Thermal Flying Height Control Research Articles

Investigations of Adhesion under Different Slider-Lube/Disk Contact States at the Head–Disk Interface

Adhesion is the key factor influencing the failure of the hard disk drive operating under ultra-low flying height. In order to mitigate the negative effects of adhesion at the head–disk interface (HDI) and promote further development of the thermal flying height control (TFC) technology, an adhesive contact model based on the Lifshitz theory accounting for the thermal protrusion (TP) geometry of TFC slider, the layered structures of the head and disk, and the operation states of the slider was proposed to investigate the static contact characteristics at the HDI. The simulation results demonstrated the undesirable unstable regions during the transitions between different operation states and the necessity of applying TFC technology. The reduction in the head–media spacing (HMS) was found to be achieved by properly increasing the TP height, decreasing the thickness of the lubricant layer or the thickness of the diamond–like carbon (DLC) layer during the flying state or the TP–lube contact state. At the TP–DLC contact regime, the attractive interaction was stronger than other states, and the strong repulsive interaction made the HMS difficult to be further reduced through the increase in the TP height or the decrease in the lubricant thickness.

Investigation of heat transfer across a nanoscale air gap between a flying head and a rotating disk

Understanding nanoscale heat transfer at the head–disk interface (HDI) is necessary for thermal management of hard disk drives (HDDs), especially for heat-assisted magnetic recording and microwave-assisted magnetic recording. To accurately model the head temperature profile in HDDs, it is imperative to employ a spacing-dependent heat transfer coefficient due to the combined effects of pressurized air conduction and wave-based phonon conduction. Moreover, while flying at near-contact, the fly height and heat transfer are affected by adhesion/contact forces in the HDI. In this study, we develop a numerical model to predict the temperature profile and the fly height for a flying slider over a rotating disk. We compare our simulations with touchdown experiments performed with a flying Thermal Fly-Height Control (TFC) slider with a near-surface Embedded Contact Sensor (ECS), which helps us to detect the temperature change. We incorporate the effects of disk temperature rise, adhesion/contact forces, air and phonon conduction heat transfer, and friction heating in our model. As the head approaches the disk with increasing TFC power, enhanced nanoscale heat transfer leads to a drop in the ECS temperature change vs TFC power curve. We find that the exclusion of the disk temperature rise causes the simulation to overestimate the ECS cooling drop. The incorporation of adhesion force results in a steeper ECS cooling drop. The addition of phonon conduction in the model causes a larger ECS cooling drop. The simulation with friction heating predicts a larger ECS temperature slope beyond contact. The simulation with these features agrees with the experiment.

Investigation of Bias Voltage and Relative Humidity on Wear and Pit Formation at the Head–Disk Interface

Wear and pit formation of a thermal-flying height-control (TFC) slider was studied as a function of bias voltage and environmental relative humidity (RH; 5%–85%). Burnishing tests were performed by applying an “over-push” heater power to the heater element, causing slider–disk contacts. The experimental results showed that both bias voltage and RH have a large effect on the power-to-contact change and the wear of the slider. Atomic force microscopy (AFM) was used to study the wear and pit formation as a function of bias voltage and RH. The wear of the slider surface was found to be less at the positive disk bias voltage compared with the negative disk bias voltage at all RH conditions. Pit formation was observed on the slider in the vicinity of the read–write transducer area when a positive bias voltage was applied to the disk at high RH conditions (85%). Pit formation was not observed at low RH.

Numerical Simulation of Thermal Fly-Height Control Slider Touchdown Test for Achieving Ideal Subnanometer Clearance Head Slider Touchdown Behaviors

Understanding the dynamics and instability of a thermal fly-height control (TFC) head slider close to and at touchdown (TD) on a magnetic recording disk is essential for developing a head slider with a clearance of ~ 0.5 nm to enhance the recording density of hard disk drives. This paper presents a theoretical simulation of a practical TD test, performed by continuously increasing the head protrusion according to the TFC power, and demonstrates ideal TD behaviors to be established in design guidelines for an effective head slider–suspension–disk system. The effects of the air-bearing stiffness and magnitude of disk waviness (DW) on the TD behaviors and their repeatability were extensively investigated. Comparison with the experimental TD behaviors of effective head sliders demonstrated that the different TD behaviors at different radial disk positions could be explained by the numerical TD simulation of the DW-excited response of a single-degree-of-freedom slider model. Possible reasons for the discrepancies between the experimental and simulation results in the light contact state were analyzed in detail, and a method of designing the head–disk interface conditions to achieve a head spacing of ~ 0.5 nm was developed. In addition, the deviation from the ideal TD behavior according to the DW magnitude was examined. The TD behavior of a head slider on a disk with a perfectly bonded lubricant was theoretically clarified, and the potential for realizing a head slider with reduced clearance was assessed.

