Modeling laser induced lubricant depletion in heat-assisted-magnetic recording systems using a multiple-layered disk structure

In this paper, we model the depletion of lubricant from a disk surface subject to heating by a scanning laser in a heat assisted magnetic recording (HAMR) system. A multi-layer disk structure is used consisting of the substrate (either glass or aluminum), the CoFe based soft magnetic under- layer, a Ru based intermediate layer, a CoCrPt based recording layer, the diamond-like-carbon layer, and the lubricant film. The thickness and material properties of the different layers are shown to play an important role in the conduction of heat from the top layer to the bottom layer and, consequently, in the lubricant depletion process due to heating by a scanning laser. The results show that it is critical to include realistic multi-layer disk structures in HAMR lubricant depletion modeling.     

Experimental study of lubricant depletion in heat assisted magnetic recording: different lubricants on HAMR media

Heat assisted magnetic recording (HAMR) is a promising choice to surpass the super-paramagnetic limit in magnetic recording and to allow continued increase in the areal recording density of hard disk drive. However, lubricant depletion on disk surface might be a problem in HAMR due to high temperature during writing process. In this work, depletion of four types of commercial lubricants on our HAMR media has been evaluated under equivalent HAMR conditions. Linear relationships between the lubricant depletion depth and the logarithm of the equivalent laser heating time have been established for these lubricants. The depletion depths of the lubricants at different laser heating durations are predicated. It is found that the performance of the four evaluated lubricants is acceptable in respect of lubricant depletion depth by hypothesizing the total heating duration for a given point on the media over the lifetime of the drive is 10⁶ ns.     

Experimental study of lubricant depletion in heat assisted magnetic recording: effects of laser heating duration and temperature

Heat assisted magnetic recording (HAMR) is a promising approach to overcome the superparamagnetic limit in magnetic recording and enable large increases in the storage density of hard disk drives. However, it is expected that HAMR causes lubricant depletion problem on disk surface under the high temperature in the heating assisted writing process. In this study, the effects of laser heating duration and laser heating temperature on lubricant depletion are studied experimentally. It is found that lubricant depletion depth is linearly proportional to the logarithm of laser heating duration. Lubricant depletion can be mitigated effectively by lowering laser heating temperature. Lubricant depletion depth over the life-time of the drive is also predicted.     

Numerical simulation of molecularly thin lubricant film flow due to the air bearing slider in hard disk drives

In this paper we numerically study the evolution of depletion tracks on molecularly thin lubricant films due to a flying head slider in a hard disk drive. Here the lubricant thickness evolution model is based on continuum thin film lubrication theory with inter-molecular forces. Our numerical simulation involves air bearing pressure, air bearing shear stress, Laplace pressure, the dispersive component of surface free energy and disjoining pressure, a polynomial modeled polar component of surface free energy and disjoining pressure and shear stress caused by the surface free energy gradient. Using these models we perform the lubricant thickness evolution on the disk under a two-rail taper flat slider. The results illustrate the forming process of two depletion tracks of the thin lubricant film on the disk. We also quantify the relative contributions of the various components of the physical models. We find that the polar components of surface free energy and disjoining pressure and the shear stress due to the surface free energy gradient, as well as other physical models, play important rolls in thin lubricant film thickness change.     

Depletion of monolayer liquid lubricant films induced by high-frequency pulsed-laser heating in thermally assisted magnetic recording

In this study, lubricant depletion due to high-frequency pulsed-laser heating was investigated for lubricant films with thicknesses of both more than and less than one monolayer. A conventional lubricant, Zdol2000, was used. It was found that the critical temperature at which the lubricant begins to deplete owing to laser heating was strongly dependent on the lubricant film thickness. In the case in which the thickness of the lubricant film was less than one monolayer, this temperature was approximately 170?°C higher than it was when the thickness was more than one monolayer. To analyze the lubricant depletion mechanism, we examined the tested lubricant film using temperature programmed desorption (TPD) spectroscopy. It was found that the lubricant depletion characteristics due to laser heating could be explained using the experimental TPD results for the tested lubricant film, and that the depletion mechanism involves the desorption or decomposition of the lubricant molecules, which interact with the diamond-like carbon thin films when the lubricant film thickness is less than one monolayer. Further, the results of TPD and of a thermogravimetric analysis (TGA) of the lubricant were compared. The thermal robustness of the ultra-thin liquid lubricant films was found to be greater than that of the bulk lubricant materials.     

Investigation of lubricant depletion under a continuous heat source using molecular dynamics simulation

Microsystem Technologies - In this study, a model for investigating lubricant depletion in heat assisted magnetic recording slider/disk system was developed using molecular dynamics simulation. The...     

