Head-disk Clearance Research Articles

A method for monitoring head media spacing change in a hard disk drive using an embedded contact sensor

Growth in the demand for higher capacity hard disk drives has pushed the requirement for head-media spacing to sub-nanometer levels. The drop in operational clearance makes a head-disk interface more susceptible to potential head-wear and contamination related issues. Such degradation processes are often accompanied by a noticeable shift in the head-disk clearance. Hence monitoring an interface for a spacing change can be helpful in early detection of its imminent failure. In this paper, we present a method to detect the change in head-disk spacing using an embedded contact sensor (ECS). This technique involves the analysis of ECS dynamic response for an interface that is subjected to heater induced spacing modulations. As the head moves closer to the disk surface, the magnitude of the ECS frequency components can be used to determine the ‘characteristic spacing’ which can be used as a metric to detect any physical change for a given interface.

Sensing and Modeling Heat Transfer Between a Recording Head and Medium for Clearance Setting in HAMR

A thermistor-based proximity sensor enables identification of head-disk contact in a hard disk drive, allowing the setting of head-disk clearance. This sensor also provides thermal information, permitting validation of computational models used to design recording heads. We describe here a set of models, and a related set of experiments to probe thermal and mechanical characteristics at the head-disk interface. Example results are shown, leading to the conclusion that models with diffusive-ballistic equations are capable of simulating heat transfer through the gas bearing.

Head flying characteristics in heat assisted magnetic recording considering various nanoscale heat transfer models

The thermal issues in heat-assisted magnetic recording (HAMR) technology have drawn much attention in the recent literature. In this paper, the head flying characteristics and thermal performance of a HAMR system during the touch-down process considering different nanoscale heat transfer models across the head-disk interface are numerically studied. An optical-thermal-mechanical coupled model is first described. The coupling efficiency of the near field transducer is found to be dependent on the head disk clearance. The shortcomings of a constant disk-temperature model are investigated, which reveals the importance of considering the disk temperature as a variable. A study of the head flying on the disk is carried out using an air conduction model and additional near-field heat transfer models. It is shown that when the head disk interface is filled with a solid material caused by the laser-induced accumulation, the heat transfer coefficient can become unexpectedly large and the head's temperature can rise beyond desirable levels. Finally, the additional head protrusion due to the laser heating is investigated.

Measurement and Simulation of Nanoscale Gap Heat Transfer Using a Read/Write Head With a Contact Sensor

Heat-assisted magnetic recording (HAMR), microwave-assisted magnetic recording, and other new technologies are being developed to boost the storage capacity of current hard disk drives (HDDs). Understanding the mechanisms of heat transfer across the head-disk interface is of major importance, because it is closely related to the design of HDDs, including lubricant flow and contact issues, especially for the reliability design of HAMR drives. In this paper, we report on a series of experiments and simulations with a perpendicular media recording (PMR) head to better understand the head temperature change and to validate the wave-based phonon conduction theory. From the static touchdown experiments, it is found that even without the air bearing, the head temperature of the thermal flight-height control (TFC) slider reduces when the clearance between the head and disk is less than 1 or 2 nm. The effect of lubricant on the disk surface is also studied. When the head temperature is relatively low, repeated touchdowns show that the starting point gets closer to the disk, suggesting that meniscus forces from the lubricant on the thermal fly-height control protrusion gradually reduce the head-disk clearance. But when the head temperature is relatively higher, the head can “melt through” the lubricant layer. The wave-based phonon conduction theory is integrated into an ANSYS simulation to study the steady state temperature distribution and protrusion profile of the head in the static touchdown experiments. The simulation result shows the same trend as the experiments of cooling before the head is in contact with the disk.

An <italic>In Situ</italic> Measurement Method for Electric Potential at Head-Disk Interface Using a Thermal Asperity Sensor

The electrostatic force at head–disk interface (HDI) becomes increasingly significant when the head–disk clearance is dropping into 1 nm scheme. Therefore, detecting and eliminating the electric potential in the HDI is a critical task for hard disk drives. To that end, this paper proposed an in situ measurement method for electric potential at HDI using a thermal asperity (TA) sensor. When calibrating the electric potential, both dc and ac components of the external voltage were supplied to the disk. As a result, the output of the TA sensor to the first harmonic of the ac component was linearly increasing with the decrease in the dc components. Experimental results show that this method is more efficient and flexible than the traditional methods.

