Narrow Track Pitch Research Articles

Bidirectional Decision Feedback Modified Viterbi Detection (BD-DFMV) for Shingled Bit-Patterned Magnetic Recording (BPMR) With 2D Sectors and Alternating Track Widths

Shingled bit-patterned media (BPM) recording is a promising candidate to improve the magnetic recording density above 1 Tb/in2 while keeping the conventional writer design. However, shingled magnetic recording (SMR) leads to a narrower track pitch, and thus increased inter-track interference motivating the investigation of array readers and 2-D sectors. In this paper, we investigate SMR on BPM with 2-D sectors at the channel density of 3.71 Tb/in2. Use of 2-D sectors allows the possibility of using either hard or soft information from already detected tracks in that 2-D sector to improve the detection performance on the current track. An important question in this context is whether all tracks in a 2-D sector should be of the same nominal width or if there is a potential benefit to using different widths for adjacent tracks. Based on our investigation, we show that alternating track widths [i.e., one less-trimmed (fat) track followed by one more-trimmed (narrow) track] are beneficial to a bit error rate (BER) perspective. In particular, we propose and investigate the use of a bidirectional decision feedback modified Viterbi (BD-DFMV) detector to improve the BER performance. Our simulations indicate that for a hypothetical 2-D sector with 13 tracks and 5% media noise and at the target BER of 10−2, using alternating track widths and readback with a single reader, the BD-DFMV algorithm can provide about 1.6 dB signal-to-noise ratio (SNR) gain, compared with using the same detection algorithm but on a 2-D sector with uniform track widths. When using a reader array instead of a single reader, this SNR gain improves to about 2.0 dB. The alternating track width structure also appears to provide more robustness to track mis-registration compared with the 2-D uniform track width case.

Investigation for Mechanism of Far Track Interference in Hard Disk Drives

In hard disk drives (HDDs) using perpendicular magnetic recording, an adjacent track interference (ATI) is a major barrier to increasing areal density because both strong writing and narrow track pitch are required. Recent head/disk combinations have exhibited not only ATI, but also a phenomenon we call far track interference (FTI). FTI is a gradual magnetic damage to written data on nearby but non-adjacent tracks that accumulate with increasing number of writes. We developed HDD level measurement ways to detect FTI and investigate the FTI mechanism considering skew angle and external field effects. We also confirmed that the minor shape change of wrap around shield (WAS) causes difference of magnetic domain structures on WAS and FTI incidence. The results support the hypothesis that the FTI is created by structures of magnetic domains and walls on a WAS of a single-pole head on the HDD.

Super-Resolution Optical Disc with Radial Density Increased by Narrowed Track Pitch Corresponding to Diffraction Limit

In–Sb-based super-resolution near-field structure read-only-memory discs with radial density increased by introducing a narrow track pitch corresponding to the diffraction limit of an optical system were developed. Using an optical system with a laser diode with a wavelength of 405 nm and an objective lens with a numerical aperture of 0.85, we confirmed that differential phase detection (DPD) could detect track errors from disc samples recorded random data including a minimum pit length of 75 nm in a 240 nm track period. It has higher capability of track error detection than push–pull detection at a narrowed track pitch. Moreover, bit error rates satisfying the criterion of 3.0×10-4 were experimentally obtained for 66.7-GB-capacity disc samples with a 240 nm track pitch through signal processing with the partial response maximum likelihood of the (1,2,2,1)-type, by applying DPD to tracking servo control. The feasibility of increasing the track density of the Blu-ray DiscTM physical format by 1.33 times was indicated.

Combined Feedforward/Feedback Control for Tape Head Track-Following Servo Systems

AbstractIn order to remain competitive with other data storage technologies, the tape industry is planning to significantly increase track densities on tape. Higher track densities require narrower track pitch and more precise positioning of the read/write heads. Hence, improved track-following servo systems for the head assembly are needed. Longitudinal transport of the tape along the tape path is subject to lateral disturbances that cause displacement between the head and the desired track and hence increase position error. This paper proposes a combined feedforward/feedback control scheme to improve the track-following performance of the tape head positioning system in the presence of lateral tape motion (LTM). In this architecture, the feedback controller is designed to guarantee system stability and take into account the lower frequency components in the disturbances. The feedforward controller requires as its input a prediction of the lateral tape motion displacement (LTMD) at the head assembly. We propose a least squares based algorithm to predict the LTMD at the head from a history of upstream and downstream LTMD measurements. Compared to using feedback-only control, the combined feedforward/feedback controller enables the head assembly to be positioned more accurately over the desired track.

