Performance of dual-stripe giant magnetoresistive heads on tape ...

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Jan 12, 1999 - recorded on tape media, using a current bias of J = 2.8 x 10'. Nema on each GMR. The DSGMR responds to the dibit with a bipolar pulse ...
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IEEE TRANSACTTONS ON MAGNETICS, VOL. 35. NO. 5, SEPIBMBER 1999

Performance of Dual-Stripe Giant Magnetoresistive Heads on Tape Susana Cardoso and Paul0 P. Freitas

Abstract-Dual-stripe giant magnetoresistive (DSGMR) devices with an insulating spacer were fabricated and tested on tape media. The head design consists of two mutually biased, unshielded GMR elements based on [Cn 19 h i F e 13 kCoFe 4 A],Io antiferromagnetically coupled multilayers, separated by a 600-Athick Si02 spacer. The sensor is patterned into a U shape, with leads connected away Prom the active region to avoid topography. Simultaneous etching of hotb GMR elements avoids misalignment problems along or across the track width. A DSGMR tape head operating in differential mode (track width of 2 pm and height of 0.8 pm) was tested versus high density dibits (0.05 < hit < 4 pm) recorded on tape media, using a current bias of J = 2.8 x 10‘ Nema on each GMR. The DSGMR responds to the dibit with a bipolar pulse independently of bit separation even when the individual GMR elements are partially saturated. Peak-to-peak output is 1.7 mVIpm, for bit sizes 2 0.4 pm. Experimental pulse amplitude and shape are in agreement with a two-dimensional micromagnetic simulation. The head response to a dibit signal loses half its maximum amplitude for a recorded hit size of 0.2 pm. Pulse broadening (read hit cell is 0.8 pm larger than recorded bit, for bit sizes < 0.4 pm) is correlated with sensor geometry.

In a conventional dual stripe design [4], topography occurs in the active region coming from superposition of the top GMR element over the bottom GMR element and contact leads [see Fig. l(a)]. Furthermore, this geometry can lead to misalignment between the two elements along or across the track width direction. A different architecture was developed allowing the electrical connections to he made away from the active region, avoiding topography. Both GMR elements are patterned in the same step, therefore eliminating all misalignment problems. The new sensor layout is shown in Fig. l(h). Fig. l(c) shows a pictorial representation of the transverse (Mu) and longitudinal ( M z )components of the magnetization in the quiescent state

U].

The individual response of each GMR sensor under static fields is analyzed (transfer curves), at current densities up to 5.7 x lo7 Ncm2. In these samples no transfer curve distortion at high current densities is observed. The head response to dibit signals recorded on tape is analyzed, with recorded hit size down to 0.05 pm. The dibit Index Terms-Dual stripe giant magnetoresistive (GMR) heads, GMR multilayers, magnetic recording heads, mutual current wave form with small bit size was chosen in order to minimize biasing, pulse shape analysis. the flux coming from the tape media, and therefore test the head in the linear regime. The signal recorded on tape is first analyzed with a shielded spin-valve head and the transition I. INTRODUCTION width parameter “a” is obtained. The DSGMR pulse signal NSHIELDED single-element GMR heads, based on self- is compared with a two-dimensional (2-D) micromagnetic biased antiferromagnetically coupled NiFe/Cu multilay- simulation [3]. Both the individual GMR element signal and ers have been proposed as possible candidates for high density the differential output were monitored. Pulse shape is correrigid media recording (410 Ghit/in2) [l], [Z].The dual stripe lated with the mutual bias field and magnetization distribution GMR [3] consists of a three-terminal, dual-stripe configura- both at the quiescent state and under the transition. The head tion, where the two mutually biased GMR elements operate in response to dihit pulses is discussed, as well as read bit cell ,differential mode. Higher output is expected, together with size versus recorded bit cell. Tape media fields are calculated cancellation of common thermal noise and common even- for longitudinal magnetization (no vertical magnetization comorder distortion in the pulse shape. A PWm of 0.16 pm was ponent), with both thin tape and thick tape approximations [51. predicted for a head height of 0.5 pm, tested on hard disk Pulse broadening is discussed in terms of sensor geometry. media with Mr.t = 0.5 memu/cm2 and a transition width a = 0.015 Fm. 11. DUAL,STRIPE GMR SENSOR FABRICATION This paper reports on the experimental demonstration on tape of the operation mode of a dual stripe GMR (DSGMR) A. Materials head with a 600 8, Si02 gap. Head design follows a previous The dual stripe GMR structure consists of two GMR elemicromagnetic simulation done for a 10 Ghit/in2 demonstraments separated by a 600 8, spacer of sputtered SiOz. Each tion [3]. GMR element is an antiferromagnetically coupled NiFe 80 Manuscript received January 12, 1999: revised June 1, 1999. This work A/[Cu 19 h i F e 13 &CoFe 4 A],,o multilayer deposited was supported in part by the PRAXIS/313.1/MMA/1751195 project and by by high-rate magnetron sputtering from 3” diameter Cu, the PRAXIS XXIIBDII 1533197 grant. The authors are with the lnstituto de Engenharia de Sistemas e NisoFezo (NiFe), and CogoFelo (CoFe) targets onto a suhCornputadores (INESC), 1000 Lisbon, Portugal and the lnstituto Superior strate table rotating at 5 rpm [6]. This method deposits one Tknico (IST), 1000 Lisbon, Portugal (e-mail: susana~eniac.inerc.pt; [Cu/(NiFe/CoFe)] bilayer per rotation. The magnetrons were [email protected]). rnn at 3.0 W/cm2 (dc), 2.8 W/cm2 (dc), and 1.3 W/cm2 (RF), Publisher Item Identifier S 0018-9464(99)07351-3.

