Drive integration challenges for perpendicular recording

Journal of Magnetism and Magnetic Materials 235 (2001) 245–252

Invited paper

Drive integration challenges for perpendicular recording

Steven E. Lambert*, Bruce M. Lairson, Hai Nguy, Linda Nguyen, Ton Huang,Jason Adler, Srini Gopalaswamy

Quantum Corporation, 500 McCarthy Drive, Milpitas, CA 95035, USA

Abstract

Integration of heads and media for perpendicular recording into disk drives will require new developments in severalareas. This paper discusses the methods we use to characterize error rate and servo performance of perpendicularrecording components, and highlights the impact of skew angle and external field on recording performance.

We also report our first efforts to incorporate perpendicular components into disk drives. r 2001 Published byElsevier Science B.V.

Keywords: Perpendicular magnetic recording; Skew; Servo

1. Introduction

The recent enthusiasm for perpendicular record-ing is the result of concern about decay of recordedinformation due to the superparamagnetic effect,often called media thermal decay. Perpendicularrecording offers several possible advantages overconventional longitudinal recording for resistingmedia thermal decay. However, there are manybarriers to practical implementation of perpendi-cular recording in disk drives, as have beendescribed well by others [1,2]. Many of thepublications on perpendicular recording appro-priately focus on the challenges of producingheads and media that will function well in arecording system. Without minimizing the effortrequired to achieve good performance of headsand disks, this paper will focus on our strategy forintegrating perpendicular heads and media includ-ing component-level evaluations. We address

several integration concerns including shapingthe waveform by the use of differentiation, theinfluence of skew angle on recording performance,positioning the head using a servo system, and theeffects of external magnetic fields. In addition, wereport on our first drive level results usingperpendicular media.

2. Motivation

The loss of signal amplitude due to mediathermal decay is a serious challenge for continuedimprovement of areal density in hard disk record-ing using conventional longitudinal recordingcomponents. Perpendicular recording using amagnetically soft underlayer offers several possibleadvantages. The most important is that the writefield of the head is increased by about a factor of 2.The medium can also be thicker while maintaininggood writing performance. Both of these advan-tages can increase the media stability factor*Corresponding author.

0304-8853/01/$ - see front matter r 2001 Published by Elsevier Science B.V.

PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 3 5 3 - 5

KuV=kBT since a higher writing field allows anincrease in the magnetic anisotropy Ku and athicker layer increases the volume V : Temperature(T) and Boltzmann’s constant kB are unchangedby using perpendicular media! In addition, de-magnetization effects (which accelerate mediathermal decay) are most harmful to longitudinalrecording at high linear density, while low lineardensity is the worst case for perpendicular record-ing. This presents an advantage for perpendicularrecording since there are more options to over-come a limitation at low density when designing arecording system.

3. Heads and disks

The perpendicular media used in this study wasa CoCrPt alloy with a thickness of 40 nm and acoercivity of 2500Oe. The nonmagnetic interlayerbetween the medium and the soft underlayer was5 nm thick, and the soft magnetic underlayer was300 nm thick. The disks were coated with 5 nmcarbon and a thin lubrication layer.Three head types were used. The basic geometry

of the write elements is shown in Fig. 1 andrelevant dimensions are given in Table 1. Head Awas a ring head with a relatively straight trailingedge, making it useful for evaluating component

level squeeze performance at high linear densities.Head B was a product ring head for a 20GB/platter drive product, with a magnetic write widthat the gap of 0.65 mm, and at the trailing edge of0.8 mm. Head C is a probe head with a pole shapedlike a trapezoid, with a bevel angle f of about 101.

4. Readback waveform

The shape of the readback waveform from aperpendicular disk is quite different from conven-tional longitudinal media. In the case of a diskwith a soft underlayer, the waveform from isolatedtransitions is a square wave. This is illustrated inFig. 2 showing time-averaged waveforms acquiredwith a type A head. Also shown in Fig. 2 is thereal-time waveform after differentiation by a highpass RC filter. The differentiated waveform is verysimilar in appearance to the signal from conven-tional longitudinal media. As we have reportedbefore [3], this differentiated signal is easilyprocessed for error rate measurements by Quan-tum’s disk drive channel chips which have beenintegrated into the electronics for our spinstand.The channel optimization software functions welland no advantage in error rate is observed if thechannel settings are further adjusted by hand. Wehave used this methodology for three generationsof partial response channels with equally goodresults. In particular, we have achieved 13.4Gb/in2

(395 kBPI� 34 kTPI) on a spin stand, a densitysufficient for our 20GB/disk desktop drive. Ofcourse, differentiation is not the optimum signalprocessing approach for perpendicular recording[4]. However, in our work both low and highfrequency rolloff points of the RC filter are

Fig. 1. Air bearing surface view of ring head write element (left)

and beveled write pole (right). The dark downward arrow

indicates the head motion direction.

