Transition Jitter Noise Research Articles

Signal Noise Ratio and Magnetic Density in Digital Magnetic Recording System: Analytical and Numerical Investigation

In this work the analysis of noise especially the transition jitter noise is used to evaluate the areal density which are consider as one of the most important characteristics in magnetic recording systems. This is possible through the analytical evaluation of the signal noise ratio (SNR) using kink soliton as domain wall profile and assuming that, the transition noise are the dominant noise in recording media. The intrinsic parameters and experimental values for magnetic recording media especially the domain wall width are chosen to emphasize the impact of noise in a certain simple materials and compounds materials used to manufacture recording media. The comparative study on evolution of areal density is done to reinforce the fact that noise can governed the magnetic density, where the values

Deep Neural Network: Data Detection Channel for Hard Disk Drives by Learning

The application of a deep neural network (DNN) as the detection channel for hard disk drive (HDD) data recovery at high user bit density and the prominent magnetic transition jitter noise are investigated in this article. Directly trained with the un-equalized readback signals without any prior knowledge of the magnetic recording channel, the DNN can automatically learn the signal characteristics, in particular the correlations between the input signals and the impact from the noise. As a result, the DNN read channel not only adapts the inter-symbol interference (ISI) but also demonstrates strong resilience against the colored magnetic noise. Our simulation results also reveal that to fully harness the learning power of the DNN data detection channel, the neural network inputs must cover the ISI spread. In addition, the training data must be sufficiently representative so that the inductive bias learned by the DNN detection channel can be used as good prior knowledge for actual detection.

The effect of adding a magnetic oxide in the grain boundaries of HAMR media

Grain-to-grain Curie temperature (Tc) variation in media reduces the signal-to-noise ratio due to its contribution to the transition jitter noise, especially when the average grain size is decreased to increase the area storage capacity. Thermally insulating magnetic grain boundaries may suppress such grain-to-grain Tc variation, especially at small grain sizes. In this paper, we discuss the effect of adding a thermally insulating magnetic oxide, in particular, BaFexOy, to the grain boundaries of granular FePt-C heat-assisted magnetic recording media. It is found that BaFexOy is chemically inert with respect to FePt and the chemical ordering of FePt-BaFexOy-C media is similar to that of FePt-C media. By tuning the volume fraction of BaFexOy and C, well-separated FePt grains (average grain size = 6.8 nm) surrounded by BaFexOy shells enable the attainment of perpendicular Hc greater than 35 kOe. Chemical mapping by transition electron microscopy shows that the magnetic oxide appears to be crystalline and completely surrounds the FePt grains. Temperature-dependent magnetization measurements indicate an effective increase in the magnetic grain size at temperatures below the FePt Curie temperature. The pulsed laser pump-probe measurement indicates a measurable reduction of the Curie temperature variation for the FePt-BaFexOy-C media compared to the reference FePt-C media.

Construction of QC LDPC Cycle Codes Over GF(<inline-formula> <tex-math notation="LaTeX">${q}$ </tex-math></inline-formula>) Based on Cycle Entropy and Applications on Patterned Media Storage

In this paper, we focus on the construction of a type of quasi-cyclic low-density-parity-check (QC LDPC) codes called cycle codes. Based on our previous work, the maximum cycle entropy (MCE) algorithm for constructing nonbinary LDPC codes can be extended to its QC form (QC-MCE), which maintains the QC structure of the parity-check matrix. With this method employed, an elegant distribution of nonzero entries over the Galois field GF( q ) can be obtained among the cycles whose length is related to the girth. Thus, the independence of probabilistic information transferred during decoding is increased, leading to a better performance. Extensive simulation results show that the proposed QC-MCE algorithm behaves much better than the conventional random one and performs as well as the existing method over a pattern media channel with both additive white Gaussian noise (AWGN) and transition jitter noise (TJN). The decoding complexity of our proposed codes is reasonably low due to the QC structure of the codes. The codes constructed with the proposed method can be well applied over the patterned media storage.

Determination of Transition Jitter Noise by Time Domain Analysis

Transition jitter noise can be caused by a variety of factors such as contributions by the medium, head, and drive electronics. In this work, transition jitter noise in magnetic recording is measured based on the time domain analysis. An example of transition jitter noise in perpendicular media is investigated using captured noisy replay signals compare with a reconstructed ideal signal. It is found that transition noise increased with increasing linear density and dropped sharply after the maximum point.

