The effect of intertrack intersymbol interference in a magnetic patterned medium is studied. A two-dimensional (2-D) channel code is proposed, dedicated to perpendicularly magnetized media without soft underlayer, which exhibit read pulses showing overshoot. Read pulse shapes were investigated using a magnetic-force microscope tip scanning the patterned medium row-by-row. To test different codes, a bit-detection simulation program was developed to generate large amounts of data on which bit error rates can be measured. Application of the 2-D channel code, which implies recording of particular dot positions with fixed bits ("ones", "zeros"), resulted in the elimination of 2-D worst-case bit patterns and a subsequent reduction of detected-bit errors. The accompanying redundancy of 22% is inevitable for this type of 2-D code

Abelmann, L.

Groenland, J.P.J.

English

Groenland, Hans

Abelmann, Leon

NARCIS 

Two-dimensional coding for probe recording on magnetic patterned media

University of Twente Research Information

IEEE TRANSACTIONS ON MAGNETICS, VOL. 43, NO. 6, JUNE 2007 2307Two-Dimensional Coding for Probe Recordingon Magnetic Patterned MediaJ. P. J. Groenland and L. AbelmannMesa+ Institute for Microsystems and Nanotechnology, University of Twente, NL-7500 AE, Enschede, The NetherlandsThe effect of intertrack intersymbol interference in a magnetic patterned medium is studied. A 2-D channel code is proposed, dedicatedto perpendicularly magnetized media without soft underlayer, which exhibit read pulses showing overshoot. Read pulse shapes wereinvestigated using a magnetic-force microscope tip scanning the patterned medium row by row. To test different codes, a bit-detectionsimulation program was developed to generate large amounts of data on which bit error rates can be measured. Application of the 2-Dchannel code, which implies recording of particular dot positions with fixed bits (“ones,” “zeros”) resulted in the elimination of 2-D worstcase bit patterns and a subsequent reduction of detected-bit errors. The accompanying redundancy of 22% is inevitable for this type of2-D code.Index Terms—Intertrack interference, magnetic force microscopy (MFM), patterned media, perpendicular magnetic recording, two-dimensional coding.I. INTRODUCTIONI N order to continue the path of continuously increasingareal data densities in magnetic mass storage devices, datarecording in magnetic patterned media is considered one ofthe contingencies to push bit densities beyond the superpara-magnetic limit of magnetic recording in continuous media [1].Essentially, the patterned medium consists of a 2-D arrange-ment of rows and columns of magnetized dots (islands) whereeach dot represents one single data bit. The phenomenon ofcrosstalk during reading will show itself in both directions andcan be taken as a combination of the conventional intersymbolinterference (ISI) along the rows and the so-called intertrackinterference (ITI) along the columns. The difference withcontinuous media is that now there is a predetermined phaserelation between the tracks, which can be exploited. In order tocombat ITI, some extensions of traditional 1-D channel codingand detection methods have been proposed [2], includingrun-length limited (RLL) codes for two-track systems, 2-Dpartial-response maximum likelihood, and the application ofturbo coding.This paper focuses on methods for future storage devicesbeing composed of a large number of independent and simul-taneously working probe-recording units. These units are ex-pected not to be individually supplied with complicated elec-tronics [3], prohibiting the application of relatively laboriouscoding methods. Additionally, it may be noted that patternedrecording media are not suited for application of conventionalRLL codes as, unlike in continuous media, the bit period cannotlocally be adapted in the channel bit pattern. In view of theseconsiderations, we examine a method for 2-D channel codingwhich is adapted to magnetic data storage in patterned media.II. MAGNETIC RECORDING SETUP AND SIMULATIONSIn order to analyze magnetic recording in patterned media,we used an experimental recording setup which can handle alimited number (100–1000) of dots. The recording mediumDigital Object Identifier 10.1109/TMAG.2007.893137Fig. 1. Measured MFM image (upper) and selected MFM-signal values fol-lowing a line crossing seven dots (lower).is a CoNi/Pt multilayer patterned by laser interference lithog-raphy with a periodicity down to 75 nm [4]. The read–writemechanism is based on a magnetic force microscope scanningthe