Spin tunneling heads above 20 Gb/in/sup 2/ - Magnetics ... - IEEE Xplore

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IEEE TRANSACTIONS ON MAGNETICS, VOL. 38, NO. 1, JANUARY 2002

Spin Tunneling Heads Above 20 Gb/in2 Sining Mao, Janusz Nowak, Dian Song, Paul Kolbo, Lei Wang, Member, IEEE, Eric Linville, Doug Saunders, Ed Murdock, and Pat Ryan, Member, IEEE

Abstract—Spin tunneling recording heads above 20 Gb/in2 have been fabricated using a bottom tunneling junction stack. The spin tunneling stack is made of Ta/PtMn/CoFe/Ru/CoFe/AlO/NiFe/Ta and stabilized by a permanent magnet abutted junction. The effective junction width is about 0.4 m wide and lapped to the junction with an optimum stripe height. The barrier has resistance area m2 , leading to a typical head resistance of product of 15–20 around 50 . Isolated pulses during the spin-stand test shows large signal up to 10 mV. On track error rate floor is better than 10 9 and the head signal-to-noise ratio is also better than that of a conventional spin valve GMR head. The areal density estimated (using BER of 10 5 ) is above 20 Gb/in2 .

Index Terms—Areal density, micromagnetic modeling, spinstand test, spin tunneling heads, TGMR.

I. INTRODUCTION

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N ORDER to meet the challenge in sensitivity and amplitude requirement for recording heads in the magnetic data storage industry, spin tunneling heads (or tunneling valve or tunneling GMR) have been considered as one of the promising candidates after spin valve/GMR heads [1]–[4]. In this work, a tunneling head design is proposed and fabricated. The electrical testing data indicate that the head is capable of areal density of 20 Gb/in . In addition, the performance is compared with standard spin valve heads. On-track error rate floor is better than 10 and the head signal-to-noise ratio (SNR) is also better than that of a conventional spin valve GMR head. The structure of a spin tunneling junction is similar to the spin valve except that the nonmagnetic Cu spacer is replaced by an insulating tunnel barrier (typically Al O ) [4]–[7]. The tunnel barrier has a thickness much less than a typical Cu spacer layer product which is critical for a in order to achieve the low recording head application. Another major difference between these two apparatus is that the current in a tunneling giant magnetoresistance (TGMR) head junction is applied perpendicular to the plane of the film (so called CPP mode—current perpendicular-to-plane). In addition, the CPP head design is simpler than a spin valve because the MR ratio is not directly related to lead resistance, head geometry, and film thickness. The tunneling heads discussed in this paper utilize improved design and testing methods based on our previous 10 G prototypes of TGMR heads. Changes were made to accommodate the top and bottom lead electrodes and eliminate the shield as electrodes. This reduces the magnetic interaction between the magnetic shield and heads. The objective of this work is to demonManuscript received June 25, 2001. The authors are with Seagate Technology, Bloomington, MN 55435 USA (e-mail: [email protected]). Publisher Item Identifier S 0018-9464(02)01283-9.

Fig. 1. Wafer layout of a TGMR head. Not to scale.

strate the technology feasibility of a recording head using a spin tunneling junction, as compared with a standard spin valve head. Fabrication processes have been advanced recently for low rejunctions [8]–[11]. This low sistance-area product junction process enables the TGMR head to be more extendible for ultrahigh areal density head application, but it is not the focus of this paper. While much effort is focused on further lowering product and many additional questions remain on the the shot noise and reliability of thin tunnel barriers, we explore the possibility of implementing a spin tunneling head to the current spin valve head scheme without adding system level complexity. It was found in this work that at a certain areal density the spin tunneling head may be used in place of the spin valve head to give better performance. II. HEAD DESIGN AND FABRICATION The TGMR head construction is based on a Seagate 10 G-Gb/in spin tunneling head design with changes to accommodate the top and bottom lead electrodes and eliminate the shields as electrodes to reduce the magnetic interaction between the magnetic shield and heads. The spin tunneling stack is made of Ta/PtMn/CoFe/Ru/CoFe/AlO/NiFe/Ta and stabilized by a permanent magnet abutted junction. The reader processing begins with ion milling of the TGMR stack trilayer, namely the free layer, the barrier, and the pinned layer to form the bottom electrode. Then a junction is defined by timed ion mill stopping in the pinning layer. Then we use a self-aligning process to lift off an insulating layer to isolate the top electrode from the bottom (Fig. 1). In the end, a top electrode is deposited and connected to the recessed lead (left contact pad in Fig. 1).

0018–9464/02$17.00 © 2002 IEEE

MAO et al.: SPIN TUNNELING HEADS ABOVE 20 Gb/in

Fig. 2. Wafer photographic view of a TMR head.

