A Theoretical Study of Vapor Lubrication for Heat Assisted Magnetic Recording

Heat assisted magnetic recording (HAMR) is a promising technique to overcome the superparamagnetic limit to further increase the areal recording density of hard disk drives. However, HAMR brings about serious problems to the slider-disk interface, such as lubricant depletion on disk surface caused by laser heating. It is proposed to overcome the lubricant depletion problem by using vapor lubrication. The lubricant film formation process on disk surface in vapor lubrication is studied theoretically based on fundamental adsorption and desorption theories. The controlling parameters of lubricant film thickness and film formation time are identified. It is found that the lubricant film thickness is controlled mainly by lubricant vapor pressure and molecular weight. The film formation time can be shortened by using low molecular weight lubricant and high temperature lubricant vapor.     

Experimental Study of Lubricant Depletion in Heat Assisted Magnetic Recording over the Lifetime of the Drive

Heat assisted magnetic recording (HAMR) is a promising choice to overcome the superparamagnetic limit in magnetic recording and further increase the areal recoding density of hard disk drive. However, it is expected that HAMR causes lubricant depletion problem on disk surface under the high temperature in the heating assisted writing process. Experimental studies of the lubricant depletion under HAMR conditions are still very limited so far. Lubricant depletion over the lifetime of the drive still remains unaddressed. In this study, a self-developed HAMR tester is introduced. The methods to control the repeatability of laser heating temperature, to determine laser heating temperature, and to adjust laser heating temperature are explained. Laser heating time in the test is correlated with that in the drive. Lubricant depletion is determined quantitatively and the relationship between lubricant depletion depth and laser heating time is established. Then, lubricant depletion depth over the lifetime of the drive is predicated. It is found that almost all lubricant on the disk surface will be depleted over the lifetime of the drive.     

A Model for Lubricant Transfer from Media to Head During Heat-Assisted Magnetic Recording (HAMR) Writing

One of the challenges in heat-assisted magnetic recording (HAMR) is the creation of write-induced head contamination at the near-field transducer. A possible mechanism for the formation of this contamination is the transfer of lubricant from the disk to the slider (lubricant pickup) due to temperature-driven evaporation/condensation and/or mechanical interactions. Here we develop a continuum model that predicts the head-to-disk lubricant transfer during HAMR writing. The model simultaneously determines the thermocapillary shear stress-driven deformation and evaporation of the lubricant film on the disk, the convection and diffusion of the vapor phase lubricant in the air bearing and the evolution of the condensed lubricant film on the slider. The model also considers molecular interactions between disk–lubricant, slider–lubricant and lubricant–lubricant in terms of disjoining pressure. We investigate the effect of media temperature, head temperature and initial lubricant thickness on the lubricant transfer process. We find that the transfer mechanism is initially largely thermally driven. The rate of slider lubricant accumulation can be significantly reduced by decreasing the media temperature. However, as the amount of lubricant accumulation increases with time, a change in the transfer mechanism occurs from thermally driven to molecular interactions driven. A similar change in transfer mechanism is predicted as the head–disk spacing is reduced. There exists a critical value of head lubricant thickness and a critical head–disk spacing at which dewetting of the disk lubricant begins, leading to enhanced pickup.     

Experimental Study of Lubricant Depletion in Heat-Assisted Magnetic Recording: Effect of the Duration of One Laser Heating

Heat-assisted magnetic recording (HAMR) is a technique to overcome the superparamagnetic limit and enabling large increases in the storage density of hard disk drives. The performance of lubricant on disk surface under the high temperature in the heating assisted writing process is a big concern. Laser heating in HAMR is quite different from conventional slow heating. Laser heating duration in one heating and cooling process in HAMR is as short as 1?ns. It is believed that lubricant depletion caused by the nano-second pulse laser heating in HAMR is much less severe than that caused by long time continuous laser heating. In this study, a method to compare the laser heating temperature at different laser heating conditions is developed. Lubricant depletion caused by nano-second and continuous laser heating in one heating and cooling cycle in HAMR is determined quantitatively based on test results. It is found that laser heating duration in one heating and cooling cycle in HAMR is not important to lubricant depletion. No matter laser heating is in nano-second or continuous in one heating and cooling cycle, lubricant depletion caused by the laser heating is comparable provided that the laser heating temperature is comparable.     

