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.     

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.     

Modeling laser heated thin film media for heat assisted magnetic recording

In the present study, we concern laser induced heat generation and its effect on lubricant depletion on the multilayer disk structure. A mathematical model is applied to explore the local laser heating from the point of view of the interaction between the disk media and the electromagnetic wave. The absorption rate of optical energy in each layer of disk structure is derived from the Maxwell equations, which is incorporated into the energy equation by a source term. We simulate the unsteady heat transfer and lubricant depletion processes on a multilayer disk structure. The effect of multi-cycle heating on the lubricant depletion is investigated. Comparison between the present results and those obtained by applying a heat flux boundary condition is also made. The present simulations provide useful information on the media structure design of hard disk drives in heat assisted magnetic recording.     

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.     

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...     

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.     

TOF-SIMS analysis of perfluoropolyether lubricant smear transferred from disk surface following laser heating

Microsystem Technologies - The structural and molecular weight changes of perfluoropolyether lubricant picked up following laser heating in heat-assisted magnetic recording (HAMR) were analyzed...     

Numerical simulation of thermal flying height control sliders in heat-assisted magnetic recording

Heat assisted magnetic recording (HAMR) is one of the most promising techniques to extend the recording density in hard disk drives beyond 1?Tb/in². Although the diameter of the spot on the disk that is heated by the laser beam is very small, on the order of nanometers, high local temperatures on the disk and the heat dissipated in the slider during the light delivery process can cause thermal deformations of both the disk and the slider, thereby affecting the flying characteristics at the head-disk interface. In this paper, a finite element model is developed which incorporates a HAMR optical system into a thermal flying height control (TFC) slider with dual heater/insulator elements to study the effect of heat dissipation in the wave guide on the thermal deformation and flying characteristics of a HAMR-TFC slider. In addition, the power input of the laser and design parameters of the heaters are investigated.     

Experimental set-up for simultaneously measuring changes in LD temperature and flying height of HAMR-HGA

Heat-assisted magnetic recording (HAMR) is a promising technology for overcoming the performance limit of the conventional magnetic head in a hard disk drive. The HAMR-HGA consists of a HAMR-head slider, a suspension, and a laser diode (LD) mounted on the slider. An optical near-field transducer (NFT) and a waveguide are near the write-pole in the head slider. During the HAMR process, current is applied to an LD, and the laser beam is coupled into the waveguide and delivered to the NFT. The NFT further concentrate the focused optical spot and the optical spot locally heats the recording medium, thereby reducing the media coercivity. The temperature of the LD and the slider, however, increases. The slider is, moreover, locally deformed, and the flying-height (FH) changes. Therefore, an experimental set-up to simultaneously evaluate the FH change and LD temperature of the HAMR-HGA was required to develop the HAMR technology. We developed a novel experimental set-up for simultaneously evaluating laser characteristics (power, voltage, and wavelength), the increase in LD temperature, and the FH change of a HAMR-HGA. By using this set-up to measure these characteristics of our prototype, the HAMR-HGA showed that the FH decreased as the LD temperature increased. The LD temperature is directly related to the laser characteristics. The change in laser characteristics affects the read-write performance of HAMR. The FH change also affects the performance. Therefore, the developed experimental set-up should be useful in improving HAMR-HGA.     

HAMR HDD shock responses

Heat assisted magnetic recording (HAMR) and slim mobile hard disk drives (HDD) are being developed parallelly to maintain cost advantage over the solid state drive. Operational shock and non-operational shock capabilities are seriously challenged for the slim HDDs due to reduced stiffness (thickness). It is worse for slim HAMR drives due to additional laser diode (LD) and other necessities being added on slider. Shock tests are part of the key performance matrices that must be passed in HDD reliability tests, and the concerns for HAMR mobile drives are, (1) slider lift-off G-level degradation during op-shock, and (2) LD back-to-back hitting during non-operational shock. We studied a few potential HAMR HGA designs, also analyzed a design that improves drive op-shock performances.     

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.     

Patterned media for future magnetic data storage

One strategy to delay the onset of superparamagnetism and achieve magnetic storage densities approaching 1 Tb/in.² is the use of lithographically patterned magnetic media. While one of the main advantages enjoyed by magnetic recording is low cost due to use of featureless media, there are several advantages that may be realized by patterning the medium in a hard disk drive. The commercial success of patterned media will of course depend on the relative costs and gains. In particular, there are three main types of disk patterning proposed, each requiring a different length scale of patterned feature. Patterning of servo marks for maintaining the head position on-track, the fabrication of discrete tracks, and the fabrication of discrete bits have all been proposed and will be discussed. For discrete bit recording single domain magnetic islands are required, and one approach to fabricating these, by depositing magnetic films onto prepatterned substrates, is described. The switching characteristics of the islands as compared to those of the full film, along with initial recording results are presented.     

A single asperity sliding contact model for molecularly thin lubricant

Microsystem Technologies - Molecularly thin lubricants are important in protecting the recording head and the rotating disk in a magnetic storage hard disk drive from mechanical damage induced by...     

Back heating effect of media hot spot at nanoscale head–disk interface

Microsystem Technologies - Heat-assisted magnetic recording (HAMR) is one of prospective high density recording technologies in current hard disk industry due to its theoretical potential. In this...     

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...     

Study of the stiction free magnetic recording head with DLC pad

This paper presents the study of the stiction free magnetic recording head which has diamond-like-carbon (DLC) pad for use with laser zone texture disk and smooth disk. The magnetic recording head with DLC pad is one of the potential solutions to extend contact start stop (CSS) technology for further lower head-disk spacing, since it can decrease stiction and friction between magnetic recording head and magnetic recording disk during CSS. The optimization of air bearing surface (ABS) design for improving take off characteristics, the optimization of DLC pad design for improving tipping stability and the result of CSS reliability test under several environments are shown in this paper. It is found that the occurrence of tipping accelerates the wear of DLC pad, the degradation of the lubricant and the wear of carbon overcoat (COC) protective layer on the disk. Finally it is shown that the magnetic recording head, which has optimized ABS and optimized DLC pad, has small coefficients of stiction less than 2.0 and reliable CSS performance at 15 nm flying height (FH) on the laser zone texture disk having low bump height. Tipping and major wear of DLC pads are not observed under several environmental conditions with this optimized design.     

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     

The optimal helium fraction for air–helium gas mixture HDDs

Helium-filled drives have recently been commercialized to enable a high recording density. However, because the use of helium increases production costs, binary gas mixtures such as air–helium have been investigated. In this paper, the dominant performance metrics of hard disk drives (HDDs) are the windage losses, the flow induced vibration (FIV), the lubricant transfer and lubricant depletion. These were investigated for air–helium gas mixtures as a function of the helium fraction. The frictional torque was empirically derived in both the laminar and turbulent regimes. The windage loss and the FIV of a helium-filled drive were found to be similar to that using an air–helium gas mixture with a helium fraction of 0.75. On the other hand, the quantity of accumulated lubricant and the maximum lubricant depletion in a helium fraction of 0.75 were superior to those in a helium fraction of 1.0. Further investigation of performance metrics should be carried out. However the performance metrics considered here showed that a helium fraction of 0.75 was favorable to a helium fraction of 1.0.     

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.     

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.