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Fig. 4. ABS view cross section of the SDT junction head with PM bias. This is the type of head used in this paper. The head bias layer is of CoCrPt 200 A/Cr50 A.

Fig. 5. ABS view TEM cross-sectional image of a spin tunneling junction head with AFM bias. The bottom SDJ stack was stabilized by a second AFM layer on top of free layer.

Fig. 3. Cross section of the SDT junction stack for 10 G demonstration [1]. The top electrode and PM are abutted with the second free layer. Bottom shield serves as electrode as well.

A wafer level photo image is shown in Fig. 2 in which the top electrode needs to be placed. As compared with our 10 Gb/in design, where the bottom shield is used as the electrode (Fig. 3), the current one uses both gap1 and gap2 which is similar to a conventional CIP spin valve head. A TEM image from ABS view is shown in Fig. 4 and the bottom contact layer of Ta/Cu/Ta is clear and PM hard bias layer is separated by an insulation layer of Al O . Different from the previous design [1], the current head uses only a simple free layer and no second free layer is used. Both top and bottom contact leads are separated from the bottom and top shields by gaps. We have designed and fabricated two types of TGMR reader structures. A standard permanent hard bias stabilized head is shown in Fig. 4. The permanent magnet is CoCrPt and has been separated somewhat away from free layer which is similar to NEC’s structure [2]. Another optional design is to use the AFM free layer stabilization. (Fig. 5) An extra AFM layer is used to provide sufficient longitudinal bias to stabilize the free layer [12]–[14] and the results will be published elsewhere.

The structure of the SDT stack is similar to an ex-situ bottom spin valve [15]. A PtMn pinned CoFe film is first deposited and annealed. The SDT structure is then deposited on top. The tunnel barrier is made by natural oxidation of about 7 of sputtered Al. A NiFe free layer was used for better magnetic responses. Fig. 6 is the cross-sectional TEM image of a 7 SDT junction in which well-defined interfaces are evident and the barrier is relatively smooth and flat. The free layer in our current design consists of only one part as used in a standard spin valve head. This design deviated from our 10 G demonstration where two free layers were used and the second free layer serves as a flux guide to bring the flux to the junction and carry the flux deeper into the junction. For design simplicity and process robustness, we did not use a generic flux-guide type head in this work. An improved lapping process enables a successful cut through the junction and places the ABS in the barrier. Shields were made of soft NiFe layers and the total shield to shield (SS) spacing is about 1500 . The free layer was stabilized by a hard bias film which consists of 280 of collimated CoCrPt with 80 of Cr seed layer. III. MICROMAGNETIC MODELING AND VORTEX STATE In the course of TGMR head build and test it has been noticed that the vortex states become more and more important.

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Fig. 6. Cross-sectional TEM view of the SDT junction stack film.


Fig. 8. Similated transfer curves of a unshielded and unbiased test head.

Fig. 9. Magnetization map for different magnetic states on the transfer curves showed in Fig. 8.

Fig. 7. Transfer curves of a unshielded and unbiased test head with various bias current. The stack used here is Ta 50/150PtMn/CF44/9Ru/CF35/ 7Al O /40NiFe/50 Ta.

As shown in a previous paper on the 10 G demonstration (Fig. 4 and Head #2 in [1]), the head transfer curve may sometime show a large hysteresis. The signature is two large loops. From MFM measurements on standalone permalloy dots, we believe the loops indicate vortex motion in an improperly biased free layer [1], [16]. Specifically, the steps correspond to the transition between no vortex, single vortex, and the double-vortex states. We also found there is an influence of bias current on the vortex states [17]. In order to fully understand the magnetic state and its reversal, we have done a complete micromagnetic simulation and compared it to the results from test features. A standard TGMR stack was made and pattered into a square shape (1 m by 1 m). The TMR response was recorded with the applied field parallel to the pinning field direction. The transitions between the fully parallel and antiparallel states takes many steps and strongly modified by the bias current (Fig. 7). Micromagnetic simulation revealed a similar transfer curve (Fig. 8). Even though the agreement between experiment and simulation are qualitative, the physical pictures of the reversal in the free layer is quite clear. Fig. 9 shows the magnetic states for different points on the simulated transfer curve. At the very beginning, the magnetization shows counter-clockwise curling

