Fabrication and MFM study of 60 nm track-pitch discrete track media

Discrete track magnetic recording media with a 60 nm track pitch and prewritten servo patterns were fabricated and tested for read/write performance, and a feasibility analysis of the embedded servo was performed. The fabrication process consisted of ultraviolet nanoimprint lithography (UV-NIL) and sequential ion beam etching on a conventional perpendicular magnetic recording medium. Magnetic patterns were written to the fabricated tracks at 700 kilo flux changes per inch (kFCI) using a spin stand and were read using magnetic force microscopy (MFM), with a resulting signal-to-noise ratio (SNR) of 12.15 dB. The servo pattern was also visualized with MFM. These results demonstrated the feasibility of writing to a 30 nm wide discrete data track and the workability of the embedded servo pattern.     

Evaluation of test procedures for determining servo compatibilityof heads and media in magnetic disk drives

We evaluate a testing specification proposed by the National Storage Industry Consortium's (NSIC) Extremely High Density Recording (EHDR) group for evaluating head and media compatibility for servo performance in magnetic disk drives. These tests use average amplitude and average noise profile measurements across isolated tracks to predict the shape, linearity, noise, and long-term stability of position error signal (PES) patterns. We compare the predictions from these tests to measurements from null and amplitude PES patterns written on a spin-stand. Results show average PES-profile prediction errors of 1%-2% track width and noise level prediction within a factor of 2. We present data from tests for long-term stability of the magnetoresistive (MR) read element after repeated write cycles by the inductive write head. In the set of heads we tested, the MR head's center and effective width changed only slightly. Although we evaluated the NSIC specification for MR read elements, the specification should be equally valid for other read head types also, as long as the PES patterns are similar     

Study of Lithographically Defined Data Track and Servo Patterns

We report on fabrication of discrete tracks on perpendicular magnetic recording (PMR) media with an e-beam lithographical process. We studied the recording performance of the e-beam media on a spinstand in parallel with conventional PMR media. Discrete track media show significant reduction in adjacent track erasure (ATE). We studied and quantitatively measured the source of the ATE improvement, and developed a triple track geometrical model to calculate achievable track density for both discrete track recording (DTR) and continuous media. From the model, we identify two factors of DTR that contribute to reaching a higher TPI. Using the same fabrication technique, we also studied servo burst design and its playback waveform quality. At 250 ktpi, we compare DTR servo bursts with servo bursts written with a conventional method. DTR servo bursts show better edge definition, which can translate to better position error signal sensitivity and support higher TPI in the future.Discrete tracks are fabricated on conventional PMR media with an e-beam litho graphical process. The recording performance is studied on a spinstand in parallel with conventional PMR media. Discrete track media shows significant reduction in adjacent track erasure (ATE). The source of the ATE improvement is studied and quantitatively measured. A triple track geometrical model is developed to calculate achievable track density for both DTR and continuous media. From the model, we identify two factors of DTR, which contribute to reaching a higher TPI. Using the same fabrication technique, we also study servo burst design and its playback waveform quality. At 250 ktpi, we compare DTR servo bursts and servo bursts written with a conventional method. DTR servo bursts show better edge definition, which can translate to better PES signal sensitivity and support higher TPI in the future.     

Distortion effects on servo position error transfer function

Two kinds of nonlinear distortion of hard-disk-drive position error signal (PES) transfer function are examined and modeled. The first distortion is the saturation of the PES due to narrow read width. The second distortion is the on-track dead band at the center of the transfer function due to wide erase band. Several formulas are developed to describe the relationships between the distortion effects and the design parameters: read width, track spacing, and erase width. These distortions are highly undesirable for the track following sectored servo system. However, they are predictable and avoidable if certain conditions are satisfied. The geometrical analysis of the distortion phenomenon is presented here     

A track locating servo system utilizing the data heads as absolute position transducers

Magnetic transitions may be compactly written in the normally unused deadspace between the data sectors on each data surface of a moving head disc drive, which may be read by ordinary data heads to obtain absolute position information for use by the track locating servo. A track following system which complements the track locating system is briefly described. Implementation of these techniques eliminates both the need for a servo surface or external encoder, and troublesome reference surface to data surface mechanical tolerance problems.     

