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IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 6, JUNE 2010
Shingled Magnetic Recording on Bit Patterned Media Simon Greaves1 , Yasushi Kanai2 , and Hiroaki Muraoka1 , Fellow, IEEE RIEC, Tohoku University, Sendai 980-8577, Japan Department of Information and Electronics Engineering, Niigata Institute of Technology, Kashiwazaki 945-1195, Japan Shingled magnetic recording on bit patterned media with areal densities of 2–4 Tbit/in2 is examined. The use of a shingled recording scheme allows wider write poles and dots with higher anisotropy to be used. The down-track and cross-track write margins can be tailored to suit the tolerances of the drive by varying the bit cell aspect ratio. Readback waveforms are calculated for sensors of various size, including the effect of thermal noise in the sensor, and the combination of a wide sensor and bit aspect ratio (BAR) of 1:1 is found to give the highest SNR. Index Terms—Bit patterned perpendicular media, heads, magnetic recording, simulation.
I. INTRODUCTION
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HINGLED magnetic recording has been shown to be a relatively simple approach to increasing the areal density a medium can support [1], [2]. Successive tracks each partially overlap the previously written track, allowing narrow tracks to be written with a wide write head. Shingled recording allows large areal density gains to be realised through the combination of a head with a wide main pole and a continuous recording medium [3]. Bit patterned media (BPM) have several advantages which make them promising candidates for use at multiple Tbit/in densities [4], [5]. The formation of discrete dots consisting of strongly exchange coupled grains leads to high thermal stability, whilst the lack of transition noise should lead to higher signal to noise ratios (SNR). However, aside from the increased complexity of manufacturing BPM, some difficulties associated with recording on these media need to be addressed. The first problem is that as the track pitch is reduced the width of the write head must also be reduced to limit adjacent track erasure (ATE). Narrower write heads generate smaller magnetic fields and ultimately the head will no longer be able to produce a sufficient field to write on the dots. A similar problem is faced with continuous media, but the second problem, that of synchronising the switching of the head field with the position of the dots, is unique to BPM. Suppose we have a 2 Tbit/in patterned medium with down-track and cross-track dot pitches of 18 nm. If the head-medium velocity is 20 m/s, the maximum time available to write to each dot is 0.9 ns. With head field rise times of the order of 0.2 ns and dot position distributions of up to several nanometres the time available for writing a dot can easily become 0.5 ns, or less. This paper will discuss the use of micromagnetic modelling to investigate the advantages of shingled recording on bit patterned media and will attempt to show that the two problems described above can be alleviated through the use of this technique. II. MODELS AND SIMULATIONS Patterned media were modelled as a tessellation of identical bit cells, each of which contained a single magnetic dot centered Manuscript received October 28, 2009; revised February 06, 2010; accepted February 08, 2010. Current version published May 19, 2010. Corresponding author: S. Greaves (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2010.2043221
Fig. 1. Portions of media with various bit cell shapes. The target track is the second from the right in each case. (a) 25 nm 13 nm bit cells, (b) 18 nm 18 nm bit cells, (c) 13 nm 25 nm bit cells, (d) 13 nm 13 nm bit cells.
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within the cell. The areal density was determined by the area of each bit cell; track pitch and linear density were controlled by the width and length of the bit cell. Initially, a 2 Tbit/in areal density was considered. Three media with bit cell dimensions of 25 nm 13 nm, 18 nm 18 nm and 13 nm 25 nm were used. The magnetic dots were 10 nm thick and had an ellipsoidal cross section in the plane of the medium with a 8 nm major axis and a 4 nm minor axis. The major axes of the dots were inclined at 0 , 45 and 90 to the cross-track axis for bit cell widths of 25 nm, 18 nm and 13 nm, respectively. A fourth medium, with 13 nm 13 nm bit cells and 8 nm diameter cylindrical dots, was used for simulations at 3.8 Tbit/in . Each of the dots had saturation , of 750 emu/cm and uniaxial anisotropy, magnetisation, , of erg/cm . Images of portions of each medium are shown in Fig. 1. In each image, the second track from the right was the target track for recording. The media contained 12 tracks of either 24 or 48 bits. Dots in the target track and at least two neighbouring tracks were discretised into 2 nm cubes. The cubes were truncated at the edges of the dots to form the
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GREAVES et al.: SHINGLED MAGNETIC RECORDING ON BIT PATTERNED MEDIA
Fig. 2. Effective head field in the middle of the recording layer, assuming a Stoner-Wohlfarth angular variation of the switching field. Units: kOe.
