Track Squeeze Research Articles

Heat-Assisted Recording: Advances in Recording Integration

Differences between heat-assisted magnetic recording (HAMR) and conventional perpendicular magnetic recording (PMR) for signal processing of the written track by one and two reader multisensor magnetic recording (MSMR) are explored using spinstand measurements. Tracks written with HAMR heads are shown to have more curvature compared with those written with modern PMR writers. We introduce two signal processing techniques in order to determine the merits and explore the recording physics tradeoffs for this complex system. To first order, we see twice as much bit error rate (BER) gain for the HAMR written track—on readback—using two reader MSMR, than the PMR. This difference we assign to the lower electronic signal-to-noise (SNR) ratio for the HAMR written track. In order to replicate practical conditions, we stress the MSMR system by applying increased adjacent track squeeze and displacing the two readers from the center of the track. Under these conditions we can resolve differences between the PMR and HAMR systems. Increased adjacent track squeeze appears to trim the broad and curved track edges, until the on-track signal starts to degrade. Increased offset of the readers from the center of the track offers improved the media (spatial) SNR, but we fail to harvest all those gains into BER across the range. We hypothesize that this is due to increasing distortion, a combination of track edge noise and reading of the encroaching tracks.

Interpolation of Noisy Measurements in a Coarse Grid: Creating Waveforms for Intermediate Points on “747” Curves

In some situations, only a limited number of measurements under a limited number of conditions may be available for analysis. This paper describes an approach for creating additional interpolated measurements in which both measurement noise as well as signal have appropriate values. This is important, for example, when evaluating the behavior of advanced readback channels where quantities, such as sector failure rate (SFR), change very abruptly from 0% to 100% in response to relatively small parameter changes. This means that a smooth interpolation of SFR between coarse measurement points is little help in determining the actual boundary of good performance. If, instead, interpolated waveforms are fed into the channel, the boundary of the region of good performance can be clearly identified. This paper formulates a solution for the general problem of noisy measurement (waveform) interpolation and then applies this approach specifically to the situation, where a series of spin-stand waveforms were captured at various reader positions and with various written track squeeze positions (i.e., a set of “747s”). Using this approach, realistic waveforms can now be created and the SFR evaluated at any arbitrary intermediate reader and writer positions.

A Physical Model for Predicting Off-Track Capability in Perpendicular Recording

Off-track capability (OTC) after adjacent track squeeze is a critical and independent parameter for determining the track density in hard disk drives. A semi-elliptical equation is derived based on a physical model for the writer pole with an isosceles trapezoid shape. It explains the different behaviors of OTC versus track pitch from both squeeze directions, as well as the variation of OTC versus track pitch due to the change of skew angle. The calculated data of OTC versus track pitch are in good agreement with the experimental data measured from the drive with perpendicular magnetic recording media.

Dedicated Servo Recording System and Performance Evaluation

The perpendicular magnetic recording (PMR) in hard disk drives is approaching its physical limitation. The emerging technologies, such as heat assisted magnetic recording and microwave assisted magnetic recording have been proposed to record on magnetic media with thermally stable smaller size grains at higher areal density (AD). However, in the media fabrication, achieving well-isolated small size of grains is more challenging than obtaining high ${K_{u}}$ material as recording media. Reducing the number of grains per bit is a major path for keeping AD growth of PMR in recent years. To minimize the SNR penalty at a smaller grain number per bit, pushing more on track density is the right approach. With the 2-D magnetic recording (TDMR) readers for inter-track interference cancellation, the off-track read capability is improved significantly for allowing a narrower track read. In the drive working environment, when the external vibration or other mechanical disturbance happens during the writing process, it creates more track squeeze at adjacent tracks and leaves a very narrow track at some locations of the track. When the track width is narrower than the squeeze to death width in the 747 curve, it causes hard failure in the channel. To solve the track squeeze problem, this paper proposes to add an additional magnetic recording layer in between the data recording layer and the soft underlayer of conventional PMR media. This additional recording layer is used to record servo information only. The continuous positioning error signal is able to improve the servo performance and to provide the real-time monitoring of the positioning error. When it is under bad servo conditions, the writing process can be stopped to avoid nontolerable track squeeze. The continuous servo signals are designed to be of moderate intensity at very low frequency, and its impact on data signal has been minimized. The linear density gap between the dedicated servo media and the conventional PMR media is able to be controlled within 3%. As the dedicated servo system keeps only around 100 wedges of track ID and sector ID at the data layer, the surface area saving at the data layer can break even in capacity. The dedicated servo technology together with TDMR readers is the key technology to achieve ultrahigh track density during both writing and reading processes.

