IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 10, OCTOBER 2006
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Side Shielded TMR Reader With Track-Width-Reduction Scheme Y. K. Zheng1 , G. C. Han1 , K. B. Li1 , Z. B. Guo1 , J. J. Qiu1 , S. G. Tan1 , Z. Y. Liu1 , B. Liu1 , and Y. H. Wu2 Data Storage Institute, Singapore 117608, Singapore Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576 Singapore A track width reduction and an area resistance (RA) reduction scheme are presented to meet the requirement of the reader for ultrahigh density recording. The side reading effect worsens the performance of the track-width-gradient-reduction design even if a side-shield layer is applied due to the overlap between the backend and the side-shield. However, the side-reading effect can be significantly suppressed in the long-height design after the introduction of the side-shield. Using the long-height sensor design, the RA, the track width and the signal level may meet the requirement of reader for ultra-high density recording. The back flux-guide design can further improve the sensitivity of the tunneling magneto resistance (TMR) sensor. Index Terms—Flux-guide, reader, side-shield, tunneling magneto resistance (TMR), area-resistance.
I. INTRODUCTION
A
S the recording density of the hard disk increases, smaller reader track width is required. In the current-in-plane (CIP) spin-valve (SV) sensor, the output signal is inversely proportional to the sensor track width. However, in the current-perpendicular-to-plane (CPP) sensor, the signal can be maintained as the size of the sensor scales down. The large giant-magneto-resistance (GMR) ratio of the tunneling GMR (TMR) sensor makes it an ideal candidate for application in the hard disk drive [1]. The TMR reader which has been used in the hard disk drive has the potential for 1 Tbits/in storage because of its large MR ratio [2]–[6]. To maintain the SNR and match the impedance of the preamplifier, low RA of less m and MR of 27% are required for the density of than 0.5 m is 1 Tbits/in [7]–[11]. However, low RA of less than 1 difficult to achieve due to both the thickness and barrier height m have limitation. In [6], GMR of 138% and RA of 2.4 been achieved by inserting a 0.4 nm Mg metal layer between the amorphous CoFeB bottom electrode layer and the MgO barrier layer. Further reducing the RA by means of reducing the thickness of the barrier layer is possible. However, the reduction of the thickness of the barrier will not only result in the reduction of MR ratio, but also worsen the uniformity of the senor, thus lowering the production yield. On the other hand, as the track width reduces, the side reading effect will increase the magnetic track width. The side shielding layer [12] can be introduced to suppress the side-reading effect efficiently. II. TRACK WIDTH REDUCTION SCHEME In order to reduce the resistance of a sensor, a track width reduction scheme is shown in Fig. 1 (top). The sensor comprises two parts. The front part TMR1 defines the track width; the backend TMR2 plays the role of reducing the resistance of the sensor. The in-stack hard bias is adopted in this structure. The
Digital Object Identifier 10.1109/TMAG.2006.880463
Fig. 1. (Top) Schematic of the track width reduction and (bottom) equivalent circuit.
side-shield is also adopted to suppress the side-reading effect. If the width of the backend is larger than that of the front part, we call it the track width gradient reduction (TWGR) scheme. If the width of the backend is equal to that of the front part, we call it the long height (LH) scheme. The equivalent circuit is shown in Fig. 1 (bottom). If the backend has no MR contribution to the sensor, it will play the role of back flux guide only. But we found that the small MR of backend can improve the total MR significantly. Fig. 2 shows the total MR dependence on the resistance of the backend with backend’s MR of 10% and 0 respectively. In order m should be about 0.35 times to achieve the RA of 0.5 of if m . The MR of 23% can be achieved if and %. This couldn’t meet the requirement of MR for 1 Tbits/in . However, we found that the small efficiency improvement (MR1 from 0 to 10%) of the backend
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 10, OCTOBER 2006
Fig. 2. Magneto-resistance dependence on the back end resistance with MR2 = 125%, and MR1 = 10% (rectangle) and 0 (circle).