dot pattern ( direction). Perpendicular recording (writing)is achieved by approaching the magnetic force microscopy(MFM) tip to the recording medium in the presence of anexternal magnetic field in the direction (perpendicular to themedium surface). An MFM read signal (Fig. 1) results from thevibrating MFM tip (probe) in the stray field of the magnetizeddots [5] and is proportional to the field’s second derivativeintegrated over the tip volume.The MFM response was simulated (Fig. 2) using a Fourier-based method on an effective surface charge distribution [5], [6],but again a limited number of magnetic dots can be treated.Both experiments and simulations support the idea that thelocal MFM signal can also be computed by taking the linear su-perposition of the weighted individual MFM responses of sur-rounding dots. Therefore, a second simulation program has beenrealized, producing the MFM response for reading the recordeddata in a large matrix of magnetized dots. This technique allowsus to study statistical effects when analyzing coding and bit de-tection techniques and determine bit-error rates.The recording media in this paper incorporate dots (bits)grouped in a regular 2-D pattern with equal row and column0018-9464/$25.00 © 2007 IEEE2308 IEEE TRANSACTIONS ON MAGNETICS, VOL. 43, NO. 6, JUNE 2007Fig. 2. Three configurations of computed magnetic dots scanned by MFM probe (upper). Simulated MFM-probe read signals scanned along dotted lines (lower).x axes are in nanometers. (a) Single bit. (b) “One” bit surrounded by eight “zero” bits. (c) Nine “one” bits.pitches. The magnetic stray fields from the dots, and thus themagnetization states, are read out row by row. Using a thresholddetector, the originally recorded bit sequence is recovered from(voltage samples of) this analog read signal. Basically, positiveread pulses (for instance associated with positive magneti-zation) will result in logical ones, negative pulses (negativemagnetization) in zeros.III. 2-D CODINGFrom our recording experiments on media without soft un-derlayer, and supported by simulations, a single dot was foundto produce an elementary read pulse showing overshoot whichmay extend over a distance larger than the dot’s dimensionsFig. 2(a). The particular pulse forms result from the compo-sition of the dot’s stray field combined with the fact that theMFM response is proportional to the field’s second derivative.For “realistic” MFM system/media designs, read pulses withovershoot maxima located “just on the centers of neighboringdots” showed overshoot amplitudes up to about 7%, but for rel-atively thin media, small tip heights and large dot surfaces, sometens of percents were computed.The emergence of 2-D ISI might lower the reliability of thedata-bit recovery (detection) process. In this paper, the MFMtip geometry was taken symmetrical with respect to the tip axis,leading to identical ISI impacts along the respective row andcolumn directions. For an elementary read pulse with 7% over-shoot, an MFM read signal was computed for two particularbit configurations [see Fig. 2(b) and (c)]. Due to the form ofthe elementary pulse, every “one” bit lowered the signal at itsneighbor-bit positions, while “zero” bits enhanced the signals atsurrounding positions. The effect of ISI is concluded to be “al-most worst case” for the central bit in the situation of Fig. 2(c),as a bit surrounded with identical bit values will result in a rel-atively low amplitude for this center position.Our approach to lower the amount of ISI during recording insuch a patterned medium is to prevent the particular bit con-figuration of Fig. 2(c) from occurring by applying a suitable2-D channel-coding scheme. This could be realized by speci-fying particular dot positions as predefined, fixed bit values (seeFig. 3. 2-D channel code with geometric constraint (fixed “one” and “zero”bits) imposed on 2-D bit patterns. Empty positions are available for recordingof user-data bits.Fig. 3). This scheme stops the square nine-bit combination ofFig. 