HAMR Noise Mechanism Study with Spin Stand Testing - IEEE Xplore

This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TMAG.2015.2448114, IEEE Transactions on Magnetics

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HAMR Noise Mechanism Study with Spin Stand Testing Hai Li1, Michael Alex2, Jian-Gang (Jimmy) Zhu1, Fellow, IEEE 1

Data Storage Systems Center, Carnegie Mellon University, Pittsburgh, PA 15213 USA 2 Western Digital Technologies, San Jose, CA 95131 USA

In this work, we present a spin stand testing study of heat-assisted magnetic recording (HAMR). Motivated by understanding the major recording noise mechanisms predicted by previous modeling, experiments were made to investigate the recording field / write current dependence of signal-to-noise ratio (SNR). The two significant noise mechanisms in HAMR, incomplete switching and eraseafter-write, have been observed in the experiments and shown to be similar to the previous modeling results. Previous simulations indicate system performance strongly correlates with recording time window (RTW). Therefore, head/media relative velocity was varied to change the RTW, and the effects of linear velocity are also compared. Further analysis has been applied to cases with different writer and media components. These results help understand the field / media requirements to optimize HAMR recording system performance. Index Terms—HAMR, noise mechanism, testing, FePt.

I. INTRODUCTION

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eat-assisted magnetic recording (HAMR) has been widely touted as a leading candidate for enabling the next generation of ultra-high density data storage [1]. To fully assess the advantage and potential of HAMR technology and to understand the fundamental physics of the HAMR recording process, extensive Landau-Lifshitz-Bloch (LLB) equation-based micromagnetic modeling has previously been conducted [2, 3]. Fundamental recording behavior and system performance dependence have been simulated. In the current work, we present and discuss several observations in HAMR recording experiments, and these testing results qualitatively agree with behavior predicted by previous simulations [3-5]. II. TESTING SPECIFICATION For both previous modeling [3-5] and experiment in this paper, 1T data patterns have been recorded, where T is the bit cell length. The modeling and experimental SNR are extracted using different methods, but qualitative trends are the same. In a previous study, a modified Voronoi pattern was generated to mimic the distributions of granular media [6]. The magnetization dynamics are described by the LandauLifshitz-Bloch equation. After data writing, a reader is used to scan the downtrack position. The specific SNR characterization method is described in [3]; the primary idea is to take the mean bit profile to calculate signal power and to take the variance of profiles to calculate noise power. To conduct a comparative study experimentally, HAMR head gimbal assemblies (HGAs) and assorted HAMR media types were measured on a spin stand tester. After recording the 1T data pattern, the waveform was read back. The rms signal at the recording frequency was taken as the signal power while the other frequency components were regarded as noise after removing the higher-order harmonics of the signal. The logarithmic ratio of signal power S, and noise power P, 10log10(S/P), gives SNR. It should be mentioned that for all the experimental data, the track width and head-media-spacing (HMS) were kept the same and the

laser power was tuned slightly to achieve this. The thermal gradient of various head/media combinations was also measured and found to be relatively constant. This suggests the thermal profile was similar for all recording conditions. III. TESTING STUDY RESULTS The spin stand testing study originates from the noise behaviors seen in previous simulations and the experimental results are utilized to validate the model. Based on the understanding of fundamental recording characteristics in terms of the recording time window, velocity effects have been studied. Various HAMR writer / media components were also tested and the optimization conditions are compared. Although there are differences between testing results and previous simulations [3-5], the measured trends qualitatively match the model predictions. All the results shown in the following graphs are from experimental measurements. A. Basic Recording Mechanisms

Fig. 1. SNR versus writing current. At low writing current the media is poorly saturated, and SNR is low. When the writing current exceeds 55 mA, erase after write effects reduce the SNR.

