IEEE TRANSACTIONS ON MAGNETICS, VOL. 30, NO. 6, NOVEMBER 1994 3797

Outlook for Maintaining Areal Density Growth in Magnetic Recording

Edward Grochowski and David A. ‘I’hompson, IBM Research Division, Almaden Research Center, 650 I Iarry Road

San Jose, California 951 20-6099

ABSTRACT4 Iistorical trends and advanced component work are cited as reason for optimism that the current progress in storage areal density will continue smoothly to at least 5 Gigabits per square inch by the year 2000.

INTRODUCTION

This paper is intended to be a non-specialist’s over- view of the prospects for continued areal density improvement, in order to help place in perspective the following papers on high density magnetic recording. The discussion will be limited to rigid disk magnetic recording, but much of the technology relates to flexible rnedia storage as well. At the present time, the state of the art in hard disk storage density is approximately one half gigabit per square inch (80 mb/sq.cm.). This paper considers thc outlook for attaining an order of magni- tude greater density in products, by the year 2000.

Fig. 1 illustrates the historical trend of areal density in rigid disk drives. The change in slope in 1990 can be ascribed to a number of important factors, including the shift to smaller disk diameters (which allows more rapid design and integration), the use of thin fdm disks and advanced heads that posses intrinsic signal-to-noisc advantages over earlier types, improvements in design productivity for disk drive electronics and attachment methods, and a profound change in the competitive environment. During the next order of magnitude areal density increase, thc only factor which is likely to slow the current rate of improvement is our ability to produce a combination heads, disks, and electronics that function reliably at the new densities. That is the subject of this paper, which considcrs only normal evo- lution of the tcchnology. Many investigators have a vision for the future similar to Fig. 2, which addresses radical innovations to replace magnetic recording as practiced today. None of these candidates is likely to produce commercial products by the year 2000.

‘I’lie fivc and a half orders of magnitude improvement shown in Fig. 1 are almost cntirely thc result of scaling thc recording process; thc present stored pattern on the disk is composed o f the same altrrnating transitions as were seen on the RAMAC, but thcy occupy 250,000 times lcss area per bit. At each step in the evolution, the essential prohlems being solvcd were shrinking the physical dimensions of thc head and storage mcdium, boosting thc readback signal to tnairitain an acceptahlc error rate, reliably achieving a decreasing hcad-to- mediutn spacing, and positioning the head with evcr- increasing specd and precision

S I I R I N K I N G r i m S I O R A O F MFDIIJM

The ultimatc limit for room temperature magnetic storage is thcrmal demagnctization of the storcd bits. Each bit can be thought of as being composed of a

magnetized assembly of magnetic particles (Attempts to provide a continuous tnediutn of high coercivity and high moment have yielded unacceptable media noise due to domain phenomena). In order to keep the media noise resulting from this granularity at acceptable values, if the particlc boundaries are not aligned with the bit locations, the number of magnetic particles per bit should be maintained above some statistical minimum. ‘This implies scaling the mean particle volume with the area of the stored bits. For an isolated magnetic particle, the characteristic time for thermal randomization is an astoundingly stccp function of par- ticle volume: A factor of two reduction in particle diameter (all other parameters being the same) will result in a factor of about ten to the sixteenth reduction of storage life, c.g., from about 100 years to about 300 nanoseconds [I]. Since this process is driven by kl’ in comparison to the energy barrier for switching, a factor of two rcduction in s i x scalcs as a factor of eight in the energy barrirr, which i y what determines the storage life. ‘Io some extent, par tick s i x reduction can be compen- sated by increascs in magnetic moment and coercivity, (which increase thc encrgy harrier), by decreasing the demagnetizing field at a transition (which is acting to dccrease the cncrgy barrier), and by irnproving particle uniformity (which dccrcases the fraction of particles having substandard barriers) Although thermal demagnctization will ultimately limit the effective par- ticle size, and hencc the attainable media S/N, that limit is scveral ordcrs of tnagnitudc away from our present densities. ‘I he pr xtical pi oblcms of obtaining suitably stable low noise Incdia are well addressed in the papers from the recent I’MRC’ [2]. The theory of thermal sta- bility is explorcd further at this conference [ 3 ] .

