3D modeling and inductance measurements were used to design an ultra-high frequency perpendicular system. Kerr microscopy and spin-stand experiments with focused ion beam (FI-B) trimmed perpendicular heads and perpendicular media directly verified the high frequency concepts

Bain, J.

Chantrell, R.W.

Chomko, R.

Khizroev, S.

Litvinov, D.

Svedberg, E.

White, R.

Wolfson, J.

English

D. Litvinov

R. Chomko

J. Wolfson

E. Svedberg

J. Bain

R. White

R. Chantrell

S. Khizroev

Crossref

Dynamics of perpendicular recording heads

White Rose Research Online

This is a repository copy of Dynamics of perpendicular recording heads.White Rose Research Online URL for this paper:http://eprints.whiterose.ac.uk/1834/Article:Chomko, R., Wolfson, J., Svedberg, E. et al. (5 more authors) (2001) Dynamics of perpendicular recording heads. IEEE Transactions on Magnetics. pp. 1376-1378. ISSN 1941-0069 https://doi.org/10.1109/20.950846eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse Items deposited in White Rose Research Online are protected by copyright, with all rights reserved unless indicated otherwise. They may be downloaded and/or printed for private study, or other acts as permitted by national copyright laws. The publisher or other rights holders may allow further reproduction and re-use of the full text version. This is indicated by the licence information on the White Rose Research Online record for the item. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. 1376 IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 4, JULY 2001Dynamics of Perpendicular Recording HeadsD. Litvinov, R. Chomko, J. Wolfson, E. Svedberg, J. Bain, R. White, R. Chantrell, and S. KhizroevAbstract—3D modeling and inductance measurements wereused to design an ultra-high frequency perpendicular system.Kerr microscopy and spin-stand experiments with focused ionbeam (FIB) trimmed perpendicular heads and perpendicularmedia directly verified the high frequency concepts.Index Terms—Focused ion beam (FIB), next generation mag-netic recording systems, perpendicular recording dynamics.I. INTRODUCTIONALTHOUGH perpendicular recording has been proposedsince the beginning of magnetic recording, it has notreplaced conventional longitudinal recording, because, untilrecently, it has been at best equal in capability [1]. This situationis changing today when conventional longitudinal recording isapproaching its fundamental limit due to superparamagneticinstabilities at approximately 100 Gbit/in density [2]. Per-pendicular recording might extend the limit to significantlyhigher densities due to the use of a thicker recording layerand exploiting the intrinsically higher write fields and strongerplayback signal of perpendicular heads due to the use of a softunderlayer [3]–[5]. However, performance of perpendicularrecording at high frequencies is a subject that has not beensufficiently explored. The demand for high data rate systemsis a consequence of the increasing linear density. Therefore,direct study of perpendicular media dynamics is of fundamentalimportance. Major open questions to achieving high data ratein perpendicular recording were believed to be magnetizationswitching in a hard layer with the easy axis oriented perpendic-ular to the plane of the disk (due to a relatively low torque) andthe influence on dynamics of a soft underlayer [6]. Recently,it was shown that the characteristic time for the magnetizationswitching in the hard layer is the orders of magnitude smallerthan the characteristic time for the magnetization switching insoft materials of a system, such as the head material and thesoft underlayer. Therefore, the soft materials dominate the timedependent roll-off.The write driver speed raises a serious concern at data ratesat which the characteristic time response is of the order of onenanosecond or less. Nevertheless, the question of the writedriver speed limitation is not going to be a subject of this paper.Because this question is as critical in perpendicular recording asis in longitudinal recording, several solutions have already beenproposed previously. The purpose of this paper is to study theManuscript received October 3, 2000.D. Litvinov, E. Svedberg, and S. Khizroev are with Seagate Research, Pitts-burgh, PA, USA (e-mail: Sakhrat_Khizroev@notes.seagate.com).R. Chomko is with the University of Miami, Coral Gables, FL, USA.J. Wolfson, J. Bain, and R. White are with the Data Storage Systems Center,Carnegie