Effect of Rheology and Slip on Lubricant Deformation and Disk-to-Head Transfer During Heat-Assisted Magnetic Recording (HAMR)

The high-temperature laser heating during heat-assisted magnetic recording (HAMR) causes the media lubricant to deform and transfer to the head via evaporation/condensation. The ability of the lubricant to withstand this writing process and sufficiently recover post-writing is critical for robust read/write performance. Moreover, the media-to-head lubricant transfer causes a continuous deposition of contaminants originating from the media at the head near field transducer, challenging the reliability of HAMR drives. Most previous studies on the effects of laser exposure on lubricant depletion have assumed the lubricant to be a viscous fluid and have modeled its behavior using traditional lubrication theory. However, Perfluoropolyether lubricants are viscoelastic fluids and are expected to exhibit a combination of viscous and elastic behavior at the timescale of HAMR. In this paper, we introduce a modification to the traditional Reynolds lubrication equation using the linear Maxwell constitutive equation and a slip boundary condition. We study the deformation and recovery of the lubricant due to laser heating under the influence of thermocapillary stress and disjoining pressure. Subsequently, we use this modified lubrication equation to develop a model that predicts the media-to-head lubricant transfer during HAMR. This model simultaneously determines the deformation and evaporation of the viscoelastic lubricant film on the disk, the diffusion of the vapor phase lubricant in the air bearing, and the evolution of the condensed lubricant film on the head. We investigate the effect of viscoelasticity, lubricant type (Zdol vs Ztetraol), molecular weight, slip, and disjoining pressure on the lubricant transfer process.     

Material Transfer Inside Head Disk Interface for Heat Assisted Magnetic Recording

Heat assisted magnetic recording (HAMR) promises to deliver higher storage areal density than the current perpendicular magnetic recording products. Laser heating is implemented in HAMR to achieve magnetic writing of the very high coercivity media. However, the high temperature environment creates several reliability challenges for the head disk interface (HDI). In this paper, material transfer within the HDI under HAMR recording conditions is studied. The mechanisms of material transfer are explored via experiments and modeling. This study revealed that temperature difference and mechanical interaction between the head and media are the main mechanisms for material transfer inside the HDI. Possible methods to remove the material are also discussed in this paper.     

Laser-Heating-Induced Damage to Ultrathin Carbon Overcoat in Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording (HAMR) is a technique for overcoming the superparamagnetic limit and enabling large increases in the storage density of hard disk drives. The performance of the disk carbon overcoat under the high temperature in the heating-assisted writing process is a concern. Laser heating in HAMR is quite different from conventional slow heating. Laser heating temperature and total laser heating duration over the lifetime of the drive are two dominant factors in the experimental study of laser-heating-induced damage to the carbon overcoat, which must be carefully controlled. In this study, a rough estimation of the total laser heating time for a given point on the media over the 5-year lifetime of the drive is given. It is expected to be only 0.1 ms. The methods of controlling laser heating temperature and total laser heating time in experimental studies are explained in detail. Laser-heating-induced damage to the a-C:Nx and a-C:Hx overcoats on HAMR media are studied. Surface topographical changes caused by the laser heating are evaluated with atomic force microscopy and structure changes by visible Raman spectroscopy. It is found that laser heating induces surface topographical and structure changes, especially for the a-C:Nx overcoat.     

Kinetics of laser induced desorption and decomposition of Fomblin Zdol on carbon overcoats

Heat assisted magnetic recording (HAMR) on magnetic hard disks is being explored as a means of increasing the areal density of stored data beyond the limits of current technologies. HAMR will subject the magnetic media, the overcoat, and the lubricant on its surface to temperatures in the range 400–650 °C for periods of a few nanoseconds per pass of the read-write head. During such rapid heating events the lubricant is prone to decomposition and desorption from the surface, either of which lead to degradation of the lubricant film, jeopardizing the integrity of the stored data. Rapid laser annealing is known to bias the reactions of small molecules adsorbed on surfaces to favor desorption over decomposition. Analysis of the desorption and decomposition kinetics of perfluoropolyalkylether lubricants such as Fomblin Zdol shows that rapid heating to high temperatures favors desorption over decomposition for molecules with molecular weights of less than 3000. For higher molecular weight Fomblins decomposition is favored at the temperatures to be used for HAMR.     