Table 1

Heads used for the present study

Head A B C

Pole thickn., L 3 mm 3mm 0.3mmWrite width, W 1.09mm 0.8mm 0.45mmRed width 0.61mm 0.30mm 0.25mmBevel angle, f 21 101

Nom. fly hgt. 30 nm 22nm 22nm

S.E. Lambert et al. / Journal of Magnetism and Magnetic Materials 235 (2001) 245–252246

adjusted for the best performance, so in fact this isthe first step in the equalization process ratherthan a simple differentiation. This straightforwardapproach allows significant progress in driveintegration studies without requiring special chipsor using off-line signal processing of digitizedwaveforms. In addition, the standard parametricmeasurements on our spinstand function well withthe differentiated waveform, again facilitatingintegration studies.

5. Impact of skew angle on error rate

One problem with recording on soft underlayerperpendicular media with a standard head issidewriting. This difficulty can be avoided by usingstandard heads at a skew angle of zero degrees,and we have made significant progress on mediaevaluations by adopting this approach on aspinstand. However, drive integration requiresattention to sidewriting which is illustrated bythe MFM image in Fig. 3 showing a track writtenusing a type A head at 151 skew angle. The middleof the track shows the square wave magnetization

expected from this system. However, the image ofthe poletip is reproduced in the medium during thewriting process since the soft underlayer accepts

Fig. 2. Time averaged waveforms acquired with a type A head. Top shows raw signal, bottom is waveform after differentiation.

Fig. 3. MFM image of a track written at 151 skew angle with

standard head A on perpendicular media with a soft underlayer.

The shape of the poletip is indicated by the white rectangle. The

medium moves upward during recording.

S.E. Lambert et al. / Journal of Magnetism and Magnetic Materials 235 (2001) 245–252 247

perpendicular flux from the entire poletip, not justthe gap or trailing edge. The result is that theslanted side of the poletip leaves a magneticpattern as shown on the right side of the track.We have quantified the effect of this sidewritingboth with and without data on adjacent tracks.The effect without data on adjacent tracks inshown in Fig. 4a where track profiles are shownfor the cases of 01 and 7151 skew angles. Thetrack profile for 01 looks like data from alongitudinal disk since the sidewritten featuresare absent when the head is at 01 skew. For7151 asmall additional peak is observed as indicated bythe arrows in the figure. This corresponds toreadback from the slanted features written on theside of the track. In practice, this readback effecthas little influence on the error rate since the peak

in sidewritten magnetization is rather far from thetrack center (about 1.25 mm (50 m00)). In addition,the side of the track will resemble demagnetizedmedia at reasonable linear densities. However, theincreased erase width due to the sidewrittenfeature can strongly degrade BER when adjacenttracks are present, as will be shown next.The results of error rate both with and without

data on adjacent tracks is shown in Fig. 5 wherethe width of a bathtub curve is shown vs. skewangle for four different cases. The top curve inFig. 5a labeled ‘‘No Squeeze’’ is the case acquiredusing a standard head with no data on the adjacenttracks. The width of the bathtub curve isindependent of skew angle. This rather surprisingresult confirms that the sidewritten features aresimilar to demagnetized media at reasonable data

Fig. 4. Track profile for 01 and 7151 skew for (a) ring head

type A, and (b) beveled probe head of type C.

Fig. 5. (a) Bathtub width vs. skew for ring head writer A at

20 ktpi. (b) Bathtub width for beveled pole head C at 50 ktpi.


densities despite the peaks visible in Fig. 4a atlower density. Note also that the half-width of thebathtub curve is only 0.25 mm (10 m00).Very strong degradation of BER is observed

when data are written on the adjacent tracks witha standard head as shown in Fig. 5a by the datalabeled ‘‘With Squeeze’’. The width of the bathtubcurve is about the same for skew=01 and751, butdecreases quickly as the skew angle increases. Thisis caused by an increase in the erase width of astandard head due to the sidewriting shown inFig. 3. The result is that data are overwritten onone side of the test track even when the gap of thewrite head is some distance away from the edge ofthe test track. This influence of excess side writingmust be eliminated to enable drive integration ofperpendicular media.There are several ways to address this concern.

