Perpendicular Recording System Research Articles

A Graded Precompensation Scheme by Pattern Classification on Nonlinear Transition Shift for Perpendicular Magnetic Recording

To achieve higher magnetic recording areal densities, signal distortions caused by the crosstalk among consecutive magnetized transitions that lead to nonlinear transition shifts (NLTS) must be eliminated. We calculated and sorted the transition shifts of different recorded patterns and classified corresponding patterns based on an analytical method. We proposed a graded write precompensation scheme according to the classified patterns and compared the detecting performance of a perpendicular magnetic recording system using the graded compensation and traditional pattern-aware compensation during the writing process. Simulation results show our proposed graded precompensation can effectively mitigate the nonlinear distortion effect caused by NLTS and outperforms the traditional pattern-aware compensation method. The detecting BER decreases by 14.7% when the SNR of electronic noise is only 20 dB and the bit length is 10nm.

Perpendicular Interlaced Magnetic Recording

Perpendicular magnetic recording is the current generation hard disk drive technology, which enables the continued and significant growth of large storage systems in data centers. There are currently two common architectures for the layout of tracks in perpendicular hard disk drives: conventional magnetic recording (CMR) and shingled magnetic recording (SMR). In CMR, any track can be written at any time and neighboring tracks do not intentionally overlap. In SMR, the tracks are written sequentially in bands with the tracks intentionally overlapped like shingles on a roof. In this article, interlaced magnetic recording (IMR) architecture is applied with a perpendicular magnetic recording system. With Perpendicular Interlaced Magentic Recording (PIMR), we observed 2%–3% increase in areal density for single writer PIMR and 7%–20% increase in areal density for two writer PIMR over CMR whereas with SMR, we observed 19% increase in areal density over CMR.

Baseline Popping Detection and Correction Algorithms for Perpendicular Magnetic Recording System

Hard disk drives (HDDs) employ the magneto-resistive (MR) head to sense the change in magnetic flux via the transitions of magnetization pattern, resulting in a readback signal. Thus, head instability plays an important role on the reliability of HDDs because it can deteriorate the system performance considerably. Baseline popping (BLP) is one of the crucial problems caused by the head instability, whose effect can distort the readback signal to the extent of causing a sector read failure. This paper proposes three BLP detection and correction algorithms for a perpendicular magnetic recording (PMR) system. Specifically, to suppress the BLP effect experienced in the readback signal, the first algorithm is based on an averaging filter and a threshold detector; the second one relies on the estimated BLP signal obtained from a linear curve fitting technique; and finally the third one uses two sequence detectors running in parallel. Experimental results indicate that the third algorithm performs better than the other schemes because it can detect and correct the BLP better than the others, especially when the peak BLP amplitude is large.

Reducing the Thermal Asperity Effect in Perpendicular Magnetic Recording System

In this paper, we propose a new method to estimate and reduce effectively thermal asperity (TA) in the perpendicular recording channels with state trellis. The proposed method comprises of recursively applying a previous noise estimate to a calculation of a current noise estimate to eliminate the effects of TA from the equalized signal. With partial response a maximum-likelihood detector base on partial-response (PR) target with DC-full component can improve the system performance by more than 50% when TA occurs. Unlike previous studies of TA cancellation or suppression, this method is different because we dont use PR targets with non-dc components and high pass filter.

Cross-track pulse shape and nonlinear loss as a function of frequency and side track erasure in perpendicular magnetic recording systems

Many previous publications have shown signal amplitude of magnetic recording systems decrease as cross-track run-out or linear density increases. This study expands on this work by characterizing cross-track transition width (T50) and nonlinear partial erasure (NLPE) changes with linear density, read-back location, and track edge erasure. Both T50 and NLPE data shown in this paper were calculated from filtered odd harmonic amplitude cross-track profiles. This analysis is novel, in that it establishes a model which describes why NLPE and T50 change by read-back location, linear density, and track-edge erasure due to non-uniform first and third harmonic widths.

Head Instability Detection for Testing Process in Perpendicular Magnetic Recording System

During hard disk drive (HDD) testing process, the magneto-resistive read (MR) head is analyzed and checked if the head is defective or not. Baseline popping (BLP) is one of the crucial problems caused by head instability, whose effect can distort the readback signal to the extent of causing possible sector read failure. Without BLP detection algorithm, the defective read head might pass through HDD assembling process, thus producing an unreliable HDD. This situation must be prevented so as to retain customer satisfaction. This paper proposes a simple (but efficient) BLP detection algorithm for perpendicular magnetic recording systems. Results show that the proposed algorithm outperforms the conventional one in terms of both the percentage of detection and the percentage of false alarm, when operating at high signal-to-noise ratio.

