Thermal Asperity Research Articles

Thermal Elasto-Plastic Asperity Contacts of Layered Media

A thermal elasto-plastic asperity contact model is investigated in this paper, which takes into account the steady-state heat transfer and the asperity distortion due to thermal elasto-plastic deformations. A hard coating and a soft coating are applied to study the correlations between contact area and contact pressure, average gap and contact pressure, coating thickness and contours of the contact stress distribution et al. The effects of material properties, the coating thickness, frictional coefficient, and the heat input combinations on the stress distribution are investigated and discussed. The results may help to make an appropriate choice of the hard coating thickness.

シリンダ状媒体対応傾斜パッドスライダの浮上安定性に関する検討

We studied a magnetic recording head for a cylindrical perpendicular magnetic storage system with regard to its stability when a slider is flying over a rotating 1-inch-diameter cylindrical medium. We also designed an inclined-pad-type slider for the cylindrical magnetic storage system. This slider has an inclined pad formed on its leading edge and a trailing pad formed on its trailing edge, to generate an air-bearing force. The inclined pad is protruded in the thickness direction of the slider relative to the trailing pad. A waveform of an acoustic-emission (AE) sensor and an envelope of a reproduced signal were obtained from the combination of the cylindrical perpendicular medium and fabricated this slider. These waveforms did not show any large pulse and thermal asperity associated with contact. These results confirm the flying stability of the inclined-pad-type slider.

Thermal asperity suppression in perpendicular recording channels

In this paper, we present an approach for detection and suppression of thermal asperity (TA) induced signal distortions in perpendicular recording channels. When the MR read head comes in contact with the disk surface, the resulting thermal transient causes a change in the resistance of the MR read element. This change in the resistance manifests as an additive voltage transient in the readback signal. Typically, a TA signal has a short rise time (60-150 ns) with a longer decay time (1-5 /spl mu/s). The peak TA amplitude can be as large as 250%. If precautions are not taken, such transients could cause sector read failure. As the flying height continues to decrease and as the recording density continues to increase, the adverse effects of TA will become even more serious in future drives. Thus, it is particularly important to develop means to handle TA effectively in perpendicular recording channels. The study shows that the proposed method is robust with respect to the changes of the peak TA amplitude and to the mismatch between estimated and actual TA durations. Thus, in conclusion, we have developed an easily implementable and effective TA detection and mitigation approach for perpendicular recording channels.

Uniform latin square interleaving for correcting two-dimensional burst errors

A Latin square is widely used as an interleaving algorithm with an error-correcting code to correct burst errors in storage and communication systems. In a storage system, there are two types of burst errors; successive errors occurring on one track, i.e. a 1-dimensional burst error, and 1-dimensional burst errors occurring over successive tracks, i.e. a 2-dimensional burst error. The 1-dimensional burst error is usually caused by surface contamination or thermal asperity in the read head and the 2-dimensional burst error is usually caused by defects in the recording medium. We introduce here a new class of Latin square, a uniform Latin square, that can improve 2-dimensional error-correction capability compared with that of the conventional Latin squares.

Measurements of particle size distribution and average particle charge in operating a hard disk drive

As the flying height of a slider in a hard disk drive decreases, the slider and disk are more likely to come into contact and generate contamination particles. Since particle contamination can cause serious problems including thermal asperity, it must be prevented to increase storage capacity. In this work, first we measured the size distribution of the particles according to the disk rotational speed by using an APS (aerodynamic particle sizer) and DMA (differential mobility analyzer)–CPC (condensation particle counter). The results showed that mainly submicron particles were generated and particle generation rate increased as the disk rotational speed increased. Then, the electrical current due to particle charge was measured by a Faraday cup. Calculated from experimental data, the average number of elementary charges per particle was from 1.9 to 5.2, and it decreased as the disk rotational speed increased. Finally, SEM (scanning electron microscopy) images of particles were obtained by using a particle sampler designed in our laboratory. With these images, we were able to verify the relation between particle generation rate and disk rotational speed.