elliptical shape. Truncation reduced the area of the elliptical dots from 100.5 nm to 97.9 nm . Dots in other tracks were only discretised into 2 nm layers to reduce the calculation time. The main purpose of these dots was to generate a magnetostatic field in the target track. From hysteresis loop calculations in a spatially uniform field, the switching fields of discretised and non-discretised single dots were found to be within 2.5% of one another at applied field angles between zero and 81 . A write head with a triangular air bearing surface, a maximum width of 70 nm and a length of 130 nm was used for writing. The head had side and trailing shields 10 nm from the right edge and trailing edge of the main pole, respectively. The headmedium spacing was 6 nm, the dots were 10 nm thick and the non-magnetic interlayer between the dots and the soft magnetic underlayer (SUL) was 1 nm thick. The head-medium velocity was 20 m/s and the head field rise time was 0.12 ns (zero—90% of peak). The effective field of the head in the middle of the recording medium, 11 nm from the air bearing surface (ABS), line passes through the apex of is shown in Fig. 2. The line runs along the trailing edge the main pole and the of the main pole. The effective field was calculated assuming a Stoner-Wohlfarth angular dependence of the dot switching field and reached a maximum value of 26.1 kOe above the edges of the main pole. The maximum down track effective field gradient was 661 Oe/nm and the maximum cross track gradient was 689 nm and nm lines, Oe/nm, evaluated along the respectively. To calculate readback waveforms, sensitivity functions of magneto-resistive (MR) heads with MR sensor widths of 12 nm, 18 nm and 24 nm were used. The sensor length was 6 nm and the sensor height was the same as the width. The shield-shield spacing was 25 nm and the spacing between the bottom of the sensor and the top of the recording layer was 6 nm. III. RESULTS A. Recording Tracks were written in the patterned media for various initial cross-track and down-track positions of the write head. Prior to
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Fig. 3. Write probability, P , versus initial position of the write head for media with 25 nm 13 nm bit cells. P = 1 indicates error-free recording.
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writing a track the dots were randomly magnetised up or down. A target track was chosen, either the eighth or ninth track from the left out of twelve, depending on the bit cell width, and an alternating pattern (up/down) was written. After writing, the target track was evaluated to determine the write probability and the track to the right of the target track was compared with the initial state to ensure no ATE occurred. The write probability, , was calculated using
(1) where is the magnetisation of the ’th dot in the track and is for a dot with and 1 for a dot with . was 24 for the 2 Tbit/in media and 48 for the 3.8 Tbit/in media. The boundary of the two-dimensional set of write head positions for which the write probability, , was equal to unity (error-free recording) was determined for each of the 2 Tbit/in media shown in Fig. 1. The result of the calculations for media with 25 nm 13 nm bit cells is shown in Fig. 3. The and axes show the distances between the point at (0,0) in Fig. 2, which is in the centre of the trailing edge of the main pole, and the centre of the dot. Negative values indicate the head is to the left of the dot centre, or the trailing edge of the main pole is behind the dot centre at the start of writing a bit. The region for which is referred to as the write window: if the initial position of the centre of the write head is within this area and the head moves along the down-track axis without lateral deviation, the written tracks will be error free. When the centre of the write nm ATE occurred, hence the head was at a position nm in Fig. 3. Moving the write head to the cut-off at left (increasingly negative ) reduced the head field in the target nm it was no track and once the head centre was at longer possible to write error-free tracks due to insufficient head field. The dashed white line in Fig. 3 is a fitted curve passing through the centre of the write window at each cross-track position. The curvature of the line is largely due to the head
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 6, JUNE 2010
Fig. 4. 14 kOe effective head field contours at two cross-track positions and dot in a 25 nm 13 nm bit cell. If the write head moves to the left the initial position must be moved backwards to compensate for the lower head field applied to the dot.
