Cross-track Profile Research Articles

Topology optimization of a heat-assisted magnetic recording write head to reduce transition curvature using a binary optimization algorithm utilizing the adjoint method

In this work, the possibility to reduce transition curvature in heat-assisted magnetic recording, using a conventional write head design, by shaping the recording field to counteract the circular profile of the heat pulse is investigated. Topology optimization of the head tip is performed in order to create the desired cross-track field profile for increasing distances from the write head tip. For the topology optimization, the adjoint method is utilized to calculate the necessary gradients and a binary optimization scheme is proposed. The optimizations are performed considering linearized material parameters reducing the computational complexity and the results are compared to optimizations incorporating the full non-linear material behavior. The optimized field profiles are evaluated for their influence on the read-back process. To do so, switching probability phase diagrams are calculated and the curvature parameter, the signal to noise ratio and the channel bit density are extracted. The presented results show that while transition curvature can be reduced by shaping the cross-track profile of the write field, this alone does not consequently lead to an improvement of the read back process. Therefore, completely new head designs, considering additional parameters have to be investigated.

Joint Multi-Level Readback for Array-Reader-Based Interlaced Magnetic Recording

In this study, a novel joint multi-track multi-level (MTML) read channel signal processing scheme is proposed to enhance the data transfer rate in an array-reader-based interlaced magnetic recording (ARIMR). Wide individual readers capture mixtures of multiple track contributions and proposed MTML read channel effectively disaggregates individual track data for joint track data retrieval in a single revolution. Numerical evaluations with the micro-pixelated magnetic channel model shows improvements of the cross-track profile of bit error rate with the proposed MTML-ARIMR approach for a dual-reader and triple-track readback configuration.

Road traffic crash characteristics of drivers who take prescription medicines that carry a risk to driving

BackgroundThe specific features of crashes involving an alcohol-intoxicated driver have been extensively characterized, but no such data are available for crashes involving a driver who has used a prescription medicine, which could help to plan and target prevention and control initiatives. This study aimed to describe the characteristics of crashes involving drivers under the influence of prescription medicines. MethodsWe took advantage of CESIR, a French record linkage study for which data were extracted and matched from three French national databases: police reports, the national police database of injurious crashes and the national health care insurance database (HCI database). The drivers included in the study were those involved in an injurious road crash in France from July 1, 2005 to December 31, 2015, whose national identity number, date of birth and sex allowed matching. Prescription medicines considered were those with the two highest levels of warning. ResultsSimilar crash profiles were found when drivers used alcohol or medicines, particularly with respect to injury severity, type of vehicle, type of collision, type of road and cross-track profile. Alcohol-related crashes were over-represented during weekends and in low-density areas and medicine-related crashes were over-represented during weekdays and in cities of fewer than 300 000 inhabitants. While a much higher strength of association with responsibility was found for alcohol than for medicines, the proportion of crashes with drivers using medicines was twice as high as crashes with drivers using alcohol. ConclusionThe lower risk carried by medicines is therefore in part compensated by a higher prevalence of use, making medicines one of the hidden factors of road risk. Characterizing these crashes will therefore be useful to better focus road safety intervention on the prevention of driving under the influence of psychotropic medicines.

A Study of TDMR for Magnetic Tape Systems

Multiple-input single-output two-dimensional magnetic recording gain in a magnetic tape system was studied under the condition where the bit aspect ratio is 22%-37% compared to current tape systems. The performance was mainly evaluated in terms of signal-to-noise ratio (SNR) cross-track profile utilizing a 2-D read-back signal synthesized from the waveform acquired with a conventional single-reader tape head. The results show that, depending on the reader pitch selected, either significant gain in tracking margin up to 215% compared to the traditional 1-D signal-processing scheme or an additional SNR gain accompanied with a moderate increase in approximately 30% in tracking margin can be achieved by utilizing the 2-D equalization.

2-D Equalization With Location Diversity and Pre-Adaptation to Handle Off-Track Variation in Array-Reader-Based Hard Disk Drives

The hard disk drive industry is transitioning into array-reader-based magnetic recording (ARMR) to provide continued growth in recording density. A 2-D equalizer acts to electronically steer the array reader to provide optimal signal pickup from the track, thereby resulting in wider and deeper cross-track profile in error-rate performance. To realize this performance, the equalizer coefficients must be matched to the location of array reader, which are not known precisely due to position errors during write and read. In this paper, we present a 2-D equalization strategy that uses location diversity and pre-adaptation to mitigate the mismatch between the reader location and equalizer coefficients. The data decisions driving the various loops are generated using three 2-D equalizers (i.e., loop equalizer) that cover the range of expected offtrack. A delayed equalization strategy that permits pre-adaptation of the equalizer coefficients is used to mitigate the effect of off-track variation in the 2-D equalizer (i.e., back-end equalizer) that feeds the iterative detector. The proposed approach shows significantly improved read performance in the presence of large offtracks.

