29.5- $hbox{gb/in}^{2}$ recording areal density on barium ferrite tape

$: 29.5- $hbox{Gb/in}^{2}$ Recording Areal Density on Barium Ferrite Tape$
IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 1, JANUARY 2011 137

29.5-Gb/in� Recording Areal Density on Barium Ferrite TapeGiovanni Cherubini�, Fellow, IEEE, Roy D. Cideciyan�, Fellow, IEEE, Laurent Dellmann�,

Evangelos Eleftheriou�, Fellow, IEEE, Walter Haeberle�, Jens Jelitto�, Venkataraman Kartik�, Mark A. Lantz�,Sedat Ölçer�, Fellow, IEEE, Angeliki Pantazi�, Hugo E. Rothuizen�, David Berman�, Wayne Imaino�,Pierre-Olivier Jubert�, Gary McClelland�, Peter V. Koeppe�, Kazuhiro Tsuruta�, Takeshi Harasawa�,

Yuto Murata�, Atsushi Musha�, Hitoshi Noguchi�, Hiroki Ohtsu�, Osamu Shimizu�, and Ryota Suzuki�

IBM Research-Zurich, 8803 Rüschlikon, SwitzerlandIBM Research-Almaden, San Jose, CA 95120 USA

IBM Systems & Technology Group, San Jose, CA 95120 USAIBM Systems & Technology Group, Yamato 242-8502, Japan

Recording Media Research Laboratories, FUJIFILM Corporation, Odawara 250-0001, Japan

The recording performance of a new magnetic tape based on ultra-fine, perpendicularly-oriented BaFe particles was investigated.Specifically, using a low lateral tape motion demonstration platform, a new servo pattern written on the advanced perpendicularly ori-ented BaFe medium, a new low friction head technology, a novel synchronous servo channel design, and advanced servo control concepts,we were able to demonstrate a record closed-loop track-follow performance with a 23.4 nm standard deviation of position-error signal,roughly one order of magnitude better than in current tape products. In addition, using read back waveforms captured on the same ad-vanced perpendicularly oriented BaFe medium with a 0.2- m-wide data reader, we demonstrated write/read performance at 518 kbpiusing advanced noise-predictive maximum likelihood (NPML) detection schemes. Combining these two results, we estimate that the newmedium can support an areal recording density of up to 29.5 Gb/in�. This result demonstrates the scalability and extendability of tapetechnology using low-cost particulate media.

Index Terms—Magnetic recording, magnetic tape recording, signal detection, tracking.

I. INTRODUCTION

T HE volume of digital data produced each year is growingat an ever increasing pace. According to an International

Data Corporation study [1], 264 exabytes of data were createdin 2007. In the future, this staggering volume of data is pro-jected to grow at a 57% compound annual growth rate (CAGR),faster than the expected growth rate of storage capacity. More-over, new regulatory requirements imply that a larger fractionof this data will have to be preserved. All of this translatesinto a growing need for cost-effective digital archives. State-of-the-art linear-tape products found on the market achieve an arealstorage density of about 1 Gb/in and a cartridge capacity onthe order of one to two terabytes. In this paper, we describea recording demonstration at an areal density of 29.5 Gb/instored on a new, perpendicularly oriented barium ferrite (BaFe)tape medium. This demonstration clearly indicates that tape re-mains a very attractive technology for data archiving with a sus-tainable roadmap for the foreseeable future.

Fig. 1 compares the evolution of the areal densities ofhard-disk and tape products over time, including recent tapeareal density demonstrations. The plot indicates that eventhough the gap between the areal densities of HDD and tapeproducts has remained essentially constant in recent years,tape areal density demonstrations exhibit a slope of about 60%CAGR, indicating the potential to reduce the areal density gapbetween tape and HDD. We can gain insight into how this

Manuscript received July 09, 2010; revised August 24, 2010; accepted August31, 2010. Date of current version December 27, 2010. Corresponding author:M. A. Lantz (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMAG.2010.2076797

is achieved by comparing the bit aspect ratios (BAR) of tapedrives and HDDs. The typical BAR (bit width to bit length)in recent HDD technology demonstrations is on the order of6:1 [2]. In contrast, the latest Linear Tape Open generation-5(LTO-5) tape drives operate at an areal density of 1.2 Gb/inwith a BAR of about 123:1 [3]. This comparison indicates thatthere is considerable room to reduce the track width in tapetechnologies. In this paper, we demonstrate a more than 18-foldincrease in track density relative to LTO-5 by dramatically im-proving the performance of the track-follow servo system. Toachieve this, we used an optimized servo pattern in combinationwith a new servo channel, an based track-follow controllerand a new low-friction head technology. This aggressive scalingof the track density necessitates the use of much narrower readelements, leading to a reduced readback amplitude. To compen-sate for this we used a new magnetic tape based on ultra-fine,perpendicularly-oriented BaFe magnetic particles that enableshigh density without using expensive vacuum-coating methodsand provides a much better signal-to-noise-ratio (SNR) thanstandard metal-particle tape media. Finally, to further enhancethe areal density through linear density increases, we imple-mented novel advanced noise-predictive maximum-likelihood(NPML) detection methods supporting linear densities of up to518 kbpi with an ultra narrow, 0.2- m data reader. In this paper,we describe each of these new technologies and show how theycan be combined to achieve an areal density of 29.5 Gb/in .The paper is organized as follows. In Section II we describethe new BaFe media, in Section III the low-friction head tech-nology, in Section IV the tape path and servo technologies,and in Section V the read channel and recording performance.Finally, in Section VI, we summarize the results and presentour conclusions.

