Performance of GPRML-AR system in perpendicular magnetic recording channel with thermal decay

The bit-error rate (BER) performance of the generalized partial response maximum likelihood with autoregressive (GPRML-AR) channel model system in perpendicular magnetic recording (PMR) channel with thermal decay is obtained. The 128/130(0,16/8) run-length-limited (RLL) code is used as a recording code. The GPR channel consists of the PR1 channel followed by the reduction circuit of predicted noise. The BER performance is evaluated by computer simulation using a thermal decay model. The model has been obtained by using an approximate equation that represents amplitude degradation of the reproducing waveform with elapsed time based on the experimental data for CoPtCr-SiO₂ PMR media. The Viterbi detector with an AR channel model is employed. Furthermore, long-term degradation of the required SNR to achieve a BER of 10⁻⁴ with elapsed time is obtained and the performance is compared with that of PR1ML system. The results show that the poorer the thermal stability of the medium becomes, the larger the SNR gain of the GPR1ML-AR system over the PR1ML system becomes. The SNR gain also increases with elapsed time.     

Dependence of error patterns and BER performance on write pre-compensation in GPRML channels

Nonlinear transition shift (NLTS) influences the performance of PRML channels in perpendicular magnetic recording. For NLTS, it is thought that write pre-compensation is useful to write the transition at the intended position. In this paper, we study the influence of NLTS on the BER performance of GPRML channels and show the appropriate amount of pre-compensation for GPRML channels both with and without a post-processor (PP) to achieve better BER performance. The results show that the widening the shortest bit length by NLTS brings the BER improvement to GPRML channel and the write pre-compensation is useful for a GPRML channel with PP.     

Micromagnetic modeling for heat-assisted magnetic recording

Heat-assisted magnetic recording (HAMR) is one of the candidate systems beyond the perpendicular recording technology. Here, a micromagnetic model and a heat transfer model are introduced to study the heating and cooling processes in the HAMR media; then, by integration of the SPT head and the laser heating source, the recording performance is simulated and investigated on a single track at an area density of 1 Tb/in². In the HAMR system, the temperature in the medium under the laser wave guide is increased by heating, and decreased by air bearing and heat conduction when the write process really occurred. The target of this study is to find the proper design of the head-laser assembly for optimum recording. It is found that the proper distance between the laser wave guide and the head's main pole rear/front edge is only 41.4/1.4 nm for optimum recording performance.     

Iterative decoding algorithm using attenuated information in PMR with bit-patterned medium

In this paper, a low-density parity check (LDPC) coding and iterative decoding system is studied in perpendicular magnetic recording (PMR) with a bit-patterned medium. We propose the efficient iterative decoding algorithm attenuating reliability information, and evaluate the sector error rate (SER) performance of the LDPC coding and iterative decoding system by R/W computer simulations. It is clarified that the proposed systems provide better SER performances than that of the conventional system at an areal density of 1 Tbit/in² and can also prevent the error floor which appears in the performance of conventional system.     

Read channel simulation based on microtrack and micromagnetic modeling for 1 Tb/in magnetic recording

Channel design for ultra-high-density perpendicular magnetic recording requires fast and precise modeling to generate readback signals corrupted by media noise. In this paper, we present a geometric-dependent approach to model random readback pulses, where a three-dimensional head and media combination for 1 Tb/in² density design is considered in the micromagnetic simulation. A systematic approach is developed to extract media noise statistics from micromagnetic modeling and generate readback pulses based on a fast microtrack model incorporating nonlinear effects. The feasibility of the proposed model is demonstrated through bit-error-rate (BER) simulation of a turbo equalization scheme over a low-density parity-check (LDPC) coded, general-partial-response (GPR) equalized perpendicular recording channel.     