A Study of Nonlinear Partial Erasure Versus Frequency and Side Track Erasure in a Modern Perpendicular Magnetic Recording System

The following study examines nonlinear signal losses in a perpendicular magnetic recording (PMR) system using tunneling magneto-resistive heads (TuMR) and exchange coupled multi-layered media capable of operating at recording density greater than 400 GB/in2. This study differs from previous total nonlinear distortion (TNLD) and nonlinear transition shift (NLTS) studies in that it focuses on nonlinear partial erasure (NLPE) using constant tones. Constant-tone NLPE is often considered negligible compared to pattern-dependent NLTS, but this study shows this is not the case at high linear densities. This study will further show that NLPE behavior at low linear density appears similar when using high density writes combined with track edge trimming. This novel finding suggests on-track NLPE and cross-track data width may simply be different aspects of track edge curvature and/or percolation.

Transition Noise Analysis of Recording Media With a Soft Underlayer (SUL) and an Antiferromagnetic Soft Underlayer (AF-SUL)

A perpendicular magnetic recording system has been modeled for the purpose of analyzing transition noise, particularly “jitter.” Conventional perpendicular media and “soft” exchange coupled composite (ECC) media with a conventional soft underlayer (SUL) and an antiferromagnetic soft underlayer (AF-SUL) have been investigated using micromagnetic simulation. The fast Fourier transform (FFT) technique and graphics processing unit (GPU)-based computing have been used to reduce the intensive computation time for magnetostatic interactions between the head, SUL, and recording layer. Interestingly, the jitter is always less dependent on reader offset from track center with the AF-SUL. Jitter for ECC media is also shown to depend less strongly on reader offset than for conventional media. The transition center deviation at the optimal anisotropy for both recording layers is lower with the AF-SUL at both linear densities considered. We further find that the track center moves alternately with direction of fringing field as expected from magnetostatic considerations.

Writing Process Induced Media Noise Measurement

Writing processes can cause extra medium noise, which is the dominant noise in the perpendicular recording system. This work describes the methodology of using averaging methods to measure the writer footprints and its noise profiles, which are recorded at different writing currents on a Spin-stand. Two different commercial heads have been used for the evaluation. The test is performed on a 2.5 inch commercial disc at the middle diameter (MD) location, rotating at 5400 RPM spindle speed, and at 0 skew angle. The center track is DC erased over an AC erased background and then recorded with a repetitive footprint data pattern and an alignment data pattern. Multiple revolution averaging and down track footprint averaging are used to retrieve the noise free footprint profile of the writer. Using the noise free footprint profile, the statistical characteristics of noise can be derived from the individual noisy footprint which shows the media noise characteristics induced by writing process. Using footprint and noise profile data, signal to noise ratio (SNR) for regions near the trailing edge of the writer profile are calculated and suggests that the ideal writing condition for writer A and B to be 55 and 25 mA respectively.

The Limits of the Wallace Approximation for PMR Recording at High Areal Density

This paper discusses the validity of the Triple Harmonic method and the Wallace readback model for the determination of head-disk spacing variations in today's perpendicular recording (PMR) systems at >; 500 Gb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> areal density. For the recording system investigated here, and in the limit of high frequency, approximately beyond the frequency corresponding to the 2 T (0011) pattern, this method fails, as head and electronic noise start to interfere. In the low-frequency range of PMR systems, the Wallace equation does not apply any longer. Accurate magnetic spacing variations can be obtained by using the FFT of the intensity ratio of the readback signals, and fitting with the appropriate algebraic expression. Using this method, a repeatability of less than 0.1 nm was obtained, by using a random data pattern of only four 512 bytes data sector long.

Performance evaluation of detectors for perpendicular recording systems

This paper evaluates the performance of detectors with d = 1 constraint for perpendicular recording systems under assumptions of nonlinear distortion. Nonlinear distortions considered are transition jitter and DC-offset. Detectors used in this study are the partial response maximum likelihood (PRML), noise-predictive maximum likelihood (NPML), fixed delay tree search with decision feedback (FDTS/DF), dual decision feedback equalizer (DDFE) and multilevel decision feedback equalizer (MDFE). Their performances are discussed in terms of mean squared error (MSE) and noise power spectra of equalizer at discrimination point and similarity of frequency response between overall system and partial response (PR) target polynomial.