One is to reduce the range of skew angles, forexample, by using a linear actuator or by using alonger arm. Another option which requires nochanges in the drive mechanics is to make a squarepoletip with widthElength (dimensions WEL inFig. 1). This greatly reduces the amount of sidewriting since the length L is much shorter than inconventional heads. A better solution is to use apoletip with a trapezoidal shape like head C shownin Fig. 1. The ‘‘bevel’’ angle f can be chosen toeliminate writing by the sides of the poletip byremoving the protruding corners. The optimumdesign will depend on the actual skew angle in thedrive and process tolerances in the mechanicalassembly.We have made measurements on a spinstand

using type C heads with trapezoidal poletips toinvestigate the recording performance of thisdesign. The data in Fig. 4b show track profiles ofdata written with a trapezoidal poletip for severalskew angles. The shape of the track profile is thesame in all three cases, unlike the data in Fig. 4awritten with a standard poletip. MFM imagesconfirm that the sidewriting observed in Fig. 3 iseliminated. The beneficial impact on BER is shownin Fig. 5b where the width of the bathtub curve isplotted vs. skew angle both with and without dataon adjacent tracks. As expected from the previousdiscussion of the data in Fig. 5a written with astandard head, the bathtub width is not affected by

skew angle when no data are present on adjacenttracks. However, the bathtub width ‘‘WithSqueeze’’ also shows no change with skew angle,quite different from the data in Fig. 5a for astandard head. This shows the benefit of eliminat-ing sidewriting by using a trapezoidal poletip.It is reasonable to ask if a square or trapezoidal

poletip can be extended as a solution for side-writing to the narrow trackwidths B0.1 mmrequired for densities of X100Gb/in2. Certainlymore invention will be needed to ensure that thepoletip remains magnetically soft and can produceadequate magnetic field. Additional studies ofboth poletip shapes are underway to help withanswering these questions.

6. Drive integration and servo

The data presented so far have focussed onvarious aspects of error rate. Equally importantfor drive integration is the performance of theservo system which positions the head on theappropriate track. We used a component-levelservo writer to study many aspects of servo withstandard heads and perpendicular media includingverification that our channel chip could decode theservo data after differentiation of the waveform.This gave us confidence that we should take thenext step of building drives with a perpendiculardisk. We used a mature platform, our recentlyintroduced 20GB/disk product with a maximumareal density of 13.4Gb/in2. One product disk andone perpendicular disk were built into each drive.Standard longitudinal heads to type B were used,although they were of a type with a flat trailingedge on the poletip to minimize writing distortionof the transitions. Servo was written at 35 kTPIusing a standard servowriter starting at the OD.An RC differentiator was added to the circuitboard as the first step in the equalization process.We were able to operate the drive in engineeringmode and characterize both the servo and BERperformance. This confirms our spinstand datawhich show that both servo and user data can beprocessed with standard chips when the signal isfirst passed through a differentiating circuit.


The servo performance was quite robust for aperpendicular disk, with servo address mark(SAM, indicating the beginning of servo informa-tion) and wedge number detected without errorsover most of the disk surface. The performancedegraded near the ID due to partial erasing of theservo information caused by the sidewriting effectdiscussed earlier. This was not important at theOD where the sidewriting occurred at the ID sideof the head and so did not degrade information ontracks already written. An MFM image of positionbursts written on a component level servowriter isshown in the top of Fig. 6. The sidewriting effect isclearly visible.The drive was able to seek and position the head

reliably (except near the ID, as mentioned above).

The positioning performance is shown in Fig. 7with histograms of 4096 consecutive positionvalues from both a perpendicular disk (withdifferentiation) and a standard disk (withoutdifferentiation) in the same drive. The heads werelocated at track 23,000 (out of 33,000 where track0 is at the OD). The head skew was �71. The twohistograms are nearly identical, showing accepta-ble performance of head positioning on perpendi-cular media. The RMS non-repeatable runout(NRRO) for both surfaces was about 1% of thetrack pitch for skew angles of �141 to +131.The difficulties we observed with servo near the

ID should be eliminated if a square or trapezoidalpoletip is used. An indication of this is shown inthe MFM image at the bottom of Fig. 6 showingservo position bursts written with a trapezoidalhead of type C. The contrast in this image is ratherpoor due to inadequate overwrite with thisparticular head. However, there is no indicationof significant sidewriting features, unlike the topimage written with a standard head. We expectimproved overwrite in future heads, giving us

Fig. 6. Servo burst patterns written at 50 ktpi for head A (top)

and head C (bottom). The scan area is 7.5mm� 7.5mm for

each image.