Mellon University, Pittsburgh, PA, USA.R. Chantrell is with the University of Wales, Bangor, UK.Publisher Item Identifier S 0018-9464(01)05990-8.Fig. 1. A diagram showing a closed magnetic flux path in (a) a longitudinalsystem and (b) a perpendicular system with a soft underlayer.intrinsic dynamic response of a perpendicular system with theunderlayer and compare it with the intrinsic dynamic responseof a typical longitudinal system. Realizing that the principaldifference is caused by the presence of the soft underlayer inperpendicular recording, the study accentuates on the role ofthe underlayer.Three-dimensional (3D) finite element modeling, includingmicromagnetics, was developed to define the design guide-lines for high-speed perpendicular systems. Kerr imagingmicroscopy was utilized to directly study the magnetic fluxdynamics of a recording system with a soft underlayer [7].II. DESIGN OF HIGH-SPEED PERPENDICULAR SYSTEMSDiagrams showing typical longitudinal and perpendicularsystems are shown in Fig. 1(a) and (b). Considering thatrecording layers are typically orders of magnitude fasterthan soft materials in a system, the dominant contributionto the frequency roll-off is going to be caused by the headinductance. The head inductance consists of two parts, thewrite coil inductance and the inductance of the yoke plus theinductance of the soft underlayer in case of perpendicularrecording. Effective inductance calculations, incorporatingthe eddy currents effect, were performed using 3D finiteelement modeling (FEM). The narrow portions of the recordingsystems, such as the leading and the trailing pole tips in thelongitudinal mode and the trailing pole tip and the underlayerin the perpendicular mode, were modeled micro-magneticallyusing LLG equation. The head and the soft underlayer wereassumed to be made of a FeAlN compound with a of20 kGauss and a of 10 Oe. A longitudinal ring head designwith a trackwidth of 100 nm and a gap length of 60 nm wasmodeled. An equivalent perpendicular head modeled had a100 nm wide trailing pole with a 30 nm separation between theair bearing surface (ABS) and the underlayer. The calculatedswitching time versus the closed magnetic path length of theperpendicular system with different numbers of the coil turns isshown in Fig. 2. The closed path is defined as the yoke length[measured at the inner yoke surface, as shown in Fig. 1(a)]0018–9464/01$10.00 © 2001 IEEELITVINOV et al.: DYNAMICS OF PERPENDICULAR RECORDING HEADS 1377Fig. 2. The switching time versus the magnetic closed path length forperpendicular systems with 4 different numbers of coil turns: 2, 5, 10, and 20.Fig. 3. The switching time versus the underlayer thickness for a 5-turn coilsystem with a 20 m flux path length.plus the length of the soft underlayer under the head, as shownin Fig. 1(b). The underlayer was modeled to be 300 nm thick.The characteristic switching time, , was defined as the timenecessary to generate the magnetic field sufficient to overcomethe anisotropy field of the recording layer, . In this work,. of 7 kOe was considered. It is evident that the inductanceof the system should decrease with the reduction of the numberof coil turns. However, as the number of turns is reduced,the recording field generated is also reduced. Fortunately, thecurrent ultra-compact recording systems, being intrinsicallymore efficient, require a smaller number of write coil turns[8]. This is especially true for a perpendicular system, whichis usually even more efficient due to the soft underlayer [9].For the particular system design, the minimum number of turnsnecessary to generate a recording field larger than the hardlayer anisotropy field, , was calculated to be two (two turnsgenerate approximately 14 kOe). (Although, the total circuitimpedance can be optimized externally, still the number ofturns around the yoke is an important factor in determining theimpedance of the head itself.) It can be seen that even a five-turnsystem is capable of the switching time less than 1 nsec if themagnetic path length is less than 20 m (see Fig. 2).The switching time versus the underlayer thickness for a5-turn coil system with a 20 m thick underlayer is shown inFig. 3.To compare longitudinal recording and perpendicularrecording, the switching time versus the magnetic closedpath for an equivalent longitudinal system with a 5-turn coilFig. 