The Instability of Angstrom-Scale Head-Disk Interface Induced by Electrostatic Force

When the storage density is coming into several Tb/in2, the head-disk spacing will drop into angstrom-scale regime [1]. At such small clearance, the electrostatic and intermolecular forces have become increasingly significant. It was reported that the electrostatic force produced by electronic potential difference between head and disk can be larger than the van der Waals force even if the slider and disk are both grounded[2],[3]. However, the electrostatic induced head disk interface instability and the relevant reliability were not well addressed in past. Therefore, this work investigates the instability of angstrom-scale head-disk interface induced by electrostatic force. First, the hysteresis effect of the touchdown-takeoff was investigated with/without eliminating the electronic potential difference. The electrical powers of thermal flying-height control (TFC) [4] head was supplied and then released to capture the hysteresis effect. It is found that a significant improvement of the hysteresis effect has been achieved when the electrostatic potential was eliminated. Second, a wear test over 60 hours when the head-disk clearance was set at an angstrom scale was performed. It is also found that the wear robustness has been greatly improved by eliminating the electrostatic potential. This study will help the design of the head-disk system for better reliability, especially for wear and flying stability.

Study of Head-Disk Interface Characterization Using Touchdown Sensor and Electromagnetic Signal in Hard Disk Drives

With the introduction of thermal fly-height control sliders, the local head-disk clearance can be reduced to a range from $\sim10$ nm to contact. This actively controlled touchdown (TD) has been used as a way to study the head–disk interface (HDI), the characteristics of which greatly affect the areal density and reliability of drives. In this paper, a high spatial resolution TD study is performed at the drive level using TD sensor signals and head-media spacing signals. Different flying stages, including passive flying, TD transition, and over push modulation, are identified from the experimental results. A correlation between the aforementioned signals at the passive flying stage is found to represent the disk surface property. The passive flying stage and over push modulation stage of TD are further analyzed in both the time and frequency domains. By establishing a correlation between the fly height and the temperature response, we can provide insight into the heat transfer model for the HDI.

Electrostatic Force Manipulation Methodology: Principles, Mechanisms, and Setup for Head–Disk Interactions Monitoring

To achieve higher hard disk drive areal density, the head–disk clearance had been reduced to $F_{\mathrm {es}}$ ) across the head–disk interface were investigated. It was shown that the distribution of the applied $F_{\mathrm {es}}$ was different for the two setups. This led to further studies of the effects of the $F_{\mathrm {es}}$ distribution on interactions monitoring. The results showed that when an alternating $F_{\mathrm {es}}$ was distributed at the pole region, the magnitude of the electrodynamically excited first-harmonic vibration increased significantly as the gap fly height was reduced. It was very sensitive to spacing change and useful for the head–disk interactions monitoring at the near-contact regime. The second harmonic was found sensitive to head–disk contact. When the alternating $F_{\mathrm {es}}$ was distributed at the slider body, the magnitudes of the first and second harmonics were larger due to larger effective surface area. However, the fly height of the slider body was mainly affected by the protrusion push-up effect, and hence the first and second harmonics showed a reverse trend as compared with the case where $F_{\mathrm {es}}$ was distributed at the pole region. In addition, slider pitch motion was also significantly affected. The effective EFM setup was constructed to demonstrate its effectiveness in head–disk interactions monitoring and contact detection.

An Experimental Study of Head Stack Assembly Dynamics Impact on Head Disc Interaction

As head-disk clearance has been reduced to nearly 1 nm, air bearing slider dynamic stability becomes increasingly important. This paper presents an experimental study about the coupling effects of Head Gimbal Assembly (HGA) dynamics and arm vibration on Head Disk Interaction (HDI). HDI dynamics was evaluated on a disc with defect. It was found that a much bigger vibration of the slider was observed when suspension local bending mode and arm bending mode are closed to each other than the situation that the two modes are separated. It is proved that both HGA and arm are related to HDI dynamics in a hard disc drive. This study is helpful to characterize system dynamics for robust HDI design.

Passive Vibration Absorption for Extremely High Density Recording

A method is proposed for passive vibration absorption in hard-disk drives during transient events such as the coming into proximity of the rotating disk within the context of thermal fly-height control nanotechnology or external shock. The method uses a nonlinear energy sink at the center of mass of the slider that can absorb energy over a wide range of excitation frequencies. Its feasibility and performance are investigated through a 4-degree-of-freedom dynamic model of the head-disk interface used to predict head-disk clearance and vibrations.