Molecular dynamics studies of lubricant depletion under moving laser heating: Effects of laser power and film thickness

Molecular dynamics simulation is employed to study the depletion behaviors of perfluoro-lubricants under scanning laser heating for heat-assisted magnetic recording hard disk drives. A partial lubricant near the substrate is irradiated by the laser beam to mimic nano-scale heat transfer from disk to lubricant. The lubricant surface morphology and thickness profiles are examined to reveal the dynamic depletion behaviors. The localized temperature evolution is also evaluated to illustrate the direction-dependent ridge formation around the depletion zone. In addition, the effects of laser power and film thickness on lubricant depletion are explored. Although evaporation is enhanced significantly at high laser powers or for lubricant with thickness around one monolayer, thermodiffusion is the primary mode of lubricant depletion under scanning laser heating.     

Static and Dynamic Slider Air-Bearing Behavior in Heat-Assisted Magnetic Recording Under Thermal Flying Height Control and Laser System-Induced Protrusion

The air bearing’s response to regions of elevated temperature on its bounding surfaces (the slider and disk) may be an important consideration in the head–disk interface design of heat-assisted magnetic recording (HAMR) systems. We implement the general non-isothermal molecular gas lubrication equation into an iterative static solver and dynamic air-bearing solver to evaluate the effect of localized heating of the air-bearing surface (ABS) due to the near-field transducer (NFT). The heat-dissipating components in our simplified HAMR design are the NFT, laser diode, and thermal flying height control (TFC) heater. We investigate the effect of each HAMR slider component on ABS temperature and thermal deformation and the slider’s flying height. The NFT induces a localized thermal spot and protrusion on the larger TFC bulge, and it is the location of maximum temperature. This ABS temperature profile alters the air-bearing pressure distribution, increasing the pressure at the hot NFT location compared with predictions of an isothermal air-bearing solver, so that the center of the pressure acting on the ABS is slightly closer to the trailing edge, thereby decreasing the pitch angle and increasing the minimum flying height. Other researchers have shown that the NFT’s thermal response time may be much faster than its protrusion response time (Xu et al. in IEEE Trans Magn 48:3280–3283, 2012). The slider’s dynamic response to a time-varying NFT thermal spot on the ABS while the combined TFC and NFT induced thermal protrusion remains constant is investigated with our dynamic air-bearing solver. We simulate the slider’s step response to a suddenly applied ABS temperature profile and a pulsed temperature profile that represents laser-on over data zones and laser-off over servo zones. The sudden (step) or rapid (pulse) increase in ABS temperature induces a sudden or rapid increase in pressure at the NFT location, thereby exciting the air bearing’s first pitch mode. For the slider design and simulation conditions used here, the result of the pitch mode excitation is to alter the position of the center of pressure in the slider’s length direction, thereby changing the pitch moment. In response, the pitch angle and minimum flying height change. The step response decays after approximately 0.15 ms. Because the laser duty cycle is much shorter than this response time, a periodic disturbance is predicted for the center of pressure coordinate, pitch angle, and minimum flying height. The peak-to-peak minimum flying height modulations are relatively small (only up to 0.126 nm); more significantly, the time-averaged minimum flying height increases 0.5 nm for the NFT that reached 208 °C compared to simulations of the isothermal ABS at ambient temperature.     

The Change in Surface Properties of Magnetic Recording Media Under Pulsed Laser Application

The effects of laser heating on surface properties of magnetic recording media were systematically investigated through novel experiments and analytical simulations. When controlled laser pulses were applied onto the surface of a perpendicular magnetic recording (PMR) media, the measured values of surface roughness, surface free energy, and surface adhesive force were significantly increased with the number of the applied laser pulses. The heat transfer modeling and simulation was performed to evaluate the change in surface temperature of PMR media by the pulsed laser. The resulting temperature was not high enough to affect the carbon film and the underlying magnetic materials, but it could change the properties of the molecularly thin lubricant film on the media surface. Based on the thermal stability of the perfluoropolyether lubricant, it was found that the change of surface properties in experiments could be attributed to the thermal degradation of the lubricant through desorption process.     

Investigation of Lubricant Transfer between Slider and Disk Using Molecular Dynamics Simulation

A model for lubricant transfer from a rotating magnetic recording disk to a magnetic recording slider is developed using molecular dynamics simulation. The combined effect of disk velocity and local air-bearing pressure changes on lubricant transfer is investigated. The simulation results indicate that local pressure changes in the absence of disk circumferential velocity can cause lubricant redistribution on the disk, while local pressure changes on a moving disk can result in lubricant transfer from the disk to the slider. The amount of lubricant transferred from the disk to the slider and the lubricant buildup on the disk are a function of the local pressure change and disk velocity. The amount of lubricant transferred from the disk to the slider and the height of lubricant buildup on the disk surface decrease with an increase in the number of functional groups of the disk, a decrease in the local pressure change, and a decrease in the disk circumferential velocity.     