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138 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 1, JANUARY 2011

Fig. 1. Recording areal density of hard-disk drive and tape products and tapedemonstrations (adapted from [6]).

II. MEDIA DEVELOPMENT

One approach to increase the recording capacity of magnetictape is to reduce the volume of the magnetic particles used inthe recording layer. Currently available commercial magnetictape media utilize iron-cobalt-based metal particles (MP). Inthis work, we investigate the use of barium-ferrite (BaFe) parti-cles as a potential replacement to MP in future tape media. Asthe origin of the coercivity of MP media is shape anisotropy, itis difficult to maintain a high coercivity as the particle volumeis reduced. In contrast, the coercivity of BaFe particles arisesfrom crystalline magnetic anisotropy, rendering the scaling ofBaFe particles more favorable. Another issue with traditionalMP media results from the oxidation of iron-cobalt. To pre-vent this, the particles are covered with a protective, nonmag-netic “shell,” which further hinders scaling to finer particle sizes.BaFe, in contrast, is already an oxide and therefore does notneed a protective shell. Finally, another advantage of BaFe parti-cles is their unique platelet shape, which makes it easier to orientthe easy axis in the perpendicular direction of the medium sur-face. Note that perpendicular orientation results in an increasein signal amplitude and a reduction in the demagnetizing fieldat high linear densities [4]. Together, these properties make thescaling of BaFe to finer particle sizes much more favorable thanMP technologies and have led to extensive investigations of theuse of BaFe particles [5]–[13] in future generations of magneticrecording media.

To improve the performance of BaFe media relative to thatof previous generations [5]–[13] even further, three new tech-nologies were applied in developing the medium. First, we re-duced the average particle volume to 1600 nm . Second, theeasy axis of the particles was oriented in the perpendicular di-rection during the coating process, resulting in a squareness ratioof 0.86 in the perpendicular direction. Finally, we also reducedthe surface waviness, achieving a surface roughness (Ra) of 0.7nm, as measured with optical interferometry, which neglects thesmall asperities, and an Ra of 2.1 nm measured using atomicforce microscopy (AFM).

The properties of this new perpendicular BaFe tape (Tape A)are summarized in Table I. For comparison, we also present theproperties of the BaFe medium used in our earlier areal density

TABLE 1PROPERTIES OF MAGNETIC PARTICLES AND TAPES

1) barium-ferrite2) metal particle3) saturation magnetization4) with demagnetization compensation5) measured with a WYKO HD2000 instrument

demonstration of 6.7 Gb/in (Tape B) [6] and the properties ofthe latest commercial MP media (LTO-5). We also note thatLTO-5 operates at an areal density of 1.2 Gb/in with a cartridgecapacity of 1.5 TB [3]. The thickness of the magnetic layer ofall three media types is approximately 60 nm.

A. Fine Particle BaFe

In general, as the size of a magnetic particle is reduced, thethermal stability factor is also reduced. Here,is the anisotropy energy density, the particle volume, theBoltzmann constant, and the absolute temperature. For BaFeparticles, the anisotropy can be adjusted by the substitution ofFe with other elements [11]. To compensate for the reduced par-ticle volume of Tape A, we adjusted the type and amount of sub-stitution elements used in the BaFe particles to increase . Byoptimizing the formulation of the 1600-nm particles, we wereable to achieve a thermal stability factor of 75, measured usingthe method described in [10]. This provides sufficient stabilityfor long-term archival applications.

B. Perpendicular Orientation

Fig. 2 presents transmission electron microscopy (TEM) im-ages of Tape A and Tape B. Note that in Tape A, the platelet-shaped BaFe particles are well aligned, whereas in Tape B theyappear randomly oriented. The orientation of the particles inTape A results in an increased squareness ratio and an improve-ment in medium performance [12]. To achieve this improvedsquareness ratio, we used intensive dispersion prior to coatingin combination with the application of a magnetic field in thedrying zone of the coating process. – loops of these tapesare shown in Fig. 3. The – loop in the perpendicular direc-tion of Tape A is very similar to that of the LTO-5 tape in thelongitudinal direction, indicating a similar degree of orientation.