Experimental Study of the Slider-Lube/Disk Contact State and Its Effect on Head-Disk Interface Stability

As head-disk clearance has been reduced to nearly 1 nm, and a partial lube-contact head-disk interface (HDI) may even be necessary to meet future magnetic spacing needs, precise characterization of the slider-lube/disk contact state (or depth) becomes increasingly important. The currently used thermal fly-height control (TFC) sliders require a slight touchdown (or contact) to determine the reference position of the clearance, and accurate measurement and control of the penetration depth into the disk lubricant are essential to achieve a stable HDI for future lube-contact recording. In this paper, multiple techniques (e.g., acoustic emission technology, lubricant distribution mapping, slider wear detection, and slider dynamics measurement) are implemented to characterize the contact state/depth and HDI stability at different TFC actuation levels. Experimental results demonstrate the effectiveness of these methods and suggest that the clearance reference position determined through the acoustic emission technology is at the lubricant-air interface. Moreover, two dynamically stable contact states-light slider-lube contact and deep slider-disk contact-are found, but the latter causes slider wear and thereby is not practical for realizing a reliable lube-contact HDI.

Dynamic instability of thermal-flying-height-control sliders at touchdown

With the wide application of thermal flying-height control (TFC) technology in the hard disk drive industry, the head-disk clearance can be controlled to as low as ~1 nm. At this ultra-low clearance, the air bearing slider is subject to relatively large interfacial forces, and it experiences more complicated dynamics, compared with the flying case. In this study we conduct a numerical analysis to investigate the dynamics of TFC sliders during touchdown. The general trend of the slider’s motion predicted by the numerical simulation qualitatively agrees with experimental findings. The touchdown process begins with a slight intermittent contact between the slider’s trailing edge and the disk, followed by a partial slider-disk contact at the trailing edge accompanied by a large pitch motion at the 1st air bearing mode; this pitch motion gets suppressed and the slider comes into stable sliding on the disk as the protrusion is further increased.

A multidentate lubricant for use in hard disk drives at sub-nanometer thickness

We describe a second generation of multidentate lubricant structures for use on a magnetic media in a hard disk drive. Building on earlier work where a perfluoropolyether (PFPE) chain with hydroxyl bonding moieties were placed in the middle of the chain as well as on chain ends, creating a structure with two PFPE sub-units for enhanced tribological performance under very low head-disk spacing, this paper focuses on a PFPE chain composed of three, even shorter PFPE sub-units. Experimental data focusing on surface characterization of sub-nanometer thickness films, as well as tribological performance, are presented that confirm the high confinement level achieved with the lubricant structure. Molecular dynamics calculations are also discussed, that are consistent with a molecular film of high stiffness, leading to a denser, more compact structure. This approach could pave the way to achieving the sub-nanometer head-disk clearance level, presumed necessary for storage densities exceeding the terabit per square inch density landmark.

Effects of PFPE Lubricant Properties on the Critical Clearance and Rate of the Lubricant Transfer from Disk Surface to Slider

The transfer of perfluoropolyether (PFPE) lubricant from the disk surface to the slider as a function of head-disk clearance has been investigated experimentally. The effects of lubricant thickness, bonding ratio, molecular polarity, and main chain stiffness on the lubricant transfer rate and the critical clearance below which lubricant transfer gets much enhanced are clarified. The critical clearance can be effectively reduced by decreasing the lubricant thickness or increasing the number of polar hydroxyl end-groups per lubricant molecule. Increasing the film bonding ratio or using lubricants with stiffer backbone can significantly decrease the lubricant transfer rate especially below the critical clearance. The results are discussed in terms of the effective disjoining pressure and its slope with respect to the film thickness.

Humidity effect on head-disk clearance

The mechanism of head-disk-clearance change due to a humidity effect was investigated experimentally. The head-disk clearance was measured by moving the head elements towards the disk by thermal flying-height-control technology until the head touched the disk while monitoring them with an acoustic-emission sensor in an environmentally controlled component tester. Measured clearance change from 3% RH to 80% RH reached about −0.6 nm at 25°C and about −1.6 nm at 60°C. Head-disk clearance change caused by humidity was classified as the slider-flying-height change and disk-touch-down-height (TDH) change to clarify the mechanism of the clearance change. Slider FH change was dominated by absolute humidity rather than relative humidity. On the other hand, the humidity effect on disk TDH change was classified as a “water-film effect” and a “lubricant-mogul effect”, which both depend on relative humidity. Both effects caused clearance change of about 0.4 nm for lubricant A, which is a lubricant with two hydroxyl functional groups at the end of the main chain. These effects were assumed to be caused by water adsorption onto the surface of lubricant. Reduction of the number of free hydroxyl groups which attract water molecules could suppress the disk TDH change related to the humidity effect.

