Adjacent Track Erasure Research Articles

Effectiveness of a pulsed laser in heat-assisted magnetic recording

Recently, much effort has been directed towards increasing the areal density of heat-assisted magnetic recording (HAMR). Here, we use our HAMR recording simulation that employs renormalized media parameters to examine the potential use of a pulsed laser instead of a continuous laser. Proper tuning of the synchronization between magnetic and laser pulses yields improved thermal gradients and comparable (or improved) recording performance relative to a continuous laser. Importantly, it also produces less average heat in the media, which is expected to improve near field transducer lifetime. Results also show that the optimized pulsed laser reduces adjacent track erasure relative to a continuous laser, which is important for non-shingled recording.

Analysis of Adjacent Track Erasure in the HAMR Media

Writing closely spaced tracks in the cross-track direction helps increase the storage density but leads to erasure of the adjacent tracks. This phenomenon is called adjacent track erasure (ATE). This article aims to establish the presence and numerical extent of ATE in different heat-assisted magnetic recording (HAMR) media. We find that thermal exchange coupled composite (ECC) media can be more susceptible to ATE compared with single layer FePt media of equivalent thickness. We implement optimization techniques to reduce the ATE in different media and increase the recording signal to noise ratio (SNR) in this process. While the ATE in single layer FePt media seems to be relatively impervious to changes in the applied field angle, small field angles appear to reduce the ATE for the high $T_{c}$ thermal ECC media. However, small angle also reduces the SNR, leading to the conclusion that 30° actually yields the best SNR even in the presence of overwrite. Introducing a finite intergranular exchange coupling improves the writing performance of the single layer FePt and reduces the ATE. We rigorously calculate the thermal decay constant and use it to evaluate the performance of pulsed laser recording. Finally, we establish a hypothesis that helps explain the presence of ATE in different HAMR media. We believe the ATE is proportional to the thermal stability factor at a temperature just below the write temperature. The proposed hypothesis correctly predicts the ATE in the single layer FePt media assuming coherent rotation. Our results can help increase the HAMR storage density and make HAMR media resistant to the ATE.

Adjacent Track Interference in Heat-Assisted Magnetic Recording: Impact and Implications

Some aspects of the challenges to high track density heat-assisted magnetic recording (HAMR) are addressed with a focus on track interferences (adjacent as well as non-nearest neighbor erasure). Comparisons with corresponding conventional perpendicular magnetic recording (PMR) parameters are also made. Adjacent and non-nearest neighbor track erasure data from functional HAMR and PMR drives and simulation data of signal-to-noise ratio loss in HAMR are also presented.

Study of Erase Band and Write Performance for Shingled Magnetic Recording With FePt-Based Exchanged Coupled Composite Media

The shingled magnetic recording (SMR) with $\text{L}1_{0}$ -FePt-based exchange coupled composite (ECC) media is a promising way to achieve ultrahigh recording density. During the writing process, the insensitive angular dependence of switching field provided by ECC media is found to play an important role in mitigating transition noise and adjacent track erasure, furthermore the investigation indicates that the skew angle (SA) and corner angle (CA) of wide main pole inevitably affect the erase band noise in SMR. Hence, this paper studies an improved calculation method of erase bandwidth (EBW) to evaluate the erase band noise more accurately, and investigates the impact of SA and CA on write performance. Additionally, by investigating the bit error rate of adjacent track’s bits caused by the erasure, the results show that the high angular tolerance of switching field provided by $L1_{0}$ -FePt(6 nm)/[Co/Ni]5(4 nm) ECC media can effectively reduce EBW. Finally, the analyses demonstrate that an optimum combination of writer CA, tapered angle, and suitable range of SA is helpful to reduce erase band noise and improve the write performance.

2-D Visualization in Magnetic Recording

The quality of magnetic recording can be assessed from 2-D distributions of the recorded readback signal, media, and read sensor noise. The procedure for generating spatial signal and noise distributions is based on cross-track data alignment using media noise as a reference. In order to analyze signal and noise statistics, multiple data periods are processed using synchronous averaging, resulting in average readback signal and rms noise voltage maps. Using multiple signal acquisitions from the same media location allows separate measurements of media and read sensor noise distributions. Fast 2-D mapping algorithms allow high-resolution visualization of arbitrary data patterns, pole footprint imaging, and encroachment of the writer field onto adjacent tracks, providing detailed measurements of recording parameters (media jitter, dc noise saturation, signal-dependent read sensor noise, transition curvature, edge noise, and others). Recently, we have introduced a highly sensitive switching probability averaging measurement (SPAM), which allows the detection of media grain switching with unprecedented sensitivity (better than one part in 1000). This 2-D technique provides quantitative assessment of the magnetic field coming out of the write head and domain wall activity, which contributes to undesirable adjacent track erasure.