CHERUBINI et al.: 29.5 Gb/in RECORDING AREAL DENSITY ON BARIUM FERRITE TAPE 139

Fig. 2. TEM images of cross sections of (a) Tape A (perpendicular orientation)and (b) Tape B (nonoriented). Scale bars are 20 nm.

Fig. 3. �–� loop of perpendicular direction of Tape A and Tape B and M-Hloop of longitudinal direction of LTO-5 tape.

Fig. 4. Optical interferometry measurement of surface roughness: (a) Tape Aand (b) Tape B.

C. Low-Waviness Surface

To reduce the head-medium spacing, the surface of themedium should be as smooth as possible. However, the funda-mental dilemma in contact magnetic recording is that such anincrease in medium smoothness may result in increased frictionand hence may reduce the durability and runability of themedium. To resolve this dilemma, we reduced the long-rangesurface roughness, which we refer to as “waviness,” whilemaintaining a moderate roughness measured over a shorterlength scale. Fig. 4 presents surface profiles

Fig. 5. AFM images of tape surface profiles: (a) Tape A and (b) Tape B.

of Tapes A and B, measured with optical interferometry. Thedata shows a waviness of for Tape A compared to

for Tape B. In Fig. 5 we presentsurface profiles of both tapes measured with AFM. Note thatcompared with Tape B, Tape A has many small asperities. Thiscombination in Tape A of low surface waviness and moderateshort-range roughness results in an increased signal from themedium while maintaining excellent durability and runability.

III. LOW-FRICTION HEAD TECHNOLOGY

In contrast to HDDs, there is no air bearing between the tapeand head in tape drives. Instead, tape heads are designed with asharp leading “skiving” edge, which prevents air from enteringthe interface, thus creating a negative pressure, which forces thetape into contact with the head [14]. In conventional head de-signs, the head surface is smoother than the tape, so that thehead-tape interaction is dominated by the tape roughness, in-cluding the effect of asperities designed specifically for control-ling the head-tape interaction and for head cleaning.

The high linear recording densities reported here require verylow magnetic spacing, which can only be achieved with taperoughness that is low enough to cause high stiction forces ina conventional flat-head geometry [14]. Variations in the tapevelocity caused by varying friction forces can disturb the timingrecovery function in the read channel, leading to sampling phaseuncertainty and high error rates.

Although a low tape-head spacing is required at the read/writeand servo elements, these elements only span one of four databands on the tape in LTO recording, i.e., out of the 12.65 mmtape width, the elements span 2.9 mm. The entire width of thetape must be supported by the head, but the region away fromthe read/write elements can be supported just as well by an airbearing as by hard contact.

Fig. 6 presents the beveled head design used for the29.5 Gb/in areal density demonstration. As can be seen in thefigure, the skiving edge appears only near the write/read ele-ments, whereas elsewhere the contour is roughly that of a circleof radius 6 mm into which a straight section has been inserted.The edge slope is 2.2 . For a cylindrical bearing of radius , theflying height at velocity is[15], where is the air viscosity, and and are the tensionand width of the tape, respectively. For velocities between1.4 and 7 m/s, the fly height varies between 0.75 and 2.2 m.We confirmed these values by numerical simulations of theReynolds equation.

The portions of the head shaped during the beveling processwere roughened to an average roughness ranging from 6 to20 nm. Because this roughness is well below the air-bearingthickness, there is no contact with the tape flying over the air


Fig. 6. Beveled head design for reducing head-tape interaction (not to scale).(a) Illustration of a head attached to brown mounting beam. (b) Photo of centersection of head, showing the location of the cross sections in (c) and (d). (c)Cross section of flat area of head, showing tape in contact. (d) Cross section ofbeveled region of head, showing air bearing between tape and head.

Fig. 7. Recording quality versus velocity for conventional flat and beveledheads. The recording quality MbTF is the mean number of bits between errorevents, i.e. the inverse of the error rate.

bearing. However, the roughness is much larger than the taperoughness, and greatly reduces static friction. Because the tapecontacts the rough area only while starting and stopping, tapedamage is avoided and the rough areas of the head will notbe polished smooth. The 1-mm transition length between thebeveled and the unbeveled regions (Fig. 6) eliminates hardcorner contact where the bevel begins.

Compared to a conventional flat tape head, the beveled headreduces running friction from 0.3 to 0.1 N, and startup stic-tion from 2 N to 0.8 N. The reduction in running frictionresults in a reduction in high frequency variations in tape ve-locity which in turn results in an improved error rate perfor-mance. Fig. 7 shows the error performance on a worn (smooth)earlier formulation of BaFe tape (similar to Tape B) [5] writtenat 2 m/s at a linear density of 480 kbpi, and read with a 3.8-reader in a modified commercial drive. At high velocities, theerror rate with the beveled head is reduced by a factor of 2 to4, whereas at very low tape velocities, the error rate can be re-duced by up to a factor of 20 relative to the unbeveled head [8].