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IEEE TRANSACTIONS ON MAGNETICS. VOL. 35, NO. 5 , SEWEMBER 1999

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(1 1 pm), saturation effects occur in the GMR elements leading to pulse distortion and consequently, large bit shift. This large bit shift is already observed in the individual GMR response to each dibit as can be seen in the inset.

V. DISCUSSION A. Mutual Bias Field Effects on Individual GMR Elements Response

In order to understand the differential output bipolar pulse presented in Fig. 13, Fig. 17 displays the calculated response

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recorded bit [pm] Fig 16. Read bit cell size dependence on the recorded bit size (clock). Experimental DSGMR data is shown and compared with a shielded spin-valve head and the micromagnetics model. Inset shows a detail of individual GMR output near the transitions, where a large read bit cell in GMR I output can be seen.

for each GMR element. Due to opposite mutual bias fields, the calculated response of both GMR elements is similar hut reversed in time with respect to each other. When the transitions are far apart ( B 4 pm), each GMR element responds with an unipolar pulse with same polarity for isolated opposite transitions. This is related to strong head saturation (large media flux) as can he seen from the calculated ( m l )distributions (see Fig. 18), where (m,) = +l. Here, (my)is the average in each layer of the magnetization component along the direction of the bias fields [3]. For small hit sizes ( B 5 0.4 pm), media flux decreases due to the overlap of media fields, and each GMR responds to the dibit with a single bipolar pulse centered in the middle of both transitions. This bipolar response is caused by the mutual current bias that splits the (my) distributions of each GMR element with respect to the self-hias-only distribution (antisymmetric). In this low flux regime, the magnetization distribution is only slightly changed with respect to the quiescent state. This response simulates what we would expect in low Mr.t rigid media regime. Independently of the individual GMR response (bipolar, unipolar, or intermediate), the differential head response to the &bit is always bipolar.

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B. Pulse Broadening and the Thick Tape Approximation

The micromagnetic simulation that has been used to describe previous section results used a thin media approximation where the transition width n = 0.17 pm is assumed constant through recorded thickness 6. In this paragraph, a more reasonable approach is used, where the transition width increases with depth into the media “a(y)” and the transition center “zo” shifts with depth “zo(y),” with y measured from the head tape bearing surface. The tape media is discretized into n horizontal laminae, each with constant transition width a(y) and thickness 6/n. The field coming from each laminae is calculated in the arc-tangent approximation, and the superposition principle is used to compute the total field at the head. Fig. 19 compares the calculated dihit response ( B = 0.4 pm) using the thick tape approximation with a thin tape model. In a first approximation (dotted line), it is assumed that the transition center does not vary with depth into the media [zo(y) = zol, and that the transition width variation with depth can be written as [5] a(y)

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In this approximation, the transition width varies from 0.13 pm at the tape surface to 0.34 pm in the deeper laminae. The output pulse has the same bipolar shape as for thin tape media, but with smaller amplitude. In a further approximation the transition center also varies with tape depth 10 = zo(y) (dashed line). This dependence

CARDOSO AND PREITAS: DUAL-STRIPE OMR TIEADS ON TAPE

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leads to an increase of the individual pulse pwSa,while decreasing the read bit cell (the two transitions of the are closer [5]). None of these thick media approximations explain the larre