SIIRINKING 1 I I I IIfA11, W t l l l l ; MAlNIAINING I l l F S I G N A 1

It would he nice lo anticipate that improvements in signal procewing would compensate for thc rcduction of head output rcsulting from scaling. Clnfortunately, this is not a realistic cxpcctation. ‘I’hc reason for this is another extrcmely stccp function in recording, that o f error rate as a function of signal-to-noisc ratio. For example, under normal opcrating conditions, the error rate of wen an idcal Vitcrbi dctcctor will degrade by approxirnatcly an ordcr of magnitude per dB of S/N [4] I t is reasonahle to ask improved codcs and chan- nels to make up two or thrce dB of the losses due to scaling, but neither history nor theory suggest that much more than tliat u i l l I x achicved, in the next six years [5J. Ncw codcs and dctcction methods will also con- tinue to i n y o v c our ability to handle burst and defect noise. which i q more troublesome than white noise in many dtivcs S i t u thr noise powcr (other than for

0018-9464/94$4.00 Q 1994 IEEE

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media noise) increases proportionately with bandwidth, it would be a triumph for the channel to allow head sensitivity to remain at its current value of about a half millivolt peak-to-peak while providing the necessary increase in data rate. It will therefore be essential to keep the head output from decreasing as the areal density incrcases.

In the days of inductive heads, the scaling laws for head output were rather simple. However, the present state-of-the-art in areal density is achieved with parametric heads where the signal energy is derived from a power supply, through the modulation by magnetic fields of the resistance of a magnetic thin film [SI. See Fig. 3. The scaling laws in this case are more complex than in the inductive case, and depend on whether con- stant field, constant power, constant temperature rise, or some other constraint on head output is used. In addi- tion, the present sensor films are already thinner than the mean free path of the conduction electrons involved in the so-called “anomalous” magnetoresistance phe- nomenon, so the energy conversion efficiency (square of the fractional change in resistance) is further reduced by scaling. Many of the other critical dimensions that must be scaled are film thicknesses, which are far from any fundamental limits, but for which defect densities may be expected to rise with scaling. Some dimensions are determined by photolithographic resolution and alignments limits. It would be unpleasant, if not pro- hibitive, to discover that E-beam or X-ray lithography were required in the next few years. There is also the problem of scaling the domain control techniques needed for acceptable magnetic noise and stability.

Many of the above problems become easier to deal with if the head sensitivity is made larger. Head designers have avidly followed the recent discoveries in giant magnetoresistance and spin valves. The exper- imental data in the paper by ?’sang, et al. [7], showing a head with more than one KV/m (peak-to-peak output signal per unit of track width), is extremely important in est%blisling that hcads of suitable sensitivity will be available for use at 5 Gb/sq.in.

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RELIABLY SCAl .I NG 1‘1 1 E 1 l F , A l ~ - T O - M E l ~ l I I M SPACING

The head-to-mcdium spacing is commonly thought to be the principal gating factor for areal density iinprove- ment. The historical record supports the assertion that the two are closely tied (Fig. 4), and the cxponential decay of the signal with spacing over magnetization wavelength placcs a strong upper bound on the spacing that can be tolerated. Scaling the physical spacing between the head and medium has caused more grief than any other aspect of magnetic recording. This is because the factors that will cause premature tribological failure are not directly measurable at the time of manu- facture, unlike magnetic phcnomena that cause increased error rate. Today’s hard disk drives all employ a slider that flies over the disk on a self- pressurized air bearing, and have a magnetic spacing on the order of 75 nanometers. At ten times the areal density, that magnetic spacing would scale down to the

vicinity of 24 nm. Depending on any wear-resistant coatings on the head and disk, and on pole-tip recession or warped surfaces, this could be close to the rule-of- thumb for “contact recording,” which is I O nm.

The reason for optimism here is simply that there are a number of techniques proposed for achieving near- contact recordi ig, including one reported at this confer- ence [SI - [ IO]. .Although the numerical techniques used for calculating air bcarings require refinement at spacings below the mean free path of air molecules, there is no experimental data to indicate that nature has any problems with such conditions. Indeed, any labora- tory can fly heads for short periods at these small spacings, and there are tnany optimists who believe that

I 1”: I

-31 /RAMPC I Y e 0 I I I

lo 1960 1970 1980 1990 Year

I I

Pig. 1 . Areal density history of the hard disk drive. Inset: more detail of the upturn in 1990. The present

compound growth rate is about 60%.

10 Year

Fig 2 Piojcc~ions for the future.

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jeads \

Fig. 3a. A coriccptual cutaway view of a magnetoresistive read head.