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Fig. 6. Write probability, P , versus initial position of the write head for media with 13 nm 13 nm bit cells. Down-track and cross-track axes have been swapped to save space.
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Fig. 7. Vertical magnetisation of 24 nm wide, 24 nm high, 6 nm long MR sensor as it passes over a medium with of 750 emu/cm .
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Fig. 5. Write windows for media with 18 nm 18 nm and 13 nm cells. (a) 18 nm 18 nm bit cells, (b) 13 nm 25 nm bit cells.
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field distribution and shows that as the head moves to the left the down-track boundary of the write window regresses. Fig. 4 shows two 14 kOe effective head field contour lines taken from the head field distribution in Fig. 2. If the centre of the write the 14 kOe contour passes through the head is located at centre of the dot. If the write head is moved 10 nm to the left nm. The initial position the same contour line moves to of the head must be moved down-track, towards the trailing nm to make the contour pass through the edge, by about centre of the dot once more. Larger head displacements to the , result in increasingly large down-track shifts, , as left, can be seen in Fig. 3. Fig. 5 shows write windows for media with 18 nm 18 nm and 13 nm 25 nm bit cells. It is clear that the bit cell shape has a strong influence on the shape of the write window. For example, the down-track write margin for the medium with nm, 25 nm 13 nm bit cells is around 11 nm at nm for the medium whereas it is around 21 nm at with 13 nm 25 nm bit cells. Conversely, the cross-track write margin is much smaller in media with narrower bit cells than it is in media with wider bit cells. There is a trade-off between the cross-track and down-track write margins which can be controlled through the bit cell aspect ratio. Depending on the relative difficulties of synchronising the write process or following a track, the bit cell aspect ratio can be tailored
to optimise the system performance. Such flexibility is not possible with non-shingled recording since the width of the write head must be changed roughly in proportion to the width of the bit cells to prevent ATE. The down-track margin for the 13 nm 25 nm bit cells was sufficiently large that the bit cell size could be reduced to 13 nm 13 nm, giving an areal density of 3.8 Tbit/in . The write window was calculated for 8 nm diameter cylindrical dots with a thickness of 10 nm and the same head field distribution as before. Fig. 6 shows that error-free recording was possible, but the maximum cross-track or down-track write margin was reduced to around 6 nm. Introducing a distribution of dot positions may shrink the boundary of the write window by the amount of the distribution. If we assume that a dot position distribution does not significantly change the nature of the magnetostatic interactions between dots, it can be seen from Fig. 6 that a position distribution of only 2 or 3 nm will make the write window almost entirely disappear. B. Readback Readback waveforms were calculated using MR head sensitivity functions. To calculate the signal from each MR sensor and the effect of thermal (magnetic) noise the sensors were discretised into 3 nm cubes with of 1400 emu/cm of 1000 erg/cm and K. A cross-track bias field, , of 1 kOe was applied and the magnetisation of the sensor was monitored as it passed over an isolated transition in a continof 750 emu/cm . An example is shown uous medium with in Fig. 7 for a sensor at 4.2 K. The maximum sensor signal,
GREAVES et al.: SHINGLED MAGNETIC RECORDING ON BIT PATTERNED MEDIA
TABLE I COMPARISON OF SNR VALUES (IN DB) FOR DIFFERENT MR SENSOR WIDTHS, , AND BPM MEDIA. NO THERMAL NOISE
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TABLE II COMPARISON OF SNR VALUES (IN DB) FOR DIFFERENT MR SENSOR WIDTHS, , AND BPM MEDIA. WITH THERMAL NOISE AT 300 K
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18 nm wide sensor gave the highest SNR. Increasing the sensor width from 12 nm to 18 nm significantly reduced the thermal noise, resulting in higher SNR. Further increasing the sensor width to 24 nm lowered the SNR as the reduction in thermal noise was outweighed by an increase in cross-talk noise and the decrease in SNR was greater for the media with smaller track pitches. Media with smaller bit aspect ratios tended to have higher SNR due to their higher signal (lower linear density), particularly when narrow sensors were used. When the sensor width was increased to 24 nm the maximum SNR was found in media with 18 nm 18 nm bit cells due to the higher rate of increase of signal with sensor width and lower rate of increase of cross-talk noise than the 13 nm 25 nm bit cell media. IV. CONCLUSION