Experiments on slider lubricant interactions and lubricant transfer using TFC sliders

Future magnetic storage density targets (>4 Tb/in. ²) require subnanometer physical clearances that pose a tremendous challenge to the head disk interface (HDI) design. A detailed understanding of slider-lubricant interactions at small clearances and contact is important to not only address magnetic spacing calibration and long term HDI reliability but also to meet additional challenges imposed by future recording architectures such as heat assisted magnetic recording (HAMR). In this work, the behavior of the disk lubricant is investigated through controlled tests using TFC sliders which are actuated to proximity (i.e. backoff) and into contact (i.e. overpush) on one specific half of the disk per rotation by synchronization with the spindle index. Observations for lubricant distribution in contact tests (i.e. overpush) reveal an accumulation of lubricant on the disk near the onset of contact suggesting a migration of lubricant from the slider to the disk as the slider approaches the disk. Experiments also reveal that there is a similar deposition of lubricant even in the absence of contact for backoff tests. Furthermore, light contact tests result in significant lubricant rippling and depletion with associated slider dynamics. The lubricant rippling frequencies correlate well with the slider’s vibration frequencies. Interestingly, strong overpush may lead to stable slider dynamics (for certain air bearing designs) that is also associated with noticeably lower lubricant distribution (compared to the light contact case), and the greatest lubricant changes are observed only at the onset and the end of contact. This paper reveals the complex nature of slider-lubricant interactions under near-contact and contact conditions, and it highlights the need for further studies on the topic to help design a HDI for recording architectures of the future.     

Experimental study of lubricant depletion induced by pulsed laser irradiation heating in heat-assisted magnetic recording

Microsystem Technologies - In this study, we experimentally investigate the lubricant depletion as well as the temperature distribution caused by pulsed laser irradiation heating and discuss the...     

Direct Monte Carlo simulation of air bearing effects in heat-assisted magnetic recording

Heat-assisted magnetic recording (HAMR) is a promising high density recording technology in current hard disk industry. It is proposed to use a heat source from the slider system for heating up the recording media in order to increase its storage density. The heat generated from a heat spot on the disk and/or the higher slider body temperature in HAMR system could affect the slider air bearing and flying height. This paper studies the heat effects on slider air bearing characteristics by using the direct Monte Carlo simulation (DSMC) method. The simulation results show that the heat spot less than 50?nm in diameter could not affect much to the air bearing; however, its location should be away from the bearing pressure peak to minimize the heat spot effect. Furthermore, high temperature slider could increase the bearing pressure and force and the trend of force increment is independent of the flow channel length.     

Lubricant transfer caused by breakage of liquid meniscus bridge

In recent magnetic storage systems, the spacing between the flying head and the disk has been decreased remarkably to less than 5?nm in order to realize ultra-high density recording. Lubricant on the disk is picked up by the flying head slider as a result of intermittent contact between the slider and the disk, or due to condensation of the lubricant vapor. In the present study, the basic characteristics of lubricant transfer (lubricant pick-up) caused by the breakage of a liquid meniscus bridge are investigated experimentally and theoretically. An experimental method by which to measure the volume fraction of the lubricant pick-up has been established. The theoretical results obtained from a simple model proposed by the authors showed good agreement with the experimental results. The validity of the theoretical model was verified based on the experimental results.     

激光脉冲作用引起的梁形微谐振器的振动

在激光脉冲作用下,有两个因素需要考虑:一个是热传导的非傅立叶效应,另一个是温度场与应变场的耦合导致能量耗散,使物体的机械能转化为热能,并且这是不可逆的.本文综合考虑上面两个因素,研究了激光脉冲作用下微米尺度梁谐振器的热弹性耦合问题.采用Fourier变换与Laplace变换相结合的方法求解梁的横向振动,并分析不同环境温度与能量吸收深度的影响.     

Reduction in lubricant pickup by bias voltage between slider and disk surfaces

We studied the effect of bias voltage between slider and disk surfaces to reduce lubricant pickup by the slider. A perfluoropolyether (PFPE) lubricant film, which is coated on the disk surface, has been considered to be charged to a negative voltage by the airflow on the rotational disk surface. Because the PFPE lubricant film is negatively charged, the lubricant pickup should be reduced by a bias voltage with a negative voltage on the slider surface. We confirmed changes in the lubricant pickup in a lubricant pickup test conducted at different bias voltages. A positive voltage of the slider accelerated the lubricant pickup, whereas a negative voltage reduced it.     