Simulation of the Performance of Various PFPE Lubricants Under Heat Assisted Magnetic Recording Conditions

Heat assisted magnetic recording (HAMR) is proposed for the next generation of hard disk drives. In HAMR systems, a laser beam heats the disk magnetic layer to the Curie temperature. This may cause the thin film lubricant coating the disk to deplete due to evaporation and surface tension gradient. In this study, we perform simulations for the Z-tetraol family of lubricants with four hydroxyl end-groups, including Z-tetraol 1200 as a low molecular weight member of the family and Z-tetraol 2200 as a high molecular weight of the family, and also for ZTMD (2,200 Da) with eight hydroxyl groups as a multi-dentate lubricant, which is manufactured based on the Z-tetraol family. All studies are performed for four cases of lubricant thicknesses including 5, 7, 12, and 14A. These numbers are chosen in order to provide a fair comparison with a previous study for Z-dol. We also investigate the relative effects of evaporation with respect to the thermocapillary shear stress. It is found that after a 2 ns illumination of the laser, a trough and two side ridges across the down-track direction can be seen in the lubricant. The performances of the lubricants can be ranked mainly based on the trough depth and also evaporation such that better lubricants show less deformation and trough depth under equal conditions of thermal spot size and peak temperature. We also found that all of the lubricants deplete rapidly and their depletion speed decreases gradually.     

Raman spectroscopy of disk coatings in a working magnetic disk drive

Raman spectroscopy is a very sensitive indicator of the structure and history of hard carbon films which have been recently used as wear-resistant coatings in thin film magnetic disk drives. In this first application of Raman spectroscopy to a working disk drive we obtain a sensitivity of 0.5 nm in the thickness of a carbon layer with respect to spatial variations. To within this sensitivity, no wear is detected in the slider track after 43000 start-stop cycles. The effect of laser heating is observed on the carbon overlayer while monitoring the disk temperature through the Raman spectrum.     

Lubricant-Induced Spacing Increases at Slider–Disk Interfaces in Disk Drives

In this article, we explore the physical mechanisms for lubricant migration on recording head slider surfaces and how this migration leads to increased slider–disk spacing during disk drive operations. This is done using both a new experimental methodology, called the “droplet stress test,” and through simulation. In our simulations, we compare the air shear-induced lubricant migration modeled either as viscous flow of a continuum liquid film with zero slip or as wind driven slippage of molecules across the surface. The experimental data are best fitted using the viscous flow model to determine an effective viscosity for the sub-nanometer thick lubricant films. This effective viscosity tends to be somewhat less than the lubricant bulk viscosity due to air shear promoting the slippage of lubricant molecules across the surface. Our experimental results also indicate that the potential spacing increase from the pickup of disk lubricant on the slider is limited by the mobile fraction of the dewetting thickness of the lubricant film on the slider.     

The Role of Slider/Disk Roughness in 1 Tb/in.2 Magnetic Recording

To achieve 1 Tb/in.² magnetic recording areal density, the head/disk spacing, or the flying height of the slider, has become so small that both the disk surface roughness and the slider air-bearing surface roughness need to be considered. In this region, the intermolecular force and the contact force become more significant due to the roughness of the two surfaces. This article targets two points: 1) slider/disk roughness effects on intermolecular force and 2) slider/disk roughness requirement for 1 Tb/in.² areal density. A probability model is built to simulate the intermolecular force and the contact force, and these two forces are introduced into the modified compressible Reynolds equation governing the air-bearing pressure of the slider. The equation is solved by the finite volume method based on an unstructured triangle-based mesh. The simulation results show that in 1 Tb/in.² areal density magnetic recording the effects of slider/disk roughness on the intermolecular force are negligible. Smaller R _a values will have fewer effects on flying performance.     