Micromagnetic simulations for terabit/in head/media systems

Bit-patterned (BP) recording is a candidate for extending magnetic data storage towards 10 Tb/in² bit densities. An analysis of the design tolerances is carried out using dynamic micromagnetic simulations and statistical models. The effects of distributions of the magnetic material properties on phase margin and addressability error-rate during writing are investigated. At 1.3 Tb/in² a rapid increase of the error-rate is observed when the write-synchronization deviates from the optimum phase φ₀. Estimates of the fabrication and write-synchronization tolerances are derived from the phase margins. It is shown that the switching-field distribution (from intra-island variations and inter-island interactions) as well as the fabrication and synchronization tolerances must be tightly controlled for Tb/in² applications. At ultra-high densities, BP media may need to be combined with energy-assisted writing, which is referred to as second-generation BP recording.     

Figures of merit of magnetostrictive single crystal iron-gallium alloys for actuator and sensor applications

Energy density, magnetomechanical coupling factor and a dimensionless sensing gage factor of 〈1 0 0〉 oriented single crystal iron-gallium alloys with 16, 17.5, 19, 24.7 and 29 at% gallium were studied as functions of stress and magnetic field. To estimate these quantities, the samples were characterized under different quasi-static stress and magnetic field conditions. The experimental behavior was modeled using an energy-based non-linear approach. Both the experimental data and the model simulations were used to calculate material parameters such as magnetic permeability, piezo-magnetic strain coefficient (d₃₃), inverse piezo-magnetic coefficient (stress sensitivity, d*₃₃) and Young's modulus in the material. These quantities were used to obtain energy density, magnetomechanical coupling factor and sensing gage factor as functions of magnetomechanical conditions. Maximum energy density of around 3 kJ/m³, magnetomechanical coupling factor higher than 0.75 and sensing gage factor on the order of 10³ were calculated.     

Error rate analysis of AF-relay wireless networks under different SNR levels

This paper presents a new method for analyzing the Bit Error Rate (BER) performance of two-hop Amplify-and-Forward Multiple Relay (AF-MR) networks. This paper considers, a flat-fading channel and a relay selection scheme to select a relay with the highest Signal-to-Noise Ratio (SNR). The method aims to unify the BER calculation under low, high and optimal SNR levels. Asymptotic BER (ABER) performance at high SNR value is calculated first, and standard expressions for exact BER (EBER) performance at low and optimal SNRs are then derived. The analytic method depends on the conventional BER (CBER) approach of one-hop communication systems. The optimal SNR is obtained by balancing energy efficiency and spectral efficiency. The proposed method is found to be effective for calculating the BER of AF-MR network performance under any SNR conditions. Moreover, it improves the accuracy of ABER performance by reducing disparity computation errors between ABER and EBER performances and this allows the BER of AF-MR networks to be accurately calculated using either ABER or EBER. The outcome expressions for the method are validated by simulation results.     

Constructive ITI-coded PRML system based on a two-track model for perpendicular magnetic recording

In this paper, we study not only the new constructive inter-track interference (CITI) code based on the equalized level but also Viterbi detection algorithm taking into account the ITI from adjacent tracks for the perpendicular magnetic recording channel without a differentiator of two-track model. Although the investigation of this paper was not practical but theoretical, the result shows that the permissible percentage of ITI for conventional Viterbi detection to attain better performance compared with the case of single track is 26%, while the percentage for the proposed one is improved up to 50%. Further investigation has to be required under a more realistic system and moreover may be expanded into patterned media perpendicular magnetic recording.     

Preparation and recording characteristics of granular-type perpendicular magnetic recording media with thin intermediate layer

Granular-type media with thin Ru intermediate layer were prepared on a highly oriented high-B_s FeCo soft underlayer (SUL). A CoPt–TiO₂ recording layer on a Ru intermediate layer of only 2 nm had high-crystal orientation, high H_c of 6.5 kOe, and a high squareness ratio (SQ) of 0.99. The magnetic property of the SUL was also good. The recording performance was measured for the media with different Ru intermediate thicknesses by using a single-pole-type (SPT) head. The media had large reproduced output even for the Ru intermediate layer thickness of 2 nm.     

