Two-dimensional Magnetic Recording Research Articles

Comparison of Two-Reader and Three-Reader 2-D Magnetic Recording Systems

Two-reader two-dimensional magnetic recording (TDMR) system is expected to improve areal density by about 10%, although practical considerations may limit the realization of all of the available gain. As a result, for further growth, hard-disk drive industry has started to develop the second-generation TDMR system, consisting of three readers. Although a natural evolution of the first-generation, it brings its own issues and challenges. In this paper, we study the behavior and optimization of the three-reader system with a particular focus on reader placement and cross-track error-correction coding gains. In addition, we consider a lower cost two-reader solution with streaming intertrack interference cancellation as an alternative to employing three readers.

Relaxing Media Requirements by Using Multi-Island Two-Dimensional Magnetic Recording on Bit-Patterned Media

We introduce and investigate a new multi-island bit representation scheme with bit-patterned media (BPM) to increase the tolerance to the reader noise and the media noise compared with the standard BPM with a single island/bit and to granular media. The key idea is to use multiple islands in two dimensions to represent one bit. The redundancy in such a multi-island representation reduces the impact of the write in errors and decreases the effect of the media noise. We show that these multi-island representations offer improved bit error rate (BER), although only one readback sample per bit is used for equalization and detection. Here, a micromagnetic writing model and a read channel including media noise are used for the simulations, where optimized writer and reader architectures with one, two, and three read elements are investigated. Simulations at a 1 ${\rm Tb}/{\rm in}^{2}$ channel bit density, for the target BER of $10^{-2}$ and readback with two readers, indicate that the readback of BPM recording (BPMR) based on 1 ${\rm Tdot}/{\rm in}^{2}$ (1 dot/bit), 2 ${\rm Tdots}/{\rm in}^{2}$ (2 dots/bit), and 4 ${\rm Tdots}/{\rm in}^{2}$ (4 dots/bit) island density can tolerate 1, 3.1, and 3.3 dB more reader noise compared with the granular media. For readback with two readers, at $10^{-2}$ target BER, writing with 4 ${\rm Tdots}/{\rm in}^{2}$ and 2 ${\rm Tdots}/{\rm in}^{2}$ island density can tolerate the media noise (modeled by island position jitter and island size fluctuation) levels of 11% and 8.5% compared with the 1 ${\rm Tdot}/{\rm in}^{2}$ with the media noise level of 5%. When the channel bit density is increased to 1.5 ${\rm Tb}/{\rm in}^{2}$ , such a multi-island approach can still provide improved reader noise and media noise tolerances compared with the standard BPMR with one bit per island and to granular media at the same density.

An Intertrack Interference Subtraction Scheme for a Rate-4/5 Modulation Code for Two-Dimensional Magnetic Recording

The performance of a rate-4/5 modulation code is evaluated in two-dimensional magnetic recording (TDMR) channels, where a magnetic medium is described by a discrete Voronoi model, and the two-dimensional sensitivity function of the reader is adopted to generate the TDMR readback signal. Since the read-head sensitivity function covers many tracks, it causes intertrack interference (ITI) that can deteriorate the system performance. Therefore, this letter proposes an ITI subtraction scheme in conjunction with the rate-4/5 modulation code in a coded TDMR channel to mitigate the ITI effect embedded in the readback signals before performing an iterative decoding process. We also investigate the data-dependent readback amplitude distributions and evaluate the TDMR system performance via computer simulation. Results show that the proposed scheme helps improve the TDMR system performance, especially when the areal density is high.

Investigation Into Harmful Patterns Over Multitrack Shingled Magnetic Detection Using the Voronoi Model

Two-dimensional magnetic recording 2-D (TDMR) is a promising technology for next generation magnetic storage systems based on a systems-level framework involving sophisticated signal processing at the core. The TDMR channel suffers from severe jitter noise along with electronic noise that needs to be mitigated during signal detection and recovery. Recently, we developed noise prediction-based techniques coupled with advanced signal detectors to work with these systems. However, it is important to understand the role of harmful patterns that can be avoided during the encoding process. In this paper, we investigate the Voronoi-based media model to study the harmful patterns over multitrack shingled recording systems. Through realistic quasi-micromagnetic simulation studies, we identify 2-D data patterns that contribute to high media noise. We look into the generic Voronoi model and present our analysis on multitrack detection with constrained coded data. We show that the 2-D constraints imposed on input patterns result in an order of magnitude improvement in the bit-error rate for the TDMR systems. The use of constrained codes can reduce the complexity of 2-D intersymbol interference (ISI) signal detection, since the lesser 2-D ISI span can be accommodated at the cost of a nominal code rate loss. However, a system must be designed carefully so that the rate loss incurred by a 2-D constraint does not offset the detector performance gain due to more distinguishable readback signals.