Digital processing of dual-frequency servo burst in hard disk drives

The trend of increasing areal density, i.e., the number of bits recorded in unit area of the media, continues in the hard disk drive (HDD) industry. This growth is sustained by increasing both the track density and bit density. The track density that can be achieved in a drive depends on many factors such as the dimensions of read/write head and the performance of the head positioning servomechanism. The higher the servo bandwidth, the better the precision achieved in controlling the position of the head. Most of the drives use an embedded servo scheme, which puts a physical constraint on the frequency at which the position error is sampled, and hence, on the achievable bandwidth. The need for a better position error sensing (PES) scheme is the motivation behind exploring alternative methods of servo encoding. This paper addresses different aspects of decoding position error from the dual-frequency servo bursts. It is shown, using both simulation and experimental results, that the proposed scheme offers a feasible solution for generating the PES signal in high-performance HDD.     

Repetitive controller design for minimum track misregistration in hard disk drives

Repetitive control is a widely used technique for the compensation of repeatable error in systems that contain rotating mechanisms or repeat a trajectory. Generally, it includes delay chains and a low-pass filter in the positive feedback loop, which generate a periodic signal. The controller has typically been implemented in a plug-in fashion and designed heuristically with the simplest form of the filter. However, this design approach is somewhat ambiguous in the selection of controller parameters because of its influence over nonharmonic frequencies. Also, it leaves the possibility for further improvement. This paper presents an improved design method for the repetitive controller that provides minimum track misregistration (TMR) in a hard disk drive (HDD). For TMR prediction, the method identifies disturbances acting on an HDD and estimates servo performance, using the identification result. We have confirmed the identification and estimation procedure through experiments. In our method, first the basic tracking controller is designed and later the repetitive controller is designed in conjunction with a Q filter. A cost function based on Parseval's theorem, reflecting the servo performance as TMR, is defined. Then the servo performance is estimated from the identified disturbance, and the plant and designed controller's frequency response are modified as necessary by changing the parameters of the controller, whose optimization is carried out with a commercial nonlinear optimization tool. The design strategy facilitates the controller design by providing an accurate estimation for the attainable servo performance and design criteria under the optimization framework.     

A proximate-time-optimal-control design and its application to a hard disk drive dual-stage actuator system

We propose a compensator-based strategy for design of a track-seeking and track-following control system for a dual-stage servo actuator in hard disk drives. A well-known decoupling structure is employed to disconnect the control of the primary voice coil motor (VCM) actuator from the loop for a secondary high-bandwidth actuator. The compensator is placed in the secondary loop and suitably combined with a saturation nonlinearity in order to obtain actuator signal boundedness. The design procedure consists of four steps: 1) design of an established nonlinear seek-settle-track following controller for the VCM; 2) design of a linear track following controller for the secondary actuator; 3) observer design; and 4) design of a compensator to retain global stability and to improve performance. The proposed control system improves performance of both long-span seeking (proximate-time-optimal controller) and short-span seeking. In addition, it achieves high-bandwidth track following performance. The experimental results show good track-following performance, and short-span/long-span-seeking performance with fast settling time. The overshoot during track seeking can be made negligible for a suitably tuned VCM-actuator control loop.     

Detection signal-to-noise ratio versus bit cell aspect ratio athigh areal densities

As areal density in hard drives increases, the bit aspect ratio (BAR) becomes an increasingly critical design issue. In this paper, we use the detection signal-to-noise ratio (SNR) as a system-level indicator to establish the relationship between the optimal BAR and all major recording system parameters that relate to the head/medium combination and servo as well as the read channel. We address practical and fundamental issues including thermal stability, medium and additive system noise, and equalization and track misregistration. The results of our analysis generally agree with the recent consensus to move toward lower BARs but also caution that the optimal BAR is a strong function of the servo capability. The results of our BAR analysis also indicate that at high areal densities, the ratio of PW50 to the bit cell is considerably smaller than found in today's drives     

Design of a high performance rotary positioner for a magnetic disk memory

The design described in this paper covers a non-traditional, rotary positioner for a 14 inch, mid-range magnetic disk memory. It was intended to provide high performance and track density while retaining traditional advantages found in rotary positioners such as high reliability, and low cost, power, weight, and size. The paper describes positioner motor architecture and design, head support arm architecture and design, performance achieved by the total servo system including positioner, and finally, conclusions reached as a result of this design approach. Extensive use of computer modelling and computer assisted testing and analysis is highlighted. In particular, modal analysis results obtained from theoretical studies are described and compared with positioner performance measurements.     