Channel Modeling and Capacity Bounds for Two-Dimensional Magnetic Recording

Two-dimensional magnetic recording (TDMR) differs from traditional track recording in that the bit-size-to-grain-size ratio is drastically reduced so that the channel bits are roughly of the same sizes as those of the magnetic media grains. This is envisioned to be achieved by shingled writing, a write-process in which the corner-writer partially overwrites the previously written adjacent track, effectively writing very closely spaced narrow tracks with no guard bands. Since the tracks are very narrow, they can be read by an array of read-elements whose spacing is equal to the narrow track pitch, creating a two-dimensional (2-D) array of readback signals (the two dimensions being the cross-track and the down-track dimensions). In the absence of an array of read elements, the same 2-D readback can be obtained by progressive scans of a single read element that reads one narrow track at a time and stores the readback signal in a 2-D array. In this paper, we present simple magnetic grain media models and shingled-write process models that capture the essence of TDMR. We assume that the granular recording medium consists of randomly shaped tiles (each tile represents a grain), randomly covering the medium plane. We then derive a suitable 2-D read/write process model. Using proper information-theoretic inequalities and bounding techniques, we derive methods to bound the capacities of TDMR channels. The results reveal that information capacities in excess of 0.6 user bits per grain are possible to attain over TDMR channels.

Long‐span seek control system for hard disk drive without mode‐switching

AbstractIn hard disk drives (HDD) there are two control modes: the head positioning control mode and the other is the seek control mode. In the head positioning control mode, a feedback controller is optimally designed to suppress disturbances. In the long‐span seek mode, a velocity feedback control system is applied in order to move the heads fast. Thus, an HDD has multiple control systems, and the head is moved to the target position while changing from one control system to the other. However, changing the control system causes a discontinuous control signal, which activates the resonant mode of an actuator. Past methods can only decrease discontinuous control, and therefore a single control system that can be used for both a seek control mode and a head positioning control mode is necessary for a narrow track pitch. In the proposed method, the feedback controller is decomposed into an integrator and a phase compensator. The VCM model is updated by the output of the phase compensator, and the integrator and the output of the velocity feedback controller control the VCM. The validity of the proposed method was confirmed by numerical and experimental results using a miniature 2.5‐inch hard disk drive. © 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 171(3): 51–60, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20935

Estimation of Maximum Track Density in Shingled Writing

In this paper, the feasibility of two-dimensional magnetic recording is discussed from the viewpoint of high track density. 747 curves of shingled track were measured using existing heads and media and were interpreted with a squeezing model dominated by the SN ratio of the read head output. As long as a reader can be placed on the median line of the squeezed data area, wider OTC than that of conventional writing will result. The modeled 747 curves were in good agreement with the experimental data for both conventional writing and shingled writing. The feasibility of narrow track pitch shingled writing was shown, however, write-to-write misregistration should be minimized to maximize the track density.

Long-Span Seek Control System for Hard Disk Drive without Mode-Switching

In Hard Disk Drive (HDD), there are two control modes. One is a head positioning control mode, the other is a seek control mode. In the head positioning control mode, a feedback controller is optimally designed to suppress a disturbance. In the long span seek mode, a velocity feedback control system is applied in order to move a head fast. Thus, a HDD has plural control systems, and the head is moved to the target position while changing from one control system to the other. However, the changing of the control system causes a discontinuous control signal, which activates the resonance mode of an actuator. The past methods only can decrease the discontinuous control. Therefore, a single control system that can be used for both a seek control mode and a head positioning control mode is necessary for narrow track pitch. In the proposed method, the feedback controller is decomposed to an integrator and a phase compensator. The VCM model is updated by the output of the phase compensator, and the integrator and the output of the velocity feedback controller control the VCM. The validity of the proposed method was confirmed by numerical and experimental results using a miniature 2.5-inch Hard Disk Drive.

Numerical Analysis of Detected Signal Characteristics from a Blue Laser Optical Disk Model with Random Rough Surfaces by FDTD Method

In order to realize a higher density version of the conventional optical disk, shorter wavelength laser and narrower track pitch have been put to practical use. However, using narrow track pitch can cause the increase of the crosstalk from the adjacent tracks. Moreover, the use of narrow pitch and short wavelength can also give rise to the increase of deterioration of the detected signal characteristics due to the microscopic roughness of disk surface. In this paper, in order to estimate the effect of surface roughness theoretically, we try to analyze the light-beam scattering and detected signal characteristics of a blue laser optical disk model with random rough surfaces by the Finite Difference Time Domain (FDTD) method.