Simulation of Expected Areal Density Gain for Heat-Assisted Magnetic Recording Relative to Other Advanced Recording Schemes

In this paper, we review our latest progress for heat-assisted magnetic recording (HAMR) systems. The temperature distribution has been computed using the finite difference time domain technique and by solving the heat diffusion equation. Magnetic behavior has been calculated using a renormalized cell technique (granular media) and atomistic simulation [bit-patterned media (BPM)]. We find that the ultimate density of HAMR on granular media depends greatly on grain size, with a 5 nm grain pitch yielding $\sim 4$ Tb/in2. We were unable to exceed $\sim 5.8$ Tb/in2 on BPM owing to excessive heating of the adjacent track, causing adjacent track erasure to date. Shingled recording on granular media, using two dimensional magnetic recording (TDMR) detection depends greatly on the mechanical systems, with likely user densities reaching $\sim 2.5$ Tb/in2. Finally, shingled recording without heat assist on BPM yielded an unexpectedly high density of $\sim 8$ Tb/in2.

Optimization of Magnetic Read Widths in Two-Dimensional Magnetic Recording Based on Micromagnetic Simulations

Optimization of magnetic read widths (MRWs) and reader positions in two-dimensional magnetic recording (TDMR) is explored in order to maximize an areal density (AD) gain over shingled magnetic recording (SMR). The optimization is based on micromagnetic waveforms from 511-bit maximal-length pseudorandom binary sequence (PRBS) patterns. The identical MRWs for two readers are initially considered and the best reader offset is found in order to achieve the largest AD gain. Given the best reader offset, the MRWs for the two readers are changed to further enhance the AD gain. A software channel is utilized to identify the major contribution of the TDMR AD gain.

Generalized Partial Response Equalization and Data-Dependent Noise Predictive Signal Detection Over Media Models for TDMR

Two-dimensional magnetic recording (2-D TDMR) is an emerging technology that aims to achieve areal densities as high as 10 Tb/in2 using sophisticated 2-D signal-processing algorithms. High areal densities are achieved by reducing the size of a bit to the order of the size of magnetic grains, resulting in severe 2-D intersymbol interference (ISI). Jitter noise due to irregular grain positions on the magnetic medium is more pronounced at these areal densities. Therefore, a viable read-channel architecture for TDMR requires 2-D signal-detection algorithms that can mitigate 2-D ISI and combat noise comprising jitter and electronic components. Partial response maximum likelihood (PRML) detection scheme allows controlled ISI as seen by the detector. With the controlled and reduced span of 2-D ISI, the PRML scheme overcomes practical difficulties such as Nyquist rate signaling required for full response 2-D equalization. As in the case of 1-D magnetic recording, jitter noise can be handled using a data-dependent noise-prediction (DDNP) filter bank within a 2-D signal-detection engine. The contributions of this paper are threefold: 1) we empirically study the jitter noise characteristics in TDMR as a function of grain density using a Voronoi-based granular media model; 2) we develop a 2-D DDNP algorithm to handle the media noise seen in TDMR; and 3) we also develop techniques to design 2-D separable and nonseparable targets for generalized partial response equalization for TDMR. This can be used along with a 2-D signal-detection algorithm. The DDNP algorithm is observed to give a 2.5 dB gain in SNR over uncoded data compared with the noise predictive maximum likelihood detection for the same choice of channel model parameters to achieve a channel bit density of 1.3 Tb/in2 with media grain center-to-center distance of 10 nm. The DDNP algorithm is observed to give $\sim$ 10% gain in areal density near 5 grains/bit. The proposed signal-processing framework can broadly scale to various TDMR realizations and areal density points.

Soft Intertrack Interference Cancellation for Two-Dimensional Magnetic Recording

We propose a detection strategy for the two-dimensional magnetic recording channel with multiple read heads, in which intertrack interference (ITI) is mitigated by a combination of linear combining and soft interference cancellation. The proposed strategy reduces the detection problem to a series of traditional one-dimensional detection problems, so that the existing iterative detection strategies based on turbo equalization may be leveraged. The performance of the proposed detector is compared with a previously reported detector, in which the ITI is suppressed linearly via a multiple-input equalizer. Numerical results demonstrate that the proposed detector outperforms the linear detector regardless of whether or not media noise is dominant, and further that the proposed detector is able to recover data from more tracks than is otherwise possible.