Analytical Expressions for the Readback Signal of Timing-Based Servo Schemes

Timing-based servo (TBS) is a powerful technology widely used in state-of-the-art tape systems for track following. To optimize TBS systems for future operating points with aggressive track-density scaling, an accurate model is needed to characterize how the servo read head geometry, servo format parameters, and magnetic media properties affect the readback signal. In this paper, we derive new analytical closed-form expressions for the readback signal of shielded magneto-resistive (MR) heads scanning over the magnetic transitions of TBS patterns. The expressions, derived via the reciprocity principle, are applicable to arctan-shaped transitions between arbitrarily magnetized areas of a thin-film magnetic medium and are given in the displacement and wavevector domain. We apply these new expressions to analyze isolated transitions as well as multi-transition TBS patterns on longitudinal, non-oriented or perpendicular media. Comparison between experimentally captured servo readback signals and the analytical expressions show an excellent match. By adding electronics and transition jitter noise, the readback signal expressions can be extended to a servo channel model suitable for TBS performance prediction by means of Monte Carlo simulations and/or bounds on the variance of the position-error signal.

Effect of track asymmetry and curvature on shingle writing scheme

Written transition curvature in perpendicular magnetic recording is generally understood to result in increased transition jitter noise and degraded signal to noise ratio or byte error rate (BER) performance. For the shingle writing scheme, asymmetry or curvature in written tracks is considered inherent due to the erasure and track edge writing characteristics. It is proposed that such a track asymmetry is more prominent at high track density/smaller track pitch recording conditions. In this report we present spin stand experimental results to study the effect of the possible track asymmetry or curvature by shingle writing and reading back in different skews. By comparing shingle writing BER bathtub profiles in different writing skew conditions 0°, +/−2°, +/−4°, +/−6°, the effect of varying shingle track asymmetry and curvature is analyzed via subsequent skewed reading process. The shingle writing BER bathtub profiles as well as the read back amplitude cross track profile are generally symmetric upon one sided erasure at different track pitches. We found that the 0° skew writing and reading process provides both the maximum BER and amplitude.

Mean-Adjusted Pattern-Dependent Noise Prediction for Perpendicular Recording Channels With Nonlinear Transition Shift

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 propose a modified pattern-dependent noise predictive (PDNP) detection algorithm for use on channels with electronics noise, transition jitter noise, and NLTS. We show that this detector can offer significant improvement in bit-error-rate (BER) compared to conventional Viterbi and PDNP detectors. Moreover, the system performance can be further improved by combining the new detector with a simple write precompensation scheme.

On the Use of Averaging to Estimate the Dibit Response

In the magnetics literature, the dibit response is typically estimated assuming knowledge of the written bit sequence and noiseless readback signal. However, magnetic recording systems suffer from various noise sources, and the readback signals are noisy. One way to obtain an estimate of the noiseless readback signal from noisy measurements is via spatial and temporal averaging. In this paper, transition position jitter and additive noise sources are considered in the readback signal. Using this model, we show that in the asymptotic regime of an infinite number of track locations and re-reads, the estimate formed via averaging has a bias. The effect of spatial and temporal averaging on the variance of the estimate is quantified without assuming an explicit distribution for the transition jitter and additive noise.

Fe–Pt–MgO stacked films for perpendicular magnetic recording

Fe–Pt–MgO stacked storage layer constructed by [Fe–Pt/Fe–Pt–MgO/Fe–Pt] trilayered structure was proposed for a next-generation high-density perpendicular magnetic recording medium. The Fe–Pt–MgO composite middle layer was prepared by sputtering the Fe–Pt–MgO composite-type target including relatively large MgO content of 50 vol%. The Fe–Pt(0 0 1) seed layer deposited on MgO underlayer was effective in forming the ordered fct(0 0 1) phase for the Fe–Pt–MgO composite film. The reduction of transition jitter noise and the suppression of signal overlap were observed in the stacked-type medium with the Fe–Pt–MgO middle layer of 1 nm thickness. The improvement of recording properties is attributed to the pinning effect of magnetic domain wall by the Fe–Pt–MgO composite layer inserted into the middle of pure Fe–Pt storage layer.

