Adjacent track interference analysis of shielded perpendicular writers

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 320 (2008) 2955–2958

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Adjacent track interference analysis of shielded perpendicular writers Akira Morinaga a,, Masafumi Mochizuki a, Wataru Kimura a, Tomohiro Okada a, Norifumi Miyamoto a, Youji Maruyama a, David Hsiao b, Yimin Hsu b a b

Hitachi Global Storage Technologies Japan Ltd., 2880 Kozu Odawara-shi, Kanagawa 256-8510, Japan Hitachi Global Storage Technologies Inc., 5600 Cottle Road, San Jose, CA 95193, USA

a r t i c l e in fo

abstract

Available online 6 August 2008

In order to achieve high track density in perpendicular recording, write-field leakage from the main pole to adjacent tracks needs to be minimized. We used a 3-D finite-element method to investigate the optimal writer design, which minimizes fringing fields. We also confirmed the results using experimentally by studying adjacent track interference (ATI) on a spinstand. According to both modeling and experimental data, we found that ATI performance can be clearly improved with a wraparound-shielded (WAS) writer while maintaining enough write-ability. In order to reduce the fringing field, a WAS having thick throat, high permeability and high saturated magnetic flux density was required. We also confirmed that variations of ATI performance due to flare length distributions can be effectively suppressed with the WAS writer. & 2008 Elsevier B.V. All rights reserved.

Keywords: Wrap-around-shielded (WAS) writer Adjacent track interference (ATI) Fringing field Flare length control

1. Introduction Perpendicular recording technology has become the mainstream for commercial HDD products in recent years. The current industry requires much larger data capacity for storage devices, however, conventional longitudinal recording systems cannot fully respond to such a demand. The major limits of the longitudinal recording system are low write-ability of narrow track writers and poor thermal decay of the media. On the other hand, perpendicular recording system has a lot of merit compared with the conventional one. In terms of write-ability, we expect superior characteristics due to the presence of a soft underlayer (SUL) underneath a recording layer and a capped layer on the recoding layer. In terms of thermal decay, better KuV/kT can be obtained with a perpendicular media because of relatively large grain volume. In addition, magnetostatic coupling between the recoding bits makes the written pattern more stable. However, in spite of these advantages, many challenging aspects are yet to be overcome in order to achieve much higher recording density in the perpendicular recording system. To maintain enough writeability, the flare length, which is neck height of the main pole, needs to be controlled carefully. But this control is extremely difficult. In PMR writer design, there are several structure proposed in the literature: single pole tip (SPT), trailing shield (TS) and wrap-around-shielded (WAS) [1–3]. The WAS writer as well as the TS writer has sharper magnetic transition in the  Corresponding author. Tel.: +81 465 48 1111; fax: +81 465 47 5387.

E-mail address: [email protected] (A. Morinaga). 0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.08.016

medium because a shield located at the trailing side of a main pole increases the gradient of the write-field. For a WAS head, we can expect it to have a steep magnetic field profile along the cross-track direction because shield drapes down to the sides of the main pole. The steep cross-track field profile reduces write-field leakage from the main pole to adjacent tracks, which improves adjacent track interference (ATI). In this paper, we use a 3-D finite-element model to investigate the optimal writer design for ATI to minimize a fringing field, and confirm the results by using a spinstand experimentally.

2. Results and discussion We used the commercial 3-D finite-element software [4] to solve Maxwell’s equations for models of heads and SUL. Fig. 1(a) and (b) shows the schematic view of the calculation model for the WAS writer and the TS writer, respectively. The WAS writer has a shield placed in the proximity of the trailing side of the pole, separated by a gap and the shield drapes down to the sides of the pole. On the other hand, the TS writer has a shield placed on the main pole separated by the gap but does not have the shields on both sides. Table 1 shows the calculation parameters for all cases. The main pole width was set to 90 nm and the side gap length, which was the distance between the top corner of the trapezoidal main pole and side-shields, was 100 nm. These settings were for the 254 ktpi (100 nm) track pitch, which was approximately 300 Gbit/in2. Main pole thickness was 200 nm. The flare length, which was the neck height of the main pole, was 100 nm. The

ARTICLE IN PRESS 2956

A. Morinaga et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 2955–2958

Flare length

θa

trailing shield gap, which was the distance between the top of the main pole and at the bottom of the trailing-shield, was 40 nm. The saturated magnetic flux density and the permeability of the main pole were assumed to be 2.4 T and 500, respectively. Fig. 2 indicates the simulated static write-field contours at ABS for the SPT, the TS and the WAS writers. The SPT writer has the largest field strength at the trailing edge but field spreads widely. The TS writer and the WAS writer have smaller on-track field due to the shields but fringing field becomes much less than that of the SPTs. To facilitate better comprehension of this, the calculated static write-field distributions along the cross-track direction for all three cases are shown in Fig. 3. The SPT and the TS structure have large write-fields at the adjacent track position. In this case,

