Shingle magnetic recording assessment with spinstand measurement

Journal of Magnetism and Magnetic Materials 324 (2012) 321–326

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Shingle magnetic recording assessment with spinstand measurement Hiroshi Kiyono n, Osamu Nakada, Takahiro Mori, Taro Oike Data Storage & Thin Film Technology Components Business Group, TDK CORPORATION, 543 Otai, Saku-shi, Nagano 385-8555, Japan

a r t i c l e i n f o

abstract

Available online 19 January 2011

Shingle Magnetic Recording (SMR) is a candidate to realize recording density of over 1 Tbit/in2. At the cost of complex HDD firmware, we can expect many gains such as higher write-ability to break through the famous tri-lemma. In this paper, promising empirical data and concerns found on spinstand were introduced and discussed. More than 18 dB of better reverse overwrite (ROW) than conventional write recording (CWR) was confirmed at 40 nm of track pitch. Areal density capability (ADC) was improved by 25.5% by SMR. Another gain from SMR was observed at inner diameter (ID) and outer diameter (OD) that ADC loss due to skew effect was improved by using preferable side write pole edge. The dependence of ADC on magnetic read width measured with micro-track method (MRWu) indicates that side reading is limiting ADC. Reduction in side reading through narrower MRWu and inter-track interference cancellation technology are necessary to further improve ADC. BPI capability improvement by steeper down track field gradient at the track edge needs to be considered. & 2011 Elsevier B.V. All rights reserved.

1. Introduction Perpendicular magnetic recording has recently enabled mass production of 500 Gbit/in2 products together with the tunneling magnetoresistive reader and the dynamic fly height control technology, and thus the technologies are contributing to memorize several hundreds Exa bytes of precious information that human being has created. As a candidate to realize recording density of over 1 Tbit/in2, Shingle Magnetic Recording (SMR) was introduced [1,2]. Encouraging modeling results were attained [3] and good performance on spinstand was demonstrated [4]. At the cost of complex HDD firmware, we can expect many gains such as higher write-ability to break through the famous tri-lemma, usage of continuous media, less influence of erase band, less pole width related yield losses, less adjacent track erasure concern, less pole erasure concern and less skew sensitivity. Fig. 1 shows the dependence of ROW on magnetic write width (MWW) including both side ease band width (EBW) of latest head and media combination. The graph indicates that current heads cannot overwrite on current Ku media at 500 kTPI (50 nm track pitch). On the other hand, due to wide write width, SMR will be able to have rich write-ability, and furthermore the rich writeability can be converted to areal density increase by head design adjustment. As a write head design, narrow main pole to side shield gap length (SDG) and wide pole width (PW) were suggested by modeling [5] to improve track edge written quality

n

Corresponding author. E-mail address: [email protected] (H. Kiyono).

0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.01.001

and write-ability. To understand the detail of the technology and to optimize heads for shingle write, we characterized shingle write with several kinds of heads and media. In this paper, promising empirical data and concerns found were introduced and discussed.

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ROW (13T/2T) [dB].

Keywords: Shingle magnetic recording SMR Erase band Side reading

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Fig. 1. Dependence of ROW on MWW+ 2EBW. Current heads cannot overwrite on current Ku media at 500 kTPI (50 nm track pitch).

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2. Experiments Shingled tracks were written on commercially available media for 500 Gbit/in2 by GUZIK read write analyzer, of which positioning error was 7 nm 0-peak. Shingled magnetization on the media was observed by GUZIK WDM5000 waveform digitizer and 3D profile software. Areal density capability (ADC) based on bit error rate through Marvell 88C8801 channel was measured for both SMR and CWR with 100 times adjacent track writing (Adjn100) and one time adjacent track writing (Adjn1). The procedure of ADC measurement is explained in Fig. 2. BPI, TPI and write current were swept to find the best attainable ADC. Attainable TPI was

defined as the track pitch where bit error rate was 10  2.5. To quantitatively analyze the result, parametric such as EBW, side reading (SR) and ROW were measured. Since EBW is a very important parameter to quantitatively characterize track edge written quality, we chose step squeeze method, which can measure EBW accurately even if MRWu is wider than MWW. The procedure of step squeeze method is explained in Fig. 3. Since worst case ROW was considered and needed to be distinguished from side reading, we measured ROW as follows: (1) AC Band Erase, (2) Write 2 T, (3) Write 1 T on next track, (4) Search the 2 T

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CWR WG = 28 nm SMR WG = 20 nm

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Shingle Pitch [nm] Fig. 2. Procedure of ADC measurement. BPI, TPI and write current were swept to find the best attainable ADC.

Fig. 5. Comparison of write-ability between CWR and SMR. Triangles are data of CWR, previously shown in Fig. 1, and circle shows the data of SMR written by a head.

Plotting the integral (Area) of the track profile as a function of erase offset. Take micro track profile at several erase offset.