Fig. 7. Histogram of PES signal for (a) longitudinal media and

(b) perpendicular media in the same disk drive.


confidence that the servo will perform well overthe entire stroke.The ultimate criterion for drive performance is

error rate, and preliminary measurements areencouraging. We observed raw error rates of10�4–10�5, with no errors when ECC was on.No special precautions were taken for magneticshielding of the drive or to attenuate stray fieldsfrom the magnets in the actuator or motor. Weused a standard drive except for the perpendiculardisk and the addition of a differentiator. Morework is needed to improve the BER performancesince spinstand measurements of similar compo-nents show error rates of 10�7–10�8. The differ-ence can be attributed to differences in the specificheads used as well as the need to further improvethe tuning of the channel in the drive. However,this initial drive level BER result shows thatsignificant progress can be made in drive integra-tion of perpendicular media without elaborateprecautions.

7. Sensitivity to magnetic fields

A major concern with integrating perpendicularmedia with a soft underlayer is the sensitivity tostray magnetic fields. [1,2] We have made BERmeasurements on a spinstand while applyingmagnetic fields in all three directions (radial,circumferential, and perpendicular) to both long-itudinal and perpendicular media. The samestandard type A GMR head and electronics wereused for all measurements except that a differ-entiator was added for the perpendicular media.The greatest change in BER was observed with thefield applied in the perpendicular direction. Thedata are shown in Fig. 8 where error rate is plottedvs. applied field for several different data rates.Note that the x axes have different scales in thetwo plots. In both cases the error rate is degradedby the application of a magnetic field, with theperpendicular case significantly more sensitive. Adirect comparison can be made in Fig. 9 whichshows two of the data sets from Fig. 8 chosen tohave comparable BER. Degrading the BER by oneorder of magnitude requires about B2.5Gaussfor the perpendicular media compared with

B18Gauss for the longitudinal disk. Note alsothat the best BER is observed for applied field notequal to 0 and that the curve is asymmetric in field.We continue to study the causes of these effects,but there is not a detailed understanding at thistime. Overall, the observed sensitivity to externalfields must be improved before mass production ofdrives with perpendicular media. However, it is

Fig. 8. Spinstand error rate vs. magnetic field applied in the

perpendicular direction for (a) perpendicular and (b) long-

itudinal media at various clock frequencies. Note that the x axes

have different scales.


also consistent with our experience that no specialshielding for magnetic fields is needed on either thespinstand or for laboratory studies of drives withperpendicular media.

8. Conclusions

Integrating perpendicular media with soft un-derlayers into disk drives presents many challengesincluding writer design and signal processing forboth servo and data. Writers using either square ortrapezoidal poletips are a possible solution tosidewriting problems. We find that standardchannel chips can be used for both servo and data

if differentiation is used as the first step in theequalization process. We followed up our spin-stand evaluations by building drives incorporatingperpendicular media with standard heads and findthat the drives can function using a productplatform designed for 20GB/disk (13.4Gb/in2).Servo performance is equivalent to longitudinal atthis design point, and raw error rates of 10�4–10�5

are observed. Spinstand measurements show thatperpendicular media are about five times moresensitive to external magnetic fields compared witha longitudinal disk. While this is a concern forfuture mass production drives, we are able toevaluate drive integration in the lab withoutspecial precautions or magnetic shielding.

Acknowledgements

The authors wish to acknowledge RaymondHsiao for the MFM images, and Mike Mallaryand Nelson Cheng for helpful discussions of theresults presented here. We especially wish toacknowledge the outstanding support of Quan-tum’s head and media suppliers.

References

[1] W. Cain, A. Payne, M. Baldwinson, R. Hempstead, IEEE

Trans. Magn. 32 (1996) 97.

[2] D. Thompson, J. Magn. Soc. Jpn. 21 (1997) 9.

[3] K. Ho, B.M. Lairson, Y.K. Kim, G.I. Noyes, S.Y. Sun,

IEEE Trans. Magn. 34 (1998) 1854.

[4] Y. Okamoto, H. Osawa, H. Saito, H. Muraoka, Y.

Nakamura, PMRC 2000, Paper 24aA-05

Fig. 9. Two data sets from Fig. 8 showing spinstand error rate

vs. magnetic field at comparable BER: 190MHz (perpendicu-

lar) and 260MHz (longitudinal).


Documents

Drive integration challenges for perpendicular recording