was determined when the sensor was centred over a saturated region of the continuous medium. At this point the maximum vertical field in the bottom layer of the sensor was about 1.2 kOe. increased from 0.584 in the 24 nm sensor to 0.593 and 0.609 was most in the 18 nm and 12 nm sensors. The increase in likely due to an increase in the average vertical field in the sensor from the medium as the sensor height decreased. Thermal noise, was calculated from the fluctuations of sensor magnetisation at 300 K in the absence of a medium and was characterised by the R.M.S. value of the vertical component of the the thermal sensor magnetisation. For a sensor of volume , giving signal to noise ratios noise varied according to of 15.72 dB in the 24 nm sensor, 13.96 dB (SNR) in the 18 nm sensor and 10.87 dB in the 12 nm sensor. The MR head output was calculated for patterned media consisting of nine tracks of 1000 dots. Dots in the fifth, centre track were magnetised alternately up and down, whilst dots in the other tracks were randomly magnetised up or down. Signals from the readback waveforms obtained from the reciprocity calvalues for each sensor calcuculation were divided by the lated from continuous media. Table I shows the calculated SNR values in the absence of sensor thermal noise. For each sensor the signal increased as the bit cell length increased (linear density decreased) and the signal was 50% larger for media with 13 nm 25 nm bit cells than for media with 25 nm 13 nm bit cells. However, cross-talk noise from adjacent tracks was more severe in the media with narrower bit cells, resulting in lower SNR. Where the sensor width was similar to the bit cell width the SNR values were also broadly similar, ranging from 15.2 dB to 16.7 dB, with the wider bit cell media having the higher SNR. Table II shows the SNR for each combination of medium and read head when the thermal noise spectrum with a high-frequency cut-off of 6 GHz was included. For all four media the
By modifying the aspect ratio of the patterned media bit cells the cross-track and down-track margins can be controlled, although one increases at the expense of the other. Such an approach is not possible with conventional recording. "Therefore, shingled recording on bit patterned media offers extra flexibility, in addition to increased write fields. It would also be extremely difficult to use a BAR of less than 1:1 at these areal densities without using the shingled recording scheme due to the narrow track widths. At 2 Tbit/in the best overall performance was obtained from 18 nm 18 nm bit cells and a 24 nm wide MR sensor. The stray field in a MR sensor from magnetic dots can be much less than 1 kOe at high areal densities and obtaining adequate SNR will be a major challenge. Very small MR sensors are highly susceptible to thermal noise which can exceed cross-talk noise from the media. Increasing the cross-track bias field and reducing the head-medium spacing may help reduce the effect of the thermal noise. ACKNOWLEDGMENT This work was supported in part by the Research and Development for Next Generation Information Technology project of the MEXT, Japanese Government, and by a Grant in Aid from the Japan Society for the Promotion of Science (#21 560 374) and the Storage Research Consortium, Japan. REFERENCES [1] R. Wood, M. Williams, J. Kavcic, and J. Miles, “The feasibility of magnetic recording at 10 terabits per square inch on conventional media,” IEEE Trans. Magn., vol. 44, pp. 917–923, Feb. 2009. [2] S. J. Greaves, Y. Kanai, and H. Muraoka, “Shingled recording for 2–3 Tb/in ,” IEEE Trans. Magn., vol. 45, pp. 3823–3829, Oct. 2009. [3] K. Miura, E. Yamamoto, H. Aoi, and H. Muraoka, “Estimation of maximum track density in shingled writing,” IEEE Trans. Magn., vol. 45, pp. 3722–3725, Oct. 2009. [4] M. E. Schabes, “Micromagnetic simulations for terabit/in magnetic recording head/media systems,” J. Magn. Magn. Mater., vol. 320, pp. 2880–2884, 2008. [5] S. J. Greaves, Y. Kanai, and H. Muraoka, “Magnetic recording in patterned media at 5–10 Tb/in ,” IEEE Trans. Magn., vol. 44, pp. 3430–3433, Nov. 2008.
