James Dillon Kiely

HAMR Drive Performance and Integration Challenges

The commercialization of heat-assisted magnetic recording (HAMR) presents some significant technical challenges that need to be resolved before the widespread adoption of the technology can begin. In this paper, we present some HAMR data from prototype drives and discuss some of the challenges related to protrusion management, recording performance optimization, and drive power requirements within the drive.

Three-Dimensional Motion of Sliders Contacting Media

Characterization of slider motion induced by contact is becoming a critical aspect of developing advanced head-disk interfaces. While vertical motion induced by contact has been studied, very little is known about off- and down-track motions. We have applied three separate laser Doppler vibrometers to measure slider movement in three orthogonal directions simultaneously. We have measured the position of a slider as it undergoes a transition from flying to making full contact with the media surface. We find that slider motion varies considerably with varying levels of contact and that motion in all three directions is considerable. Spectral decomposition is used to identify the vibration modes that are excited in each direction, and we find that for most of the test velocities, modes excited in the vertical direction give rise to motion in the two orthogonal directions. In addition, we present a depiction of the vertical, down-track, and off-track position changes by plotting the position of the slider in real space coordinates to help visualize more completely how the slider moves in space. These trajectories depict the periodic, elliptical path the slider takes and identify how the paths change with contact. Analysis of motion identifies that at some levels of contact, a majority of motion is repeatable, but that nonrepeatable components increase with the amount of contact. Additionally, down-track motion is the only component whose magnitude increases monotonically with increasing contact.

Head–Disk Lubricant Transfer and Deposition During Heat-Assisted Magnetic Recording Write Operations

Lubricant accumulation was found on the media surface the instant when the laser is turned OFF during heat-assisted magnetic recording write operations. By changing the write cycles, laser ON/OFF duration, media, and head temperatures, we find that this lubricant accumulation is related to the change in head-media temperatures. The observed lubricant deposition process is restricted to a short time window (1-2 μs) after the laser is turned OFF. An equilibrium model of thermal displacement due to evaporation and condensation processes is presented and used to discuss the effect of the head-media temperature changes on the lubricant accumulation. Possible solutions to minimize the lubricant transfer and deposition are discussed.

Wear Analysis of Head-Disk Interface During Contact

To achieve a higher storage density in a hard disk drive, the fly height of the air bearing slider, as part of the magnetic spacing, has to be minimized. At an ultralow fly height, the intermittent-continuous contact at the head-disk interface (HDI) is unavoidable and directly affects the mechanical and magnetic performance of the hard disk drive, and is of great interest. The HDI wear has a nonlinear and time-varying nature due to the change of contact force and roughness. To predict the HDI wear evolution, an iterative model of Coupled Head And Disk (CHAD) wear, is developed based on the contact mechanics. In this model, a composite transient wear coefficient is adopted and multiple phases of the wear evolution are established. A comprehensive contact stiffness is derived to characterize the contact at the HDI. The abrasive and adhesive wear is calculated based on the extended Archards wear law. The plastic and elastic contact areas are calculated with a three-dimensional (3D) sliding contact model. Based on the CHAD wear model, for the first time, the coupling between head and disk wear evolutions is thoroughly investigated. Accelerated wear tests have also been performed to verify the disk wear effect on the slider wear. A wear coefficient drop with time is observed during the tests and it is attributed to a wear mechanism shift from abrasive to adhesive wear A shift in the type of contact from plastic to elastic accounts for the wear mechanism change.

Independent measurement of air-bearing pitch, roll, and vertical modulation

We describe a method of measuring the dynamic changes in pitch and roll angle of air-bearing sliders used in hard disk drive magnetic recording. The technique provides simultaneous, single-point measurements of the dynamic changes in pitch angle, roll angle, and vertical position. We apply the technique to characterize the dynamic responses of a slider reacting to an idealized disk defect and demonstrate how different translation and rotational modes get excited sequentially. Spectrogram analysis is then applied to illustrate the changes in frequency content of the three different modulation modes. We model the slider response as an impulse response to provide experimental determination of response frequencies and damping factors. We perform these analyses as a function of rotational velocity and find that response frequencies increase and damping factors decrease with decreasing velocity. These analyses provide insight into factors controlling slider dynamics and demonstrate the utility of the technique.