Design Curves and Information-Theoretic Limits for Perpendicular Recording Systems

To optimize a high-density magnetic recording system, one needs to know the tradeoffs between various components of the system including the read/write transducers, the magnetic medium, and the read channel. In this paper, we consider a channel model characterized by three parameters: the replay pulse width T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> , the transition jitter noise standard deviation sigma <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</sub> , and the signal-to-electronic-noise ratio SNR <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">WG</sub> . We utilize information-theoretic tools to determine the acceptable region for the channel parameters T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> and sigma <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</sub> when optimal detection and linear coding techniques are used. This paper is an extension of a similar analysis for a system that utilized a minimum mean-squared error (MMSE) equalizer, a Viterbi detector, and a Reed-Solomon (RS) code. Our main conclusion is that there is a considerable potential gain to be achieved by using improved detection and coding schemes as compared with the present system

Magnetic and recording characteristics of perpendicular magnetic media with different anisotropy orientation dispersions

A systematic investigation of the magnetic and recording characteristics of CoPtCrO media with different Co grain magnetic anisotropy orientation dispersion (AOD) is performed. AOD is characterized by Δθ50 of the x-ray diffraction Co peak. AOD was systematically varied between 3.3° and 10.5° by varying the growth condition of Ru layer. With increasing AOD, more grain-boundary segregation occurs, resulting in reduced exchange coupling. Micromagnetic simulation and experimentally measured time dependence of coercivity indicate that magnetic switching of well-isolated Co particles is affected by exchange coupling strength as well as Co AOD. Higher coercivity, narrower switching field distribution, and good thermal stability are achieved with lower Co AOD. Reduced Co AOD leads to a substantial decrease of both transition jitter and dc noise.

Advancements in PMR thin-film media

The purpose of this paper is to review advancements in the design, characterization, and optimization of PMR thin film media for magnetic recording output, resolution, signal-to-noise, and thermal stability at ultra-high densities from 100 to 1000Gbpsi. In particular, the single and double-layer PMR thin-film media to be discussed include HCP CoCr-alloys, HCP CGC CoPt/CoCrPt, FCC Co/Pd multilayers, HCP CoCrPt–SiO2 granular oxides, FCT FePt, and tilted media. This paper reviews the materials processing techniques used to optimize physical microstructures with seed layers and intermediate layers to decrease grain size, narrow size distribution, dope with elements to inhibit grain growth, segregate elements to grain boundaries to enhance magnetic isolation, and increase coercivity, magnetization squareness, negative nucleation field, narrow easy-axis orientation, switching field distribution, and reduce hysteresis loop slope at coercivity, magnetic cluster domain size, DC reversed domain noise, and recording transition jitter noise.

신호패턴 종속잡음 채널을 위한 신호검출

고밀도의 저장기록장치 채널의 주요 신호검출 오류의 원인은 천이 지터잡음이다. 이러한 채널환경에서 최적의 신호검출기 구현을 위해서는 고도의 복잡도가 요구되는데 이는 지터잡음이 신호와 상관관계가 있고 잡음간에도 상관관계가 존재하기 때문이다. 본 논문에서는 계산량과 하드웨어 복잡도 관점에서 효율적인 두 가지 종류의 신호검출기에 대해서 설명한다. 이는 전통적인 비터비 복호기의 가지값을 변화시킨 형태이며 같은 이진데이터 값의 반복을 제한하는 부호와 함께 결합하여 일반적인 PR 채널에 적용된다 기존의 비터비 알고리즘의 복잡도와 비교하면 비터비 트렐리스에서 각각의 가지값을 계산할 때 추가적으로 하나의 곱셈기 혹은 덧셈기의 증가가 요구된다. Transition jitter noise is one of major sources of detection errors in high density recording channels. Implementation complexity of the optimal detector for such channels is high due to the data dependency and correlated nature of the jitter noise. In this paper, two types of hardware efficient sub-optimal detectors are derived by modifying branch metric of Viterbi algorithm and applied to partial response (PR) channels combined with run length limited modulation coding. The additional complexity over the conventional Viterbi algorithm to incorporate the modified branch metric is either a multiplication or an addition for each branch metric in the Viterbi trellis.

Low-noise mechanism in exchange-coupled perpendicular magnetic recording media

The noise in perpendicular magnetic recording was studied. Noise analysis based on a model with position fluctuations of sharp transitions suggested that the saturation in integrated noise at high linear densities represents recording resolution of the recording media. Recording simulation study of the transition jitter noise for media with increased exchange coupling between adjacent grains indicated no enhancement in transition jitter as long as the recording resolution is increased. It was suggested that a higher recording resolution could be compatible with a low-noise property in perpendicular magnetic recording media.

Coupled granular/continuous perpendicular recording media with soft magnetic underlayer

Fabrication, magnetic properties, and read/write characteristics of coupled granular/continuous (CGC) perpendicular media are described. The media consist of continuous Co/Pt multilayers, which have a strong in-plane exchange coupling, and a granular CoCrPt layer on top of a soft magnetic underlayer. These continuous and granular layers are magnetically coupled, thus the switching field distribution becomes effectively narrower. Recording measurements using a single-pole writer and a giant magnetoresistive reader show substantial improvement in the head output and resolution of the CGC medium, compared to the plain granular medium. Transition jitter and medium noise are reduced in the CGC media. As a result, the CGC media have 3–5 dB higher signal-to-noise ratio (SNR) and better thermal stability than the plain granular medium. The concept of CGC perpendicular media is a viable way to achieve both the thermal stability and medium SNR requirements for high-density recording.