generated by the tunnel current (point 1) and a three-domain state forms when the free layer is about to switch (point 2). A three-domain state is formed which has two 90 walls and one 180 wall. At point 3, the three-domain state transfers into a vortex state. From point 4 to 6, the transitions repeat like from 1 to 3. The understanding of the free layer magnetization reversal is very important to head design and longitudinal bias optimization to realize the linearity and magnetic stability. IV. TRANSFER CURVES Since the heads are lapped into the junction to form the ABS surface, we have to pay careful attention to the process optimization. Furthermore, the stability of the head must be carefully examined due to the reasons discussed in Section III. Fig. 10 shows a typical slider transfer curve of a good head. It has very smooth transition between 300 Oe and 300 Oe. This corresponds to Oe and an offset field is an effective anisotropy of present. The offset field indicates that this particular head is over biased, i.e., the free layer magnetization is not perfectly orthogonal to that of the reference layer and they form an acute angle in the remnant state. Moreover, the high field scan up to 2000 Oe shows that the pinned layer remain fixed. It should be mentioned that the apparent TMR ratio in the finished head is not as high as the test features (or the intrinsic TMR response). This is primarily caused by the fact that in the current design both the top and bottom electrodes are made of thin metal layers. The use of shields as electrodes will recover the intrinsic TMR and the head performance is expected to be further improved.


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Fig. 12. Typical bias current effect on amplitude on TGMR heads. No saturation observed up to 7 mA.

Fig. 10.

Slider-level transfer curves measured after lapping.

Fig. 13.

Bathtub curves for a TGMR head.

B. Bit Error Rate Test and Areal Density Capability

Fig. 11.

Isolated pulses for a TGMR head used for demonstration.

V. SPIN-STAND TESTS A. Head Performance Using a standard inductive writer, we write approximately 0.9 m wide tracks with different bit density on a 0.36 memu/cm disk with coercivity of 4300 Oe at a radius of about 0.9 in. A spin-stand testing system consists of a Philips 5362 preamplifier and NEC Juno channel with EPRML architecture. Typical head media spacing (HMS) is about 0.5 in (fly height around 0.3 in). The speed is about 4950 rpm (or 466 ips). Isolated pulses for a typical TGMR head are shown in Fig. 11. The head shows large signal of about 4.5 mV peak to peak. There is nearly no base line shifting in the head, indicating a good magnetic stability. Furthermore, the bias current influence on the amplitude at spin-stand was measured up to 7 mA. As shown in Fig. 12, the output voltage increase monotonically with the sense current from 3 to 7 mA. Below 5.5 mA, it is also linear with the bias current and starts to show some deviation away from linear response. This is similar to NEC TGMR head [2].

Bit error rate (BER) testing was also done using the 0.9 m-wide written track at various linear densities. On-track BER was below 9 at 430 kbpi and remained below 6 even at 500 kbpi. Off-track capability (OTC) at 5 BER was measured with background interference present but without adjacent tracks. An example of the partial bathtub produced by the OTC measurement is shown in Fig. 13. In this example, the half-width of the bathtub at 5 BER is 9.2 in and the measured BER at the bottom of the tub is 7.1. From OTC and the known written width, the method of [18] was used to estimate the maximum track density supportable by the reader as a function of linear density. The track density and linear density were then combined to determine the areal density capability of the reader. Fig. 14 shows that, at 430 kbpi, the readers were capable of greater than 20 Gb/in with better than 9 BER on-track. Given that the reader gap is wide around 1500 , the high linear density capability of the spin tunneling head is not fully understood yet though the low fly height (7 nm) contribution may be partly responsible (indicated by narrower PW50). C. Comparison of SV and ST Heads In the last demonstration, we used both voltage sensing and current sensing schemes to handle the heads’ high impedance.

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VI. CONCLUSION

Fig. 14.

Areal density capability from two heads.

TABLE I PERFORMANCE COMPARISON OF SPIN VALVE AND SPIN TUNNELING HEADS.

We have designed and fabricated an improved spin tunneling head based on our early work. The head was tested in a reader-only mode, showing a 20 Gb/in capable areal density. The head uses a bottom tunneling junction with PtMn pinning layer and SAF pinned layer. Natural oxidation was used for a high quality barrier formation. A simple free layer was used and no flux guide involved. Separated top and bottom leads were used to mimic the standard spin valve head structure. Using a conventional spin-stand test system for spin valve heads, we have evaluated the current head performance. Isolated pulse of some heads during the spin-stand test shows large signals up to 10 mV. For some selected parts, on-track error rate floor is better than 10 and the head SNR is also better than that of a conventional spin valve GMR head. It has been clearly demonstrated that the spin tunneling head technology is feasible and practical. We have confirmed the extendibility of TGMR head technology beyond 50 Gb/in . Further development is needed to make the head capable for ultrahigh areal density head applications. ACKNOWLEDGMENT

The values are the average of many heads. Spin valve heads used in this study are of Seed/PtMn/CoFe/Ru/CoFe/Cu/CoFe/NiFe/cap layer. Free layer has NiFe A. The shield-to-shield spacing for spin valve head equivalent thickness of 40 is typically around 1100 A. The GMR ratio for spin valve stack is 14%. Spin tunneling used in this comparison is made with free layer of pure NiFe of 40 A and shield-to-shield spacing of 1500 A. LFA/DC noise is used for a measure of SNR. DC noise (voltage) was obtained with head reading the DC erased disc over 20 MHz bandwidth.