Fig. 8. (a) Photograph of tape path and (b) block diagram of prototype setupfor track-follow experiments.

The dynamic effect of head tape-friction is primarily to excite acompressional sound wave in the tape, which reflects back andforth between the two rollers on either side of the head at a fre-quency of 32 kHz. The effect of the resulting timing change isgreatest at low velocities, where the 32-kHz time scale is fastestcompared to the bit rate.

IV. TAPE PATH AND SERVO DESIGN

A. Tape Path and Hardware Platform

To investigate the maximum track density achievable with thenew BaFe media, we have developed a flexible experimentalplatform for closed-loop track-follow experiments. The setupconsists of a commercial experimental tape transport (MountainEngineering II Inc., Longmont, CO, USA), the electronics cardand voice-coil motor (VCM) from an IBM1 TS1130 tape drive,a prototype beveled giant magnetoresistance (GMR) tape headequipped with 2.5- m-wide servo readers, an FPGA board, aDSP/FPGA board, and a host computer. A photograph of thetape path and a block diagram of the setup are shown in Fig. 8.

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Fig. 9. Demo servo pattern layout and parameters.

The tape path consists of ten porous ceramic externally pres-surized air-bearing tape guides that utilize hard-edge guiding.These guides in combination with a long tape path result in lowvalues of lateral tape motion (LTM) that vary with the edgeroughness of the tape sample. For the prototype BaFe mediasamples used in this study, we typically observed a lateral tapemotion with a standard deviation of m with tape transportvelocities of 2 m/s. The tape transport also includes two ten-sion sensors and performs velocity and tension control indepen-dent of the servo information written on the tape. The TS1130electronics card is used as the front-end for the read/write headand provides bias currents to the head as well as amplificationand analogue-to-digital conversion of the signals from two servoand 16 data readers in each of the two head modules. Sam-ples from the two servo readers are fed from the drive cardto the FPGA board in which the servo channel, described inSection IV-C, has been implemented. The servo channel pro-vides estimates of the lateral position of the head to the DSP/FPGA board, which in turn runs the track-follow controller de-scribed in Section IV-D. Synchronous operation of the track-follow controller is achieved by running the DSP in an inter-rupt-driven mode with an interrupt provided by the FPGA boardwhenever a new head lateral position estimate is available. TheDSP/FPGA board also hosts a digital-to-analogue converter anda current amplifier to produce the control signal that is fed backto the voice-coil actuator. Data and control parameters are trans-ferred between a host PC and the DSP/FPGA board using aFireWire™ interface and a graphical user interface implementedin MatLab™.

B. Servo Pattern Design

In a tape system, the servo pattern geometry of the dedicatedservo bands is of crucial importance for the overall servo perfor-mance, and hence for the achievable track density. In this work,we have adopted the same basic servo principle as described inthe LTO standard [16] and illustrated in Fig. 9, but have modifiedthe pattern geometry to enhance the performance of the servosystem. To analyze the impact of the individual servo patternparameters on the overall servo performance, we developed amodel of the servo readback signal that includes the servo readerwidth , the azimuth angle , and the pulse width at 50% am-plitude as parameters [17], [18].

Our primary design goal was to support the best possibletrack-follow performance. This is achieved by improving thequality of the head lateral position and the tape velocity esti-mates for a given quality of the servo write process and by in-creasing the update rate of the estimates. In reference to Fig. 9,

Fig. 10. Simulated dibit waveform shape as a function of azimuth angle, readerwidth, and servo stripe width.

the geometry of the servo pattern can be described by the fol-lowing parameters: azimuth angle , servo stripe distance ,servo stripe width , servo subframe distance , the number ofservo bursts (four in the LTO standard, designated A, B, C, andD), and the number of stripes per servo burst. The geometry ofthe pattern impacts the shape of the readback waveform. In ad-dition, the shape of the servo signal is influenced by the mediumproperties and the servo reader width . The lateral position ofthe head is determined by measuring a distance between adja-cent servo bursts along the longitudinal tape direction, and thenestimating the lateral position using the relationship [19]

where is the estimated distance between two adjacentservo bursts at a lateral position , is a pattern-dependentconstant, and is the azimuth angle of the servo stripes. Theterm determines the amplification of the measure-ment error associated with the measurement of . Hence,the quality of the parameter estimates can be enhanced byincreasing the angle . For example, by increasing the anglefrom 6 to 18 , the error amplification is reduced by a factor ofapproximately 3.

Such an increase in the azimuth angle has several further im-plications. First, a larger angle results in a longer servo frameif the remaining pattern parameters are unchanged. In order tocompensate for the pattern extension, the pattern length can becompressed by reducing the servo stripe width and spacing .Second, a narrower servo reader width is needed to avoid thedispersion of the readback waveform, as illustrated in Fig. 10,where the dibit shapes have been obtained using the dibit re-sponse described in [17] and [18]. A narrow servo reader widthresults in a smaller servo signal amplitude, and hence in a lowersignal-to-noise ratio (SNR).