Cross-Track Resolution of Readers With Shield Tabs

We study the effects of patterned shield tabs on the cross-track resolution of TMR read heads. Patterning tabs in the cross-track direction expands the design space in which to tune the magnetic width of the reader. We use a finite-element micromagnetic code to explore separately the effects of both the lower and upper shield tabs' thicknesses and intrinsic moments. Cross-track profiles can be narrowed by increasing the moment of a thin upper shield tab. The slight widening of profiles due to a bottom shield tab may not be detrimental to overall bit density and balanced by an overall gain in down-track resolution.

Satellite Payload Motion for Remote Sensing

Abstract The paper presented below discusses the scanning motion of payloads on satellites in the context of multi body dynamics, the control systems used to control the scan motion and their effect on image stability, jitter and pointing accuracy. Remote Sensing Satellites usually have a number of payloads and each of them scans in a specific manner as determined by its purpose. This scanning motion has an effect on the spacecraft itself, and the attitude motion produced will thus disturb the payload's motion causing jitter and change in the image being recorded in the payload. The satellite must have adequate attitude control in order to cancel out these motions and meet the pointing accuracy and stability requirements as demanded by the payload. The geometry of the scan mechanisms are presented in order to derive their ideal ground track (representing image recorded by the payload), and to calculate the ideal in-track, cross-track or other angle profile that must be generated by the scan mechanism to obtain that ground track. A simulation of the multi-body system is presented, using the equations of motion on the satellite and its payload. A control system is then added to the payload which is articulated to the satellite in order to produce the above motion and another control system is added to the satellite in order to maintain its stability and compensate for all the disturbing torques produced by satellite scan mechanisms. We investigate if the motion produced meets the requirements for pointing accuracy and jitter stability for the payload, and if not, develop strategies to modify the control systems in order to accomplish this.

Cross-track pulse shape and nonlinear loss as a function of frequency and side track erasure in perpendicular magnetic recording systems

Many previous publications have shown signal amplitude of magnetic recording systems decrease as cross-track run-out or linear density increases. This study expands on this work by characterizing cross-track transition width (T50) and nonlinear partial erasure (NLPE) changes with linear density, read-back location, and track edge erasure. Both T50 and NLPE data shown in this paper were calculated from filtered odd harmonic amplitude cross-track profiles. This analysis is novel, in that it establishes a model which describes why NLPE and T50 change by read-back location, linear density, and track-edge erasure due to non-uniform first and third harmonic widths.

Exploring the Benefits of Using CryoSat-2's Cross-Track Interferometry to Improve the Resolution of Multisatellite Mesoscale Fields

Abstract Sea surface height (SSH) measurements provided by pulse-limited radar altimeters are one-dimensional profiles along the satellite's nadir track, with no information whatsoever in the cross-track direction. The anisotropy of resulting SSH profiles is the most limiting factor of mesoscale SSH maps that merge the 1D profiles. This paper explores the potential of the cross-track slope derived from the Cryosphere Satellite-2 (CryoSat-2)'s synthetic aperture radar interferometry (SARin) mode to increase the resolution of mesoscale fields in the cross-track direction. Through idealized 1D simulations, this study shows that it is possible to exploit the dual SARin measurement (cross-track slope and SSH profile) in order to constrain mesoscale mapping in the cross-track direction. An error-free SSH slope allows a single SARin instrument to recover almost as much SSH variance as two coordinated altimeters. Noise-corrupted slopes can also be exploited to improve the mapping, and a breakthrough is observed for SARin errors ranging from 1 to 5 μrad for 150-km-radius features in strong currents, and 0.1–0.5 μrad for global mesoscale. Although only limited experiments might be possible with the error level of current CryoSat-2 data, this paper shows the potential of the SAR interferometry technology to reduce the anisotropy of altimeter measurements if the SARin error is significantly reduced in the future, and in particular in the context of a prospective SARin demonstrator optimized for oceanography.