하드디스크 드라이브 동작 시 발생하는 입자 크기분포와 입자당 평균 대전량 측정

As the flying height of a slider in a hard disk drive decreases, the slider and disk are more likely to come in contact and generate contamination particles. Since particle contamination can cause serious problems including thermal asperity, it must be prevented to increase storage capacity. When particles are generated in a HDD, particles can be charged and have a few number of elementary charges. In this paper, the size distribution of particles and electrical current due to particle according to the disk rotational speed were measured. Also, the average number of elementary charges was calculated from experimental data. SEM images of particles were obtained by using a particle sampler designed in our laboratory.

傾斜パッド型ヘッドを用いたＣｏＰｔＣｒ‐ＳｉＯ２垂直二層膜シリンダ状媒体の検討

A cylindrical magnetic storage system was developed, using inclined-pad-type heads and perpendicular double-layered media with a CoPtCr-SiO2 recording layer. The inclined ABS (air-bearing surface) slider has an inclined leading pad formed on the leading edge of the slider and a trailing pad formed on the trailing edge of the slider to generate an air-bearing force. The fabricated head exhibits flying stability on a rotating cylindrical medium. A composite recording layer, CoPtCr-SiO2/Ru/Pt/Ta deposited on a rotating cylindrical substrate with a soft magnetic underlayer (SUL) shows high coercivity over 5 kOe, and squareness ratio of around 1, which are almost the same characteristics as the same composite recording layer on a conventional disk substrate with SUL. The fundamental read/write characteristics of CoPtCr-SiO2/CoZrNb perpendicular double-layered cylindrical media were measured using a merged-type GMR head with an inclined-pad-type slider designed for use with cylindrical media. The envelope of the head output did not show any modulation, large spike pulses, or thermal asperity associated with contact with the cylindrical medium. Isolated waveforms with sufficient symmetry were also obtained.

하드 디스크 드라이브 회전수 변화가 드라이브 내 나노 오염 입자 발생에 미치는 영향

In high-density hard disk doves, the slider should be made to fly close to the magnetic recording disk to generate better signal resolution and at an increasingly high velocity to achieve better data rate. The slider disk interaction in CSS (contact-start-stop) mode is an Important source of particle generation. Contamination particles In the hard disk drive can cause serious problems including slider crash and thermal asperities We Investigated the number and the Sizes of particles generated in the hard disk drive, operating at increasing disk rotational speeds, in the CSS mode CNC (condensation nucleus counter) and PSS (particle Size selector) were used for this investigation In addition, we examined the particle components by using SEM (scanning electron microscopes), AES (auger electron spectroscopy), and TOF-SIMS (time of flight-secondary Ions mass spectrometry) The increasing disk rotational speed difectly affected the particle generation by slider disk interaction The number of particles that were generated increased With the disk rotational speed. The particle generation rate Increased rapidly at motor speeds above 8000 rpm Tins Increase may be due to the Increased slider disk Interaction. Particle Sizes ranged from 14 to 200 nm. The particles generated by slider disk Interaction came from the lubricant on the disk, coating layer of the disk, and also slider surface.<br/>

Thermal Asperity Detection and Cancellation in Perpendicular Magnetic Recording Systems

During the defect scanning process on the production line, when asperities are detected on the surface of a magnetic storage medium, that location is marked, and no data are stored to marked locations. However, some asperities go undetected, and cause problems during data recovery. In this paper, we introduce a new method to detect and cancel the thermal asperity effects for perpendicular magnetic recording systems. We compare the new method to the existing methods in terms of the bit error rate after Viterbi detection.

Performance of Efficiently Encodable Low-Density Parity-Check Codes in Noise Bursts on the EPR4 Channel

We consider in this paper the possibility of using a low-density parity-check (LDPC) code as the complete error control system in a magnetic recording channel. We compare the performance of selected LDPC codes with two Reed-Solomon (RS) code schemes on an extended partial-response class 4 (EPR4) channel, with particular emphasis on their performance in noise bursts (induced by media defects or thermal asperities). We quantify the performance of LDPC codes in noise bursts via a maximum-burst-length parameter, and we present a simple algorithm for computing this parameter. We find some very promising initial results: the LDPC codes are very robust against large noise bursts (128 bits long or more), and are superior to the RS schemes examined in the measurable error rate region. The extent to which they are superior depends on the particular LDPC scheme involved, and the results provide motivation for further investigation in this area.