Fig. 3. Microtrack profile of the TMR (a) with side shield (rectangle) and (b) without side shield (circle).
can improve the total signal (increment of 30%). Now the MR of more than 27% can be achieved. The sensitivity can be improved by further introducing the back flux-guide structure. The back flux-guide has no contribution to the resistance of the sensor [13], [14]. III. MICROMAGNETIC MODELING RESULTS As the backend will collect the flux from the neighboring track, the side shield is required to make sure that the flux can only pass through the front part. In order to evaluate the magnetic track width and the sensitivity of the sensor, the microtrack profile is calculated. The detailed reader structure is shown in Fig. 1. In the simulation, the physical track width of the sensor of bias is 40 nm. In-stack bias is used to suppress the noise. of the free layer. The width of the bias layer is 2.5 times of layer is 44 nm, which is slightly larger than the width of the free layer so as to suppress the noise effectively. The gap between the sensor track edge and the side shield is set to be 10 nm. The recording track width of 4 nm and bit length of 10 nm of the perpendicular media is used. As no soft-under layer is applied, the thick media with thickness of 100 nm is used in the simulation for simplification. The energy minimization method is used in the simulation tool [15]. In order to obtain the TAA signal, both the positive signal and the negative signal are calculated. The TAA value is the subtraction of the negative signal from the positive signal. Fig. 3 shows the microtrack profile a) without side shield and b) with side shield. No MR effect of the backend is considered. The sensor dimension is also shown in the inserted picture. From Fig. 3, one can see that the side shield can prevent the signal of the neighboring track effectively. After introducing the side shield, both MT50 and MT10 reduce. However, after considering the backend’s MR, only the MT50 reduces. The effect of the backend on the total sensor is shown in Fig. 4. Here, the side shield also serves as the flux-guider of the backend due to the overlap between the side shield and backend. There is trade-off between the MT10 and the efficiency of signal enhancement after considering the existence of the backend MR. The overlap between the side-shield and the backend results in the side-reading effect in the TWGR structure, so reducing the overlap area could improve the side-reading effect.
Fig. 4. Micro-track profile after considering the backend MR, with (rectangle) and without (circle) side shield.
Fig. 5. Microtrack profile of the LH sensor with (circle) and without (rectangle) side shield.
In the special case, there is no over-lap between the side-shield and the backend if the backend has the same width as the front part of the LH structure. Fig. 5 shows the micro-track profile of this type of sensor with and without side-shield. Now, the side-reading effect has been improved. As the sensor’s aspect ratio (width/height) is less than 1, the larger hard bias field would apply on the LH sensor. Subsequently, the sensitivity
ZHENG et al.: SIDE SHIELDED TMR READER WITH TRACK-WIDTH-REDUCTION SCHEME
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provement can be achieved after the introduction of the flux guide. The back flux guide can use the free layer of the TMR sensor, while the reference layer has been removed. This structure can remove the gap between the flux-guide and the sensor so as to improve the efficiency of the flux-guide [14]. IV. CONCLUSION
Fig. 6. Microtrack profile of the LH sensor (circle), normal-height sensor (rectangle) and normalized signal of the LH sensor (triangle).
An area-resistance (RA) scheme is presented to meet the RA requirement of TMR reader for the high density recording. In the TWGR scheme, reader sensitivity can be improved due to the lower sensor height, but the side-reading effect is difficult to suppress. In the long-height sensor scheme, the side-reading effect can be suppressed by the introduction of the side-shield. The of the original one. Though there is much signal is only signal drop, the structure may meet the final signal requirement of reader for 1 Tbits/in2 recording due to the large GMR ratio of the TMR. We can achieve the effective RA and effective MR m and 28%, respectively. The back flux-guide can of 0.43 further enhance the signal by about 10%. The lapping process is also less critical for the LH sensor design than for the normal sensor design in high density recording. REFERENCES
Fig. 7.