In previous modeling results, SNR is seen to strongly correlate with the magnetic writing field. At low fields, incomplete switching noise dominates, since the field is insufficient to orient the magnetization in the presence of strong thermal agitation during cooling. Consequently, this

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results in a great amount of DC noise. On the other hand, at high fields, the predominant noise is caused by “erase-afterwrite.” This occurs when the medium is exposed to the reversed external field for too long, and the previously recorded bits are erased. Erase-after-write noise generally leads to an increase in transition jitter. Figure 1 shows SNR versus writing current. Here we assume the magnetic field is linearly proportional to the current, which drives the magnetic writer. The results indicate the SNR first quickly increases with writing current and then saturates at around 55 mA. This is because incomplete switching noise is avoided with higher writing current and media saturation improves. When the current is increased further, the SNR starts to decrease gradually. In testing, although the write current from the amplifier is maximized, the field generated is still insufficient [4, 7] to cause extreme erase after write. Additionally, the other noise sources such as Tc variations, may make the curve flatter, as shown in modeling [5]. The curves observed experimentally for various head and media vary from each other somewhat, but the primary trends are similar, as shown in this representative curve.

window. For a specific media Hk (anisotropy field) versus T (temperature) curve, a certain recording field gives a corresponding recording temperature Trec. The recording time window is proportional to the range defined by Tc and Trec [3, 4]. With higher writing field / current, Trec will move further from Tc, and the time window would be broadened. This allows for recording with a longer time and avoids incomplete switching. It should also be mentioned that the noise here mainly appears within each bit and therefore, this type of incomplete switching noise would manifest itself as DC noise.

Fig. 3. Experimental signal variance in the down-track direction for media written at high write current.

Fig. 2. Readback signal level versus downtrack position using both low and high write currents for recording. The red trace was recorded at 35 mA while the blue trace was recorded at 65 mA.

To better observe the recording behavior in detail, waveforms written at low and high currents were captured as shown in Fig. 2. The red trace here was recorded with lower current, 35 mA, while the blue trace was recorded with higher current, 65 mA. Both curves have similar shapes but the magnetization (signal) values are significantly different. It can be noted that the zero-crossing of each curve is not located at 0 volts. This is because the reader has some nonlinear characteristics. As indicated by the arrows on the right hand side, the range between the maximum and minimum level is defined as the saturation level. Clearly shown here, the saturation level is much higher for the case with higher writing current. On the other hand, it should be mentioned that the concave shape of the signal is because that the preamplifier used in measurements does not go to extremely low frequencies (DC). This ac-coupling results in a “boost” in the signal at the transition, followed by a rapid “droop” in the long-wavelength bit-cell. This makes the transition look high. This could also be interpreted in terms of recording time

On the higher writing current side of Fig. 1, erase-afterwrite is believed to be the dominant noise mechanism [3]. To validate this prediction, experimentally-measured signal variance is plotted versus the down-track position in Fig 3. Two variance peaks can be seen. They sit at the transition positions and represent the transition jitter noise. Ideally, the transition jitter profiles should be symmetric, i.e. the variance peaks should be symmetric. However, a “bump” can be noticed at the left foot of each peak. It should be mentioned that the magnetic writer is moving to the right relative to the recording medium in Fig. 3. After one bit cell has been written, the polarity of the recording field will be reversed. But the previously recorded bit cell will also feel this reversing field. Additionally, even thermal profiles with very high thermal gradients will still have tails extending into the previous bit cell during the cooling process. The combined effect of heat and field will cause the previous bit cell to suffer from partial erasure and induce erasure noise. This explains why the bumps in Fig 3 are a clear signature of erase-afterwrite noise. As mentioned, the ac-coupling of the preamplifier makes the transition signal look relatively high and manifests less erasure effect. Instead, variance shows more clear erasure. Additionally, erase-after-write noise can also be explained in terms of the recording time window. As before, when the recording field is too high, Trec would be quite far from Tc and give a wide temperature range for recording. Therefore, the recording time window will be too wide and the grains will remain switchable even after being properly switched. Only when the optimal time window is set will the erase-after-write process be avoided. B. Velocity Effect on Field Dependence In previous modeling, the system performance shows strong correlation with recording time window. Changing the disk



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rotational velocity can directly change the recording time window. In this section, the writing current dependence of SNR will be investigated at different disk rotational velocity values.

the DC noise has been plotted as a function of the disk linear velocity. The results indicate that the DC noise increases with increasing velocity. This agrees with Fig. 4 since at 20 mA, the SNR is lower at higher linear velocities. C. Velocity effect with Various Components In previous sections, the basic HAMR recording noise behaviors have been investigated and interpreted in terms of the recording time window. In this section, multiple recording components will be tested and compared in order to understand optimization requirements.

Fig. 4. SNR versus writing current at different head/media linear velocities. The blue curve was measured at 7.95 m/s while the red one was taken at 18.54 m/s.