High Density Perpendicular Recording With Wrap-Around Shielded Writer

Wrap-around shielded writers combining both trailing shield and side shield have been studied both by modeling and by experiments. WAS design is demonstrated to have superior performance over its predecessor trailing shielded writer in perpendicular magnetic recording systems. While maintaining the down-track performance, the side shield significantly reduces the side fringing field, thus enabling high track densities. In this paper, various aspects of WAS design are investigated. Issues unique to side shielded design such as near and far track erasure are also discussed in detail. An outlook of future PMR writer and recording systems design is presented at the end.

Trapping Electron Assisted Magnetic Recording

Moving towards 10 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> areal density, finding a proper recording scheme with enough write-ability is the most challenging task of a magnetic recording system. Some recording schemes with enhanced write-ability, such as HAMR, MAMR, graded media, etc., have been proposed to achieve higher recording density. Here we propose a new alternative approach for enhanced writing-trapping electron assisted magnetic recording (TEAMR). In the TEAMR configuration, an electrical bias is applied to the main pole of the write head with the disk media and the other parts of head slider grounded. As the main pole area is very small, the electrostatic force produced by electrical potential is a few orders smaller than the air bearing force at the rear pad. Therefore, it will not affect the flying performance of the head slider. At the nanometer head media spacing, a very strong electrical field is produced in the head media interface. This strong electrical field will cause free electrons to accumulate (be trapped) at the interfacial surfaces of metallic magnetic grains. These trapped electrons are localized in the surface atoms of magnetic grains and will alter the valance-electron band filling of those surface atoms. For many magnetic materials, the extra band-filling electrons reduce the magnetic anisotropy energy and make it easier to be magnetically switched. In this work, the TEAMR effect was proved by the experiment study on Co alloy based commercial disk media. The first principle calculation on L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ordered FePt crystal shows that the magnetic anisotropy can be reduced to zero with around 0.38 electrons trapped into 1 unit cell of FePt. Further increase in trapped electrons will change the magnetic easy axis from out-of-plane to in-plane, which is considered as a negative magnetic anisotropy. With the magnetic anisotropy reduction at the surface atoms of each grain, micromagnetic simulation result shows that the effective switching field can be reduced to around 11% of anisotropy field for a 1.6 ? 1.6 ? 3.2 nm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> FePt grain. Thus TEAMR can be another good candidate for energy assisted recording requiring very little modification to the current perpendicular magnetic recording system.

Analysis of Nonlinear Transition Shift and Write Precompensation in Perpendicular Recording Systems

In high density perpendicular magnetic recording channels, nonlinear transition shift (NLTS) is one of the distortions that can degrade the system performance. Write precompensation is a standard method used to combat the negative effect of NLTS. In this paper, we present an analysis of the bit-error-rate (BER) for perpendicular recording systems with NLTS and write precompensation. Media jitter noise and additive white Gaussian noise are also considered in the model. A BER lower bound is derived, as well as a more easily computed estimate of the bound. The write precompensation values that numerically minimize the estimate of the BER lower bound prove to be very close to those found using Monte-Carlo channel simulation. We then apply these methods to the design of multilevel precompensation schemes, for which the optimization of precompensation values by Monte-Carlo channel simulation is computationally infeasible. The results show that for higher recording densities subject to increased ISI and noise, the use of more complex precompensation schemes does not significantly improve the system performance.

Development of high density magnetic recording media for hard disk drives: materials science issues and challenges

<title/>The tremendous increase in areal density of hard disk drives is mainly ascribed to harmonic development between magnetic recording media and heads in their scaling, especially allowing a commercial transition from the longitudinal to perpendicular recording system. This paper reviews the main features, recent breakthroughs and future potentials of both the longitudinal and the perpendicular media from a viewpoint of materials science. Special attention is firstly paid to the ‘trilemma’ problem for the media, i.e. the compromise among writability, thermal stability and signal to noise ratio (SNR). The evolution of media materials these years are then addressed with emphasis on the thermodynamic origin of magnetically induced phase separation of Co-Cr based alloys, which governs media noise and coercivity, and its applications to the current longitudinal media. The materials challenges for media to achieve 500 Gb in.−2 and above are further predicted from the viewpoints of thermal stability improvement and microstructure control of media materials, and their engineering issues have been discussed for the current Co-Cr based alloys, potential FePt and CoPt ordered, phase separated Co-W based alloys and magnetic rare earth compounds. Finally, the future media approaching 1 Tb in.−2 and beyond are addressed with respect to the principles, progress, engineering challenges and future directions.

