IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 10, OCTOBER 2006 2489

Air Bearing Simulation of Discrete TrackRecording Media

Maik Duwensee1, Shoji Suzuki2, Judy Lin2, David Wachenschwanz2, and Frank E. Talke1

Center for Magnetic Recording Research, University of California at San Diego, La Jolla, CA 92093-0401 USAKOMAG, Inc., San Jose, CA 95131 USA

Sliders flying over patterned or discrete track media are exposed to an entirely different pressure field from the air bearing than inthe case of present day “smooth” disks. In this first study, we investigate the influence of discrete track media parameters such as groovewidth, groove pitch, and groove depth on the steady-state flying characteristics of five different slider designs using a finite-element-basedair bearing simulator.

Index Terms—Air bearing, discrete track media, patterned media, simulation.

I. INTRODUCTION

HEAT ASSISTED MAGNETIC RECORDING (HAMR)or patterned media recording is expected to be used in

future disk drives in order to increase the areal density above500 Gbit/in . In HAMR, laser light is used to heat individualbit cells, thereby lowering the media coercivity during the writeprocess. In patterned media recording, individual tracks or indi-vidual discrete bits are fabricated on the disk surface. Fig. 1(a)illustrates typical discrete track media and Fig. 1(b) typical dis-crete bit media, respectively. In bit patterned media (BPM), eachbit forms a single surface entity that is physically separated fromneighboring bits in the circumferential, as well as radial direc-tion. Thus, transition noise between adjacent bits is absent. Indiscrete track recording (DTR) technology, the bits are storedon single tracks which are physically separated from each other.Thus, the transition noise is eliminated in the radial direction butnot in the circumferential direction.

Sliders flying over BPM or DTR media “see” a disk surfacethat consists of ridges and grooves in the case of DTR, or in-dividual island-like regions in the case of BPM. The flying be-havior of a slider over such a surface is different from that forflying over a “smooth” disk. A reduction in the steady-stateflying height coupled with a different flying attitude can be ex-pected for DTR or BPM media unless planarization of the disksurface is performed.

To investigate the effect of discrete tracks, we have modi-fied an existing finite-element-based air bearing simulator [1]that takes the characteristics of the grooved disk surface into ac-count. A finite-element approach was chosen, because of theease of implementation of mesh size variations for differentareas of the air bearing surface. In areas of high pressure andlow spacing, a very fine mesh is selected to investigate the ef-fect of the ultra fine grooves appropriately. In areas of higherspacing, we have chosen a coarser mesh, assuming that the in-fluence of surface features is negligible at large spacing. Thisapproach keeps the overall model size small enough to be solv-able on a PC. A uniform high mesh density would result in a

Digital Object Identifier 10.1109/TMAG.2006.878617

Fig. 1. a) Discrete track recording. b) Bit patterned media.

Fig. 2. Pico slider design.

forbiddingly large numerical problem that could not be handledon present day computers.

II. MODELING

Five different slider designs were simulated, one pico andfour femto form factor designs. Fig. 2 shows the subambientpressure pico slider design used. On a nongrooved smooth sur-face this design has a flying height of 4 nm.

Fig. 3(a)–(d) shows the four femto designs used in this inves-tigation. The femto designs 1–4 have flying heights of 30, 14,15, and 19 nm, respectively. Design femto 2 was derived fromdesign femto 1 by decreasing the size of the center pad, design3 is a modified version of design 2 as can be seen from the dif-ferently shaped center pad. In design number 4, we moved thetwo side pads closer to the trailing edge, resulting in additionalair bearing pressure support for the slider. All other air bearingdesign parameters were kept constant.

For the pico slider design, we created a finite-element meshwith an element edge length of 1 m. A uniform mesh densitywas maintained. For the femto designs, we used nonuniformmeshes, with extremely small elements in the area of the trailingedge and center pad. On the trailing edge part of the center padan element size of 40 nm was defined. The center pad surfacewas meshed with elements 100 nm in size. The same element

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2490 IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 10, OCTOBER 2006

Fig. 3. Femto designs 1–4 [a)–d), respectively].

TABLE IRESULTING MODEL SIZES

Fig. 4. DTR parameter.

size was realized on the two side pad surfaces. The rest of theair bearing surface was meshed with elements 1 m in size.The resulting model sizes for all air bearing designs used in thisstudy are summarized in Table I.