Overall, an increased azimuth angle, a reduced servo stripewidth, and a reduced servo reader width result in a nonnegli-gible energy loss in the servo waveform, which limits the gainsobtained by the reduced error amplification described above. Tofind the optimal tradeoff between azimuth angle, servo reader


Fig. 11. (a) Optical micrograph of new servo pattern decorated with ferro-fluid.(b) Servo waveform of demo servo pattern read back from perpendicular BaFemedium (Tape A) with a 2.5-�m-wide servo-reader.

width, and servo stripe width, we performed extensive VHDLlevel simulations of the servo channel and track follow con-troller as well as extensive experiments. This work indicated thatdespite the energy loss, the track-follow performance is best foran azimuth angle of 18 and a servo pattern that was compressedby a factor of 5/3 relative to LTO.

The compression of the servo pattern by a factor of 5/3 is notsufficient to compensate for the pattern-length extension intro-duced by an azimuth angle of 18 . To compensate for the in-creased angle and to further increase the servo update rate, wehave chosen a servo pattern height of 23.35 m, a reduction bya factor of 8 relative to the LTO specification. This servo pat-tern height enables a reduction of the servo frame length from200 m in LTO to 100 m for the demo pattern, hence doublingthe update rate of the estimates.

The final step in optimizing the servo pattern was an opti-mization of the servo formatting process. The main aspects in-clude the selection of a suitable pre-erase method before servoformatting the perpendicularly-oriented BaFe medium and theoptimization of the write gap width of the servo writer andthe applied servo write current in order to achieve a symmetricreadback waveform with high SNR. An optical micrograph ofthe servo pattern decorated using ferro-fluid is reproduced inFig. 11(a), and the details of the overall geometry are summa-rized in Fig. 9.

C. Synchronous Servo Channel

A typical servo readback waveform captured with the demosetup is presented in Fig. 11(b). Estimates of the tape velocityand of the lateral position of the head reading the servo pat-terns are derived from the relative timing of the readback servobursts. The computation of the estimates is performed by a dig-ital servo channel that processes the samples of the servo signalobtained by an analog-to-digital converter (ADC) operating at

Fig. 12. Block-diagram of the synchronous servo channel.

a fixed clock frequency. The sequence of head lateral-positionestimates is used to generate a position signal that is fed backto the track-follow servo mechanism. To achieve high-perfor-mance track-follow servo operation, the measurement noise inthe lateral position estimates must be minimized.

A synchronous servo channel architecture was adopted forrealizing the servo channel in our experimental system [20].Optimum filtering of the servo signal, which is necessary tominimize measurement noise in the position signal, is achievedby a digital matched-filter interpolator/correlator. The interpo-lator/correlator is included in the synchronous servo channelprior to tape velocity estimation, head lateral-position estima-tion, and detection of the longitudinal position (LPOS) infor-mation that is encoded in the servo pattern, as shown in theblock diagram of Fig. 12. The interpolated signal samples areobtained at a predetermined rate, defined in terms of samplesper unit of length, independent of the tape velocity. This prede-termined rate is different from the ADC sampling rate, whichis given in samples per unit of time. A timing-base referenceunit yields the interpolation instants for a desired step interpo-lation distance and the computed tape velocity estimate. Con-sequently, the matched-filter interpolator/correlator achieves aclose approximation of the optimum filter for the detection ofthe servo signal, independent of the tape velocity.

The time instants at which the correlator output signal ex-hibits maximum values are used to compute the tape velocityand head lateral-position estimates. The correlation methodadopted in the synchronous servo channel results in a substan-tially improved quality of the estimates compared with estimatecomputations based on the peak arrival times of the readbackservo signal.

For an ideal servo signal, the resolution or noise floor of theservo channel is on the order of 1 nm for the considered detec-tion algorithms and their finite-arithmetic implementation. Thistranslates into a position error signal with a standard deviation ofless than 10 nm obtained by simulating closed-loop track followcontrol, assuming the controller used in the demo, a tape for-matted with the 18 pattern, as described in Section IV-B, a SNRmeasured at the servo channel input of 28 dB, as measured fortape A, and negligible lateral tape motion.


Fig. 13. Schematic diagram of the track-follow control system.

Fig. 14. Head actuator frequency response.

D. Track-Follow Controller

The main task of the track-follow controller in the tape driveis to ensure the accurate positioning of the write/read elementsin the head at the center-line of the data tracks during writeand read operations. The main challenge the control systemhas to overcome originates from lateral tape motion (LTM),which leads to a misalignment of the read/write transducerswith respect to the track locations [21]. The track-follow feed-back controller utilizes the position estimates generated by theservo channel to control the lateral position of the head usinga voice coil actuator to follow the disturbances and to keep theread/write transducers centered on a given track location.