Micro track profile with (circle) and without (rectangle) flux-guide.
would reduce. In addition, less flux can reach the backend, so the total sensitivity would decrease further. Fig. 6 shows the micro-track profile of the LH sensor and the normal height sensor. The sensor’s height of the LH and the normal sensor is 160 nm and 35 nm, respectively. In order to compare the side-reading effect, the normalized signal of the long-height sensor is also shown in Fig. 6. In this case, the total output of the normal one. If signal of the LH sensor is only m , the MR and RA of the normal sensor are 125% and 2 respectively, the effective MR and effective RA of the LH m , respectively. It may meet the sensor are 28% and 0.43 requirement of the reader for 1 Tbits/in density. We also noted that even if the side-shield is applied in the LH sensor, the side-reading effect in the LH sensor is still larger than that of the normal one. Further suppression of the side-reading effect is required for the LH sensor. In order to further increase the reader’s efficiency, the back flux-guider can be introduced. Since the back flux-guider is not a TMR, there is no MR and resistance contribution to the total sensor. But the flux guider can improve the sensitivity [14]. Fig. 7 shows the microtrack profile with back flux-guide (rectangle) and without flux-guide (circle). Sensitivity of 10% im-
[1] S. Mao, Y. Chen, F. Liu, X. Chen, B. Xu, P. Lu, M. Patwari, H. Xi, C. Chang, B. Miller, D. Menard, B. Pant, J. Loven, K. Duxstad, S. Li, Z. Zhang, A. Johnston, R. Lamberton, M. Gubbins, T. McLaughlin, J. Gadbois, J. Ding, B. Cross, S. Xue, and P. Ryan, IEEE Trans. Magn., vol. 42, no. 97, pp. 97–102, 2006. [2] S. S. P. Parkin, C. Kaiser, A. Panchula, P. M. Rice, B. Hughes, M. Samant, and S. Yang, Nature Mater., vol. 3, pp. 862–867, 2004. [3] S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, and K. Ando, Nature Mater., vol. 3, pp. 868–867, 2004. [4] D. D. Djayaprawira, K. Tsunekawa, M. Nagai, H. Maehara, S. Yamagata, N. Watanabe, S. Yuasa, Y. Suzuki, and K. Ando, Appl. Phys. Lett., vol. 86, pp. 092502–1, 2005. [5] K. Tsunekawa, D. D. Djayaprawira, M. Nagai, H. Maehara, S. Yamagata, N. Watanabe, S. Yuasa, Y. Suzuki, and K. Ando, “CoFeB/MgO/CoFeB magnetic tunnel junctions with high TMR ratio and low junction resistance,” presented at the Intermag. Asia 2005. Paper FB-05. [6] K. Tsunekawa, D. D. Djayaprawira, S. Yuasa, M. Nagai, H. Maehara, S. Yamagata, E. Okada, N. Watanabe, Y. Suzuki, and K. Ando, IEEE Trans. Magn., vol. 42, no. 1, pp. 103–107, 2006. [7] R. Wood, IEEE Trans. Magn., vol. 36, pp. 36–42, 2000. [8] R. W. Wood, J. Miles, and T. Olsen, IEEE Trans. Magn., vol. 38, pp. 1711–1718, 2002. [9] M. Mallary, A. Torabi, and M. Benakli, IEEE Trans. Magn., vol. 38, pp. 1719–1724, 2002. [10] J. Zhen, N. Bertram, B. Wilson, and R. Wood, IEEE Trans. Magn., vol. 38, pp. 1429–1435, 2002. [11] J. J. Miles, D. M. A. McKirdy, R. W. Chantrell, and R. Wood, IEEE Trans. Magn., vol. 39, pp. 1876–1890, 2003. [12] C. Haginoya, M. Hatatani, K. Meguro, C. Ishikawa, N. Yoshida, K. Kusukawa, and K. Watanabe, IEEE Trans. Magn., vol. 40, pp. 2221–2223, 2004. [13] L. Abelmann, J. Zhu, J. A. Bain, K. Ramstock, and C. Lodder, J. Appl. Phys., vol. 87, pp. 5538–5540, 2000. [14] Y. K. Zheng, D. You, and Y. H. Wu, IEEE. Trans. Magn., vol. 38, pp. 2268–2270, 2002. [15] Y. Zheng, Y. Wu, and T. Chong, IEEE. Trans. Magn., vol. 36, pp. 3158–3160, 2000.
Manuscript received March 7, 2006; (e-mail:
[email protected]. edu.sg).
