Bernhard E. Knigge

Will the numbers add up for sub-7-nm magnetic spacings? Future metrology issues for disk drive lubricants, overcoats, and topographies

To achieve disk drive recording densities greater than one terabit per square inch, future head-media spacings (HMSs) will need to be less than 7 nm. This will place severe demands on the metrology tools used to measure the thickness and topographies of the contributors to the head-media spacing. Here, we first review some of the metrologies used for characterizing overcoats, lubricants, and topographies and discuss some of the limits that will make it difficult to achieve a sub-7-nm HMS. We next show new results for measuring lubricant redistribution on disk surfaces after contact with a small pad on the slider and present a new method for determining wear rates of these slider contact pads.

Numerical and Experimental Analyses of Nanometer-Scale Flying Height Control of Magnetic Head With Heating Element

Hard drives featuring sliders with active flying-height (FH) control using thermal expansion of a heating element have been recently introduced in products. This approach allows to actively compensate for static FH variations and achieves sub-3-nm clearance during read/write operation. This paper describes a nonlinear numerical model of a perpendicular magnetic recording head for accurate simulation of pole-tip protrusions and their effect on FH change under various conditions, such as at an elevated drive temperature, with the heater activated or during write operation. The model integrates an electrical-thermomechanical finite-element model of slider and a full air-bearing solver, and includes lapped pole-tip recession and slider/disk deformation due to air-bearing pressure. We are able to predict key parameters that are not easily measurable (e.g., minimum/reader/writer FH, different protrusion profiles for ambient temperature, heater actuation, and during writing). We also present novel experimental methods for measuring protrusion and clearance delta profiles with angstrom-level resolution. The modeling results are compared to experimental data under various test conditions showing excellent agreement. From this method, we are able to quickly evaluate and optimize different heater, head, and ABS designs.

Head challenges for perpendicular recording at high areal density

To explore recording head challenges for perpendicular recording at 200 Gb/in/sup 2/ and beyond, the design, fabrication and performance of narrow track dual-element heads were studied using an ABS trailing shield writer design and a conventional CIP-GMR reader design. Parametric recording tests of these heads on low noise CoCrPt/SUL media show that, with the trailing shield design, good writability and low disk transition jitter around 2.5 nm were achieved at narrow write trackwidths down to 120 nm. In addition, peak-to-peak signal amplitudes around 1 mV and T/sub 50/ widths around 28 nm were also achieved at read trackwidths around 60 nm. The areal density potential of these heads was studied using a PRML channel at /spl sim/50 MB/s data rate. Results show linear densities around 1000 Kbpi at ontrack byte error rates of 10/sup -4/, and track densities around 200-240 ktpi using a criterion of 15% offtrack to trackpitch ratio. In all, areal densities of 210-230 Gb/in/sup 2/ were achieved with these head and disk components.

Dynamics of contacting head-disk interfaces

We have successfully designed, fabricated, and tested contact recording sliders where most of the suspension load is supported by an air-bearing surface with only a small contact force (<5 mN) acting on the rear contact pad. To understand the contact dynamics, we have developed an integrated approach where experimental results from friction and laser doppler vibrometry are modeled using an air-bearing code modified to include contact forces. A low bounce (<1 nm mean-to-peak) is achieved in our designs by reducing the real area of contact to minimize friction, by increasing disk roughness, and/or by reducing the width of the slider contact pad. Due to the reduced magnetic spacing, these contact recording heads have bit-error rates several orders lower than conventional flying heads.

Influence of contact potential on slider-disk spacing: simulation and experiment

As slider-to-disk spacing becomes smaller than 10 nm, electrostatic and intermolecular forces become increasingly important. Even if the slider and disk are both grounded, a potential difference can exist between them due to the contact potential, which, as we show here, can generate an electrostatic force greater than the van der Waals force. We have developed a method for measuring the contact potential between the slider and disk by monitoring the induced slider motion with a Laser-Doppler-Vibrometer (LDV) and lock-in amplifier while applying ac and dc bias voltages. We find that the first harmonic of the ac driving frequency is minimized when the dc bias voltage cancels the contact potential. We have preformed air bearing modeling of the slider-disk interfaces that also incorporates electrostatic forces. From simulation, we find that the flying height is reduced and the pitch angle increased as the potential difference between the slider and disk is increased.

Nonlinear dynamic effects at the head disk interface

In this paper, we report slider lubricant interaction phenomena as a function of lubricant thickness and disk velocity on smooth disks. Airbearing vibrations as well as torsional and bending mode vibrations of pico sliders were investigated using a high bandwidth laser Doppler vibrometer (LDV) and a high bandwidth acoustic emission (AE) system.

A novel wear-in-pad approach to minimizing spacing at the head/disk interface

Here we demonstrate a new approach to bring the disk drive recording head close to the disk media, while achieving tight control of the final head-media separation. The slider in this approach has a miniature Wear-In-Pad (WIP) at its trailing edge, encapsulating the read/write elements. As the slider flies over the disk, the slider overcoat and head recession are quickly worn off, resulting in the bottom of read and write elements clearing the disk roughness by only a few nanometers. We show that the Wear-In-Pad is a viable way to achieve the sub 7 nm head-media separations believed to be necessary for Tb/in/sup 2/ recording.

Time evolution of lubricant-slider dynamic interactions

In this paper, we report slider dynamics changes as a result of lube-slider interactions. Lubricant moguls as well as ripples are observed, and they change in amplitude and in frequency, following the slider dynamics.

Slider Vibration Analysis at Contact Using Time-Frequency Analysis and Wavelet Transforms

Time-frequency analysis and wavelet transforms are employed to investigate transient contact dynamics at the head/disk interface of computer hard drives. Wavelet transforms are used to resolve multiple short consecutive contacts at high time resolution at high frequencies. The reassignment method is applied to the time-frequency distribution to enhance the time-frequency resolution, thereby allowing to resolve air-bearing and slider body frequencies simultaneously. The results indicate that strong impacts between slider and disk can lead to excitations of slider body and suspension vibrations. Finite element modal analysis of nano and pico glide sliders is found to be in good agreement with experimentally measured frequencies.

Contact force measurement using acoustic emission analysis and system identification methods

Abstract Acoustic emission (AE) signals were used to investigate the contact force occurring at the head/disk interface of a computer hard drive. The AE sensor was calibrated directly using the “ball drop method” and indirectly using system identification. For the indirect calibration, a high bandwidth laser Doppler vibrometer (LDV) was used. The transfer function was established from the harmonic response derived at different vibration modes and frequencies. A finite element method based transient response simulation of impact was used to estimate the velocity and stress response of the slider. In our experiments, contact forces were found to be in the range of 5–25 mN for nano sliders and 2–10 mN for pico sliders.