This study aims to investigate the impact that factors such as skew, radius, and transition curvature have on areal density capability in heat-assisted magnetic recording hard disk drives. We explore a “ballistic seek” approach for capturing in-situ scan line images of the magnetization footprint on the recording media, and extract parametric results of recording characteristics such as transition curvature. We take full advantage of the significantly improved cycle time to apply a statistical treatment to relatively large samples of experimental curvature data to evaluate measurement capability. Quantitative analysis of factors that impact transition curvature reveals an asymmetry in the curvature profile that is strongly correlated to skew angle. Another less obvious skew-related effect is an overall decrease in curvature as skew angle increases. Using conventional perpendicular magnetic recording as the reference case, we characterize areal density capability as a function of recording position.

Areal density growth in modern hard disk drives (HDD) is becoming increasingly difficult to achieve as Perpendicular Magnetic Recording (PMR) approaches the super paramagnetic limit of ∼1Tb/in2. Heat-Assisted Magnetic Recording (HAMR) is on the verge of becoming the next generation of high-density magnetic recording technology. By using a laser to provide temporary localized heating of the media during the recording process, HAMR enables media designs with smaller magnetic grains than PMR while maintaining thermal stability.

A deeper understanding of the unique recording characteristics in an HDD environment is a critical step in the maturity of HAMR as it continues to make progress towards production. Minimizing transition curvature is understood to be crucial for improving recording performance,1,2 and the increase in HAMR curvature relative to PMR has drawn significant attention in recent studies.2 This study investigates the interaction between transition shape and recording position, and the impact it has on recording performance.

All recording measurements were performed in HDDs with the disks spinning at 10.5k RPM in a 2.5-inch form factor in an environment that minimizes disturbance from external mechanical vibrations. HDD case temperature is consistent at approximately 1-2°C above ambient room temperature, and held within +/- 1°C for all measurements. The spacing between the recording head and media surfaces is actively controlled to maintain a constant clearance that is consistent with typical drive operation. The laser power in HAMR drives is selected by finding the best bit error rate (BER) in the presence of adjacent track writes in a manner similar to previous HAMR HDD studies.3 Bit length and track pitch are approximately equivalent at all locations across the recording surface. The edges of a written track are defined where the signal amplitude falls below 50% of the peak amplitude on either side of the track.

Reconstructed scanline images of media magnetization patterns are acquired using the “ballistic seek” approach to micro-track footprinting.4 This approach minimizes sources of error that are inherent to in-situ footprinting due to the convolution of the reader sensitivity function with the media magnetization pattern, and the cross-track averaging effect that results from the relatively large free layer width in the magnetoresistive read-back sensor. This technique uses a single, continuous microtrack that is trimmed at a slight angle to the trajectory along which it was written, eliminating the opportunity for scan-to-scan timing error and reducing the total measurement time. A 2D transition profile is extracted from the footprint images by fitting a quadratic curve to the down-track phase shift as a function of cross-track position. The second-order term of the resulting equation is used to quantify the curvature of the transition profile in units of nm-1, and the first-order term quantifies profile asymmetry with respect to the track center. An assessment of the measurement capability under nominal conditions found that with a 95% confidence interval and 80% statistical power, reporting the average of 10 repeated curvature measurements resulted in a measurement uncertainty of 0.00137 nm-1 and a minimum detectable difference between two samples to be 0.0029 nm-1, or approximately 10%.

Areal density capability (ADC) measurements are performed using the ASTC criteria for conventional (non-shingled) versions of PMR and HAMR.5 This criterion requires that a test track is first written to the media, and then the adjacent tracks on both sides of the test track are written 32 times each, using different encoded random data patterns under the same recording conditions as the test track. The written data must include a minimum of 50 sectors that are 4 kB in length, which are re-read from the test track without any additional error correction until 10,000 consecutive sectors have been read. The highest areal density where this procedure can be performed without any erroneous sectors determines the maximum ADC.