Magnetic force microscopy study on wide adjacent track erasure in perpendicular magnetic write heads

We used a phase-contrast magnetic force microscopy (MFM) to observe and analyze the failure of magnetic write heads due to the WATEr problem, which limits the off-track performance. During MFM imaging, the magnetic write head was energized by a DC current. The induced out-of-plane magnetic field was then detected by scanning a MFM probe across the surface of the magnetic write head. MFM images were then mapped with WATEr measured results from a spin stand method. Results showed that WATEr effect can be generated by several factors, i.e. the structure of magnetic domains and walls from material discontinuities and the magnetic field leakage at different locations on magnetic write heads. Understanding WATEr mechanisms is useful for design and process development engineers.

Microwave-Assisted Magnetic Recording on Dual-Thickness and Dual-Layer Bit-Patterned Media

This paper describes how microwave-assisted magnetic recording can be used to reduce adjacent track erasure and increase areal density in dual-thickness and dual-layer bit-patterned media. By tuning the high-frequency magnetic field, selective switching of dots in different layers or dots with different thicknesses can be achieved. Offsetting arrays of dots in different layers makes it easier to detect the signal from each layer, and areal densities of 5 Tb/in2, or more, may be possible.

Adjacent and Far Track Erasure Dependence on Media Soft Underlayer Permeability in Shielded Head Recording

The permeability of the magnetic soft underlayer (SUL) was systematically varied by changing the antiferromagnetic exchange coupling, and the effect on recording performance and adjacent track erasure (ATE) and far track erasure (FTE) was investigated. Perpendicular magnetic recording performance highly depends on SUL permeability. Highly permeable SUL can reduce the head erasure field located at far tracks from a stressing track but generates more erasure at adjacent tracks near the side shield. Optimum SUL permeability can improve on-track performance with tolerable ATE or FTE. Finite element method models of a shielded write head and media with different SUL permeabilities were performed. A high-permeability SUL has a larger impact on short-time high FTE fields and potentially affects the erasure at 3-6 tracks from main pole. This shows agreement with experimental results.

Simulation of Expected Areal Density Gain for Heat-Assisted Magnetic Recording Relative to Other Advanced Recording Schemes

In this paper, we review our latest progress for heat-assisted magnetic recording (HAMR) systems. The temperature distribution has been computed using the finite difference time domain technique and by solving the heat diffusion equation. Magnetic behavior has been calculated using a renormalized cell technique (granular media) and atomistic simulation [bit-patterned media (BPM)]. We find that the ultimate density of HAMR on granular media depends greatly on grain size, with a 5 nm grain pitch yielding $\sim 4$ Tb/in2. We were unable to exceed $\sim 5.8$ Tb/in2 on BPM owing to excessive heating of the adjacent track, causing adjacent track erasure to date. Shingled recording on granular media, using two dimensional magnetic recording (TDMR) detection depends greatly on the mechanical systems, with likely user densities reaching $\sim 2.5$ Tb/in2. Finally, shingled recording without heat assist on BPM yielded an unexpectedly high density of $\sim 8$ Tb/in2.

Skew Effect-Induced Track Erasure of Shingled Magnetic Recording System

One of the feasible methods to achieve ultrahigh areal density of magnetic storage system is shingled recording with bit patterned media technology. The adjacent track erasure (ATE) may take place under the impact of the skew footprint, which is associated with the initial slider state, the skewed pole, and the writing corner angle. A sequence of shingled trapezoidal shaped writing heads is modeled to explore the skew effect versus the writing performance. The results show that larger corner angle and narrow skew range are recommended to enhance the writing field and avoid the ATE. The analysis also suggests that hot-spot disk data in the region where the skew angle ranges from -20° to 0° has the lowest possibility to be potentially disrupted.