Capacity Increase in Radial Direction of Super-Resolution Near-Field Structure Read-Only-Memory Disc

We propose a novel super-resolution near-field structure (super-RENS) read-only-memory (ROM) disc with a narrow track pitch using a group tracking technique. The group tracking technique enables the tracking control of an optical disc because it consists of track pitch sizes that are less than the diffraction limit. The group tracking technique to increases capacity in the radial direction of the super-RENS ROM disc. By using this technique, carrier-to-noise ratio (CNR) was obtained at 33 dB and the minimum crosstalk level was -23 dB at the 100 nm pit length. A readout track was selected by controlling the offset level of the tracking error signal in a high definition digital versatile disc (HD-DVD) system. It was confirmed that the capacity of the super-RENS ROM disc with the group tracking structure was improved in the radial direction.

Design of Domain Wall Displacement Detection Disk with Groove Recording for High Density and Wide Recording Power Tolerance

Domain wall displacement detection (DWDD) magneto-optical (MO) recording is the most useful method for achieving high linear density recording without using a higher numerical aperture (NA) objective lenses. Securing a wide recording power tolerance is very important to realize a high density optical storage system. From this point of view, we designed the DWDD disk with groove recording using blue laser and NA 0.60 optics. We optimized the heat sink structure and the displacement layer for groove recording, and we could improve the bit error rate with a track pitch of 0.44 µm and a high linear density of 0.07 µm/bit, which corresponds to 21 Gbit/inch2. Additionally, we developed groove substrates using reactive ion etching (RIE) and O2 plasma treatment. We found that the land shape was very important for the recording power tolerance, and we could obtain a wide recording power tolerance with a bit length of 0.09 µm. We confirmed that we could achieve higher density by using a narrower track pitch while maintaining sufficient recording power tolerance.

High-Density Groove Mastering Using an Electron Beam Recorder and Plasma Etching Process

An electron beam recorder (EBR) has been confirmed to have high-resolution performance and we think it is a promising recorder to produce a master stamper for the next-generation high-density optical disks. Because the high-density groove recording disk requires a narrower track pitch, the improvement of the recording resolution of the mastering recorder is necessary. However, owing to electron scattering, fabricating a groove structure was difficult even when we use a finely controlled conventional EB mastering process. A plasma etching process is an expected process as one of the solutions. Consequently, we adopted the plasma etching process for high-density groove disk mastering. A data bit length of 112 nm/bit was successfully recorded on the groove disk at a wavelength of 405 nm and an objective lens of 0.85 numerical aperture.

High-Track-Density Optical Disc by Radial Direction Partial Response

We propose a new recording method, the radial direction partial response (RPR), with which we are able to use a narrow track pitch. This method can be used only for optical read-only-memory discs. We present experimental results of this method combined with the multilevel recording technique called “single carrier independent pit edge recording” (SCIPER). (The combined method is called SCIPER-RPR). When a track pitch of 0.45 µm and 0.28 µm/bit in linear recording density is read out by a digital versatile disc (DVD) equivalent optical pickup, we confirmed a bit error rate of 1.5×10-4.

TbFeCo as a Perpendicular Magnetic Recording Material.

TbFeCo perpendicular magnetic recording media were developed by employing a NiP underlayer to control the magnetic properties of TbFeCo magnetic layer. With the NiP underlayer the magnetization reversal changed from the wall motion to the rotation mode. Dynamic write-read characteristics of a single-layer medium were examined using a conventional merge-type GMR head with a write core width of 1 μm that was designed for longitudinal magnetic recording. A clear magnetic pattern of 450 kFCI, high media SNR and acceptable overwrite properties were obtained. It was shown the thermal stability of the TbFeCo medium is good enough for practical applications. A double-layer medium with a soft-magnetic backlayer exhibited a practical overwritability for a narrow track pitch of 0.4μm.

Crosstalk canceling for laser-assisted magnetic recording

A promising method of crosstalk canceling is proposed for laser-assisted magnetic recording, which makes it possible to record and read with a narrow track pitch limited to nearly the size of a focused laser spot. In this method, a track pitch narrower than the laser spot size is obtained by utilizing the canceling of the leakage flux from an adjacent track. We achieved a crosstalk of −24 dB or less with a 0.7 μm track pitch in laser-assisted magnetic recording by using a laser spot size of 1.07 μm in diameter and a magnetoresistive head with a track width of 1.4 μm.