Investigation of Mechanical Clearance Change with Thermal Flying-height Control Slider at High Altitude

Clearance change between a magnetic head with a thermal flying-height control (TFC) function and a magnetic disk at high altitude was investigated. To clarify the change of head-disk clearance in the case of TFC actuation at high altitude, head-disk clearance change was experimentally evaluated as a function of altitude and head-disk clearance by using a TFC function. The evaluation results show that the head-disk clearance change is larger in the case with TFC actuation at high altitude than that in the case without TFC actuation.

A-3 ヘッドディスククリアランスに与える湿度の影響(口頭発表:ヘッド・ディスク・インターフェイス)

To increase the areal recording density of hard disk drives, the magnetic spacing between the head and magnetic disk must be reduced. Therefore, the head-disk clearance also has to be reduced. Thermal flying height control technology has been applied to move the head element close to the disk in recent hard disk drives. However, it is necessary to control flying height properly to keep the same clearance because it changes in various environmental conditions. We have focused on studying the effect of environmental conditions on the clearance. In a humid environment, it is generally observed that a slider loses its flying height possibly due to the pressure drops caused by water vapor condensation in the air bearing. In our experiment, the effect of humidity on clearance was investigated quantitatively with an environmentally controlled component tester. Our finding was that the clearance variation due to humidity is because of not only flying height decrease but also disk touch-down height increase.

Simulation of Lubricant Behavior by Using the Particle Method

Lubricant behavior within a head-disk clearance in recent hard disk drives was simulated by using a particle method, which is based on continuum dynamics. The lubricant was simulated as a group of particles. The surface tension and the wall-adhesion models used in the particle method were verified before applying the particle method to the lubricant behavior simulation first. That is, simple benchmark problems like liquid-droplet deformations under unbalanced surface tension and wall adhesion force were used to confirm the accuracy of the models. In the lubricant simulation, the effects of viscous stress and surface tension on the lubricant behavior were studied. The study found that viscous stress is the dominant factor in comparison with surface tension on bulk condition of viscosity; that is, the lubricant bridge is deformed by the viscous stress, not by the surface tension.

Electric-Field-Assisted Dip Coating Process of Ultrathin PFPE Lubricant Film for Magnetic Disks

In this paper, the influence of the electric field on the dip-coating process was studied by using lubricants Z-tetraol and TA-30. The electric field assisted the adsorption of both lubricant molecules on the disk surface. This electric field also resulted in an increase in the lubricant film thickness. A positive electric field increased the bonded ratio of the lubricant film. The surface-free energy, step height, and head-disk clearance of the Z-tetraol lubricant film were not affected by the electric field; however, those of the TA-30 lubricant film were improved by the electric field. By molecular dynamics simulation, we confirmed that the molecular mobility of the solution increased by applying the electric field. The lubricant molecules in the solution, which show higher mobility, should have more opportunity to adsorb onto a disk surface. These results show the possibility of reducing the head flying height by the electric-field-assisted dip (EFAD) coating process.

HDI-15 ELECTRIC FIELD ASSISTED DIP COATING PROCESS OF ULTRA-THIN PFPE LUBRICANT FILM FOR MAGNETIC DISKS(Head/Disk Interface and Tribology IV,Technical Program of Oral Presentations)

The influence of the electric field on the dip-coating process was studied by using lubricants Z-tetraol, D-4OH, TA-30, and QA-40. The electric field assisted the adsorption of the lubricant molecules on the disk surface. This electric field also resulted in an increase in the lubricant film thickness. The surface free energy, step height, and head-disk clearance of the Z-tetraol lubricant film were not affected by the electric field; however, those of the other lubricant film were improved by the electric field. As the these results, we estimated that the molecular conformation of the lubricant changed from a random coil shape to a loose shape by the field, and the OH end-group functions aligned themselves along the direction of the field. These results show the possibility of reducing the head flying height by the EFAD coating process.