Perpendicular magnetic recording technology at 230 Gbit/in

A perpendicular recording system that allows areal densities beyond 200 Gbit/in² has been designed and tested to investigate the major challenges in perpendicular magnetic recording. The integrated write/read head has a trailing shield to improve the write head field gradient and a conventional CIP–GMR reader. The medium is a low-noise CoPtCr-based oxide medium with a CoTaZr soft underlayer. On track byte error rates at ∼ 50 Mb/s are better than 10⁻⁴ at ∼1000 kbpi. Using a 15% off-track criterion at 10⁻² byte error rate, track densities between 200–240 ktpi are realized, yielding areal densities of 210–233 Gbit/in². High-resolution magnetic force microscopy (hrMFM) has been employed to investigate the write characteristics of these heads with improved cross-track resolution. Using a quantitative analysis method, many parameters, such as transition curvature and transition width, are estimated from the hrMFM image. Significant transition curvature is found, which increases the width of the read head response to a transition, T₅₀, by 2–3 nm.These results give insights into the recording physics of perpendicular recording and in particular point out improvements required for achieving even higher areal densities.     

Adjacent-track interference in ultrahigh-density perpendicular recording system

In perpendicular recording system, the increase of track density is crucial to achieve ultrahigh areal density. At higher track densities, the adjacent-track interference (ATI) arises. In this work, ATI is studied by micromagnetic simulation. Two adjacent tracks are written successively. The track–track distance (TTD) and head–medium spacing are varied to analyze the write and read performance of these two tracks and to investigate the influence of ATI on recording performance. Simulation results indicate that when a track is written first, it is less vulnerable to ATI. ATI is stronger in a track with higher linear recording density. The head–medium spacing plays a significant role in the achievement of low ATI in perpendicular recording system. If the head–medium spacing is reduced to 5 nm, areal recording density above 540 Gb/in² could be realized.     

High SNR approximation for performance analysis of two-way multiple relay networks

Signal-to-Noise Ratio (SNR) is the key parameter in the performance evaluation of Two-Way, Amplify-and-Forward, Multiple-Relaying (TAF-MR) networks. This paper introduces new methods that use high SNR (HSNR) level approximations to analyze accurately the Bit Error Rate (BER), optimize the Relay Location (RL), and balance the energy efficiency (EE) and spectral efficiency (SE) of such networks. We consider a flat-fading channel and a strategy for selecting a relay with the highest SNR. Standardized BER expressions are obtained for a wide range of SNRs (low, high, and optimal) to yield more accurate predictions of the BER performance. Thus, by using HSNR, a unified analysis for calculating the Asymptotic BER (ABER) and the exact BER (EBER) performance is developed. The optimal SNR level is obtained by optimizing the power of network sources, which include the relay and user powers. Further, we propose a method for integrating the RL with the balancing of the EE and SE optimally to achieve the best EE improvement. The derived expressions for the methods are validated by simulation.     

Recording simulation beyond 2 Tbit/in on bit-patterned media with shielded planar head

A simple technique for bit-patterned media was proposed to increase achievable areal recording densities beyond 2 Tbit/in². Introduction of longitudinal magnetic anisotropy to the media indicated reduced effect of magnetostatic interaction between the dots. Recording simulation with a shielded planar pole head exhibited increased write shift margins in both down and cross track directions compared with that of the perpendicular anisotropy media. It was suggested that recording of an areal density of 2.5 Tbit/in² would be realized with a down and cross track margins of 3.5 and 4.0 nm, respectively. Better recording performance at high areal densities is expected if suitable head could be designed.     

Performance of a Chase-type decoding algorithm for Reed–Solomon codes on perpendicular magnetic recording channels

Algebraic soft-decision Reed–Solomon (RS) decoding algorithms with improved error-correcting capability and comparable complexity to standard algebraic hard-decision algorithms could be very attractive for possible implementation in the next generation of read channels. In this work, we investigate the performance of a low-complexity Chase (LCC)-type soft-decision RS decoding algorithm, recently proposed by Bellorado and Kav?i?, on perpendicular magnetic recording channels for sector-long RS codes of practical interest. Previous results for additive white Gaussian noise channels have shown that for a moderately long high-rate code, the LCC algorithm can achieve a coding gain comparable to the Koetter–Vardy algorithm with much lower complexity. We present a set of numerical results that show that this algorithm provides small coding gains, on the order of a fraction of a dB, with similar complexity to the hard-decision algorithms currently used, and that larger coding gains can be obtained if we use more test patterns, which significantly increases its computational complexity.     