Write-Induced Head Contamination in Heat-Assisted Magnetic Recording

One detrimental by-product of heat-assisted magnetic recording writing is the creation of head contamination. Here, we present the current understanding of the driving forces, growth mechanisms, and growth rates of write-induced head contamination. The combination of an evaporation and condensation model with shear forces suggests a flow of lubricant on the head may precipitate contamination. The contamination is observed to grow in the head–media gap until it contacts the media surface, at which point an additional material pickup mechanism can be activated. Evidence of contact-induced transfer and a chemical reaction of the contamination is presented, and the impacts of contamination on head temperatures and thermal gradient is presented. Depending on the contamination properties, head temperatures may be increased substantially, leading to increased reliability risk. Consistent with previous analyses, we find that contamination may increase media thermal gradient.

Sub-critical telephone cord delamination propagation and adhesion measurements

Thin film delamination can occur when the stored elastic energy per unit area in the film due to the residual stress exceeds the interfacial toughness. Telephone cord morphology is commonly observed in delaminating thin films under compressive stresses. Here, the biaxial film stress is partially relieved by film buckling in the direction perpendicular to the telephone cord propagation, and by “secondary” blister buckling in the direction of telephone cord propagation, which results in the sinusoidal fracture patterns. A superlayer indentation test, in which additional stress is supplied to the crack tip using a nanoindenter, can be used to measure the interfacial toughness. Estimates of the energy release rate for diamond-like carbon (DLC) films on magnetic media were obtained using the superlayer indentation test, as well as the delaminated buckling profiles. The results obtained by these two independent methods are in good agreement with each other. We find the average adhesion energy to be 6 J/m 2 for DLC films on magnetic media. Normally telephone cord blisters “run out of steam” and stop once the interfacial toughness exceeds the strain energy release rate. It is possible to make blisters propagate further by either putting mechanical energy into the system, or by introducing liquids at the crack tip, thus reducing the film interfacial toughness. Environmental species can assist cracking and contribute to thin film delamination, which is readily observed in vintage mirrors. Crack propagation rates on the order of microns per minute were measured for DLC films in different fluid environments. We identify how telephone cord buckling delamination can be used as a test vehicle for studying crack propagation rates and environmentally assisted cracking in thin films.

Writer and Reader Head-to-Media Spacing Sensitivity Assessment in HAMR

We collate multiple experimental measurements of heat-assisted magnetic recording (HAMR) near-field transducer (NFT) and reader sensitivity measurements on spinstand to compare and contrast with the conventional perpendicular magnetic recording (PMR). The readback process shares many similarities, but differences appear due to the increased measured curvature of the prewritten track, which increases the observed pulsewidth (Pw50), lower readback amplitude due to reduced flux from the media transition (MrT), and increased coating thicknesses. We find that the reader head-to-media spacing (HMS) sensitivities and requirements converge toward the conventional scaling requirements. The HAMR write process is more complex due to the uncertainties associated with the optical properties and protrusion position of the NFT. However, accumulating multiple studies varying write HMS with coatings, NFT changes, and media changes, we consistently observe lower sensitivities compared with the conventional writer HMS, in line with modeled comparisons, leading to more relaxed requirements on the NFT clearance than the PMR write HMS. A 1.5 Tbpsi basic technology demonstration demo is shared using the HMS numbers in the bounds of the claims.

Media Roughness and Head-Media Spacing in Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording involves the transfer of energy to the recording medium via optical means. To enable high areal density, the recorded track must be smaller than the diffraction limit of focused light, which is accomplished by using a near-field transducer (NFT) with a corner or peg with small dimension. Energy transfer using such a transducer is a near-field effect, and therefore is highly sensitive to the spacing between the NFT and the medium. Since the recording medium has some surface roughness, there will be a variation in the NFT-to-medium spacing and this will impact the amount of energy transferred from the NFT. We model the effect of Gaussian surface roughness on NFT energy transfer and predict surface temperature variations for a rough surface. In addition, we illustrate how changing the head-medium spacing changes the impact that roughness has on surface temperature variation. We combine these modeled predictions with spinstand measurements of recorded data and conclude that the effect of media roughness results in only limited temperature excursions above the nominal recording medium temperature.

Sliding Contact Micro-Bearing for Nano-Precision Sensing and Positioning

Applications involving sub-nanometer, relative, in-plane motion between two substrates require precise control of gap-spacing between substrates for, both position-sensing as well as for signal transduction between the substrates. A method of passive gap-spacing control using MEMS-fabricated rigid spacers is proposed. A model to design a low-friction and low-wear interface between the sliding substrates is developed. Prototype parts with hard-coated interfaces and with and without lubrication were fabricated and tested. Sliding friction coefficients of 0.1-0.15 or less and wear life of millions of sliding cycles were achieved on prototype parts. Better results are predicted for MEMS-scale devices.