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.     

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.     

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.     

Lubricant Flow and Evaporation Model for Heat-Assisted Magnetic Recording Including Functional End-Group Effects and Thin Film Viscosity

The lubricant covering a hard disk in a heat-assisted magnetic recording drive must be able to withstand the writing process in which the disk is locally heated several hundred degrees Celsius within a few nanoseconds to reduce the coercivity of the media and allow writing of data. As a first step in modeling a robust lubricant, we have developed a simulation tool based on continuum theory that incorporates previously proposed variations of viscosity and an additional component of disjoining pressure due to functional end-groups with film thickness. Here we apply this simulation tool to a conventional perfluoropolyether lubricant, Zdol 2000, for which there exists experimental data. The simulation tool can be used equally well for other lubricants once their properties become known. Simulations at small length and time scales that are unobservable with current experimental capabilities are performed. We investigate the effect of the total disjoining pressure and thin film viscosity on evaporation and lubricant flow for different initial thickness. For films thicker than 1 nm, the inclusion of polar disjoining pressure suppresses the lubricant thickness change due to evaporation and thermocapillary shear stress compared with cases without this component. Thin film viscosity is an important property to consider for thinner lubricants. We also consider how lubricant depletion depends on laser spot size and thermal spot maximum temperature. The smaller spot profiles exhibit side ridges due to thermocapillary shear stress while the larger spot profiles show no side ridges, only a trough due to evaporation. The lubricant depletion zone width and depth increase with increasing thermal spot maximum temperature.     

A Model for Laser Induced Lubricant Depletion in Heat-Assisted Magnetic Recording

The lubricant evaporation caused by the rapid laser heating is always a big concern in heat-assisted magnetic recording. In this article, we develop an empirical equation based on the existing measurement data to describe the relation between the evaporation coefficient of lubricant and temperature on the disk surface. The evaporation coefficient of lubricant is found to decrease from ~1.0 to ~0.003 for the temperature range from 406 to 512 K and follow the trend given by the Arrhenius formula. By incorporating this formula into a previously established evaporation model, we can get a new model, which enables us to predict the lubricant evaporation and depletion caused by the rapid laser heating more accurately than ever.     

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.     

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.     

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.     

Contact Stress-Induced Micromagnetic Behavior in Magnetic Recording Disk

Stress plays an important role in the magnetic properties of ferromagnetic materials. Sliding contact in hard disk drives can lead to tribological failures of the disk in terms of data loss and demagnetization. However, the relationship between contact stress-induced magnetization changes and tribological failures of magnetic recording disk is rarely discussed. In this study, the contact stress-induced micromagnetic behavior in magnetic recording disk was investigated using micromagnetic simulation. A micromagnetic model including the magnetostriction effect into the Landau–Lifshitz–Gilbert equation was developed to simulate the stress effect on the magnetization changes. Then finite element analysis was used to calculate the critical stresses for the occurrence of data loss and demagnetization of perpendicular magnetic recording disk under sliding contact according to our previous experimental results. Based on these simulation results, it was found that the magnetic moment decreased by 8.9 % under the critical stress for data loss, and it rotated 55.7° under the critical stress for demagnetization. In addition, the simulated static domain structures when data loss and demagnetization occur were in agreement with the previously reported experimental results. Finally, the relationship between the contact stress-induced tribological failures and micromagnetic behavior of the magnetic disk was illustrated. It was proposed that data loss is caused by the magnetization reduction, while demagnetization is caused by the magnetization rotation.     

Effect of Functional End-Groups on Lubricant Reflow in Heat-Assisted Magnetic Recording (HAMR)

In the developing heat-assisted magnetic recording technology, a laser heats up the magnetic media to the Curie temperature of a few hundred degrees celsius for a few nanoseconds. Accordingly, the thin-film lubricant coating on the disk experiences thermo-capillary and evaporation effects followed by its depletion. In order to maintain a reliable head–disk interface, the lubricant needs to return to the initial uniform profile, in a process known as lubricant reflow. We performed numerical simulations of the lubricant reflow and compared the recovery times for widely used lubricants in hard disk industry including Z-dol, Z-tetraol, and ZTMD lubricants with similar molecular weights. We modeled the lubricant reflow on the disk for a wide range of film thicknesses and laser spot sizes, based on a classical lubrication theory and material properties reported by experiments. The results show that the recovery times for Z-tetraol 2200 and ZTMD are significantly greater than that for Z-dol 2000, while the recovery time for ZTMD is close to that for Z-tetraol, despite its higher viscosity value. It is also shown that all lubricants have an optimum film thickness for recovery time, and this optimum point largely depends on the dewetting behavior of the lubricant.     

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.     

Nearmiss Count Between Flying Head and Magnetic Disk by Detecting Electrical Potential of Voltage-Applied Slider

A simple in-situ measurement of flying head dynamics is developed, making use of weak conductive sliders and media as capacitance and resistance sensors. This method is confirmed to detect such abnormal functioning occurring in the head-medium interface as nearmiss, contact or crash. Flying head dynamics are quantitatively studied, using a simple and effective method wherein the slider is excited by externally pressurized air flow. Without this disturbance, the nearmiss count is almost zero, with the number tending to increase up to a resonant frequency governed by the slider inertia and air film stiffness with increasing disturbance, presenting a maximum value at a disk velocity of 3 to 5 m/s.     