Investigations of stacking fault density in perpendicular recording media

In magnetic recording media, the grains or clusters reverse their magnetization over a range of reversal field, resulting in a switching field distribution. In order to achieve high areal densities, it is desirable to understand and minimize such a distribution. Clusters of grains which contain stacking faults (SF) or fcc phase have lower anisotropy, an order lower than those without them. It is believed that such low anisotropy regions reverse their magnetization at a much lower reversal field than the rest of the material with a larger anisotropy. Such clusters/grains cause recording performance deterioration, such as adjacent track erasure and dc noise. Therefore, the observation of clusters that reverse at very low reversal fields (nucleation sites, NS) could give information on the noise and the adjacent track erasure. Potentially, the observed clusters could also provide information on the SF. In this paper, we study the reversal of nucleation sites in granular perpendicular media based on a magnetic force microscope (MFM) methodology and validate the observations with high resolution cross-section transmission electron microscopy (HRTEM) measurements. Samples, wherein a high anisotropy CoPt layer was introduced to control the NS or SF in a systematic way, were evaluated by MFM, TEM, and magnetometry. The magnetic properties indicated that the thickness of the CoPt layer results in an increase of nucleation sites. TEM measurements indicated a correlation between the thickness of CoPt layer and the stacking fault density. A clear correlation was also observed between the MFM results, TEM observations, and the coercivity and nucleation field of the samples, validating the effectiveness of the proposed method in evaluating the nucleation sites which potentially arise from stacking faults.

Skew angle effects in shingled magnetic recording system with double/triple reader head array

Shingled Magnetic Recording (SMR) is a scheme used to extend the life of the current perpendicular magnetic recording technology. SMR enables writing narrow tracks with a wide writer. Currently, SMR employs a single reader and will suffer inter-track interference (ITI) as the tracks become comparable in width to the reader. ITI can be mitigated by using narrower readers; however, narrower readers suffer from increased reader noise. Another approach to combat ITI is to process 2D readback and use ITI cancellation schemes to retrieve the data track. Multiple readbacks can be obtained either with a single reader and multiple revolutions or with a reader array. The former suffers from increased readback latency. In this work, we focus on the latter. When using a reader array, the skew angle poses major challenges. During writing, there is increased adjacent track erasure, and during readback the effective reader pitch varies and there is an increase in the 2D intersymbol interference caused by the rotated reader profile. In this work, we run micromagnetic simulations at different skew angles to train the grain flipping probability model, and then evaluate raw bit channel error rate performance at skew. In particular, we investigate the performance degradation caused by skewing of the 2 or 3 read head array for various read-head geometries.

Noise Characterization in High-Areal Density Perpendicular and Shingled Magnetic Recording

In this paper, we present a technique of noise analysis for perpendicular and shingled magnetic recording that is variance centric, i.e., with analysis focused on noise power. Measured noise from a home track viewpoint is decomposed into electronic noise, transition noise, nontransition noise, and inter-track interference components. The separation of media noise into transition and nontransition noise components is accomplished by a variance deconvolution technique. Experimental noise analysis is shown for cases of track squeeze, adjacent track erasure, and read head offset. Potential capacity gains are also assessed for the case of electronic noise improvement.

HAMR Performance and Integration Challenges

Heat-assisted magnetic recording (HAMR) is a fast evolving technology, and has been established as the next enabler of higher areal density in magnetic recording. After achieving high areal density capability, HAMR drive integration was recently demonstrated. In this paper, we discuss some of the recent learning from component performance and drive integration. We identify a key challenge in HAMR integration: erasure due to thermal background heating. The background heating was introduced to improve near-field transducer reliability and reduce laser power requirement. We present experimental and modeling data on the impact of thermal background, both in the cross-track (adjacent-track erasure) and down-track (self-erasure) dimensions.

Micromagnetic Model Analysis of Planar-Type Write Head Field Response and Dependence on Pole Tip, Return Yoke, and Shield Structure

We used a micromagnetic model to investigate the high-frequency (1-5 GHz) response, peak recording field strength, recording field gradient, and adjacent track erasure (ATE) field for various main pole tip, shield, and return yoke structures in planar type heads. The effect of the pole tip area at the air-bearing surface (ABS) and the return yoke structure is also discussed. It was found that the pole tip and shield arrangement had a major effect on the peak recording field strength, recording field gradient, and ATE field, i.e., the trailing and side shields work to reduce the undesired field, however, they also slow the head field response due to the interaction between the main pole tip and the shield. The field responses are also dependent on the main pole tip-shield structures and pole tip structures. The return yoke thickness and area were important determinants of response speed and peak field.