Fig. 4 shows the parameters that determine DTR media. Theyare groove depth (d), groove width (w), and groove pitch (p), re-spectively. For the simulation of the pico design, we have used aconstant groove width of m and a constant groove pitchof m. The groove depth was chosen to be between zeroand eight nanometer ( nm). For the femto designs,we have chosen a groove width of nm and a groovepitch of nm, due to the very fine meshes for thesedesigns. The range of the simulated groove depth was from 0 to15 nanometer ( nm). The skew angle for all simula-tions was assumed to be zero.

III. SIMULATION RESULTS

Fig. 5(a) shows a typical pressure distribution of a sliderflying over DTR media using a uniform mesh. A magnifiedview of the pressure distribution in the area of the trailing edgecenter is shown in Fig. 5(b). We observe from Fig. 5(b) that thegrooves influence the pressure distribution in the trailing edgearea significantly. However, very little change in the pressuredistribution was found in areas of high spacing. Thus, it isapparent that a fine mesh is needed only in the areas of highpressure and low spacing, while a larger mesh size can be

Fig. 5. Pressure distribution of slider flying over DTR media. a) Entire airbearing surface. b) Detail of pressure at trailing edge pad.

Fig. 6. a) Absolute and b) normalized flying height change versus groovedepth.

used elsewhere. This knowledge was used during the meshingprocess of the femto slider designs.

Fig. 6(a) shows the absolute change in flying height as a func-tion of groove depth, while Fig. 6(b) shows the normalized lossof flying height as a function of groove depth. As can be seen, theabsolute loss in flying height (FH) is the same for all slider de-signs investigated. Analyzing the slope of the curves, we derivethe following empirical expression for the loss of flying height:

(1)

This formula is known to be a correct approximation for pos-itive pressure air bearings such as two rail slider designs. Theexpression can be derived from the Reynolds equation for aninfinitely wide inclined plane air bearing. Surprisingly, it seemsto be also a good estimate for the loss in flying height of sub-ambient pressure sliders flying over a grooved disk, considering

DUWENSEE et al.: AIR BEARING SIMULATION OF DISCRETE TRACK RECORDING MEDIA 2491

Fig. 7. Pitch angle change versus groove depth.

Fig. 8. Relative peak pressure increase. a) Just femto designs. b) All designs.

that in our simulations. This result is an indica-tion for the dominant role of the trailing edge pad in the designsinvestigated.

Fig. 7 shows the simulated pitch angle change as a functionof groove depth. In all our simulations, the pitch angle remainednearly constant.

Fig. 8(a) shows the increase in peak pressure as a function ofgroove depth for the femto slider designs investigated. We no-tice only a slight increase in peak pressure. In Fig. 8(b), we haveplotted the peak pressure versus groove depth for all designs.As can be seen from this graph, the increase in peak pressurefor the pico design is significant, up to five times the value of anongrooved head disk interface. Comparing the absolute peakpressure values for the pico design of 1.8 MPa (18 atm) (non-grooved case) with close to 9.1 MPa (90 atm) (10 nm groovedepth), this slider design may require a special lubricant thatcan withstand such high pressure values without excessive lu-bricant migration.

IV. SUMMARY

One pico and four femto slider designs have been analyzed toinvestigate the influence of DTR media on flying height, pitchangle, and peak pressure. It was found that the flying height losscan be expressed by a simple (1) involving groove width, groovepitch, and groove depth. The results show that the peak pressurecan increase significantly for sliders with a low initial flyingheight. Only a small increase in peak pressure was observed forsliders with an initial flying height of 14 nm and higher. Thepitch angle appears to be independent of DTR parameters.

REFERENCES

[1] M. Wahl, P. Lee, and F. Talke, “An efficient finite element-based airbearing simulator for pivoted slider bearings using bi-conjugate gradientalgorithms,” STLE Trib. Trans., vol. 39, no. 1, 1996.

[2] D. Wachenschwanz et al., “Design of a manufacturable discrete trackrecording medium,” IEEE Trans. Magn., vol. 41, no. 2, pp. 670–675,Feb. 2005.

[3] B. Horton, “Magnetic head flyability on patterned media,” M.S. thesis,Georgia Inst. Technol., Atlanta, GA, Jul. 2004.

[4] Y. Soeno et al., “Feasibility of discrete track perpendicular media forhigh track density recording,” IEEE Trans. Magn., vol. 39, no. 4, pp.1967–1971, Jul. 2003.

Manuscript received March 13, 2006 (e-mail: maik@talkelab.ucsd.edu).