Characterization of the Reader Width Using the Micro-Track Test in Perpendicular Recording

Micro-track test is a valuable tool used to characterize the electrical reader width in perpendicular recording. The test is based on trimming both sides of a track written with a single tone and followed by a cross-track scan of the remaining track. The reader cross-track profile or read sensitivity function is characterized by a narrow band measurement of the read-back signal as a function of cross-track position. With increasing track density, the signal amplitude decreases due to incomplete media saturation and to an increase in the off-track noise due to percolation and opposite polarity media saturation. These effects cause a lump at the edge of the micro-track profile that changes the width of the profile (MT10 and MT50). Micro-magnetic analysis indicates that the off-track demagnetization field creates a track-edge section in the media with opposite polarity relative to the originally written track. This section is coherent with the main track and decreases the read-back signal during the scan of the micro-track. Consequently, the shape of the cross-track read sensitivity function becomes distorted, causing a reduction in MT10 and a side lump. The solution to this problem is to increase the amplitude of the micro-track read-back signal by reduced trimming and by the use of a higher density (KFCI) in the main track that minimizes the off-track demagnetization field. These test improvements are shown to preserve the integrity of the micro-track profile and to enable an accurate measurement of MT10 and MT50. Optimizing reader and writer widths in perpendicular recording requires an accurate measurement of these parameters.

Off-Transition Noise Characterization During Head Thermal Protrusion

This paper characterizes the read-back off-transition noise (OTN) in spectral, spatial, and time domains during the thermal protrusion of a magnetic head. In perpendicular recording, the OTN is typically lower than the background noise, and shows bathtub-like cross-track profile. The spatial and spectral characteristics of the read-back OTN make it sensitive to the head positing change in cross-track, down-tack, and vertical directions. In contrast to the conventional on-transition signal reading, the OTN monitoring reverses the usage of signal and noise and lowers the requirement on signal quality. Monitoring OTN during head's thermal control can detect air bearing's stability, head's fly-ability, and head-lube/disk contact without Fourier transform (FT) related techniques. A single-track and single read-frequency head-lube/disk contact detection method with high simplicity, high sensitivity, and high repeatability is introduced.

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.

Comparison of Track Densities of Discrete Track Media and Continuous Media: A Numerical Model

We have compared track density capabilities of discrete track media and continuous perpendicular media by using a numerical model of off-track capability. In the model, off-track capability is obtained from the signal-to-noise ratio (SNR) cross-track profile. We have investigated the effects of read width, erase bandwidth, write width, and on-track media SNR. Our simulations show that the track density for discrete track media can be 25%-35% higher than that for continuous media. Track density advantage of discrete track media is reduced when the erase band width becomes wider or the on-track media SNR is higher.

Impact of Media Design on Track Edge in Perpendicular Magnetic Recording

As track density increases, it has become even more critical to understand the track edge. In this study, the impact of media design on the track edges in perpendicular magnetic recording has been investigated. This study shows that soft under layer thickness and intergranular exchange coupling have significant impact on both track edge erasure and write width. In the range we studied, head-to-keeper spacing and media coercivity have a strong impact on write width, but not on track edge erasure. On the other hand, media design does not impact transition curvature significantly. Experimental observations were supported by media micromagnetic modeling results and write field simulation using a finite element model. Analyzing the write bubble helps to understand the dependence of adjacent track encroachment on media design. Cross track profiles of various signal-to-noise ratio (SNR) components can provide valuable insights into different track edge mechanisms.

Integrated Heat Assisted Magnetic Recording Head: Design and Recording Demonstration

Scaling the areal density, while maintaining a proper balance between media signal-to-noise ratio, thermal stability, and writability, will soon require an alternative recording technology. Heat assisted magnetic recording (HAMR) can achieve this balance by allowing high anisotropy media to be written by heating the media during the writing process (e.g., by laser light) to temporarily lower the anisotropy. Three major challenges of designing a HAMR head that tightly focuses light and collocates it with the magnetic field are discussed: 1) magnetic field delivery; 2) optical delivery; and 3) magnetic and optical field delivery integration. Thousands of these HAMR heads were built into sliders and head-gimbal assemblies, and optical and scanning electron micrograph images are shown. Scanning near-field optical microscopy (SNOM) characterization of the HAMR head shows that the predicted ~ lambda/4 full-width half-maximum (FWHM) spot size can be achieved using 488 nm light (124 nm was achieved). SNOM images also show that wafer level fabricated apertures were able to effectively eliminate sidelobes from the focused spot intensity profile. A magnetic force microscopy image of HAMR media shows that non-HAMR (laser power off) was not able to write transitions in the HAMR specific media even at very high write currents, but transitions could be written using HAMR (laser power on), even at lower write currents. A cross-track profile is shown for a fully integrated HAMR head where the magnetic pole physical width is ~350 nm, but the written track is ~200 nm, which demonstrates HAMR. A HAMR optimization contour shows that there is an optimum write current and laser power and that simply going to the highest write current and laser power does not lead to the best recording. Lastly, some prospects for advancing HAMR are given and a few key problems to be solved are mentioned.