4. The switching time versus the magnetic closed path length for a 5-turncoil longitudinal system.Fig. 5. A system for study of dynamics of perpendicular recording.is shown in Fig. 4. The switching time for the longitudinalsystem was determined as the characteristic time necessary toovercome an equivalent longitudinal media with the coercivityfield of 7 kOe. It can be noticed that for a perpendicular systemcomparable switching times can be achieved by adjusting theflux path length.III. KERR MICROSCOPYA Kerr microscope was developed to directly study dynamicsof recording systems both with and without a soft underlayer[10]. A Kerr image was taken from a region under the recordingpole tip. In the case of a perpendicular system, the image wastaken from a region in the soft underlayer, as shown in Fig. 5.A focused ion beam (FIB) image of a FIB trimmed perpen-dicular head used for study of perpendicular recording is shownin Fig. 6.In the experiments a square pulse with a rise time of less than1 nsec was applied to study the head and underlayer response. Asequence of polar Kerr images taken at increasing times after thefield pulse on a perpendicular system and an equivalent longi-tudinal system are shown in Figs. 7 and 8, respectively. To sum-marize the results, the normalized write current pulse and theKerr angle responses at the point of maximum field change areshown for the longitudinal and perpendicular systems in Fig. 9.It can be seen that the switching time for the both perpendic-ular and longitudinal systems are approximately 0.750 to 1 nsec.1378 IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 4, JULY 2001Fig. 6. A FIB image (ABS view) of a FIB trimmed perpendicular head.Fig. 7. A sequence of polar Kerr images taken at increasing times after a fieldpulse on a perpendicular system.Fig. 8. A sequence of polar Kerr images taken at increasing times after a fieldpulse on a longitudinal system.IV. CONCLUSIONS3D Modeling indicated that using a soft underlayer in perpen-dicular recording is not going to deteriorate its high-frequencyperformance if the 0.3 m thick underlayer and head materialsare made of a FeAlN compound with a of 20 kGauss and aFig. 9. Normalized current of the write driver and Kerr angle for theperpendicular and longitudinal systems as a function of time.of 10 Oe. Kerr microscopy was developed to directly studydynamics of the recording process in such a system. The exper-iments with FIB trimmed heads with a relatively small numberof turns were in agreement with the theoretical predictions. Asthe main conclusion of this work, a perpendicular system, ifoptimally designed, should not demonstrate fundamentally de-graded dynamic performance compared to an equivalent longi-tudinal system.ACKNOWLEDGMENTThe authors would like to thank S. Tamaru and the team atSeagate Research for helpful discussions and maintenance ofthe experimental equipment.REFERENCES[1] S. Iwasaki and Y. Nakamura, “An analysis for the magnetization modefor high density magnetic recording,” IEEE Trans. Magn., vol. 13, p.1272, 1977.[2] S. H. Charap, “Thermal stability of recorded information at high densi-ties,” IEEE Trans. Magn., vol. 33, no. 1, p. 978, 1997.[3] D. A. Thompson and J. S. Best, “The extendibility of magnetic recordingfor data storage,” in IBM Executive Briefing, Jan. 1998.[4] H. N. Bertram and M. Williams, “SNR and density limit estimates: Acomparison of longitudinal and perpendicular recording,” IEEE Trans.Magn., vol. 36, no. 1, p. 4, 2000.[5] S. Khizroev, M. H. Kryder, Y. Ikeda, K. Rubin, P. Arnett, M. Best, andD. A. Thompson, “Recording heads with trackwidths suitable for 100Gbit/in density,” presented at Intermag’99, IEEE Trans. Magn., vol.35, no. 5, p. 2544.[6] A. Lyberatos, S. Khizroev, and D. Litvinov, “High speed coherentswitching of longitudinal recording media,” presented at the 8th JointMMM-Intermag’2001.[7] D. Litvinov, G. Chen, R. Chomko, Y. Liu, K. Mountfield, M. Kryder,and S. Khizroev, “Micromagnetics of a soft underlayer,” presented atIntermag’2000.[8] E. Valstyn and L. Shew, “Performance of single-turn film heads,” IEEETrans. Magn., vol. 9, no. 3, p. 317, 1973.[9] S. Khizroev, R. Chomko, G. Chen, Y. Liu, K. Mountfield, M. Kryder,and D. Litvinov, “Perpendicular systems above 100 Gbit/in density,”presented at Intermag’2000.[10] J. Wolfson et al., presented at 8th Joint MMM-Intermag’2001.

Dynamics of perpendicular recording heads

Abstract

Similar works

Full text

Available Versions

Crossref

White Rose Research Online

University of Miami: Scholarship@Miami