2(c) (nine “ones,” or nine “zeros”) from occurring. Obvi-ously, this coding scheme results in a code rate 7/9 (or redun-dancy 22%).The benefit of the introduction of such a fixed bit pattern onbit detection has been analyzed using a computer simulationprogram. The program incorporates 2-D ISI and offers, amongother things, facilities for simulation of bit-position jitter. Aftercomputation of the read signal for a serpentine scanning tra-jectory (indicated in Fig. 3), the data bit sequence was recon-structed. Fig. 4 shows the number of detection errors in a se-quence of 15 876 bits as a function of the magnitude of uni-formly distributed bit-position jitter. Here, the elementary readpulse was taken with a relatively large 16% overshoot in orderto investigate the effect of the introduction of the 2-D channelcode of Fig. 3.For noncoded data and low jitter levels we found the bit errorsgenerally occur on medium locations corresponding with thebit pattern of Fig. 2(c), i.e., a block of nine (or more) identicalbit values. Introduction of the 2-D code eliminated this class oferrors (Fig. 4). Increasing the jitter magnitude above factor 0.1resulted in new errors at locations of bits which are surroundedby both “one” and “zero” bits. However, the plotted curves inFig. 4 suggest that the 2-D channel code only modestly combatsthis second error category.GROENLAND AND ABELMANN: TWO-DIMENSIONAL CODING FOR PROBE RECORDING ON MAGNETIC PATTERNED MEDIA 2309Fig. 4. Impact of 2-D channel code on bit-detection performance.IV. CONCLUSIONIn the case of conventional recording in continuous media,the recording density in terms of user bits might increase by ap-plication of constrained (or RLL) codes. Conversely, the intro-duction of a 2-D channel code in patterned-media recording, asdiscussed, always gives rise to a lower effective user-bit densityas a result of the introduction of fixed bit positions, but mightalso result in a lowered number of bit errors. If well designed,the total results will be an effective increase in user bit density.Additionally, the fixed bit pattern of the code discussed mightbe brought in as embedded data information applicable for servocontrol and data addressing. It should be noted however that thechannel bit density in patterned media will be primarily deter-mined by the lithographic fabrication technology used to definethe bits in the medium [7].The code method as depicted in Fig. 3 can be considered a“brute force” approach, because it claims redundant bit posi-tions which are located on fixed positions which are not cor-related to local data content. Beforehand, it is indeterminatehow to compile a coding scheme requiring minimal overheadfor avoiding “worst case” bit configurations. More research isrequired for the development of effective and suitable codingmethods in view of the required immunity to 2-D ISI and (other)noise effects.ACKNOWLEDGMENTThe authors would like to thank M. H. Siekman for providingthe results of MFM recording experiments on patterned mediaand L. Alink for building the ISI simulation program.REFERENCES[1] A. Moser, “Magnetic recording: Advancing into the future,” J. Phys.D, Appl. Phys., vol. 35, no. 19, pp. R157–R167, 2002.[2] B. Vasic and E. M. Kurtas, Eds., Coding and Signal Processing forMagnetic Recording Systems. Boca Raton, FL: CRC, 2005, ch. 24,25, 39.[3] L. Abelmann, T. Bolhuis, A. M. Hoexum, G. J. M. Krijnen, and J. C.Lodder, “Large capacity probe recording using storage robots,” in Inst.Elec. Eng. Proc.—Sci. Measure. Technol., 2003, vol. 150, pp. 218–221.[4] R. Murillo, H. A. van Wolferen, L. Abelmann, and J. C. Lodder, “Fab-rication of patterned magnetic nanodots by laser interference lithog-raphy,” Microelectron. Eng., vol. 78–79, pp. 260–265, 2005.[5] L. Abelmann, A. G. van den Bos, and J. C. Lodder, , H. Hopster and H.P. Oepen, Eds., “Magnetic force microscopy—Towards higher resolu-tion,” in Magnetic Microscopy of Nanostructures. Berlin, Germany:Springer-Verlag, 2005, pp. 253–283.[6] B. Vellekoop, L. Abelmann, S. Porthun, and J. C. Lodder, “On the de-termination of the internal magnetic structure by magnetic force mi-croscope,” J Magn. Magn. Mater., vol. 190, pp. 148–151, 1998.[7] B. D. Terris and T. Thomson, “Nanofabricated and self-assembledmagnetic structures as data storage media,” J. Phys. D: Appl. Phys.,vol. 38, pp. R199–R222, 2005.Manuscript received October 30, 2006 (e-mail: j.p.j.groenland@utwente.nl).

Eds., “Magnetic force microscopy—Towards higher resolution,” in Magnetic Microscopy of Nanostructures.

Eds., Coding and Signal Processing for Magnetic Recording Systems.

Fabrication of patterned magnetic nanodots by laser interference lithography,”

Large capacity probe recording using storage robots,” in Inst.

Magnetic recording: Advancing into the future,”

Nanofabricated and self-assembled magnetic structures as data storage media,”

On the determination of the internal magnetic structure by magnetic force microscope,”

http://purl.utwente.nl/publications/64126

Two-dimensional coding for probe recording on magnetic patterned media

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