Numerical and experimental investigation of heat transfer across a nanoscale gap between a magnetic recording head and various media

With the emergence of Heat-Assisted Magnetic Recording and Microwave-Assisted Magnetic Recording, understanding nanoscale heat transfer at the head-media interface is crucial for developing reliable hard disk drives. There is a need to develop a methodology that uses a spacing-dependent nanoscale heat transfer coefficient, determined by using wave-based radiation and van der Waals force driven phonon conduction theories to predict head temperatures in hard disk drives. We present a numerical model to simulate the head temperature due to heat transfer across a closing nanoscale gap between the head and the media (nonrotating) and compare our results with static touchdown experiments performed with a head resting on three different media (Si, magnetic disks with AlMg, and glass substrates). The Thermal Fly-Height Control (TFC) heater in the head is powered to create a local protrusion, leading to contact of a resistive Embedded Contact Sensor (ECS) that is used to measure the temperature change. As the ECS approaches the media, enhanced phonon conduction heat transfer causes a drop in the ECS temperature vs TFC power curve. Our model shows that the introduction of van der Waals forces between the head and the media during computation of the head's thermal protrusion causes a steeper drop in the simulated ECS temperature curve, ensuring a good quantitative match with experiments for all of the media materials tested and different initial ECS-media spacings. We isolate the effect of air conduction on ECS cooling by comparing our simulations with experiments performed in air vs vacuum.

Spacing Control in Heat-Assisted Magnetic Recording

The spacing between the flying head and the rapidly rotating disk in hard disk drives continues to decrease in order to grow the areal density. In heat-assisted magnetic recording (HAMR), the spacing control complexity is compounded by the additional protrusion resulting from the laser heating. Compared to the writer induced and thermal fly-height control protrusion, the near-field transducer (NFT) protrusion is a faster and more local protrusion that requires new HAMR-specific spacing control techniques. In this paper, we will review the simulation and experimental studies which shed light on both the steady-state and transient characteristics of the NFT protrusion. Simulation analysis reveals the scale of the protrusion and its effects on spacing control. Experimentally, we have demonstrated a novel technique to characterize the NFT protrusion and assist in setting the spacing for each HAMR head. Based upon the knowledge gleaned from characterization, compensation schemes have been developed. Some of the schemes have already been successfully demonstrated.

Theoretical Simulation of Touchdown Dynamics of Thermal Fly-Height Control Slider

Theoretical Simulation of Touchdown Dynamics of Thermal Fly-Height Control Slider

気体軸受圧力による磁気ディスク媒体及びスライダの表面変形

The slider/disk spacing of magnetic disk drives has been reduced in order to increase the recording density. Especially, the thermal flying-height control technology, where the electromagnetic transducer is protruded into the slider/disk spacing by heating the slider partially, contributes to make the flying height less than 2nm. Because of the narrower gas bearing clearance, the bearing pressure rises up to several tens times of atmospheric pressure. This pressure rise causes the deformation of slider and disk surface. The slider and disk surface deformation causes the change of gas bearing clearance and consequently the change of air bearing pressure. The pressure change causes the flying attitude. The flying characteristics also may be different from that of expected at the design, where the effect of deformation is not included. It is important to estimate correct flying characteristics affected by the deformation to realize stable flying and accurate recording. We introduced iterative method for searching the flying attitude affected by the slider/disk deformation. We obtained the air bearing pressure by solving the molecular gas lubrication equation numerically. And the simple numerical method, which utilizes the elastic half-space assumption, was employed for calculating the deformation of pressurized disk surface. The finite element method was applied to analyze the slider deformation. And the effect of deformation on the slider attitude is evaluated.

Correlation of Disk Topography Waves with Nanometer-Scale Lubricant Moguls and Magnetic Head Media Spacing

Magnetic recording disk carbon overcoats are lubricated with nanometer-thick films of perfluoropolyether lubricant. It is well-known that lubricant thickness redistribution takes place due to air shear stress oscillation at air bearing resonant frequencies and also due to shear stress oscillation induced by disk topography waves on test tracks. We extended this work to demonstrate correlation between surface topography and lubricant redistribution on whole disk surfaces. Lubricant moguls are shown to form over regions of the disk surface which have topography waves that are half the slider length, and the lubricant thickness peak is out-of-phase down track from the topography peak height. There is a critical relative humidity above 20% beyond which moguls are readily formed by the slider flying at 10 nm without thermal fly-height control. The significance of the lubricant redistribution for drive magnetic performance has long been the subject of debate. The slider flying height modulation measured by magnetic head media spacing was in good agreement with the dynamic air bearing simulation based on the measured disk surface topography. Measured head media spacing image data on the same disk surface before and after lubricant redistribution had the same length scale as the correlation between topography and lubricant thickness variation. These results demonstrate that lubricant thickness redistribution on the order of atomic diameters can degrade magnetic performance, and that the surface topography waves alone can degrade areal density by as much as 2%.