Magnetization footprints measured at three different skew angles at the outer-diameter (OD), mid-diameter (MD) and inner-diameter (ID) are shown in figure 1(a), accompanied by their corresponding transition profiles in figure 1(b). Consistent with observations reported in a previous study,4 we see the transition profile in HAMR becoming increasingly asymmetrical as skew angle is introduced. The asymmetry of the transition profile across the stroke occurs with mirrored symmetry around the zero-skew radius, with the “nose” of the profile shifting away from the center of the track by as much as 10 nm, moving radially away from zero skew. In addition to the increased profile asymmetry, another less obvious skew-related sensitivity found in HAMR is a decrease in the amount of transition curvature that occurs as skew angle is increased.

FIG. 1.

Scan line images of magnetization footprint (a) and their corresponding transition curvature profiles (b) measured at zero skew and at +/- 14° skew angles in a HAMR HDD.

FIG. 1.

Scan line images of magnetization footprint (a) and their corresponding transition curvature profiles (b) measured at zero skew and at +/- 14° skew angles in a HAMR HDD.

Close modal

In figures 2(a) and 2(b), the relationship between transition profile metrics and skew angle are shown for 4 HAMR drives that were each measured 10 times at 7 different skew angles across the recording surface. Figure 2(a) reveals a 98.3% correlation between skew angle and the quantified profile asymmetry, with the tangent of the skew angle on the horizontal axis to show that the relationship to profile asymmetry is nearly 1:1. Figure 2(b) shows a statistically resolvable decrease in transition curvature for each drive as the absolute skew angle is increased.

FIG. 2.

Average transition profile metrics for a sample of 4 HAMR HDDs that were measured 10 times each at 7 different skew angles across the recording surface, showing the correlation between skew angle and (a) transition profile asymmetry; and (b) transition curvature.

FIG. 2.

Average transition profile metrics for a sample of 4 HAMR HDDs that were measured 10 times each at 7 different skew angles across the recording surface, showing the correlation between skew angle and (a) transition profile asymmetry; and (b) transition curvature.

Close modal

Our discussion of transition asymmetry focuses on how HAMR recording geometry changes with skew, specifically the change in spacing between the thermal spot and the leading edge of the write pole. For simplicity, we assume for now that any distortion of the thermal spot at skew is negligible, and that this embodiment of HAMR is such that the width of the write pole is much larger than the width of the thermal spot.

Figures 3(a) and 3(b) provide schematic drawings that illustrate the orientation of the write pole relative to the thermal spot when the recording head is positioned at zero skew and extreme ID skew respectively. We identify two geometric factors that contribute to a change in spacing between the write pole and the thermal spot. The first is that the distance between the pole edge and thermal spot projected along the direction of media rotation increases with skew by a factor of 1/cos(α). The second term results from the angle created between the pole edge and track center, causing a spacing change that occurs as a function of cross track position (z) that can be described by z*tan(α). The combined result of these effects is illustrated in figure 3(b).

FIG. 3.

HAMR recording geometry at (a) zero skew; and (b) extreme ID skew, illustrating how the spacing between the thermal spot and the edge of the write pole changes when skew angle is introduced.

FIG. 3.

HAMR recording geometry at (a) zero skew; and (b) extreme ID skew, illustrating how the spacing between the thermal spot and the edge of the write pole changes when skew angle is introduced.

Close modal

We establish a pair of axes along the cross-track (z) and down-track (x) directions, with an origin defined at the point where the leading edge of the write pole intersects the center of the track when the head is positioned at zero skew. We impose a dashed line along the leading edge of the write pole in figure 3(b). The equation of this line quantifies the change in location of the edge of the write pole in the down-track direction (x) as a function of cross-track position (z) for a given skew angle (α). We can see from figure 3(b) that the slope of this line is given by the tangent of the skew angle, which we have shown in figure 2(a) to have a nearly 1:1 correlation to profile asymmetry.