Anisotropy changes in magnetoresistive heads due to lapping andannealing

The elimination of Barkhausen noise in order to attain stable read performance is of primary interest in magnetoresistive (MR) read heads. One approach to making MR elements stable is to make the effective uniaxial anisotropy field large with respect to the hard axis fields experienced during operation and storage. In addition to providing a stabilizing effect, the anisotropy field determines the sensitivity of the element. A simple measurement of sensitivity at the component level is used to evaluate the effects of lapping and annealing. Analysis of sensitivity data, measured transfer functions, and calculated transfer functions shows that the stress-induced anisotropy can be reduced by magnetic annealing. Recording tests with unshielded, annealed, read-only heads reading a track recorded by a separate head show that the read signal is stable and free of Barkhausen noise for externally applied hard axis fields of up to 100 Oe. However, this stability comes at the cost of lower sensitivity hence lower readback signal level. A balance of stability and output can be achieved by adjusting the magnetocrystalline anisotropy, the magnetostriction, or the thermally induced stress and by the use of annealing after final lapping     

Head positioning servo design for the IBM 3344/3350 disk files

The 3344/3350 disk files have significantly increased both the recording areal density and total storage capacity per spindle in comparison to previous disk file products, such as the 3340. Parameters directly related to the head positioning systems are compared. Three areas of the head positioning servo system that required design changes for increased performance are described: (1) the encoding used on the servo disk to obtain position information, (2) the compensation technique employed in the track following controller to reduce steady state positioning error and (3) implementation of the phase plane trajectory for improved accuracy and settling as well as decreased access time.     

Design issues for practical rigid disk magnetoresistive heads

Some of the issues of magnetoresistive (MR) head design are discussed from the point of view of practical, high-reliability, low-cost, manufacturable heads for rigid disk files. The major advantages of MR heads over single element inductive thin-film heads are that the MR head gives an ideal, easy to equalize output waveshape, can be used on disks with high coercivities for high linear densities, and is less subject to stability problems at narrow trackwidths. The major advantages of MR heads over metal-in-gap ferrite heads are in narrow trackwidth and high-frequency potential. Disadvantages of MR heads in the rigid disk environment include concerns about reliability and, more specifically, possible environmental fragility, loss of performance due to imperfect alignment between the read and write elements and/or due to signal asymmetry, and the possibility of higher cost/price     

Evaluating Track-Following Servo Performance of High-Density Hard Disk Drives Using Patterned Media

In recent years, track densities of magnetic hard disks have continued to grow. A promising approach to continuing the trend to ultrahigh density is using bit-patterned media (BPM). However, the implementation of BPM in hard disk drives (HDDs) to achieve high recording density is challenging and requires various new techniques, such as new servo pattern designs and position error signal (PES) decoding schemes. In applying BPM in HDDs, it is important to select a servo pattern providing sufficient PES quality for head positioning. In this paper, we discuss evaluation of PES quality and servo pattern performance from a closed-loop (servo) point of view in order to evaluate the quality of several servo patterns. We consider three servo patterns (the amplitude pattern, chevron pattern, and differential frequency pattern) as case studies. We developed a PES simulation tool to provide a realistic HDD track-following simulation. Because of PES nonlinearity in the amplitude servo pattern, we considered time-based servo patterns as alternatives. For time-based servo patterns, we found that readback signal sampling and transition jitter greatly affect PES quality. Therefore, we conclude that the differential frequency servo pattern is superior to other patterns, since it is less sensitive to transition jitter and readback signal sampling.      

Synchronous servo scheme using maximum-likelihood detectors

For magnetic disk drives, we have developed a scheme for synchronous servo with maximum-likelihood (SSML) detection that is synchronous around the entire circumference of the track and utilizes ML estimation to detect head position, clock mark phase, and track addresses. Compared to the conventional asynchronous sector servo scheme, the SSML scheme has many, much shorter, servo areas-typically several hundred-distributed on the disk. This results in higher servo information rate, which will be needed for future high-track-density disk drives with a wide servo bandwidth. We analyze the performance of the SSML scheme, including the format efficiency. We discuss a limitation of the synchronous servo scheme with respect to the number of servo areas, and we show an appropriate region for its application. Feasibility of the SSML scheme has been demonstrated in prototype 2.5-in hard disk drives.     