Characterization of Adjacent Track Erasure in Perpendicular Recording by a Stationary Footprint Technique

As the perpendicular recording writer keeps scaling down to improve recording areal density and data rate throughput, the off-track performance of the writer, including both adjacent track erasure and wide area adjacent track erasure, becomes one of the key challenges in hard disk drives. Understanding writer erasure locations and mechanisms is critical. In this paper, stationary footprint technique is used to study various perpendicular writer designs. The asymmetric responses of two side shields in wrapped-around shielded writer are observed, which agree well with micromagnetic modeling results. The impacts of asymmetric side shields on cross track gradient and adjacent track erasure were qualitatively verified. The hot spot locations and mechanisms of wide area track erasure have been investigated. The impacts of recording media and recording reader in stationary footprint measurement are also discussed.

Advanced Micromagnetic Analysis of Write Head Dynamics Using Fastmag

Magnetization and magnetic field dynamics arising when switching a realistic recording head model is studied. The write head design comprises a return pole, yoke, main pole, tapered trapezoidal pole tip, tapered wrap around shield (WAS), and soft underlayer. The analysis was performed using the high-performance micromagnetic simulator FastMag, which is well suited for the write head dynamic problems due to its ability to handle complex magnetic devices discretized into many millions of elements. The head dynamics is considered for different mesh densities, switching data rates, and current waveforms. It is demonstrated that improper discretization may result in a very different magnetization behavior. This is especially pronounced for cases of high switching rates, for which meshes of insufficient density resulted in a completely incorrect behavior, e.g. absence of switching. On the other hand, sufficiently dense meshes resulted in reliable dynamics and switching behavior. Furthermore, magnetization dynamics effects in WAS and their effects on the magnetostatic fields in the media layer were studied. WAS significantly improves the head field gradients in both down- and off-track directions, which is important for high areal recording densities. However, the presence of WAS leads to reduced write fields below the pole tip and to significant undesired magnetostatic fields below the side shields in the media layer. Such undesired fields can be obtained close to the pole tip as well as far from the tip. These phenomena result from the domain wall creation, propagation, and annihilation in WAS due to the switching. The field close to the pole tip can result in adjacent track erasure, while fields far from the tip can lead to far track erasure. The existence of these fields should be accounted for when performing recording system design optimization and analysis.

Micromagnetic Analysis to Reduce Adjacent Track Erasure Field in Planar Write Heads

Micromagnetic analysis was carried out to reduce the adjacent track erasure (ATE) field in planar type recording write heads. First, a quasi-static head field analysis showed that a three-step tapered pole had advantages over one-step and two-step poles. Next, dynamic micromagnetics proved that smaller models had higher recording fields and lower ATE fields compared with larger models. Finally, it was shown that the planar head had advantages over conventional, single-pole-type (SPT) heads in respect of its larger recording field, faster response to a high-frequency current and smaller ATE field.

An Energy Barrier Model for Write Errors in Exchange-Spring Patterned Media

Bit-patterned media (BPM), in which the medium is patterned into nanometer-sized magnetic islands and each island stores one bit, offers the potential for improved thermal stability. In order to study the write-error rates of systems using BPM, a model that calculates the energy barrier of a single island in exchange-spring or exchange-coupled composite (ECC) BPM, has been developed. The model has been shown to agree well with micromagnetic simulations. A study of energy barriers showed that ECC islands can be designed to retain a greater energy barrier in the presence of an applied field than single layer islands. This indicates that such ECC islands will be less prone to thermally activated adjacent track erasure (ATE) than single layer islands.

Analysis of write-head synchronization and adjacent track erasure in bit patterned media using a statistical model

An analysis of the performance of a bit patterned media data storage system composed of nanoscale islands with variations in position and magnetic properties has been carried out. The statistical model of write errors includes adjacent track erasure and error rates are computed according to the down-track synchronization of the write head switching position and cross-track head position variations. Two-dimensional maps of bit error rates reveal that distributions of position and anisotropy have a severe impact on the performance of the system. Results show that head field cross-track gradients needs to be tightly controlled to minimize the effects of adjacent track erasure.