Noise spectrum analysis in perpendicular magnetic recording of double-layered media

Noise spectrum properties of double-layered perpendicular magnetic recording media were studied for two dominant medium noises, a transition jitter noise and a reversed domain noise. It was found that the transition jitter noise spectrum was delivered from the derivative of the transition shape, while the reversed domain noise spectrum was derived from a simple model of the impulsive reversed domain. The noise enhancement in a low-frequency region with increasing the recording density was found to be attributed to the relative increase in the jitter noise. The noise spectra for both sources were theoretically whitened when the transition width and the reversed domain size became negligibly small against reproducing resolution.

Signal space detection for recording channels with jitter noise

A delay-constrained sequence detector is considered for recording channels whose major impediments include intersymbol interference (ISI) and magnetic transition jitter noise. The jitter noise is data-dependent, and a given noise sample is correlated with neighboring noise samples. A sequence detector with a finite decision delay can be formulated in a finite dimensional vector space. For a correlated noise channel, the decision boundary is generally quadratic. We present a technique for obtaining a minimal set of hyperplanes approximating a quadratic decision boundary with a negligible performance loss. In this process, a distance measure, which is consistent with the notion of the effective SNR, is defined and used as a design parameter to trade the complexity and performance. As an achievable performance bound, we derive the effective SNR for the maximum-likelihood sequence detector (MLSD) for these channels. The performance of the partial response maximum likelihood (PRML) detector commonly adopted for current data storage channels as well as the Viterbi algorithm (VA) based on the traditional Euclidean metric, which serves as the MLSD for additive white Gaussian noise, are also analyzed and compared with that of the proposed signal space detector.

Precise noise characterization of perpendicular recording media

Perpendicular recording is one of the technology candidates to extend the areal density growth trend with the superior thermal stability and the high recording resolution. As we have experienced with longitudinal recording, noise originating from perpendicular recording also has a strong impact on the channel performance. In this study, the noise characteristics of the readout waveforms of a tri-layer perpendicular media and single-pole write/MR read heads were investigated using precise time-domain analysis. Waveform fluctuations were obtained by subtracting the ensemble-averaged signal from each measured noise-contaminated waveform. The time-domain analysis makes clear the dominant type of transition-related noise such as transition jitter, pulse breathing or dc noise, etc. It was revealed that the noise from perpendicularly recording medium concentrates at the center of written transitions. The dominant transition noise source was found to be the transition position jitter. The level of transition jitter of the perpendicular media used in this study was found to be comparable to that of 5 Gbpsi class longitudinal media. MFM measurements revealed that magnetization fluctuates in large size of about 80 nm, which results in the fluctuation of the transition centerline. Reducing transition jitter should be the focus of developing future high performance perpendicular recording media.

Bit errors due to channel modulation of media jitter

Recording media written-in transition jitter noise can limit magnetic recording systems. Read channel intersymbol interference (ISI) modulates media jitter in a head/disk/channel, attenuating jitter at low linear density due to classical ISI but amplifying at high density. This paper assesses whether jitter modulation is significant in typical disk drive peak detect (PD) and partial response (PR) channel design, where maximum linear bit density is sought at a specified bit error rate (BER). Experimental isolated pulse data, transition noise, and electronic noise measurements from a 65 mm head/disk recording design are used as inputs to a Matlab simulation of PD and PR Lorentzian channel low pass and boost filters, PD differentiator filter, PR digital filter, and PD/PR bit detectors. The goal of such models is optimizing the designable PD and PR channel parameters for realistic maximum linear density prediction. However their complexity obscures the jitter modulation contribution, which is demonstrated here by the shift in the EER versus density curves, when the jitter modulation is mathematically turned off. Jitter modulation is thereby shown to be significant but not dominant in (1,7) PD density limits. About an order of magnitude BER loss due to jitter modulation can occur at typical PD linear channel bit densities P/sub 50//T/spl ap/2. Jitter amplification in PD channels can occur at channel density 3.8 (above usable PD limits). PR channels only amplify media jitter, degrading EER by about an order of magnitude at typical (0, k) PR4 densities, and degrading timing channel accuracy twice as fast as channel density increases. Lorentzian isolated pulse (ISOP) modeling is compared to recording physics mathematical pulses, and simulation statistical validity checks are made.