Fig. 13 shows a block diagram of the one-degree-of-freedomtrack-follow controller implemented here. In this figure, de-scribes the dynamics of the VCM actuator in the lateral direc-tion. To identify the transfer function , the frequency responseof the actuator was measured by applying a chirp excitationto the actuator and measuring the position of the head usinga laser Doppler vibrometer (Polytec GmbH, Waldbronn, Ger-many) and by measuring the longitudinal tape position via thedecoded servo pattern at a tape velocity of 2 m/s. The exper-imentally obtained frequency responses are shown in Fig. 14.The plant dynamics are dominated by the fundamental reso-nance mode, which can be accurately captured by a second-

Fig. 15. Power spectral density of open-loop PES.

order model. The delay in the phase response data measuredusing the servo pattern corresponds mainly to the position mea-surement delay, which depends on the servo pattern format andthe tape speed.

The bandwidth requirements for the track-follow controlsystem are set by the frequency characteristics of the LTM.An LTM estimate can be provided by reading the signal fromthe servo channel while keeping the actuator at a fixed lateralposition. This estimate also contains the effects of measurementnoise denoted as in Fig. 13. Many factors contribute to themeasurement noise in the PES signal, including the format ofthe servo pattern, the method used for detecting and decodingthe position information, the characteristics of the servo reader,the analog front-end, and the underlying medium noise. Fig. 15shows the power spectral density of open-loop PES data foran LTO-4 tape formatted with the standard 6 azimuth angleLTO servo pattern and for the new perpendicular BaFe mediumformatted with the 18 pattern described in Section IV-B.In both cases, the tape speed was 2 m/s. The low-frequencypart of the spectrum is dominated by LTM, which is mainlydetermined by the tape transport system, the roughness of thetape edge and the tape speed. The larger azimuth angle of thedemo servo pattern combined with the higher SNR of the newBaFe medium results in an improved resolution, as can be seenby comparing the spectra in the higher-frequency regime.


Fig. 16. Magnitude response of the closed-loop transfer functions.

The track-follow controller was designed using thecontrol framework [22]. The controller was obtained bynumerical optimization using weighting functions to specifythe requirements on the closed-loop transfer functions. Theweighting functions were chosen to balance the system require-ments in terms of disturbance rejection capabilities and sen-sitivity to measurement noise. For the one-degree-of-freedomfeedback loop of Fig. 13, the transfer function that relatesthe disturbance to the error and therefore serves as ameasure of the system’s capability to follow the LTM distur-bance is . The performance weight waschosen such that the magnitude of is low in the low-fre-quency range, where LTM dominates. The transfer function

relates the noise signal to the outputand measures the impact of the measurement noise on the

output. The weight was used to shape the transfer functionin such a way that the controller rolls off sufficiently fast at

high frequencies. An additional weight was used to imposeconstraints on the control signal . The objective of theoptimal control design is to find a stabilizing controller bysolving the problem: , where is defined as

In practice, the specified requirements are met if a controlleris found such that . Using synthesis, a sev-enth-order controller was obtained. Fig. 16 shows the magnituderesponse of the closed-loop transfer functions and of the de-signed controller. The closed-loop transfer function relating theLTM disturbance to the PES exhibits a 3 dB bandwidth of ap-proximately 650 Hz.

E. Track-Follow Performance

Track-follow experiments were performed using the perpen-dicular BaFe medium (Tape A) at a transport velocity of 2 m/s.The track-follow controller described above was implementedin the DSP of the experimental setup with a sampling frequencyof 40 kHz determined by the 2 m/s tape speed and the 50 mdistance between position estimates. Fig. 17(a) depicts a 7.5-s

Fig. 17. (a) Time trace of closed-loop PES demonstrating a 23.4 nm standarddeviation. (b) �� of PES data from (a).

capture of the closed-loop PES. We used two metrics to assessthe quality of the track-follow performance: (1) the standard de-viation of the PES , and (2) the 99.9% cumulative dis-tribution function . The is a measure of thedistance within which 99.9% of the position error estimates fall,as shown in Fig. 17(b). The data exhibits a standard deviationof 23.4 nm and a of 87 nm. The minimum track width(TW) that can be supported for a single channel with a readerwidth RW and the track-follow fidelity achieved are estimatedusing the model described by the Information Storage IndustryConsortium (INSIC) [23]:

Taking a reader width of 0.2 , as used in the experimentsdescribed in Section V, and a of 87 nm, measured asdescribed above, leads to an estimated track width of 0.446and an achievable track density of 57 ktpi.