Control Section Design of a Multicomponent Microforce Measurement System

This paper presents a discussion of a new approach to the problem of measuring small forces in the presence of a large tare load with emphasis on the analysis and design of the force balance servo system. The object under study is suspended by an air bearing, and the applied forces, in the plane perpendicular to the direction of the tare load, are nulled by a two-dimensional servo system with force readout supplied by measuring the required servo motor currents. Each servo loop consists of a position sensor, a compensating network, a power amplifier, and a servo motor. The criteria governing selection of these components are discussed, and detailed design information is presented. The system was found to have excellent repeatability, linearity better than 1 percent, and cross coupling between dimensions less than 1 percent.     

Acousto-optic parallel read/write head for optical disk data storage

Parallel read and write of optical disks has traditionally used a static grating for read or a linear array of independent lasers for read and write. Depending on the implementation, these systems suffer from coherent cross talk, excessive space between spots, and an inability to independently track. We show that a dynamic acousto-optic grating can generate multiple parallel read/write spots on the disk, each of which can be independently modulated and tracked and all of which are incoherent in less that a bit period. The resulting disk pickup can potentially reach gigabit per second transfer rates with only a modest increase in the drive complexity.     

磁光盘驱动器聚焦与循轨数字伺服系统

分析了磁光盘驱动器聚焦与循轨数字伺服系统各组成部分的原理，并设计和实现了一套基于ＤＳＰ的数字伺服控制系统，仿真和实验结果表明：这种数字伺服系统的设计方案简化了电路设计，系统性能良好，是一种理想的系统实现方式。     

3 Gb/in2 recording demonstration with dual element headsand thin film disks

We have successfully demonstrated magnetic recording at an areal density of 3 Gb/in² with narrow track inductive-write MR-read dual element heads on low noise Co alloy thin film disks. In this demonstration, the write head is a ten turn thin film inductive head with thick and narrow P2 pole-tips. The read head is a shielded ~1 μm trackwidth MR sensor soft-film biased in the read region for linearization and exchange-biased at the tail regions for magnetic stabilization. During recording tests, the heads were flown over low noise Co-alloy media at a clearance similar to that in the previous 1 Gb/in² recording experiment. Results showed good writability from the narrow track write head in terms of overwrite and hard transition shift. Readback yields symmetrical signals as large as 600 μV (p-p) and rolloff measurements showed 50% densities as high as 5000 fc/mm. Track profile and microtrack profile measurements showed the write and read trackwidths to be ~1.4 μm and ~1.1 μm respectively, with tight side-writing and side-reading characteristics. An overall assessment of the parametric recording results suggested areal density feasibility up to as high as 3 Gb/in². This projection was confirmed by error rate performance testing using a PRML channel with a digital filter and write precompensation. At a data rate of 4-5 Mb/s and at very low ontrack error, a linear density as high as 185 Kbpi and an optimized track pitch as narrow as 1.5 μm were achieved, corresponding to an areal recording density of ~3.1 Gb/in²     

Comparative study of MR head designs with abutted vs. overlaidpermanent magnet bias

We have developed Soft-Adjacent-Layer (SAL)-biased MR head products with two types of permanent magnet (PM) domain stabilization schemes, namely, abutted and overlaid. Both types of MR heads are capable of achieving recording density of 500 Mb/in² with linear bit densities ranging from 100-118 kbpi. While transfer curves with a transverse sweeping field showed similar behavior for both PM schemes, the overlaid structure has a large hysteresis in its transfer curve under a longitudinal sweeping field while the abutted junction was hysteresis free. Micromagnetic modeling showed that the hysteresis for the overlaid scheme arises from the varying field response of the MR element in the active region, under the leads and under the PM. Magnetic test shows that the abutted MR head with a PM defined TW had a slower rise in sensitivity and had an effectively narrower magnetic track width. On the other hand, the overlaid MR head with an active region defined by the leads with MR under them has a steeper rise in sensitivity, a broadened magnetic read track and side readings