Data Loss and Demagnetization of Perpendicular Magnetic Recording Disk Under Sliding Contact

Data loss and demagnetization of perpendicular magnetic recording disk under sliding contact were investigated experimentally. The data loss tests of the disk against a diamond tip under normal forces (0.005–0.05 mN) and the scan of the disk with the magnetic head were sequentially carried out. Then, the demagnetization tests under normal forces (6–10 mN) were performed on the disk to examine the demagnetization behavior. After the tests, the sliding contact areas in the disk samples were observed by atomic force microscopy and magnetic force microscopy. The results showed that data loss occurred without any scratch damages on the disk and demagnetization of the magnetic medium did not occur in the data loss area. The demagnetization occurred only when the scratch depth in the disk exceeded the thickness of the diamond-like-carbon top layer. Finally, a method to study the relationship between data loss and demagnetization of the perpendicular magnetic recording disk under sliding contact was proposed and the conclusion was given that data loss of the disk was not induced by demagnetization of the magnetic medium.     

Experimental Investigation of Local Temperature Increase in Disk Surfaces of Hard Disk Drives Due to Laser Heating During Thermally Assisted Magnetic Recording

In this study, a novel method was developed to evaluate the local temperature increase under steady-state conditions on disk surfaces of hard disk drives due to laser heating during thermally assisted magnetic recording (TAMR). In particular, we evaluated changes in the refractive index and the extinction coefficient of disk substrates due to laser heating by varying the laser power, and determined the relationship between the laser power and the changes in these optical constants. Simultaneously, test disks were heated using a conventional heater, under heating conditions equivalent to those associated with laser heating, by varying the temperature. We evaluated the changes in the optical characteristics of the disk substrates due to heating, and formulated a relationship between the temperature and the changes in the optical characteristics. Then, the local temperature increase due to laser heating under steady-state conditions was evaluated by calibrating both sets of experimental results. In addition, the temperature increase was compared with the numerical simulation results, and good agreement was observed between the experimental and the simulation results. Thus, it was confirmed that, this method is useful and effective for estimating the temperature increase under steady-state conditions in disk surfaces of hard disk drives due to laser heating during TAMR. This method is also a first step toward understanding the occurrence of temperature phenomena in TAMR systems.     

硬盘磁头超薄气膜润滑研究进展

介绍硬盘磁头发展状况以及超薄气膜润滑理论的研究现状，提出超薄气膜润滑研究中存在的问题，如10nm以下一毪行间隙的润滑理论模型、超薄气膜润滑有效的数值计算方法，以及建立飞行高度试验台和开发磁头气体轴承设计分析软件模块等问题，并介绍一种新型有效的计算磁失磁盘超薄气膜的数值计算方法。     

硬盘磁头抛光中抛光液流场的研究

根据运动学分析,研究了磁头和抛光盘接触面之间速度场的分布;结合流体力学理论,将抛光盘表面的沟槽视为楔形滑块或阶梯轴承,研究了磁头和抛光盘接触面之间抛光液的流动行为.并以织构纹形抛光盘的一种抛光工艺为例,分析了抛光液膜厚度随速度变化而发生的变化.结果表明,磁头和抛光盘的转速比大于或小于1时,接触面之间形成变化的速度场;由于速度的变化,接触面之间局部的抛光液流量也随之变化;相应地,抛光液膜的局部厚度也发生变化,这种变化达到数量级;液膜承担的载荷亦发生变化,形成局部的负压或高压,从而导致磨粒的行为发生变化,影响抛光过程.     

The Effect of PFPE Film Thickness and Molecular Polarity on the Pick-Up of Disk Lubricant by a Low-Flying Slider

Lubricant pick-up by a low-flying slider is investigated for hydroxyl-terminated perfluoropolyethers as a function of the number of hydroxyl (OH) groups and of film thickness on the surface of finished rigid disks. The total number of hydroxyl (OH) groups per main chain is 2, 4, and 8 for Zdol, Z-Tetraol, and ZTMD, respectively. The amount of disk lubricant that is picked up by the low-flying slider decreases with decreasing PFPE film thickness and increasing number of OH functional groups. The results are discussed in terms of the disjoining pressure characterizing the lubricant film on the disk surface.     

磁头/磁盘表面保护膜及抗吸附分子膜研究

在中国国家自然科学基金重大项目《先进电子制造中的重要科学技术问题研究》资助下，针对2nm厚度的DLC薄膜的制备和磁头、磁盘间的吸附等问题，探索“磁头、磁盘表面润滑规律和超薄保护膜的生长机理及技术”，目标是寻找磁头、磁盘表面超薄DLC薄膜新的制备方法和制备工艺，发现超薄DLC保护膜的生长机理和生长极限，开发磁头表面抗吸附分子膜的制备技术。报告研究所取得的体系化理论成果。 为了制备厚度为2nm的超薄DLC薄膜，使用FCVA技术代替磁控溅射和CVD技术。通过优化制备参数，制备出厚度为2nm，表面粗糙度为0.128nm，并且连续均匀的DLC薄膜。探索基体形貌对薄膜生长模式的影响规律。发现在脉冲偏压幅值-100V、占空比20%条件下制备的薄膜性能最优