The characteristics of a written transition are determined by the effective switching field gradient. In the case of HAMR, the switching field gradient is a combination of the field produced by the writer (Hw), the slope of the anisotropy field (Hk) as a function of temperature (T) and the spatial variation of the thermal profile in the media.6 Most HAMR studies assume Hw in the vicinity of the thermal spot to be completely uniform, and consequently that the recording temperature occurs along a single isotherm within the thermal spot. However, because we’ve shown that the asymmetric distortion of the transition profile at skew is highly correlated to the orientation of the write pole, a reasonable hypothesis is that the underlying mechanism involves some amount of spatial variation in the effective write field within the vicinity of the thermal spot. If the effective write field strength decreases as the distance from the write pole increases, an increase in recording temperature will be required to compensate, causing recording location to shift to a higher contour of the thermal profile.

If we apply this explanation to the example of the head positioned at an extreme ID skew angle as shown in figure 3(b), the increase in spacing from the write pole at the OD side of the track would result in lower field amplitude at that location, causing the recording temperature to shift to a higher thermal contour that is closer to the peak of the thermal profile. Similarly, at the ID side of the track the recording temperature would shift to a lower thermal contour away from the profile peak, resulting in a recorded transition that has deviated asymmetrically from the true shape of the thermal spot in a manner consistent with our experimental results.

The significance of understanding the mechanisms driving transition curvature in HAMR is to minimize the limitations that curvature imposes on ADC. To investigate the impact of the shape and curvature of the transition profile, we measure ADC at various points across the recording surface and attempt to identify changes which are characteristically similar to transition curvature.

ADC measurements using the ASTC criteria5 were performed on groups of HAMR and PMR drives at multiple locations evenly distributed across the recording surface. A full range of linear density and track pitch settings are tested, and in the case of HAMR, laser power is also allowed to vary. Figure 4 shows the measured ADC normalized to the maximum value of each surface as a function of disk radius for HAMR and PMR, including a piecewise interpolated average for each group.

FIG. 4.

A comparison of normalized ADC profiles across the recording surface for groups of HAMR and PMR drives.

FIG. 4.

A comparison of normalized ADC profiles across the recording surface for groups of HAMR and PMR drives.

Close modal

The resulting profiles show a significant difference between HAMR and PMR. For PMR HDDs, the maximum ADC is achieved at a location near the MD and rolls off symmetrically as the head moves to either side, decreasing by ∼8-10% at both the ID and OD. Conversely, the ADC profile of a HAMR HDD heavily favors the ID of the disk and decreases with radius by up to ∼20% at the extreme OD, indicating a strong dependence on linear velocity. The HAMR ADC dependence on linear velocity results from a roll-off in the linear density that achieves a maximum ADC. The skew-related changes in curvature across the stroke shown in figures 2(a) and 2(b) do not explain the radial dependence of ADC. We can speculate that other factors like transition noise and broadening at the curved edges may be more dominant terms for HAMR ADC, however further work in this area is needed to fully understand these interactions.

An increase in transition curvature relative to PMR has been identified to potentially impose a significant limitation on areal density growth for HAMR. In this work, we demonstrate acceptable measurement capability of the “ballistic seek” approach for in situ footprint imaging. We show that the HAMR transition profile becomes increasingly asymmetrical with simultaneously reduced curvature as skew is introduced, and discuss how this behavior is likely related in part to changes in spacing between the write pole and thermal spot. We show that the characteristic change in ADC across the recording surface is significantly different between HAMR and PMR. The trend of decreased transition curvature with skew in HAMR does not yield a similar improvement in ADC across the stroke, which appears to be primarily dominated by linear velocity. Therefore, while transition curvature alone may still be a significant hurdle to overcome in HAMR, it does not appear to be the primary limiting factor for ADC in current HDD designs, and therefore more work is needed to fully understand this and other factors that limit areal density growth for HAMR.

HDD-based studies of this nature are only made possible through collaborative research and development efforts across a vast range of technically complex disciplines. Though it is very difficult to list each person individually, the authors would like to thank the entire Seagate HAMR team for their continued contributions towards the development and maturity of HAMR technology.

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