COntoct

Spln Valve Head

2 4 6 8 1 0 M 40 Sensing Layer Thickness. nm

Fig. 3b. ' 1 . 1 ~ active elenlent of a spin valve head, showing 11s structural similarity to an M R head, and the much

thinner liltin neccssary for optimal sensitivity.

straight - fo rward evolutionary scaling will succeed through attcntion to tolerances, and the development of suitably compliant suspensions for scaled-down slider dimensions Sce I;ig 5 .

I'OSI I lONlNG T ' I I E I1EAI)

Achieving approxitnately three times the present state-of-the-art track density (now about 5000 tpi or 200 tpmm) does not present any fundainental problems, although maintaining the historical improvement rate for seck times may not be possible without further

Head-Media Clearance, nanometers

Pig. 4. The historical correlation between head-media clearance and areal density. Clearance is a limiting factor for linear density and engineertng concerns have kept the

track density to bit density ratio at about 20:l.

1975

3

Thin Head Etched Pattern ;;gad 30 % PIC0

TiC/Alumina Slider Etched Pattern MR Head Near Contact

Integrated Slider/Suspenslon Thin FilmKomposite Structure MR Head Near Contact

Pig 5 . 'l'he evolution towards smaller sliders.

downward scaling in disk diameter. The usc of fluid bearing spindles is often advocatcd for ultra high track dcnsitics, but that decision tnust be driven by perform- ance and reliability questions ... optical storage devices already achieve the requisite track densities without them. One advantage obtained frotn the higher output and separate readiwrite transducers of an MR or spin valve head is the relaxed T M R requirements due to a reduced read track width (write-wide, read-narrow), compared to the requirement expected from simple scaling of single-transducer designs [ 1 I].

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COMMENIARY

The essential technologies necessary for achieving 5 Gigabits per square inch are progressing satisfactorily, and there appear to be no fundamental roadblocks to success. Recent advances in head sensitivity are espe- cially significant. A demonstration of this areal density under the sort of ground rules used for the 1989 Gigabit demonstration [I 121 would be the best way to validate our optimism. One significant program to achieve this goal is underway in the United States by the National Storage Industry Consortium, which combines indus- trial, university, and government activities [13].

If this density is achieved, will the resulting recording systems be just like the present ones, but with ten times the capacity? Probably not, though there certainly will be a market for very high capacity drives. If present trends continue, by the end of the decade the dominant disk size will be 65 mm, down from the present 95 mm, and each platter will hold 2.5 GB. Data rate and power consumption characteristics will cause a further splitting between the high performance and the low cost or low power versions of these drives. But those projections are beyond the scope of this overview. ‘

REFERENCES

[ I ] C. Denis Mee and Eric D. Daniel, Magnetic Recording, Volume I: Technology, New York: McGraw-Hill, 1987, pp. 127 - 130.

[2] “TMRC on recording media,” IEEE n u n s . on Magnetics, vol. 29, pp. 342 - 1223, January 1993.

[3] P. L. Lu and S. H. Charap, “Thermal Stability a t 10 Gbit/sq.in. Magnetic Recording,” paper HC-01 at this confer- ence. C. Coleman, et al., High Data Rate Magnetic Recording in a Single Channel, Proceedings No. 59 (Fifth International Con- ference on Video and Data Recording), London: IERE, pp. 151 - 157, April 1984. T. D. Howell, et al., “Advanced Read Channels for Magnetic Disk Drives,” paper AA-05 at this conference. C. Tsang, “Design and Performance Considerations in High Density Longitudinal Recording,” J . Appl. Phys., vol. 69, pp. 5393 - 5398, April 1991.

C. Tsang, et al., “Design, Fabrication, and Testing of Spin- Valve Read Heads for lligh Density Recording,” paper AA-02 at this conference. E. M. Williams, “High Areal Density Inductive Technology,” paper AA-04 at this conference. H. Hamilton, et al., “Contact Perpendicular recording on rigid media,” IEEE 72an.s. on Magnetics, vol. 21, pp. 4921 -4926, November 1991. The so-called “Visqus” technology. See US patent 5,097,386. C. Denis Mee and-Eric D. Daniel, op. cit., vol. 11, pp. 121-128. C. l s ang , et al., “Gigabit density recording using dual-element MR/inductive heads on thin-film disks,” IEEE nuns. on Mag- netics, vol. 26, pp. 1689- 1693, September 1990, and com- panion papers in that issue. J. L. Simonds, “U.S. Digital Recording Industry,” monograph available upon request from NSIC, 9888 Carroll Center Road, San Diego, CA 92126-4580 USA.

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