Performance of perpendicular magnetic recording with track squeeze using bidirectional pattern-dependent noise prediction detector

In this study, we investigated the performance of perpendicular magnetic recording with squeezed tracks. Based on spinstand signals, we developed a signal generation module to generate the readback signals for tracks of various squeezed track width and bit length. To mitigate the drop in bit error rate (BER) as squeezed track width decreases, we implemented the bidirectional pattern-dependent noise prediction (BiPDNP) detector with low-density parity check codes. To improve the BiPDNP detector over earlier work, we modified the log-likelihood ratio computation. Our results show that our proposed BiPDNP detector improves the bit error rate, and allows smaller squeezed track width and smaller bit length for the same BER performance.

Performance Evaluation of Array-Reader-Based Magnetic Recording with a Drive-Tap Measurement Platform

Array-reader-based magnetic recording (ARMR) is one of the promising candidates to achieve areal density beyond 1 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in magnetic hard disk drives. Joint signal processing of multiple readback streams from the array-reader enables reliable data retrieval under high linear densities and large track squeezes, compared with the single-reader system. In this paper, we present the development of an ARMR evaluation platform employing a single-reader-based drive-tap to expedite performance evaluation under realistic drive environments. The drive-tap interface of the platform can control track squeeze and linear bit density conditions, and optimal bit-aspect ratio-based areal density capability (ADC) can be assessed with this platform. The resulting measurements show that, for the head-media used in this paper, the ARMR ADC gain over single-reader is around 6.7% with shingled magnetic recording at outer diameter (OD) and around 4.9% with perpendicular magnetic recording at OD.

Noise Characterization in High-Areal Density Perpendicular and Shingled Magnetic Recording

In this paper, we present a technique of noise analysis for perpendicular and shingled magnetic recording that is variance centric, i.e., with analysis focused on noise power. Measured noise from a home track viewpoint is decomposed into electronic noise, transition noise, nontransition noise, and inter-track interference components. The separation of media noise into transition and nontransition noise components is accomplished by a variance deconvolution technique. Experimental noise analysis is shown for cases of track squeeze, adjacent track erasure, and read head offset. Potential capacity gains are also assessed for the case of electronic noise improvement.

Thermally Assisted Magnetic Recording at 4 Tbit/in$^{2}$

Simulations of thermally assisted magnetic recording were carried out targeting an areal density of 4 Tbit/in2. The effects of the heat spot radius, maximum temperature during recording, Curie temperature and thermal conductivity of the recording layer on the written tracks were investigated. Triple track squeeze simulations were used to determine the minimum track pitch. The target of 4 Tbit/in2 may be realized if stringent conditions on the heat spot and thermal conductivity are met.

Signal Model for Shingled Magnetic Recording Based on Data Dependent Erase Band Analysis Under Track Squeeze

In this study, a new signal model for shingled magnetic recording (SMR) is developed based on the drive waveform analysis with squeezed side tracks. Shingled recording effects are modeled by the data dependent effective write width or erase band, and the readback signals are represented by the weighted sum of the individual target and side track signals without intertrack interference (ITI). The weights of the target and side track signals, denoted by the ITI coefficients, are estimated from the drive waveforms collected from the track squeeze tests. Near the track edge location, the ITI coefficients are reduced by the erase band and the data-independent electronic noise is effectively increased. In addition, the ITI coefficients also depend on the recorded data patterns due to such effect as track width broadening by low frequency data patterns. Cross-track ITI coefficient profiles are modeled by Gaussian read sensitivity and rectangular magnetization functions and applied for various SMR conditions. Two data sets of 50% and 44% bit transition rates are investigated to take into account data dependent erase band. Based on the proposed signal model, the cross-track bit error rate (BER) performance of the SMR are evaluated with the readback signals, synthesized for various read offsets. The results show that modulating the data bits to have fewer bit transitions may improve the on-track BER performance, while its off-track performance can be limited by the increased ITI due to the low frequency data patterns. In addition, BER performance of SMR can be improved by the ITI cancellation (ITIC) scheme.

Off-Track Error Probability Due to Adjacent Track Squeeze in Shingled Writing

Statistical investigations into off-track errors for shingled and conventional writing were carried out to reveal error behavior at high track densities. The residual data width in shingled writing tended to be wider than that in conventional writing for the same average track pitch. A comparison of the distributions of the calculated residual data width revealed that for conventional writing erroneous tracks were generated when the track was squeezed by both neighboring tracks, i.e., double squeezing. Shingled writing has an advantage in that this double squeezing does not occur and therefore a higher track density can be achieved.