Effect of disk rotational speed on contamination particles generated in a hard disk drive

In high-density hard disk drives, the slider should be made to fly close to the magnetic recording disk to generate better signal resolution and at an increasingly high velocity to achieve better data rate. The slider disk interaction in CSS (contact-start-stop) mode is an important source of particle generation. Contamination particles in the hard disk drive can cause serious problems including slider crash and thermal asperities. We investigated the number and the sizes of particles generated in the hard disk drive, operating at increasing disk rotational speeds, in the CSS mode. Condensation nucleus counter (CNC) and PSS (particle size selector) were used for this investigation. In addition, we examined the particle components by using SEM (scanning electron microscopes), AES (auger electron spectroscopy) and TOF-SIMS (time of flight-secondary ions mass spectrometry). The increasing disk rotational speed directly affected the particle generation by slider disk interaction. The number of particles that were generated increased with the disk rotational speed. The particle generation rate increased rapidly at motor speeds above 8000 rpm. This increase may be due to the increased slider disk interaction. Particle sizes ranged from 14 to 200 nm. The particles generated by slider disk interaction came from the lubricant on the disk, coating layer of the disk, and also slider surface.

Head–disk contact detection in the hard-disk drives

The increasing capacity of current hard-disk drives places a great importance on reliability, as tens of gigabytes of data can be lost in an instant due to a single tribological failure event. To improve the reliability of the head–disk interface there is a need to predict, measure and monitor any interactions during manufacture. This paper discusses some different techniques that can be used for head–disk contact detection. These techniques range from a traditional acoustic emission method to some drive specific tests, such as thermal asperity (TA) detection and variable gain amplifier (VGA) signal measurements. The paper also discusses the merits of these techniques and ways to theoretically predict contact.

Low-density parity-check codes over gaussian channels with erasures

We consider low-density parity-check code (LDPCC) design for additive white Gaussian noise (AWGN) channels with erasures. This model, for example, represents a common situation in magnetic and optical recording where defects or thermal asperities in the system are detected and presented to the decoder as erasures. We give thresholds of regular and irregular LDPCCs and discuss practical code design over the mixed Gaussian/erasures channel. The analysis is an extension of the Gaussian approximation work of Chung et al. In the two limiting cases of no erasures and large signal-to-noise ratio (SNR), the analysis tends to the results of Chung et al. (see ibid., vol. 47, p.657-670, Feb. 2001) and Luby et al. (1997), respectively, giving a general tool for a class of mixed channels. We derive a steady-state equation which gives a graphical interpretation of decoder convergence. This allows one to estimate the maximum erasure capability on the mixture channel, or conversely, to estimate the additional signal power required to compensate for the loss due to erasures. We see that a good (capacity-approaching) LDPCC over an AWGN channel is also good over the mixed channel up to a moderate erasure probability. We also investigate practical issues such as the maximum number of iterations of message-passing decoders, the coded block length, and types of erasure patterns (random/block erasures). Finally, we design an optimized LDPCC for the mixed channel, which shows better performance if the erasure probability is larger than a certain value (0.1 in our simulation) at the expense of performance degradation at unerased (AWGN channel) and lower erasure probability regions (less than 0.1 in our simulation).

Reduced-complexity decoding of Q-ary LDPC codes for magnetic recording

Binary low-density parity-check (LDPC) codes perform very well on magnetic recording channels (MRCs) with additive white Gaussian noise (AWGN). However, an MRC is subject to other impairments, such as media defects and thermal asperities. Binary LDPC codes may not be able to cope with these impairments without the help of a Reed-Solomon code. A better form of coding may be Q-ary LDPC codes, which have been shown to outperform binary LDPC codes and Reed-Solomon codes on the AWGN channel. In this paper, we report on our investigation of Q-ary LDPC coded MRCs, both with AWGN and with burst impairments, and we present a new reduced-complexity decoding algorithm for Q-ary LDPC codes. We show that Q-ary LDPC codes outperform binary LDPC codes in the presence of burst impairments.