V. READ CHANNEL AND RECORDING PERFORMANCE

From a signal-processing perspective, magnetic-recordingtape channels employing BaFe particulate media exhibit fairlydifferent characteristics in their system responses and noiseproperties compared with those based on MP media. Fig. 18(a)shows the dibit response of a tape channel that employs alongitudinally-oriented MP medium, and in Fig. 18(b) it iscontrasted with the dibit responses of two recording channelsutilizing Tape B (nonoriented BaFe) and Tape A (perpendicu-larly-oriented BaFe). The relatively asymmetric dibit shape ofthe nonoriented medium and the strong initial undershoot ofthe perpendicular medium are both features resulting from thespecific orientation of the BaFe particles.


Fig. 18. Dibit responses of (a) an LTO-4 recording channel with MP mediumusing a GMR read head having 3.8 �� reader width and 0.18 �� shield-to-shield gap length, and (b) a BaFe recording channel with Tape B (nonoriented)and Tape A (perpendicularly-oriented) media using a GMR read head having0.2 �� reader width and 0.08 �� shield-to-shield gap length. In all cases, thelinear density is 343 kbpi and no write-equalization is employed. � denotes thebit duration.

Moreover, Fig. 18 also shows that dibit responses fromthe BaFe media decay faster and thus have larger high-fre-quency content then the dibit responses from the MP media.A Lorentzian approximation shows thatrepresents a best match for both BaFe particle orientations atthe linear density of 343 kbpi,2 with PW50 denoting the pulsewidth at 50% peak amplitude and being the bit duration. Ata linear density of 600 kbpi, is obtained. Notethat the write equalization scheme used with MP tape mediaturns out to be inappropriate for BaFe tape media because thespectral shaping achieved by this pre-emphasis technique isnot well-matched to the channel response of BaFe tape. Thus,the use of write equalization with BaFe would lead to a loss ofsignal energy through the recording channel and, hence, to aperformance degradation.

MP and BaFe particulate media also differ with respect to thenoise components of their readback signals. In particular, therelative contributions of stationary electronics and medium noiseand nonstationary data-dependent noise vary significantly for thetwo cases. The results of a noise decomposition [24] study areshowninFig.19.At the relative linear densityof1,correspondingto 343 kbpi, nonstationary noise for the LTO-4 MP medium con-tributes about 70% of the total noise power, whereas Tape B(nonoriented BaFe) and Tape A perpendicularly-oriented BaFe)exhibit about 25% and 30% nonstationary noise, respectively.Although in the BaFe case this proportion is relatively smaller,its nonnegligible contribution still warrants the application ofdata-dependent detection techniques, as discussed below. Notethat the increase of the data-dependent noise power for the BaFemedia at higher linear densities may partly be due to the strongerjitter in the timing phase of the signals used for noise decompo-sition. Fig. 20 compares the power spectral densities of the totalnoise for Tape B and Tape A at two linear recording densities.Tape A exhibits a significant reduction in total noise power,resulting from a combination of the perpendicular orientation ofthe particles, the reduced particle volume, the improved particledispersion, and the reduced surface roughness of the tape.

2Note that only uncoded data was used in the experiments presented in thissection and we have therefore normalized the linear densities using the LTO-4channel linear density of 343 kfci (after modulation coding) rather than the userlinear density of 323 kbpi.

Fig. 19. Proportion of data-dependent noise for the MP and BaFe particulatemedia. The recording conditions were the same as for Fig. 18.

To determine the linear recording density that can be achievedwith the most advanced formulation of the BaFe medium (TapeA), a single track of data was recorded in a loop-tester environ-ment using a thin-film-head write element having a saturationmagnetic flux density of 18 kG and write gap length of 0.17 m.The written track was wide enough to ensure that no impair-ments due to track misregistration are present when readback isperformed with a narrow 0.2- m GMR reader having 0.08 mshield-to-shield gap length. No tracking control of the read headwas applied during the readback process. The written data con-sisted of a repeating 255-bit pseudo-random binary sequence(PRBS), which was recorded at linear densities ranging from343 to 600 kbpi.

The captured readback waveforms were processed by asoftware read channel that implements all the functions of anactual in-drive read channel, employing, however, full precisionfor signal and coefficient representations. The data-detectionschemes were optimized so that a post-detection bit error rate(BER) of less than , which was our target for a successfuldemonstration, is guaranteed, despite the fairly poor detec-tion-SNR values that resulted from using an ultra-narrow readerat high linear recording densities.

To this end, it was necessary to move away from the tradi-tional extended partial-response class-4 (EPR4) signal shapingand detection, and to resort to a read-channel and detector de-sign based on “generalized” partial-response signaling. In thisapproach, the target response of the overall channel can be se-lected with the dual goal of ensuring a good match to the actualresponse of the recording channel and effectively whitening thenoise process and reducing its power at the input of the detector.This objective is achieved by noise-predictive maximum-likeli-hood (NPML) detection [25], [26].