Thermal asperity suppression based on least-squares fitting in perpendicular magnetic recording systems

Thermal asperity (TA) causes a major problem in data detection process. Without the TA detection and correction algorithm, the system performance (even with perfect synchronization) can be unacceptable, depending on how severe the TA effect is. This paper proposes a new method to suppress the TA effects in perpendicular magnetic recording channels. The TA detection is a threshold-based approach, while the TA correction is done by averaging the readback signal and applying a least-squares fitting technique to estimate the TA signal. Then, the corrected readback signal is obtained by subtracting the TA-affected readback signal by the reconstructed TA signal. Results indicate that the proposed method performs better than the existing one in terms of bit-error rate and is robust to changes in the peak TA amplitude.

Stress Induced Permanent Magnetic Signal Degradation of Perpendicular Magnetic Recording System

Model scratches of the size found in hard disk drives are produced under controlled conditions at a series of applied loads on both longitudinal magnetic recording (LMR) media and perpendicular magnetic recording (PMR) media using a diamond tip. The scratches are created at low speed, eliminating thermal considerations from the interpretation of the media response. Nanoindentations are produced as well. The scratches and indentations are characterized by atomic force microscope (AFM), magnetic force microscope (MFM), and also by the same magnetic reader and writer used in an integrated hard disk drive (HDD). A comparison of the response of PMR and LMR media shows the PMR media to have larger scratches and greater magnetic signal degradation than LMR media for a given scratch load. The extent of magnetic damage, as measured by MFM, is greater than the extent of surface mechanical damage, as measured by AFM. Analysis of scratches using the HDD reveals that the magnetic damage is irreversible and permanent damage in magnetic layer, which is confirmed by cross section transmission electron microscope image. The experiments reveal the mechanism for magnetic scratch erasure in the absence of thermal effects. This understanding is expected to lead to improved designs for mechanical scratch robustness of next-generation PMR media.

Message-passing iterative decoding between detector and RSC code decoder for PMR channel

For higher density perpendicular magnetic recording (PMR) systems, it is difficult to expect an improvement in performance when only partial response maximum likelihood (PRML) or noise predictive maximum likelihood (NPML) detection is used. Hence, we consider a system that performs better than either PRML or NPML. We concatenate a channel detector using a recursive systematic convolutional (RSC) code decoder and a message-passing algorithm. The RSC code itself is simple but its performance is inferior to other iterative decoding methods. Therefore, we used channel iteration between the channel detector and the RSC decoder in order to improve performance. We also used a precoding and random interleaver. We evaluated the performance on the PMR channel with jitter noise.

Perpendicular Recording Technology: From Research to Commercialization

Perpendicular magnetic recording was a long-awaited technology to break the areal density limit of the longitudinal recording system. Through the research on high squareness CoPtCrO/Ru recording media, we have established the intrinsic design concept of stable perpendicular magnetic recording in which the high nucleation field of perpendicular magnetic media supports the very stable magnetization against disturbance fields in recording processes. By intensive study of the perpendicular magnetic recording integration into the 2.5-in 50-80 GB/platter test hard disk drives (HDDs), we have successfully developed the first 1.8-in 40 GB/platter HDD product featuring the perpendicular magnetic recording technology at 133 Gbit/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> recording density. Thermal stability and spacing robustness in the writing process of the perpendicular recording system were also found to be great advantages of the future portable HDD systems.

Error Rate Expression for Perpendicular Magnetic Recording

An expression for estimating error rate in a perpendicular magnetic recording system is developed. The probability of error is estimated for a dominant dibit error event. Noise includes stationary white Gaussian head/electronics noise and nonstationary colored medium transition noise. Error rate is determined for a variety of parameter changes. In particular, it is shown how two systems with the same total signal-to-noise ratio can have different error rates. Expansion of the model to include additional signal and noise effects as well as the evaluation of different error events is discussed.