the Head-Media Interface During Static Touchdown

With minimum fly height of less than 10 nm in contemporary hard-disk drives, understanding nanoscale heat transfer at the head-media interface is crucial for developing reliable head and media designs. Particularly, with the emergence of Heat-Assisted Magnetic Recording (HAMR) and Microwave-Assisted Magnetic Recording (MAMR), head failure due to overheating has become an increasing concern. There is a need to develop a methodology to use theoretical curves for spacing-dependent nanoscale heat transfer coefficient to predict head and media temperatures in actual hard disk drives. In this study, we present a numerical model to simulate the head and media temperature profiles during static touchdown and compare our results with experiments performed with a magnetic head on a silicon wafer. As the head approaches touchdown with increasing TFC power, the phonon conduction heat transfer coefficient between the head and the substrate increases exponentially, causing a drop in the head temperature vs TFC power curve. Our model shows that the introduction of van der Waals forces between the head and the substrate causes a steeper drop in the head temperature curve and ensures a good quantitative match with experimental results.

Impact of Shallow Void Corrosion on HDD Initialization Failures

This paper investigates how corrosion growth that originates from shallow film voids on thin film sputtered magnetic media can impact the final HDD system performance. The impact of shallow void corrosion [1] in HDD system can be both on its behavior during initialization i.e. servo write formatting and initial defect mapping, and on its long term reliability. One of the main concerns for HDD reliability due to media is the occurrence of a condition called metallic smear. Metallic smear is a contamination of the head-pole area that causes an unstable fly height condition. Metallic smear is possibly caused by media corrosion and therefore void corrosion is a logical concern. However, our study shows that shallow void corrosion is not associated with metallic smear risk and does not contribute to HDD initialization failures.

Understanding Instabilities of TFC Femto-Slider and Pico-Slider via Surface Force Theory

We formulated the surface pressure of a disk surface that consists of three layers of diamond-like-carbon (DLC) as well as bonded and mobile lubricants based on the asperity adhesive contact theory. We demonstrated that this surface force could well evaluate the touchdown behaviors of a thermal fly-height control (TFC) slider. This theory shows that the touchdown behaviors of a current TFC femto-slider are mainly governed by the asperity meniscus force rather than the van der Waals force (vdW). However, it is shown that the static instability of a pico-slider was generated from the vdW force. Moreover, it is estimated that the dynamic instability of the pico-slider was mainly caused by the meniscus force rather than the vdW force. This surface force theory can allow us to understand consistently various phenomena of slider dynamics.