An effective way to further improve the detection performanceis to extend the noise prediction concept implemented in NPMLsequence detection to also take into account the data-depen-dent nature of the noise process. In this case, the branch-metriccomputation of the NPML sequence detector involves data-


Fig. 20. Power spectra of the noise components of the readback signals for TapeB (nonoriented BaFe) and Tape A (perpendicularly-oriented BaFe) media at (a)343 kbpi and (b) 600 kbpi linear recording density. � denotes the bit durationand � is frequency. Note that the dip observed at �0.8 and the peak between0.8 and 1 is an aliasing artifact that results from the 5/4 sampling rate used tocollect the readback signal which was then re-sampled at a 5� higher rate.

dependent noise prediction. Hence, in data-dependent NPML(DD-NPML) detection, predictor coefficients and predictionerror both depend on the local data pattern [27], [28].

For the NPML and DD-NPML detection results presented inthis paper, a target polynomial of the form

was considered, where the coefficientsand of the noise-prediction filter are possibly data-dependent.Readback signal equalization towards the class-4 PRcharacteristic have the advantage of placing a spectral null, andhence mitigating the noise components, both at dc and at theNyquist frequency. The fourth-order target polynomialimplies an NPML detector with 16 states. For DD-NPML, thenumber of prediction filters needed is determined by the lengthof the data pattern on which noise prediction is conditioned.Here, data patterns were assumed to consist of five consecutivechannel bits, hence a separate noise-prediction filter was usedfor each of the 32 branches in the detector trellis. Although stillmore powerful (DD-)NPML detection can be achieved by re-sorting to higher-order target polynomials, this avenue was notpursued in this demonstration because our aim was to evaluatethe recording performance with detectors that are realistic intheir implementation complexity.

The SNR at the detector input for an EPR4 read-channel asa function of the relative linear density for Tape B (nonorientedBaFe) and Tape A (perpendicularly-oriented BaFe) is presentedin Fig. 21. It is seen that Tape A provides a considerable im-provement of up to 2.9 dB in available SNR. In Fig. 22, theBER performance versus normalized linear density for 8-stateEPR4, 16-state NPML and 16-state DD-NPML detection andfor the two BaFe media are presented. The results show that ata relative linear density of 1.4 and using EPR4 detection, theBER of Tape A (perpendicularly-oriented BaFe) is more than40 times lower than that of the Tape B (nonoriented medium).Also, the detector performance can be significantly improved by16-state NPML and 16-state DD-NPML detection. In particular,for the post-detection BER target of , 16-state DD-NPMLallows operation at a relative linear density of up to 1.51, thatis, a linear density of 518 kbpi. Combining this result with the57 ktpi track density shown in Section IV, we estimate that thenew perpendicular BaFe media (Tape A) can support an arealrecording density of up to 29.5 Gb/in .

Fig. 21. SNR versus relative linear density at the EPR4 read-channel output.

Fig. 22. Performance of various detection schemes on Tape B (nonorientedBaFe) and Tape A (perpendicularly-oriented BaFe).

VI. CONCLUSION

Using flexible tape media based on ultra-fine, perpendic-ularly-oriented BaFe particles, an areal recording density of29.5 Gb/in was demonstrated on a single recorded channel, i.e.,an approximately 25-fold increase with respect to state-of-theart tape products. Specifically, using a 0.2- m-wide GMRreader, we demonstrated that a post-detection BER target of

is achievable at a linear density of 518 kbpi using a16-state DD-NPML read channel. In addition, we developed astate-space-based track-follow controller, a novel synchronousservo channel and a new servo pattern, which in combinationwith the improved SNR of the BaFe media and our low frictionhead technology, allowed us to demonstrate a track-followperformance with a 23.4 nm standard deviation of the track-fol-lowing error. The minimum track width supported by thisdegree of track-following fidelity and a 0.2- m-wide readeris 0.446 m, corresponding to a track density of 57 ktpi.The combination of these two results leads to the aforemen-tioned areal density of up to 29.5 Gb/in . Note that this was a


single-channel recording demonstration in which we primarilyfocused on the performance of the head/medium/read-channelcombination and on the performance of the track-follow servosystem. Effects, such as erase bands between tracks and theside-reading behavior of the read sensor which will likelybecome important at sub-micron track widths, were not consid-ered. Moreover, head tolerances and tape dimensional stability,which are important in parallel channel tape recording, werenot considered. However, what is clear from this areal densitydemonstration is that there exists plenty of room to continue toscale the areal density and cartridge capacity of tape systemsfor many years to come.

ACKNOWLEDGMENT

The IBM authors would like to thank R. Biskeborn,E. Childers, G. Decad, E. Gale, D. Hellman, M. Hill,R. Hutchins, C. Lo, H. Nagura, A. Sasaki, and K. Schirmer fortheir support of this work.

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29.5- $hbox{gb/in}^{2}$ recording areal density on barium ferrite tape

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