Effect of hard-disk drive spindle motor vibration on dynamic microwaviness and flying-height modulation

In order to achieve higher recording densities up to 1 Terabit per square inch using conventional magnetic recording technologies, the recording slider will need to be physically spaced very close to the rotating disk, possibly via the use of an air-bearing surface. However, as the recording slider is flying at such ultra-low spacing of few nanometers over a high-speed rotating disk, it is experiencing disturbances from various different sources and of a wide frequency range. These disturbances may cause the recording slider to vibrate significantly, a condition known as flying-height modulation (FHM), which may result in data loss and possibly head–disk interface failure. A significant source of slider excitation is due to low frequency surface topographical features of the rotating disk, termed dynamic microwaviness. Dynamic microwaviness is a dynamic property of the disk and differs from regular topographical microwaviness, which is a static property. Most research works on dynamic microwaviness and FHM have been focused at the component level, using somewhat idealized conditions, such as high performance air-spindle motors that exhibit very low vibration amplitudes. In this paper, actual hard-disk drive spindle motors are used to investigate the effect of spindle motor vibration on dynamic microwaviness and FHM. It is found that there is a clear connection between spindle motor vibration and dynamic microwaviness that affects FHM.     

Study on Lubricant Depletion Induced by Laser Heating in Thermally Assisted Magnetic Recording Systems: Effect of Lubricant Thickness and Bonding Ratio

In this study, we have carried out fundamental research on lubricant depletion due to laser heating in thermally assisted magnetic recording. In particular, we investigated the effects of lubricant film thickness and lubricant bonding ratio on lubricant depletion. Conventional lubricants Zdol2000 and Ztetraol2000 were used. The lubricant depletion characteristics due to laser heating were found to depend largely on the lubricant film thickness and material. That is, for films thicker than one monolayer, the lubricant depletion depth increased with the laser-irradiation duration, whereas the thickness of the lubricant after laser irradiation on the diamond-like carbon (DLC) films tended to remain at a constant film thickness of one monolayer. The lubricant depletion width gradually increased as the laser irradiation duration increased. The increasing trends for the lubricant depletion width were quantitatively very similar and almost independent of the initial lubricant film thickness. However, for lubricant films with thicknesses less than one monolayer, the lubricant depletion depth was very small. The lubricant depletion width increased remarkably to several hundred micrometers as the laser irradiation duration increased. The lubricant depletion depth and width were much smaller for Ztetraol2000 than Zdol2000. In addition, the lubricant-bonding ratio was found to greatly affect the lubricant depletion characteristics due to laser heating. In other words, the lubricant depletion depth and width decreased as the bonding ratio increased. The lubricant depletion mechanism involves the evaporation of mobile lubricant molecules when the maximum attained temperature is less than 100 °C. Another suggested lubricant depletion mechanism involves the thermocapillary stress effect, which is induced by the disk surface temperature gradient resulting from the non-uniformity of the laser spot intensity distribution.     

Inert Gas Filled Head–Disk Interface for Future Extremely High Density Magnetic Recording

Inert gas filled head–disk interface (HDI) is a possible solution in reducing the magnetic spacing between the magnetic head and the magnetic media for achieving further increased recording density of a magnetic recording system. This article investigated the flying and thermal performances of a thermal actuated slider at inert gas filled HDI by using a couple-field analysis method which consists of a finite element model of the entire slider, an air bearing model based on the generalized lubrication equation and a heat transfer model which incorporates various molecular dynamics models and considers temperature effects. The simulation studies showed that the variation of gap flying height (FH) with the heater power in the inert gas is quite similar to that in air. It is also found that the slider’s thermal actuation efficiency in helium is slightly better than those in argon and air. However, the temperature effects in a fully sealed drive are totally different to those in an open drive. As a result, the inert gas filled HDI normally requires a larger thermal actuation stroke due to the temperature effects in a fully sealed drive.     

Dynamics in the Bridged State of a Magnetic Recording Slider

A novel region of tribological interaction is explored by inducing near contact between the magnetic recording slider and disk. In this study, we performed frictional measurements over a wide range of subambient air pressure and disk rotation rate. Since the slider is supported over the disk by an air bearing, it has been found that cycling from ambient to subambient and then back up to ambient pressure over several minutes of time forms a frictional hysteresis loop. The high-friction branch of the loop, referred to as the bridged state, is characterized by an average frictional displacement and resonant vibration of the suspension mount assembly. The bridged state is currently employed for accelerated wear testing of magnetic slider/disk/lubricant systems. Future magnetic recording systems designed to operate at increasingly lower physical spacing will need to take into account these frictional forces which accompany the incipient contact between the lubricated disk and slider with finite surface roughness. A single degree of freedom model is solved to determine the equivalent dynamic friction force on the slider as an impulse series with random impulse frequency and amplitude from the measured frictional displacement in the bridged state. The mean slider-disk spacing in the bridged state is derived from the experimental friction force, the spacing probability density function, and the adhesion stress from the Lifshitz model for dispersion interaction energy.