Characterization of media cross-track thermal profile in heat-assisted magnetic recording

In the heat-assisted magnetic recording, the thermal profile of the media affects the recording performance seriously. Unfortunately, there is no direct method to measure it dynamically. In this paper, an indirect method is proposed to characterize the cross-track thermal profile with the spin-stand. The experimental results indicate that the local temperature increase has nonlinear relationship with the laser power. As the laser power increases, the thermal profile width increases. The media has a higher temperature increase and a bigger thermal profile at a slower rotation speed. The reasons for these phenomena are explained as well.

Transition Curvature Assessment for High TPI Applications

Transition curvature is a growing concern for high tracks per inch application. Due to low resolution from the playback transducer in cross track direction, the characterization of the transition edge is very difficult. This situation increases the engineering difficulty for writer pole design optimization and recording performance analysis. We have derived a simple transition shape model to study the transition curvature quantitatively. The complicated transition edge is simplified into one curving parameter. With such simplification, we are able to relate the transition curving effect with the cross track profile. In this paper, we will discuss the concept and analytical derivation of this model. A particular H/M combination is studied and its transition curvature is estimated. Compared with magnetic force microscope and micromagnetic modeling, this new approach has potential to become a practical spinstand or even Hard disk drive measurement if the W/R nonlinear effects are minimized

Direction control of anisotropy in the soft-magnetic underlayer for L1 Fe–Pt perpendicular media

Induced anisotropy of soft-magnetic underlayers (SUL) were pinned to radial and circumferential directions in double-layered perpendicular media, and effects of the directions on recording properties were studied for Fe–Pt media. A medium with the SUL of radial anisotropy showed a sharper cross-track profile than that of a medium with the SUL of circumferential anisotropy. Furthermore, signal-to-noise ratio (SNR) of the former was found to be 4dB higher than that of the latter at 500kfrpi. A SUL of radial anisotropy with an anisotropy-dispersion narrower could result in suppressing the fluctuation of write-field gradient and lead to further high SNR.

Direct measurement of the field from a magnetic recording head using an InAs Hall sensor on a contact write/read tester

At 1Tbit/in2 areal density magnetic recording dimensions, reliable magnetic field metrology does not exist. One technique to map the spatial profile of the magnetic field of a write head is to use a contact read/write tester. A magnetic recording head is brought into contact with a Hall sensor, and is subsequently scanned with nm resolution. For a 300nm track width longitudinal recording head, the magnetic field of the head was mapped. Measurements include the down track field gradient and cross-track field profile and the current-field transfer curve. These results suggest this technique offers a viable write field metrology.

Ridge waveguide as a near field aperture for high density data storage

The performance of the ridge waveguide as a near-field aperture in data storage systems is investigated. Finite element method (FEM) and finite-difference time-domain (FDTD) based software are used in the numerical simulations. To verify their accuracy at optical frequencies, the FEM and FDTD are first compared to analytical results. The accuracy of these techniques for modeling ridge waveguides at optical frequencies is also evaluated by comparing their results with each other for a plane wave illumination. The FEM, which is capable of modeling focused beams, is then used to simulate various geometries involving ridge waveguides. Near-field radiation from ridge waveguide transducer is expressed in terms of power density quantities. Previous studies in the literature consider the performance of the transducer in free space, rather than in the presence of a recording magnetic medium. The effect of the recording magnetic medium on the transmission efficiency and spot size is discussed using numerical simulations. The effect of various geometric parameters on the optical spot size and transmission efficiency is investigated and discussed. Based on our numerical simulations, a promising transducer design is suggested to obtain intense optical spots well below the diffraction limit. Numerical simulations suggest that a full width at half maximum spot diameter of 31nm in the recording magnetic medium can be obtained. The maximum value of the absorbed optical power density in the recording medium is about 1.67×10−4mW∕nm3 for a 100mW input power. In-track and cross-track profiles for this design are compared with Gaussian distributions.