Figure 4 shows the SNR versus writing current at 7.95 m/s (blue) and 18.54 m/s (red) linear velocity. Both curves shown here have similar write current dependence. The SNR first increases with write current, reaches an optimum and then gradually decreases at higher write current. At low write current, performance at lower velocity is better, while on the high write current side, the performance at high velocity is better. On the low current side, recording suffers from a narrow time window; the magnetization cannot be properly saturated, inducing noise. Slower rotation gives a wider time window and this relaxes the incomplete switching condition. On the high writing current side, the recording is degraded due to the relatively long time window. Here, higher velocity would effectively shrink the time window and prevent the medium from being exposed to a reversing field, giving a higher SNR, as seen in the figure. Fig. 6. SNR versus writing current at 7.95 m/s and 18.54 m/s linear velocities. The same hard media was used. The top graph used a strong writer while the bottom graph used a weaker writer.

Fig. 5. Medium DC Noise versus linear velocity at 20 mA.

The components we have tested mainly show incomplete switching as the dominant noise mechanism. Here, we selected the write current to be 20 mA and measured recording characteristics at different velocities. As mentioned in the previous observation of incomplete switching, the noise occurs due to the unsaturation of the magnetization level of the media, which is proportional to the DC noise. So in Fig. 5,

Fig. 6 shows SNR of the same hard media using both a strong writer and a weak writer. Here, hard media means the anisotropy field is relatively high and the strong writer means the field generated is relatively high. The results here show that the four curves all show similar trends and incomplete switching dominates. It can be noted that the top graph with the strong writer has a crossing point between the curves with different velocities. This is supposed to be the boundary between the incomplete switching dominant region and the erase-after-write dominant region. However, for the bottom graph, no crossing point is observed. Additionally, the spacing between the curves is larger for the bottom graph. These observations match up with the essence of different strengths of writers. With the same writing current, the field generated by the weaker writer is lower. Therefore, taking the left part of top graph and scaling it laterally would lead to a graph similar to the bottom graph. Next, as shown in Fig. 7, the same strong writer has been employed while different medium are utilized to record.



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Similarly, the recording has been conducted at both 7.95 m/s and 18.54 m/s. The top graph is for hard media while the bottom graph is for soft media. Here, soft media means the anisotropy field is relatively lower than that of the hard media. Again, the curves show similar trends. But here, both graphs exhibit crossing points for curves measured at different velocities. For the bottom graph with soft media, the eraseafter-write is more severe, so the crossing point is located at lower writing current. This is because the soft media has lower Hk and therefore smaller energy barrier to resist thermal erasure during the cooling process. Even relatively small magnetic field would lead to erase-after-write noise for this head / media combination.

But for high writing current, the hard media effectively reduces the relatively long recording time window and therefore erase-after-write noise is suppressed.

Fig. 8. SNR versus writing current for media with different anisotropy fields. The hard and soft media refer to high and low anisotropy field, respectively.

IV. SUMMARY A spin-stand testing study of heat-assisted magnetic recording has been presented here. The two significant noise mechanisms in HAMR, incomplete switching and erase-afterwrite, have been observed in the experiments and shown to be in qualitative agreement with previous micromagnetic modeling results. Recording at various linear velocities was also investigated in order to understand the role of the recording time window. Multiple head and media components have been compared and the experimental results qualitatively agree with the previous modeling predictions and also justify the importance of the recording time window. ACKNOWLEDGMENTS This research was supported by the Data Storage Systems Center at Carnegie Mellon University as well as its industrial sponsors. We acknowledge the assistance of Mr. Jimmy Bui of Western Digital for his help in the measurements. REFERENCES Fig. 7. SNR versus writing current at 7.95 m/s and 18.54 m/s. The same strong writer has been utilized. The top graph is for hard media while the bottom graph uses soft media.

To better compare the hard and soft media types, the blue curves in Fig. 7 have been re-plotted in Fig. 8. As indicated, on the low writing current side, SNR is relatively higher for soft media. On the high writing current side, the SNR is relatively higher for hard media. This phenomenon can be explained in terms of the recording time window. For a certain head field, the Hk versus T curve would have a corresponding Trec. With a higher room temperature Hk, Trec would move closer to Tc. Therefore, the temperature range defined by Trec and Tc would be narrower. Consequently, the recording time window, which is proportional to this temperature range, would be also narrower. For low writing currents, the medium already suffers from incomplete switching due to a limited recording time window; for hard media, the time window would decrease even more and noise would further increase.

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