T_gl

WAS throat

Pw

T_gl s_gl

Pt

1.2

θb

Main pole widht : 90nm

ya yb WAS throat (thickness) Head underlayer spacing Field calculation position Magneto motive force (MMF)

WAS

TS

SPT

90 nm 200 nm 100 nm 40 nm 80 nm 451 821 100 nm 60 nm 20 nm 0.175 AT

’ ’ ’ ’ ’ ’ ’ – ’ ’ ’

’ ’ ’ – – ’ ’ – ’ ’ ’

Normalized effective field

Trailing gap length : 40nm

Table 1 Simulation parameters

Main pole width (Pw) Main pole thickness (Pt) Flare length Trailing gap length (t_gl) Side gap length (s_gl)

Shield throat : 100nm

1.0

Fig. 1. Calculation model of (a) WAS, (b) TS and (c) SPT.

0.8 SPT

0.6 TS

0.4

0.2 WAS

0.0 0

50

100

150

200

Crosstrack displacementfrom track center (nm)

Main pole (Bs, m) ¼ (2.4, 500). WAS (Bs, m) ¼ (1.7, 1000).

Fig. 3. Static write-field distribution along the cross track direction.

SPT

WAS

TS

100

100

100

50

50

50

0

0

0

-50

-50

-50

Down-track displacement (nm)

10000

8000

6000

-100

-100

-100

-150

-150

-150

-200

-200

-200

-250

-250

-250

0

4000

2000

0

-300

-300 0

50 100 150 200 Cross-track displacement (nm)

-300 0

50

100

150

200

0

50

100

150

Fig. 2. Write-field contour comparison between the SPT, the TS and the WAS writer. A separation denotes 1000 Oe.

200

ARTICLE IN PRESS A. Morinaga et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 2955–2958

the adjacent track is directly exposed to leakage fields from the main pole. On the other hand, the WAS structure has smaller leakage field. This is because the WAS placed around the main pole effectively shunts the stray write-field at the adjacent track position. Therefore, according to these calculations, better ATI performance is expected with the WAS. Next we focused on the properties and size of the shields and investigated how much they were influenced on fringing field by using calculation quantitatively. Fig. 4 shows WAS permeability dependence on normalized fringing field. When permeability of the WAS becomes large, fringing field becomes small. When we compare ‘‘permeability (m) ¼ 100’’ case to ‘‘m ¼ 1000’’ case, 14% (from 3.1 to 2.7 kOe) fringing field reduction can be achieved with at adjacent track position. Here the adjacent track position was 100 nm, which corresponded to 254 ktpi track pitch. Fig. 5 shows the WAS throat dependence on normalized fringing field. Fringing field at adjacent track position successfully decreases as increasing the WAS throat. It indicates shield works effectively when its thickness becomes large. The field at adjacent track of the WAS writer having 205 nm throat was smaller than that of 100 nm by

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37% (from 2.7 to 1.7 kOe). On-track field also decreased as increasing the WAS throat. However, it is less affected compared with fringing field. Fig. 6 shows the Bs of the WAS dependence on the fringing field. The permeability was fixed here. We found that a small Bs WAS resulted in large fringing field. When Bs of the WAS becomes 1.7 T or less, the magnetization of WAS saturated. Then it cannot shunt extra field effectively. In other words, we can receive the benefit of the WAS with the WAS having 1.7 T or more Bs. Next focal point is robustness for flare length distribution. Considering ATI is a sensitive function of flare length, a large flare length distribution in a large population of heads results in a large ATI performance distribution. However, it is difficult to eliminate the actual flare length distribution completely. Flare length dependence on stray field intensity at the adjacent track position was calculated and shown in Fig. 7. In the case of the SPT writer, field strength increases significantly as flare length decreases. On the other hand, the field increase is less (920 vs. 470 Oe) in the case of the WAS writer. We can expect less ATI distribution with the WAS writer. Fig. 8 shows the experimental ATI performance of the SPT and the WAS writer as a function of magnetic write widths.

1.2

1.2

SPT

Normalized effective field

Normalized effective field

WAS_Bs = 2.4T

1.0

1.0

0.8 WAS μ = 100

0.6 SPT

0.4

WAS_Bs = 2.0T WAS_Bs = 1.7T WAS_Bs = 1.0T

0.8

WAS_Bs = 0.5T

0.6 0.4 0.2

0.2 WAS μ = 1000

0.0

0.0 0

50

100

150

0

200

50 100 150 Crosstrack displacement from track center (nm)

200

Crosstrack displacement from track center (nm) Fig. 6. Saturated magnetic flux density (Bs) of the WAS dependence on the fringing field.