Intertrack Interference Cancellation for Shingled Magnetic Recording

The intertrack interference in squeezed tracks of a magnetic hard disk drive is investigated based on captured waveforms. Specifically, extracted intertrack interference impulses and statistics are presented as a function of track squeeze and read offset. The bit-error-rate improvement of intertrack interference cancellation is shown based on simulations with captured drive waveforms. Architecture and implementation aspects for hard disk drives with intertrack interference cancellation are described.

Off-track error probability due to track squeeze in shingled writing

A statistical investigation of off-track errors for shingled and conventional writing was carried out to reveal the behavior at high track densities. The remanent data width in shingled writing was wider than that in conventional writing for the same average track pitch. Comparison of the distributions of the calculated remanent data width showed that for conventional writing erroneous tracks were generated when the track was squeezed by both neighboring tracks, i.e., double-squeezing. Shingled writing has an advantage in that this double-squeezing does not occur and therefore a higher track density is achievable.

Control Design for Self-Servo Track Writing Using a State-Space Disturbance Observer

Self-servo track writing is a process of writing servo bursts on a subsequent track using servo bursts from the previously written track as a reference. A fundamental obstacle in self-servo track writing is radial error propagation due to written-in repeatable runout (RRO) in the seed track and external disturbances. In this paper, we introduce a control scheme for the self-servo track writing process using a state-space disturbance observer. A linear quadratic Gaussian/loop transfer recovery controller with a state-space disturbance observer is designed for feedback control. In addition, feedforward control for the reference correction signal is designed to prevent radial error propagation. From simulation results, it turns out that the nonrepeatable runout (NRRO) component is a major source of AC track squeeze. The proposed disturbance observer is very effective in reducing the effect of the NRRO. The proposed control scheme is validated by simulation results which show improvement in the standard deviation and the AC track squeeze of the written tracks.

Track squeeze using adaptive intertrack interference equalization

Summary form only given. A new maximum likelihood sequence detection (MLSD) technique to reduce the effects of inter-track interference (ITI) is presented in this paper. As track densities increase, ITI and off-track interference (OTI) occur, and have detrimental effects on the performance of current MLSD decoders. A solution is to perform MLSD with an adaptive ITI equaliser constructed within the decoder trellis.

Magnetic recording demonstration over 100 Gb/in/sup 2/

Areal density growth has enabled hard disk drives to continually fulfill the demands from new enterprise, desktop, and consumer applications. The demonstration of 100 Gb/in/sup 2/ is another milestone that was achieved recently and is described in this paper. The recording demonstration employed fully integrated magnetic recording heads and thermally stable multilayer antiferromagnetically coupled (AFC) media through a commercially available channel chip. At a track density of 149 ktpi and linear density of 680 kbpi, the achieved off-track capability with 5% track squeeze was 10% of the track pitch, while maintaining a raw bit-error rate (BER) of 10/sup -4/ or better. This yielded an areal density of 101 Gb/in/sup 2/ with measured on-track raw BER of 5/spl times/10/sup -5/.

Disk drive integration issues for perpendicular recording

In addition to achieving high areal density, recording with perpendicular media must be compatible with other elements of disk drive recording. These include use of rotary actuation, the need for offtrack capability, stray field immunity, and servo control of the head position. Rotary actuation implies writing and reading with the head skewed at substantial angles relative to the track, while servo requires clean track edges. A soft magnetic underlayer, while improving the perpendicular write efficiency, exacerbates problems with head skew and stray fields. We have performed experiments related to drive integration, at the component level and by building drives incorporating perpendicular media. We report on the effect of head skew on the drive, particularly for servo and data recovery. We report on a beveled probe head design which minimizes the effect of skew on the servo pattern and track squeeze. In a 20 Gbit/in/sup 2/ drive, perpendicular media yields comparable servo and nearly equal areal density performance to longitudinal recording.

Magnetic signal distribution in disk drives

This work present an overview of the application of high resolution magnetic read back mapping within a functioning production disk drive. It is demonstrated how magnetic or mechanical media defects can be visualized and discriminated against each other and how track squeeze effects and servo pattern layout/integrity can be studied. Examples are given where only the native data signal is used for the analysis or how additional over writing is used to gain further information. Like in many other fields, the visualization of specific features enhances and contributes to the understanding of underlying mechanism.