Novel Joint Detection and Decoding Schemes for TDMR Channels

Two-dimensional magnetic recording (TDMR) relies on advanced signal processing and coding techniques to enable storage of data at higher densities. Various detection and decoding schemes have been proposed in the literature. In this paper, a recently proposed joint detection and decoding scheme known as the joint Viterbi detector decoder (JVDD) is presented for TDMR channels. The JVDD operates on a trellis and attempts to find a sequence in the trellis that belongs to a legal codeword with minimum metric. It does so by using a metric thresholding and parity checking technique over the same trellis structure. The iterative detector/decoder consisting of a soft-output Viterbi algorithm detector and low-density parity check decoder is used as a performance benchmark for the JVDD. In this paper, we perform the comparison with various equalization/detection techniques consisting of: 1) a 1-D general partial response equalization scheme followed by a 1-D-detector as used in today’s Partial Response Maximum Likelihood Detection (PRML) channels; 2) a 2-D equalization scheme that equalizes to a 1-D target [fully equalizing away the intertrack interference (ITI)], followed by a series of parallel 1-D-detectors to handle the intersymbol interference (ISI); and 3) a 2-D-equalization scheme followed by 2-D detector to combat both ITI and ISI.

Two-Dimensional Magnetic Recording With a Novel Write Precompensation Scheme for 2-D Nonlinear Transition Shift

This paper proposes a novel write precompensation scheme that adjusts the current magnitude, not the timing, to mitigate the serious 2-D nonlinear transition shift (NLTS) during the shingled writing process in a two-dimensional magnetic recording (TDMR) system. This scheme calculates the needed optimum write field for the current transition being written by subtracting the demagnetizing field produced by the previous written bits from a fixed write field and determines the optimum writing current based on the minimum mean-squared-error criterion between the needed field and the available fields within an established field library. Such a write precompensation scheme is tested with a micromagnetic writer and exchange coupled composite media, and readback with a rotated reader array combined with a read channel. Simulation indicates that the proposed write precompensation technique can decrease bit error rate compared with the case without write precompensation and approach the performance of a system with negligible NLTS. Correspondingly, the maximum user areal density is increased from 3.2 to 4.1 Tbits/in $^{\mathrm {\mathbf {2}}}$ for a TDMR system with 8 nm Voronoi grains due to the proposed write precompensation.

2-D Magnetic Recording: Progress and Evolution

Two-dimensional magnetic recording (TDMR) was proposed in 2008 as a means of pushing beyond 1 Tbit/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> areal-density yet staying with relatively conventional magnetic components. A density of approximately 1 Tbit/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> is considered to be the limit for conventional perpendicular magnetic recording. The original concept for TDMR has evolved greatly since it was originally proposed. This evolution has occurred as more information has become available about the characteristics of shingled tracks and 2-D read-back signals and as constraints have become apparent in the implementation of recording components and electronics. With today's technology, the projected gains are smaller than originally envisioned. As a very practical first step that introduces TDMR and provides modest gains without incurring undue complexity or cost, this paper focuses on the idea of a stacked two-element reader operating on shingled magnetic recording tracks and a two-input equalizer feeding a standard data detector. The dual-reader gains are assessed based on combining waveforms captured from a single reader at many off-track positions, track-pitches, and radii.

Analysis of magnetic noises in two-dimensional magnetic recording readers

A model is introduced in this paper to describe the effects of thermal magnetic noises in the read sensor under off-track reading conditions taking into account the influence of the magnetic field generated by the recorded magnetization patterns on the medium. Our numerical studies show that there exist additional noises in the hard biased reader, depending on recorded patterns on the magnetic media. The influence of the recorded magnetic patterns is not only on the variation in reader resistance but also on the high frequency noises caused primarily by the thermal fluctuations. The theoretical analysis is based on the micromagnetic modeling of the state of magnetization in read sensor taking into account of its external magnetic fields due to both the hard bias and the media magnetization patterns. The effect of the thermal agitation of the gyromagnetical precession of magnetizations is evaluated by considering the angular magnetic noise.

Spinstand demonstration of areal density enhancement using two-dimensional magnetic recording (invited)

Exponential growth of the areal density has driven the magnetic recording industry for almost sixty years. But now areal density growth is slowing down, suggesting that current technologies are reaching their fundamental limit. The next generation of recording technologies, namely, energy-assisted writing and bit-patterned media, remains just over the horizon. Two-Dimensional Magnetic Recording (TDMR) is a promising new approach, enabling continued areal density growth with only modest changes to the heads and recording electronics. We demonstrate a first generation implementation of TDMR by using a dual-element read sensor to improve the recovery of data encoded by a conventional low-density parity-check (LDPC) channel. The signals are combined with a 2D equalizer into a single modified waveform that is decoded by a standard LDPC channel. Our detection hardware can perform simultaneous measurement of the pre- and post-combined error rate information, allowing one set of measurements to assess the absolute areal density capability of the TDMR system as well as the gain over a conventional shingled magnetic recording system with identical components. We discuss areal density measurements using this hardware and demonstrate gains exceeding five percent based on experimental dual reader components.