MWW+EBW (Outer)

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Area of Micro Track Profile

EBW (Inner)

EBW (Outer) Erase Offset Fig. 3. Procedure of stepped squeeze method.

Fig. 4. Magnetic field image of shingled tracks and track profiles at 40, 50 and 60 nm of track pitch.

H. Kiyono et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 321–326

signal peak and measure TAA_2 T, (5) AC Band Erase, (6) Write 2 T, (7) Overwrite 13 T, (8) Write 1 T on next track, (9) Move to 2 T peak position searched in step (4), (10) Measure TAA_2 T_residual and (11) Calculate ROW ¼20log 10(TAA_2 T_residual/TAA_2 T).

3. Results and discussion Fig. 4 shows the magnetic image generated from read back signal and track profile, written by a head that has 80 nm of SDG. Even for 40 nm (635 kTPI) of tack pitch, clear track profile and 1800 CWR (Adj*100) CWR(Adj*1) CWR (Adj*1) SMR

magnetic image were observed. Fig. 5 shows the gain of SMR in write-ability. Triangles are data of CWR previously shown in Fig. 1, and circle is data of SMR written by a head. Write gap length of heads for CWR is 28 nm and that for SMR is 20 nm; therefore ROW of CWR is 13 dB better than SMR at 130 nm of track pitch, but although WG is 8 nm narrower, ROW of SMR is 18 dB better than CWR at 40 nm of track pitch. Fig. 6 shows the comparison of ADC between SMR and CWR with latest heads. TPI was improved by 31.4% by SMR, BPI dropped by 4.3% and in total ADC was improved by 25.5% from CWR with Adjn100. Approximately half of TPI gain came from the reduction in number of adjacent write from 100 to 1, and rest of the gain came from the wider date track explained in [4]. Fig. 7(a) shows the result of TPI and BPI combination sweep, and Fig. 7(b) and (c) shows schematically estimated bit shape for [high TPI/low BPI] and [low TPI/high BPI] respectively. 800 Gbit/in2 of ADC was obtained at high TPI condition. However SMR is

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Fig. 6. Comparison of ADC between SMR and CWR with latest heads. TPI was improved by 31.4% by SMR, BPI dropped by 4.3% and in total ADC was improved by 25.5% from CWR with Adj*100.

MD Skew = 0

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Fig. 8. ADC loss due to skew effect was improved by using preferable side of write main pole edge.

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Track Density [kTPI] Fig. 7. (a) Result of TPI and BPI combination sweep. (b) and (c) Schematically estimated bit shape for [high TPI/low BPI] and [low TPI/high BPI], respectively. 800 Gbit/in2 of ADC was obtained at high TPI condition.

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sensitive to track miss registration (TMR) [4] and demands for higher transfer rate are strong; so BPI push is in preferable direction for further higher areal density. To increase BPI, we can convert the rich write-ability into steeper write field for example by narrower trailing shield gap. Regarding read shield gap length, still we have room to reduce. Fig. 8 shows another gain from SMR observed at ID and OD that ADC loss due to skew effect was improved by using preferable side of write main pole edge. EBW is an important parameter that indicates written track edge quality. The so called erase band is the sum of erased band and disappeared band. Erased band depends on head fringe field strength and media Hn. We can observe it more clearly by adjacent track erasure with multiple writing. Disappeared band

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is the erase band due to interference between transitions. Transition in down track direction at track edge is less steep; so the transitions disappear from track edge at higher FCI. [4,6]. Therefore as well as lower fringe field and higher Hn, higher head field gradient at track edge in down track direction is also important to reduce EBW. Fig. 9(a) shows an example of track edge written quality comparison. We compared the dependence of EBW on FCI of two kinds of side shielded heads. Probably EBW at low FCI is mainly due to erased band. Although SDG of Head-A is 60 nm, erased band width is only 4 nm. As FCI become higher and inter-signal interference at track edge become in-negligible, disappearing of band starts to increase and total EBW become wider. Slope coefficient of EBW in Fig. 9(b) implies that track edge

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Fig. 9. Dependence of EBW on FCI. (a) Thick lines are average of 5 heads data. Although SDG of Head-A is 60 nm, erased band width is only 4 nm. (b) Slope coefficient of EBW implies that track edge magnetization in down track direction of Head-A is steeper than Head-B.

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Fig. 10. Dependence of ADC and TPI on parametric. (a) and (b) Indicates that side reading is limiting TPI and ADC. (c) Shows that write-ability is not a main cause that limits ADC for this case. (d) Unexpectedly the dependence of ADC on EBW is not clear. Probably this is due to the fact that wider EBW is improving the side reading.