Analysis of Microwaviness-Excited Vibrations of a Flying Head Slider in Proximity and Asperity Contact Regimes

The vibration characteristics of a thermal fly-height control (TFC) head slider in the proximity and asperity contact regimes attract much attention, because the head–disk spacing (HDS) must be less than 1 nm in order to increase the recording density in hard disk drives. This paper presents a numerical analysis of the microwaviness (MW)-excited vibrations in the flying head slider during the touchdown (TD) process. We first formulate the total force applied to the TFC head slider as a function of the HDS, based on rough-surface adhesion contact models and an air-bearing force model. Then, the MW-excited vibrations of a single-degree-of-freedom (DOF) slider model at TD are simulated by the Runge–Kutta method. It is found that, when the MW amplitude is less than the spacing range of static instability in the total force, the slider jumps to a contact state from a near-contact or mobile-lubricant-contact state. It then jumps to a flying state even when the head surface is protruded further by increasing the TFC power. When the MW amplitude is relatively large, a drastically large spacing variation that contains a wide range of frequency components below 100 kHz appears in the static unstable region. These calculated results can clarify the mechanisms behind a few peculiar experimental phenomena reported in the past.

Effect of Head–Disk Interface Biasing and Relative Humidity on Wear of Thermal Flying Height Control Sliders

In this investigation, we examine wear of thermal flying height control sliders as a function of DC bias voltage across the head–disk interface, relative humidity, and lubricant type. Wear tests were conducted using two different experimental setups, a load/unload tester, and a spin-stand tester with relative humidity control. Head wear was determined by measuring the change in the heater touch-down power (ΔTDP) before and after wear testing. After wear testing, selected recording heads were examined using atomic force microscopy and time of flight secondary ion mass spectrometry to investigate wear and deposit formation in the read/write region. Our results show that the polarity of the head–disk interface bias voltage and the relative humidity influence deposit formation and wear in the read/write region of recording heads.

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.

Writing Process Modeling and Identification for Heat-Assisted Magnetic Recording

Written-in signal quality for heat-assisted magnetic recording (HAMR) is dominated by the hot spot thermal profile on the media. The thermal profile is determined by the head and media design, the laser power, and the near-field transducer (NFT) tip to media spacing (NMS). NMS keeps changing during the writing process due to protrusions generated by the laser-related heat sources, on top of the usual head-to-media spacing change due to thermal fly-height control and writing current. In this paper, data pattern writing process is modeled as a dynamic system consisting of NFT protrusions of different time constants, energy transfer efficiency versus NMS, media heat transient process, and power-to-signal conversion function. The parameters of this model are identified by minimizing the differences between the model output and the measured read-back signal magnitude of an experimental data pattern written on an HAMR drive. The Gaussian-Newton method is used to find the parameters of the non-linear system. Using the identified parameters, our model is able to predict the written-in signal magnitude, given the writing conditions such as laser power, laser current overshoot, and various thermal actuator operating conditions.

Use of an embedded contact sensor to study nanoscale heat transfer in heat assisted magnetic recording

A near field transducer is employed in the heat assisted magnetic recording technology in order to focus the light energy into a nanoscale spot on the disk. This is necessary to heat the high coercivity magnetic media to their Curie temperature, so the write transducer can record the data. However, the heat transfer mechanism across the head disk interface (HDI) is still not well understood. The current perpendicular media recording systems have a thermal fly-height control means in the air bearing slider near the read/write transducers for placing the transducers within 1 to 2 nm of the rotating disk. In order to monitor this near contact spacing, this system also uses an embedded contact sensor (ECS). Here, we investigate how this ECS can be used to study the heat transfer across the nanoscale gap between the read/write transducer and the disk. This study shows that the self heating effect of the ECS is strong when its current bias is too high. But this self heating effect can be isolated from other heat sources, which allows us to use the ECS for the desired heat transfer measurements. The experiments show that the heat transfer across the HDI is a strong function of the head-disk spacing.

Near Contact Thermal Flying Height Control in Hard Disk Drives

A modeling and optimization approach is proposed and implemented for minimizing variations in flying height of thermal flying height control (TFC) sliders. The method utilizes the resistive heater element of TFC sliders for adjustment of spacing and the embedded thermal contact sensor (TCS) as a relative measurement for changes in flying height. In the modeling approach, the static and dynamic behavior of the TCS as a function of heater power is characterized. A model of the system is identified from experimental step excitation data using a realization algorithm. In the optimization approach, the optimal feedforward heater input profile is calculated via convex optimization techniques based on the static and dynamic behavior of the system. The modeling and optimization approach was verified experimentally, showing that the proposed approach reduces the difference between the maximum and the minimum value of the TCS measurements by a factor of two, indication a twofold reduction of flying height variations.