Improvement of Performance of a Tracking Servo System for an Optical Disk Drive

A tracking servo system for high-speed and precise tracking control is proposed. An optical disk camera-recorder to record and retrieve high-definition television signals is being developed. The camera-recorder must have both a high data transfer rate and a large capacity. These requirements will be fulfilled by narrowing the track pitch on the disk and by increasing the disk rotation speed. Generally, optical disks have a radial run-out of up to approximately 100 µm peak-to-peak. The tracking servo system must have precise control for the narrow track pitch and high-speed control for the high disk rotation speed. To meet these requirements, we propose a tracking servo system consisting of a feedback controller and of a feedforward controller which employs a pre-compensator applying the zero phase error tracking method. The simulation results show reduction of the residual tracking error and also an increase in the disk rotation speed.

Noise Improvement for a High NA MO System

A feasibility study of a magneto-optical(MO) disk for an NA 0.85 system is presented. The disk noise level in this system was relatively large compared to the conventional NA 0.55 system at the recording density corresponding to this spot size. Two methods were introduced for the improvement. One was the optimization of the groove structure, and the other was the reduction of the metal surface roughness. The former was found to be the typical noise source for optics for a narrower track pitch disk. A UV ashing apparatus was utilized for reduction of the noise. The latter noise source was characteristic for the surface MO recording, and the etching process was found effective for the improvement. By using both these methods, the noise level was drastically decreased, achieving high areal density of 7 Gb/inch2.

Short-marks recording characteristics of laser-pumped magnetic-field-modulation recording in a narrow track pitch and on magnetically induced superresolution disks

We evaluate laser-pumped magnetic-field-modulation (LP-MFM), continuous laser-irradiated magnetic-field-modulation, andlight-intensity-modulation recording in magneto-optical disks, usingtemperature calculations and experiments. LP-MFM is useful forrecording short marks: One can record magnetic domains as small as 0.15-mum marks with LP-MFM by using a spin-polarized scanningelectron microscope. LP-MFM is also useful for reducing the trackpitch. Experiments showed that no cross write occurred at a 0.6-mum track pitch, and calculation showed that the heatdistribution is narrow. We confirmed the advantages of LP-MFM forrecording magnetically induced superresolution disks by investigatingthe difference in the shapes of the recorded domains.

Off-Track Overwrite Characteristics of a Single-Pole Flying Head

A submicron track width of 0.4μm was observed by measuring an MFM image with a combination of a single-pole head and a perpendicular double-layered medium. Because of its sharp head field distribution, this combination is considered capable of obtaining a very narrow track-pitch. By measuring the off-track overwrite profiles with a thin-film single-pole flying head, we found that the erase band width was around 25 nm, which was acceptable for merged MR head and perpendicular double-layered medium.

Dependence of side-erasing effect and core efficiency on side taper angle of high Bs metal in gap head

The side-erasing effect is a very serious problem, especially in guardband-less recording systems with a narrow track pitch (e.g., 10 μm pitch of consumer digital VCRs). We report the dependence of side-erasing effect and output voltage on the oblique side taper plane angle of high Bs (Fe–Zr–N) metal in gap (MIG) heads. We measured the side-erasing width written by several kinds of MIG heads (gap length=0.21 μm, track width=14 μm, gap depth=12 μm) with various side taper angles (45°, 60°, and 90°) between the gap plane and the side taper plane on metal evaporated tapes and on two types of particulate tapes. Measurement conditions were 10.2 m/s relative velocity, 2 μm wave length, and 720, 960, and 1750 Oe Hc tapes. The side-erasing width decreases with increasing side taper angle in the MIG head or with increasing tape Hc. The difference between the side-erasing width for a 60° and a 90° head is small. However, the output voltage decreases with increasing side taper angle. The normalized output voltage of 45° head at 20.4 MHz is superior 0.58 and 2.38 dB to the 60° and 90° head, respectively. Simulation results of the side-fringing field and the core efficiency using a three-dimensional finite element method, incorporating the decreasing of Fe–Zr–N permeability on tilted substrates, indicate a similar trend to the experimental results. The 60° side taper angle MIG head is more suitable than the 45° and 90° heads for high density and guardband-less recording, because it is a good compromise between output voltage and side-erasing width.