Interactions between nano-spacing flying head sliders and ultra-thin liquid lubricant films with non-uniform distribution in hard disk drives

This paper describes the effect of ultra-thin liquid lubricant films on air bearing dynamics and flyability of less than 10 nm spacing flying head sliders in hard disk drives. In particular, the effect of non-uniform lubricant film distributions on head/disk interface dynamics are studied. The disks with lubricant on one half of disk surface thicker than the other half were used in this study. The dynamics of sliders is monitored using acoustic emission (AE) and the interactions between the slider and disk are investigated experimentally. The disks were also examined with a scanning micro-ellipsometer before and after each test. Complicated slider responses were observed and clarified. In addition, it was found that the periodic lubricant film thickness modulations or non-uniformity caused by the slider-disk contact interactions could be observed. It is suggested that this lubricant film thickness non-uniformity will be one of the technical issues in order to achieve ultra-low head/disk contact interface of less than 10 nm.     

Smear growth on head slider surface from siloxane outgas on heat assisted magnetic recording

Microsystem Technologies - In this study, the smear by the lubricant and siloxane pickup at the laser heating in the heat assisted magnetic recording (HAMR) was evaluated using our developed...     

Quantitative study of lubricant puddling at the head/disk interface

 This paper presents a new methodology to quantify the micro-level lubricant accumulation (lubricant puddling) on a magnetic recording hard disk for flyability testing. After flying over the disk for certain period of time, the head was parked on the disk for certain time duration to allow the lubricant picked up on the head to flow to the disk surface. Using the optical surface analyzer with proper calibration and background removal, the volume of lubricant in a lubricant puddle on the disk has been measured accurately. The effect of various factors, including lubricant type, thickness, the way of head parking, and head parking duration, on lubricant puddling has been investigated. This method is a useful tool for flyability study and other studies that need to quantify the volume of localized lubricant accumulation on the disk. Received: 5 July 2001/Accepted: 11 December 2001     

Effect of thermal pole tip protrusion and disk roughness on slider disk contacts

Ultra-high areal density for hard disk drives requires a stable head disk interface at a flying height lower than 8 nm. At such a low flying height, small flying height variations may cause slider/disk contacts. Slider/disk contacts can also occur when a write-current is applied to the write coil since the flying height between slider and disk can be affected by the thermal expansion of the pole tip. In this paper, we investigate the vibration characteristics of sliders during thermally induced contacts using laser Doppler vibrometry. We perform a parametric study of contact events using disks with different surface roughness and lubricant thicknesses, and analyze the slider motion statistically. For a given write current, we observe that the slider vibrations increase with disk roughness and lubricant thickness.     

Simulation of temperature around laser-heating media in heat-assisted magnetic recording

Heat-assisted magnetic recording (HAMR) is a new approach, which makes the head write data easily under a low magnetic field using a laser to heat the magnetic media to reduce its coercivity, thus, it is considered to be the next generation of higher recording areal density technology. In this paper, a three-dimensional HAMR finite-element model of hard disk drive (HDD) is developed. The temperature distributions around the laser-heating area on disk surface are investigated when the HDD is filled with air and helium. The cooling effects of the disk rotation and the heat convection in head-disk interface (HDI) are also analyzed.     

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.     

Experimental evaluation of a pyroelectric detector linearity used for pulsed laser energy absolute measurement

The linearity of a pyroelectric detector used for pulsed laser energy absolute measurement has been studied using an experimental evaluation. Two different methods were used to evaluate the detector linearity: by comparison to a calorimeter and using the inverse square distance law. Different detection substrates were used and tested. A heating material in the form of a thin film was integrated into the substrate. This material is not only absorbent to transform laser energy into heat but also electrically resistive to transform the electric energy delivered by a current pulses generator into heat by Joule effect. An original approach based upon an electric substitution method has been used to measure the pulses energy given by the laser system and not the pulses mean power as it is commonly used. The measurement method is then based upon the comparison of the maximum voltage responses which are proportional to the pulse energy.     

Structural stability of nanometer thick diamond-like carbon films subjected to heating for thermally assisted magnetic recording

In this study, the structural stability of ultra-thin diamond-like carbon (DLC) films subjected to heating was investigated experimentally. The thermal robustness of nanometer thick DLC films for thermally assisted magnetic recording was demonstrated, and their damage mechanisms were elucidated using chemical vapor deposition and filtered cathodic vacuum arc DLC films. In addition, the refractivity of disk substrates with heated DLC thin films was evaluated using a scanning microellipsometer. This measurement system is suggested to be an effective method for evaluating the thermal stability of DLC films. The effect of the heating duration on the thermal stability of DLC films was also investigated by this method. Further, it was suggested that DLC thin films on an air bearing surface may be affected by laser heating because this surface may be heated for a significantly longer duration owing to the magnetic head read/write operation. However, the DLC thin films on the disk substrate may not be affected as severely by laser heating.