Finite element analysis simulations of thermomechanical head-disk interface contact in thermal flying-height control slider design

In order to achieve higher recording density in hard disk drive (HDD), thermal flying-height control (TFC) slider design has been used to reduce the physical spacing between the read/write elements in the slider and the rotating disk surface. During the read/write operation in hard disk drives, the intermittent contact at the head-disk interface (HDI) directly affects its performance and reliability. In this study, the thermomechanical micro-contact between a TFC slider and a disk defect was systematically investigated through finite element analysis (FEA) modeling and simulations. The thermal actuation technology in a TFC slider was incorporated into the FEA model to enable the thermal protrusion of read/write sensors outward the air bearing surface (ABS). In order to obtain the scientific relationship between the thermal protrusion and the resulting HDI contact behavior, parametric FEA simulations were carried out based on the 23-factorial design of experiment (DOE), where the surface temperature rise and the residual deformation on the TFC slider surface were used as the output parameters. From the statistical data analysis, it could be found that the larger thermal protrusion resulted in higher temperature rise and more residual deformation.

Nanoscale heat transfer in the head-disk interface for heat assisted magnetic recording

Laser heating has been introduced in heat-assisted magnetic recording in order to reduce the magnetic coercivity and enable data writing. However, the heat flow inside a couple of nanometers head-disk gap is still not well understood. An experimental stage was built for studying heat transfer in the head-disk interface (HDI) and the heat-induced instability of the HDI. A laser heating system is included to produce a heated spot on the disk at the position of the slider. A floating air bearing slider is implemented in the stage for sensing the temperature change of the slider due to the heat transfer from the disk by the use of an embedded contact sensor, and the gap between the two surfaces is controlled by the use of a thermal fly-height control actuator. By using this system, we explore the dependency of the heat transfer on the gap spacing as well as the disk temperature.

Thermo-molecular gas-film lubrication dynamics considering boundary temperature and ambient gas (2-DOF numerical analysis by t-MGL equation)

The static and dynamic flying characteristics of a slider in a hard disk drive have become an important consideration owing to recent increases in recording density. In the present paper, the characteristics of a plane inclined slider (Case 1) and a step slider flying in either air or He (Case 2) over a running boundary wall with local temperature distributions are analyzed using the thermo-molecular gas-film lubrication (t-MGL) equation. For a plane inclined slider (Case 1), the fundamental static and dynamic characteristics are analyzed numerically and are examined through two limiting approximations: the approximation for infinite bearing number and the incompressible short bearing approximation. For a step slider (Case 2), the decreases in the minimum spacing for a slider flying in He are significant because the mean free path of He, ?He, is approximately three times that of air, ?air. The increases in the minimum spacing due to laser heating are negligible in both air and He because the heat spot size is very small. Moreover, the decrease in the minimum spacing produced by thermal deformation (projection height, dmax) by laser heating in the thermal fly-height control slider is reduced by the total additional pressure of (1) MGL pressures produced by the air-film wedge effect, (2) t-MGL pressures produced by the applied temperature distribution, and (3) van der Waals attractive pressure due to the ultra-small spacing. The spacing fluctuation in He caused by a running wavy disk is smaller than that in air, because the inlet-to-outlet spacing ratio (h1/h0) in He is larger than that in air.

境界面の温度と適応係数を考慮した分子気体潤滑（t-MGL）理論の構築

As new magnetic recording techniques, thermal fly-height control (TFC) and heat-assisted magnetic recording (HAMR) have been investigated in order to increase the recording density of hard disk drives (HDDs). When the spacing is less than about 10 nm, the flow in the lubrication region is close to the free molecular flow, since spacing h is less than mean free path λ of the gas. In this paper, we derived the free molecular t-MGL equation considering temperature distributions and accommodation coefficients at the boundary, for examining thermo-molecular gas-film lubrication (t-MGL) characteristics in several nanometers. Moreover, we show numerically the effects of accommodation coefficients on the basic characteristics of pressures and shear stresses in free molecular region.

気体軸受圧力による磁気ディスク媒体及びスライダの表面変形

The slider/disk spacing of magnetic disk drives has been reduced to less than 2nm by using the thermal flying height control system in order to increase the recording density. Because of the narrower gas bearing clearance, the bearing pressure rises up to several tens times of atmospheric pressure. This pressure rise causes the deformation of slider and disk surface, and consequently the slider-disk gap changes from the designed value, in which the effect of deformation is not included. The flying characteristics also may be different from that of expected. We employed the simple numerical method for analyzing the deformation of pressurized disk surface utilizing the elastic half-space assumption. The finite element method was applied to analyze the slider deformation. We also introduced the flying attitude search method against the case where the disk and slider deformation exists and the effect of deformation on the slider attitude is evaluated.