H. Kiyono et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 321–326

800 Media-A Media-B 750 ADC [Gbit/in2]

magnetization in down track direction of Head-A is steeper than Head-B. Fig. 10 shows the dependence of ADC on parametric. The dependence of ADC on ROW shows that write-ability is not a main cause that limits ADC for this case. Correlation between ADC, TPI and MRWu indicates that side reading is limiting TPI. Unexpectedly the dependence of ADC on EBW is not clear. Probably this is due to the fact that wider EBW is improving the side reading. I would like to describe about side reading to explain the importance of narrow reader and side reading cancellation technology. Fig. 11 shows schematic drawing of 50 nm track pitch ( 500 kTPI) of shingled tracks and a 35 nm of MRWu micro-track profile overlaid on TEM picture of 9 nm media grains size. Flare portion of the micro-track profile in the adjacent tracks corresponds to side reading (colored noise), which 35 nm of MRWu reader reproduces at 50 nm of track pitch. Reduction in side reading through narrower MRWu is necessary to further improve the TPI capability and ADC consequently. Fig. 12(a) shows the relation between the physical read width and magnetic read width. As it is well known, it is very difficult to make reader narrower physically, and this data implies that it is even more difficult to make the reader narrower magnetically. Fig. 12(b) shows the mechanism of the difficulty that reader height is much larger than media to reader magnetic spacing; so wider range signal is reproduced by the tall sensor stripe. Therefore we believe that side reading cancellation technology [7] and/or 2D technology [1] will become very important to increase TPI. Another factor that affects side reading is the steepness of track edge magnetization in crosstrack direction. Fig. 13 shows ADC

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Fig. 13. ADC comparison data among media.

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Fig. 12. (a) Relation between physical read width and magnetic read width. As it is well known, it is very difficult to make reader narrower physically, and this data implies that it is even more difficult to make the reader narrower magnetically. (b) Mechanism of the difficulty that reader height is much bigger than media to reader magnetic spacing; so wider range signal is reproduced by the tall sensor stripe.

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comparison data among media. MRWu of media-A is narrower, and ADC is better than others. Since micro-track profile contains the effect of track edge magnetization steepness in crosstrack direction, Steeper crosstrack edge magnetic transition is probably one of the main reasons making MRWu narrower. For better TPI capability, media-A like property is preferable, and steeper head field gradient in crosstrack direction is also important to improve side reading. As pointed out by Miura et al. [4], SMR is more sensitive to TMR than CWR. Fig. 14 shows the dependence of ADC on TMR. TMR was simulated by squeezing on the spinstand. As expected, ADC of SMR drops more rapidly than CWR. To enjoy the better TPI capability of SMR, TMR must be suppressed. A candidate technology to suppress TMR is Thin-film Micro-Actuator (TMA) [8] located just beside slider, which can make servo band width higher than 5 kHz.

4. Conclusion We confirmed gains from SMR on spinstand with current heads and media. More than 18 dB of better ROW than CWR was confirmed at 40 nm of track pitch, 26% of ADC improvement was observed and less effect of skew on ADC at ID and OD was confirmed. Further ADC gain is expected by converting rich writeability to head field gradient in both down track and crosstrack direction. Since EBW of current side shielded writer is already small, it seems that smaller trailing shield gap to push BPI should be the head design direction. To enjoy higher TPI capability of SMR, smaller track miss registration is needed and narrower reader and/

or side reading cancellation technology is very important. SMR is a promising technology to realize recording density of over 1 Tbit/in2, and with the technology, HDD will be able to continue to serve the role of a modern ‘‘Rosetta Stone’’ [9].

Acknowledgments I wish to thank T. Kagami, K. Hirata and K. Anagawa for discussion, N. Takahashi for sharing EBW data and K. Fukuda and K. Noguchi for their valuable advice.

References [1] Roger Wood, Mason Williams, Aleksandar Kavcic, Jim Miles, IEEE Trans. Magn. 45 (2) (2009) 917. [2] Ikuya Tagawa, Mason Williams, Dig. Intermag. (2009) FA-02. [3] Simon Greaves, Yasushi Kanai, Hiroaki Muraoka, IEEE Trans. Magn. 45 (10) (2009) 3823. [4] Kenji Miura, Eiji Yamamoto, Hajime Aoi, Hiroaki Muraoka, IEEE Trans. Magn. 45 (10) (2009) 3722. [5] Yasudhi Kanai, K. Hirasawa, Y. Jinbo, K. Yoshida, Simon Greaves, Hiroaki Muraoka, Dig. Intermag. (2009) DC-08. [6] Takayuki Ichihara, Hidekazu Kashiwase, Hiroyuki Nakagawa, Hiroaki Nemoto, Masafumi Mochizuki, IEEE Trans. Magn. 44 (2008) 3404. [7] Kazumasa Ozaki, Yoshihiro Okamoto, Yasuaki Nakamura, Hisashi Osawa, Hiroaki Muraoka, Dig. PMRC 18 (2010) aE-4. [8] Hideki Kuwajima, Hirokazu Uchiyama, Yuko Ogawa, Hiroyuki Kita, Kaoru Matsuoka, IEEE Trans. Magn. 38 (5) (2002) 2156. [9] Shun-ichi Iwasaki, Dig. PMRC 17 (2010) aA-1.