C. Sensor Gearnet*, Effect,y

In this section, the contribution of each part of the sensor (lateral parts and central region) to the DSGMR head output pulse shape is discussed. The finished (lapped) head is approximated by three rectangular regions (where the 2-D micromagnetic model is used), with different height, current densities, and distance to tape. Part A (Ir = 0.8 pm, d = 0.18 /hm, J = 2.8 x 10' A/cm2) corresponds to the central area of the sensor which is close to the tape surface and Part B ( h = 1.8 pm, d = 0.25 pm), corresponds to the lateral parts of the sensor. Fig. 20 shows the simulation of the dihit response of both parts of the head for B = 0.4 pm. In both cases, the read bit cell decreases with increasing current density. Optimum current density can be defined as that leading to maximum output and minimum read bit cell. Parts B of the sensor, with higher head-tape separation and larger height, show a very significant pulse broadening (larger peak to peak

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VI. CONCLUSIONS A DSGMR tape head with 600 b, Si02 spacer was fahricated, with an innovative design avoiding misalignment and topography effects between the GMR elements, The DSGMR head was tested with high density dibits recorded on tape. Head response to the dihit is a bipolar pulse for all bit sizes studied, in agreement with the 2-D micromagnetic model, even when the individual GMR elements show saturation effects. For D > 0.4 pm head output is 1.7 mV per micron of track width. The DSGMR head output decreases sharply for B < 0.4 bm, due to individual pulse interference (the transition width of 0.17 pm was independently characterized with a spin-valve head). The read bit cell for B < 0.4 pm is always larger than B (by 0.8 pm). This is correlated with the U-shape of the sensor. The lateral parts, being farther away from the tape surface, and carrying a lower current, respond with larger pulses (both larger read hit cell, and pulse tails away from transitions).

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For future applications, the DSGMR will he studied in shielded format, in order to seduce the contribution of the lateral parts of the sensor, leading to sharper pulses, and improving head resolution (read bit cell closer to recorded bit). performance of a DSGMR on disk media will he reported as a continuation of this work.

ACKNOWLEDGMENT The authors would like to acknowledge K. McKinstry, J. Torliue, and R. Dee, at Storage Technology Corporation, for their assistance in the mechanical processing and tape deck testing of the heads. REFERENCES

[I] N.Smith, A. Zeltser, and M. Parker, "GMR multilayers and head design for ultrahigh density magnetic recordinp," IEEE Trans. Maan., vol. 33, pp. 135-1741, la". i996: [2] N.Smith, "Micromagnetics of GMR multilayer sensors at high current densities," IEEE Trans. Magn., vol. 30, pp. 3822-3824, Nov. 1994.

131 P. P.Freitas, S. Cardoso, and N. Oliveira, "Micramqnetic analysis and current biasing of dual-stripe GMR and dual-GMR sensors for high density recording:' IEEE Trans. Magn., vol. 34, pp. 1510-1512, July 1998. [41 T. C. Anthony, S . L. Naberhuis, J. A. Brug, M. K. Bhattacharyya, L. T. Tran, V. W. Resterman, and G. G. Lopatin. "Dual stripe magnetoresistive heads for high density recording," IEEE 7hi.anr.Magn., YOI, 30, pp. 303-308, Mar. 1998. 151 D. Wei, H. N. Bertram, and F. Jeffers, "A simplified model of high density tape recording," IEEE Trans. Magn.. vol. 30, pp. 2739-2749, ' %nt 1996 I"r.. ._, .. P. P. Preitas, M. C. Caldeira, M. Reissner, B. G. Almeida, I. B. Sousa, and H. Kung, "Design, fabrication, and wafer level testing of

(NiFe/Cu)xn dual stripe GMR sensors," IEEE Trans. Magn., vol. 33, pp, 2905-2907, Sept. 1997. FastHenry: Lirer's Guide, M.Kamon, L. M.Silveira, E. Smithhisler, and

I. White, Massnchnsetts Inst. Tech"., Cambridge. MA, 1996 [Onlinel. Available: fasthenry~rle-vlsi.mit.edu. H. N. Bertram, Theory of Magneric Recording. New York Cambridge Univ. Press, 1994, ch. 617. N. J. Oliveira, P. P. Freitas, S . Li, and V. Gehanno, "Spin valve read heads for tape applications," IEEE Trans. Magn., vol. 35, pp, 734-739, Mar. 1999. H. N. Bertram, "Linear signal analysis of shielded AMR and spin valve heads:' IEEE Trans. Magn., vol. 31, pp, 2573-2578, Nov. 1995.