Viterbi detection for partial-response channels with colored noise

Summary form only given. A method proposed previously by Dorfman and Wolf (see IEEE J. Select. Areas in Commun., vol. 19, p. 662-667, Apr. 2001) for combating the Thermal Asperities (TA) required running two Viterbi detectors in parallel - one matched to an EPR4 channel (target) and the other to a PR5 channel. A PR5 channel was formed by passing the EPR4 channel output through a 1-D filter preceding the PR5 matched detector and was shown to be superior to EPR4 in the presence of TA's, but 2.7 dB worse than EPR4 with Gaussian noise only. The reason for this deficiency is that the 1-D filter colors the noise substantially. In this paper an approach is proposed to improve Gaussian performance of the PR5 detector by taking the noise correlation into account. It is also shown that EPR4 channel performance can be improved by the same method.

Retry mode soft Reed-Solomon decoding

Summary form only given. In this paper, we propose a new retry mode scheme for magnetic recording systems based on a soft Reed-Solomon (RS) decoding algorithm . Instead of using asymptotic bounds, we present realistic simulation results over partial response channels, and discuss some practical implementation issues such as system architecture and erasures due to disk defects and thermal asperities. Simulation results show that a significant performance improvement can be obtained without increasing hardware complexity.

ECC-less LDPC coding for magnetic recording channels

This paper presents a system for magnetic recording channels, using low-density parity check (LDPC) codes and a novel method of erasure compensation that can correct thermal asperity (TA) and media defects (MDs). The system does not require error-correction codes (ECC) based on the Reed-Solomon codes, and thus is ECC-less. Simulation results show that the performance of ECC-less LDPC codes is better than that of the LDPC codes with ECC. It is also shown that LDPC codes of rate 0.814 can correct 48-byte TA and 24-byte MD, which are sufficient for practical hard disk drives.

Flux guide type tunnel-valve head for tape storage applications

The feasibility of tunnel magnetoresistive read heads with flux-guide (FG) layers was investigated for helical-scan tape storage applications. In order to stabilize tunnel-valve elements, a long-range exchange coupling bias scheme was employed, consisting of a Cu layer and an antiferromagnet deposited on top of a free layer in the tunnel-valve element. The maximum output of the FG type reader was about four times larger than that of a conventional anisotropic magnetoresistive reader, and thermal asperity noise of the magnetoresistance reader could be reduced drastically. The tunnel-valve readers have symmetrical track profiles, which are suitable for nontracking tape systems.

하드 디스크 드라이브 회전수 변화가 내부 필터 입자 포집 성능에 미치는 영향

Contamination particles in a hard disk drive can cause serious problems including slider crash and thermal asperities. A recirculation filter is typically installed in the hard disk drive to remove the particles. Measurements and theoretical predictions of particle concentration decay with the filter are carried out for a commercially available HDD. Especially, the effect of disk rotational speed on the particle capture efficiency is investigated. Results show that filter efficiency is higher for higher disk rotational speed.

Sensitivity and rejection capability of thermal asperities in a hard disk drive

With the use of giant magneto-resistive (GMR) head, thermal asperity (TA) has been a big concern in drive industry. In this study, we investigated several heads and disk related factors that affect the TA sensitivity of the drive. TA experiments were conducted by introducing the particles in the drives using a particle injection chamber. It is found that the slider ABS shape can help to reduce TA or contamination at the head/media interface. However, TA sensitivity of the drive mainly depended on the intrinsic property of GMR sensor. TA sensitivity of head increases as the bias current increases. GMR head is much less sensitive to TA compared with MR head. However, if similar bias current is applied to both MR and GMR heads, TA sensitivity of GMR head became almost identical to that of MR head. TA sensitivity of different types of disks was also studied and the dominant factor affecting the TA sensitivity is observed to be the scratch resistance of the carbon overcoat layer.