This approach will require more system level modification to adopt the spin tunneling head into a drive. The purpose of this work is to explore the possible path for a drop-in approach of the spin tunneling head as in the case of AMR to GMR head technology transition. We did a statistical summary for many tested heads and found that the spin tunneling head can be as good as spin valve heads and the SNR can be even better (Table I). Regarding the noise in spin tunneling heads, the apparent magnetic noise is similar to that in a spin valve. The Johnson noise and the shot noise add more to the total head noise. Particularly, the shot noise contribution in the spin tunneling head is of great concern [2], [19]. However, the real shot noise measurement from some experiments suggests a significant deviation from theoretical estimates and the theoretical calculation may give just an upper limit [20], [21]. This is mainly due to the nature of a thin barrier, and more systematic work is needed to fully understand the shot noise in the spin tunneling head. Finally, using an optimized configuration and film stack, we have extended our TGMR technology to 50 Gb/in areal density. We have obtained on-track BER of 10 at 578 kbpi and 87 ktpi with a moderate reader resistance at 110 Ohms. Detailed results will be presented in future publication [22].

The authors would like to thank many colleagues at Seagate Recording Head operation for help and encouragement. Specifically, they thank N. Amin, Z. Gao, T. Boonstra, J. Price, J. Wolf, I. Jin, D. Olson, A. Goyal, J. Kirchberg, and D. Markuson for technical support in the head build. They are also grateful for discussions with Drs. D. Dimitrov, A. Mack, C. Hou, S. Xue, H. Wang, J. Chen, O. Heinonen, A. Johnston, W. O’Kane, J. Fernandez-de-Castro, M. Kief, S. Gangopadhyay, M. Covault, K. Ohashi, S. Araki, M. Sahashi, B. Gurney, S. Wang, M. Ho, J. J. Sun, W. Egelholf, J. S. Moodera, J. Doughton, and P. Gorge. REFERENCES [1] D. Song, J. Nowak, R. Larson, P. Kolbo, and R. Chellew, “Demonstrating a tunneling magneto-resistive read head,” IEEE Trans. Magn., vol. 36, pp. 2545–2548, 2000. [2] K. Ohasi, K. Hayashi, K. Nagahara, K. Ishihara, E. Fukami, J. Fujikata, S. Mori, M. Nakada, T. Mitsuzuka, K. Matsuda, H. Mori, A. Kamijo, and H. Tsuge, “Low-resistance tunnel magnetoresistive head,” IEEE Trans. Magn., vol. 36, pp. 2549–2553, 2000. [3] K. Shimazawa, O. Redon, N. Kasahara, J. J. Sun, K. Sato, T. Kagami, S. Saruki, T. Umehara, Y. Fujita, S. Yarimizu, S. Araki, H. Morita, and M. Matsuzaki, “Evaluation of front flux guide-type magnetic tunnel junction heads,” IEEE Trans. Magn., vol. 36, pp. 2542–2544, 2000. [4] J. S. Moodera, L. R. Kinder, T. M. Wong, and R. Meservey, “Large magnetoresistance at room temperature in ferromagnetic thin film tunnel junctions,” Phys. Rev. Lett., vol. 74, pp. 3273–3276, 1995. [5] S. S. P. Parkin, R. E. Fontana, and A. C. Marley, “Low-field magnetoresistance in magnetic tunnel junctions prepared by contact masks and lithography,” J. Appl. Phys., vol. 81, p. 5521, 1997. [6] R. C. Sousa et al., “Large tunneling magnetoresistance enhancement by thermal anneal,” Appl. Phys. Lett., vol. 73, pp. 3288–3290, 1998. [7] H. Tsuge, T. Mitsuzuka, A. Kamijo, and K. Matusda, “Magnetic tunnel junctions with low resistance, high current density and good uniformity,” in MRS Spring Meeting Proc., vol. 87, 1998, p. 517. [8] J. J. Sun, K. Shimazawa, N. Kasahara, K. Sato, T. Kagami, S. Saruki, S. Araki, and M. Mutsuzaki, “Magnetic tunnel junctions on magnetic shield smoothed by gas cluster ion beam,” J. Appl. Phys., vol. 89, p. 6653, 2001. [9] J. Fujikata, T. Ishi, S. Mori, K. Matsuta, K. Mori, H. Yokota, K. Hayashi, M. Nakata, A. Kamijo, and K. Ohashi, “Low resistance magnetic tunnel junctions and their interface structures,” J. Appl. Phys., vol. 89, p. 7558, 2001.


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