Influence of patterning fluctuation on read/write characteristics in discrete track and bit patterned media

The read/write characteristics of non-patterned media (NPM), discrete track media (DTM), and bit patterned media (BPM) are examined by modeling the magnetization distribution of NPM and patterning fluctuation of DTM. By comparing spin-stand measurement with calculation, the magnetization distribution of NPM was well characterized with a new Voronoi cell magnetic cluster model, in which the cluster size at the track edge, 〈D_edge〉, was larger than that at the track center, 〈D_center〉 by a factor of two. Based on an analysis of patterning fluctuations seen in SEM images of DTM, line-edge roughness (LER) was modeled as a long-wavelength center-line roughness (CLR) plus a short-wavelength line-width roughness (LWR). It was confirmed that the standard deviation of the patterning fluctuation was much smaller than that of the magnetic fluctuation for NPM. This allowed DTM to achieve higher off-track performance than NPM. By examining the 747 curves, it was revealed that DTM could have an advantage in track-density of up to approximately 25% assuming patterning fluctuations can be well controlled at high track density. In BPM, fabricating accurate dots is essential. The relationship between dot defect rate and patterning fluctuation was examined, and the maximum allowable standard deviation of LER was derived as 2 nm for achieving 1 Tbspi.     

Micromagnetic recording field analysis of a fast-switching single-pole-type head

Single-pole-type (SPT) heads for perpendicular magnetic recording were investigated using a Landau–Lifshitz–Gilbert (LLG) micromagnetic analysis program running on a PC cluster system. Dynamic recording fields were calculated for various driving currents, head structures and head materials. The dynamic head field response is discussed with regard to the write timing window for bit-patterned media, as described in previous papers.     

Designing magnetics of capped perpendicular media with minor-loop analysis

A novel method of analyzing the switching field distribution (SFD) and magnetic correlation length of perpendicular magnetic recording media that uses major and minor magnetization loops is proposed. By applying the analysis to a series of capped perpendicular media, we found that a thick capping layer with a low saturation magnetization effectively reduced SFD without rapidly increasing the magnetic correlation length. Transmission electron microscope observation suggests that the SFD is narrowed by the increased uniformity of intergranular exchange coupling via the thick capping layer. Evaluations of recording characteristics demonstrated a close correlation between narrower SFDs and improved recording performance. Reducing exchange coupling dispersion is a clear solution for improving the performance of recording media.     

Past and present of perpendicular magnetic recording

Past and present of perpendicular magnetic recording (PMR) is described. PMR was born at a university institute 30 year ago and hard disk drive (HDD) industry is now converting the products from longitudinal magnetic recording (LMR) to PMR. The way to the present, however, was not flat. The research of PMR experienced the death valley in the 1990s, but was overcome by its unique research system. The commercialization of PMR was much later than it was expected in the beginning, which might have affected the position of HDDs in storage systems. The market of HDDs is expected to steadily grow towards the future being supported by PMR.     

Design and recording simulation of 1 Tbit/in patterned media

Design of patterned media for an areal density of 1 Tbit/in² with thermal stability is presented based on perpendicular M–H loops of the media. Required perpendicular magnetic anisotropy was estimated to be achieved with known materials. However, it is indicated that magnetostatic interaction between the dots becomes a limiting factor for achieving higher densities. Recording simulation using a Karlqvist pole head on the designed media exhibited possibility of the recording of 1 Tbit/in². Shift margin of the write head in the cross-track direction was found to be increased with elongated dots in the down-track direction. Recording simulation with an FEM-analyzed field of a side-shielded multi-surface pole head exhibited successful recording with increased cross-track shift margins as well as the effect of the elongated dot shape.