Performance evaluation of signal dependent noise predictive maximum likelihood detector for two-dimensional magnetic recording read/write channel

Two-dimensional magnetic recording is affected by the inter-track interference (ITI) from the adjacent tracks. We investigate the improvement of partial response maximum likelihood (PRML) systems with signal dependent noise predictor (SDNP) in the bit error rate performance. The systems reduce the influence of ITI by two dimensional finite impulse response filter using the waveforms reproduced by triple readers from the adjacent tracks. The results show that the SDNP provides larger improvement to PR(2,6,1)ML system compare with PR1ML system.

Bit error rate performance for head skew angle in shingled magnetic recording using dual reader heads

The two-dimensional magnetic recording (TDMR) exploits two-dimensional signal processing using the neighboring read-back waveforms. We allocate dual readers for the intended tracks and evaluate the effects of head skew angle on the bit error rate performance in partial response class-I maximum likelihood system with a two-dimensional finite impulse response filter using two read-back waveforms under TDMR R/W channel specifications of 4 Tbit/in.2. The results show that the effect of positive skew angle is larger than that of negative skew angle, and the center of skew angle should be shifted to minus direction.

Evaluation of Multiple Reader Location for TDMR R/W Channel

The two-dimensional magnetic recording (TDMR) by shingled magnetic recording needs two-dimensional (TD) signal processing using the multiple read-back waveforms. In this paper, to allocate dual readers (reader 1 and 2) for the intended tracks, we evaluate the bit error rate (BER) performance of a low-density parity-check coding and iterative decoding system with a TD-finite impulse response equalizer under TDMR R/W channel specifications of 4 Tb/in2. Here, the TDMR R/W channel is modeled using a discretized granular medium and the writing process based on a slope model considering the media switching field angle given by the Stoner-Walfarth reversal mechanism. The result shows that to affect the good BER performance, the reader 1 should be placed slightly toward the next track from the center of the home track, and the reader 2 should be placed around the boundary between the home and following tracks.

Two-Dimensional Magnetic Recording Using a Rotated Head Array and LDPC Code Decoding

Two-Dimensional Magnetic Recording Using a Rotated Head Array and LDPC Code Decoding

Study of Two-Dimensional Magnetic Recording System Including Micromagnetic Writer

Study of Two-Dimensional Magnetic Recording System Including Micromagnetic Writer

A Study of SMR/TDMR With a Double/Triple Reader Head Array and Conventional Read Channels

Two-dimensional magnetic recording is a novel technology proposed to extend the life of conventional granular magnetic recording (CGMR). It is the readback counterpart technology to shingled magnetic recording (SMR) that is being pursued by the industry today. In SMR, a 2-D block of bits is written as a single unit: modifying any bit within the block requires a modification to the entire 2-D block. In TDMR, by comparison, a 2-D block is read and processed as a single unit. Latency during writing of a 2-D block is not a serious problem as the data is buffered in the RAM. Latency during readback, however, results in a performance degradation to the user who has to wait before getting his file. If there is a single reader head, 2-D readback necessitates multiple revolutions of the disk leading to a significant latency, which may not be acceptable to the user and is one of the reasons the industry is hesitating for 2-D magnetic recording (TDMR). Recent events have seen companies working on the development of multiple (two or three) reader heads that read multiple tracks simultaneously with a single pass of the head. Such double/triple reader head arrays could be used to support TDMR, obviating the need for multiple revolutions of the media and reducing the read latency. An alternative use for the multireader heads would be in an SMR scenario, where only a single track is readback at a time. In this case, the extra readers could be used to estimate and mitigate the intertrack interference coming from the adjacent tracks. The difference between these two modes is the number of tracks detected with each head pass. For TDMR there would be two or three tracks detected with a single pass while for SMR there would be just one. In this paper, we study and optimize the impact of parameters of the double/triple reader head array being used in the SMR and TDMR readback scenarios. The key difference between these scenarios is in the design of the GPR equalizer and we compare and contrast the recording system varying several key design parameters.

An iterative inter-track interference mitigation method for two-dimensional magnetic recording systems

At high recording density, the readback signal of two-dimensional magnetic recording is inevitably corrupted by the two-dimensional (2D) interference consisting of inter-symbol interference and inter-track interference (ITI), which can significantly degrade the overall system performance. This paper proposes an iterative ITI mitigation method using three modified 2D soft-output Viterbi algorithm (2D-SOVA) detectors in conjunction with an iterative processing technique to combat the 2D interference. The codeword of the outer code is divided and then written on three separate tracks. For every iteration, all 2D-SOVA detectors exchange the soft information to improve the reliability of the a priori information and use it in the branch metric calculation, before feeding the refined soft information to the outer decoder. Simulation results show that the proposed method outperforms the conventional receiver and the existing partial ITI mitigation method.