Fig. 4. WAS permeability dependence on normalized fringing field.

Field intensity at 100 nm offtrack position (Oe)

1.2 SPT WAS throat : 100nm

Normalized effective field

1.0

WAS throat : 135nm WAS throat : 170nm

0.8

WAS throat : 205nm

0.6

0.4

0.2

0.0

7000 SPT WAS

6000 920 Oe

5000 4000 3000 470 Oe

2000 1000 0 20

0

50 100 150 Crosstrack displacement from track center (nm)

Fig. 5. WAS throat dependence on normalized fringing field.

200

40

60 80 Flare length (nm)

100

120

Fig. 7. Flare length dependence on calculated stray field intensity at the adjacent track position.

ARTICLE IN PRESS A. Morinaga et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 2955–2958

0.5

2.0

BER degradation

1.6

WAS

BER degradation

1.8

SPT

1.4 1.2 1.0

BER_ave.: -5.4

BER_ave.: -5.4

MWW_ave.: 133nm

MWW_ave.: 130nm

0.4 0.3 0.2 0.1

0.8 0.0

0.6

Head A Head B WAS throat: 150nm

0.4 0.2 0.0 120

40 38 36 34 32 30 28 26 24 22 20

Overwrite (dB)

2958

Head C Head D WAS throat: 100nm

Fig. 9. Experimental ATI performance of the heads having 150 and 100 nm WAS throat.

130

140

150

160

170

180

Magnetic write width (nm) Fig. 8. Experimental ATI performance of the SPT and the WAS writer as a function of magnetic write widths.

A spinstand with sector servo was used for this experiment. Here, ATI performance is defined as bit error rate (BER) degradation of a track after writing an interference signal 10,000 times on the adjacent track position. In order to compensate write-width contribution, ‘‘BER after 1 time adjacent track writing’’ was utilized for reference value. Here, the main pole physical width was 120 nm, WAS throat was 170 nm, and Bs of the WAS was around 1.7 T. Comparing the WAS to the SPT writer having the same magnetic write width, BER degradation of the WAS writer was smaller than that of the SPT writer. It is consistent with the simulated results shown in Fig. 3. In this case, the average overwrite (OW) of the WAS and the SPT writer were 38.2, 35.6 dB, respectively. OW value can be got higher by increasing write-field. However, the field strength at the adjacent track also get higher because SPT writer has a wider tail of magnetic field, as shown in Fig. 3. On the other hand, the WAS writer provides relatively smaller fringing fields at the adjacent track because WAS works as a shunt of stray fields. Therefore, the WAS writer can realize better ATI performance head with appropriate writeability by controlling head dimensions such as the flare length and the WAS throat. And we can certainly find in Fig. 8 that less scattered plots for the WAS writer as expected from calculated results in Fig. 7. Finally, as stated before, one of the key factor for controlling ATI is the WAS throat. Therefore, we compared the ATI of thick WAS writer to that of thin WAS writer with the capped media experimentally. The designed main pole width was 90 nm and the trailing gap length was 40 nm. The track pitch was set to be approximately 200 ktpi. Two groups of heads were selected. Heads A and B have 150 nm WAS throat and heads C and D have

100 nm. These heads were selected so that averaged magnetic write width, bit error rate and overwrite characteristics can be almost the same shown in Fig. 9. Heads A and B having thick WAS certainly showed less BER degradation although they had same MWW and OW. It is consistent with simulated results as shown in Fig. 5.

3. Conclusion By modeling and experiment we found that ATI performance can be clearly improved with a WAS writer while maintaining enough write-ability. The WAS structure effectively shunts stray flux in the adjacent track direction. In order to reduce the fringing field effectively, a WAS having a thick throat, high permeability and high Bs was required. In addition, ATI performance variations come from flare length distributions can be suppressed with the WAS writer. We can conclude that the WAS writer has significant advantages for achieving high track density recording.

Acknowledgments The authors would like to thank Dr. Zhong-heng Lin and Dr. Mark Haertling for their help for ATI experimental technique on a spinstand. The authors would also like to express their gratitude to Dr. K. Mitsuoka, Dr. H. Fukui and Mr. S. Tadokoro for fruitful discussion and encouragement. References [1] [2] [3] [4]

M. Mallary, et al., IEEE Trans. Magn. 38 (4) (2002) 1719. Yimin Hsu, et al., IEEE Trans. Magn. 43 (2) (2007) 605. Tomohiro Okada, et al., IEEE Trans. Magn. 41 (10) (2005) 2899. /http://www.jri.co.jp/pro-eng/jmag/e/jmg/S.