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NiFe trilayers by softening the Co hard layer
Solid State Communications 131 (2004) 459–462 www.elsevier.com/locate/ssc
Enhancement of magnetoresistance sensitivity in Co/Cu/NiFe trilayers by softening the Co hard layer Xiaofang Bi*, Bai Yang, Liqing Gan, Shengkai Gong, Huibin Xu School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Xueyuan Road, Beijing 100083, China Received 17 March 2004; accepted 4 June 2004 by A. Zawadowski Available online 19 June 2004
Abstract Co/Cu/NiFe trilayers were prepared by sputtering without magnetic field applied. We have found that the Co(2 nm)Cu(1 nm)NiFe(2 nm) trilayer using Ta as buffer layer exhibits an enhanced magnetoresistance (MR) sensitivity by a factor of more than 6 and a low saturation field of 9.3 Oe. Experimental results have demonstrated that the low saturation field is attributed to the softening of the Co layer by depositing the Co(2 nm)Cu(1 nm)NiFe(2 nm) sandwich on Ta layer. The decrease of the coercivity of the Co layer also plays an important role in the enhancement of MR sensitivity by reducing the effective coercivity of the NiFe layer, which is discussed in terms of the change in interlayer coupling. q 2004 Elsevier Ltd. All rights reserved. PACS: 75.47.De Keywords: D. Trilayer; D. Magnetoresistance; D. Sensitivity; D. Coercivity; D. Interlayer coupling
1. Introduction Since the discovery of the giant magnetoresistance (GMR) in Fe/Cr multilayers [1], many other systems such as Co/Cu [2] and NiFe/Cu multilayers [3] that exhibit the GMR effect have been found. In those multilayer systems, the GMR effect is attributed to the antiferromagnetic coupling between the adjacent magnetic layers across the nonmagnetic layers. However, it is the antiferromagnetic coupling that results in a high saturation field for the multilayers and hence a low magnetoresistance (MR) sensitivity [4]. This limits their applications for use in MR sensors and high-density read-out heads [5]. On the other hand, a different type of GMR in hard/Cu/soft [6 – 10] multilayers has been of great interest both from scientific and practical points of view, because they exhibit a much lower saturation field, for example, a saturation field of 500 Oe for Cu/Co/Cu/NiFe multilayers, than those in Fe/Cr and Co/Cu multilayers. They are composed of the weakly * Corresponding author. Tel.: þ86-108-231-5999; fax: þ 86-108231-4871. E-mail address: [email protected] (X. Bi). 0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2004.06.005
exchange coupled magnetic layers with different coercive forces and a nonmagnetic spacer. An antiparallel alignment for the magnetization in the two magnetic layers can be realized at a moderate field by the spin rotation of only the soft layers with lower coercivity. Many researches have been done for the hard/nonmagnetic/soft sandwich systems mainly regarding their MR ratios [9,11] and their applications into synthetic AFM layers to replace conventional AFM materials such as NiO and FeMn [12,13]. In addition to this, however, MR sensitivity is of significant importance in terms of the applications in sensors, high-density read-out heads and other magnetic storage technology. Hence, much interest has been focused on the improvement of sensitivity for the multilayers [4,14,15]. Among those researches, it has been reported that the MR sensitivity for the Co/Cu multilayers can be improved by replacing the Co layer with a soft magnetic layer, such as CoZr/Cu [4] and NiFeCo/ Cu multilayers [15], which stimulated us to study the change of the MR sensitivity in hard/Cu/soft trilayers by changing the coercivity of the hard layer. In this paper, we report our recent results on the improvement of MR sensitivity by softening the Co hard layer in the Co/Cu/NiFe trilayers. We have found that for the
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trilayers, the MR sensitivity can be largely enhanced by a factor of more than 6 and saturation field be reduced from 43 Oe to less than 10 Oe when deposited on Ta buffer layer. Experimental results have shown that the reduction of saturation field is attributed to the magnetic softening of the Co hard layer by depositing the Co(2 nm)Cu(1 nm)NiFe(2 nm) trilayer on Ta layer. The decrease of the coercivity of the Co layer also leads to the enhancement of MR sensitivity by reducing the effective coercivity of the NiFe soft layer.
2. Experimental All the films were deposited continuously in a DC magnetron sputtering system without interruption of vacuum. No magnetic field was applied during the deposition process. Other detail deposition conditions for both magnetic layers and nonmagnetic layers have been described elsewhere [16]. Coercivities for the trilayers were obtained from their hysteresis loops measured by LDJ9600 vibrating sample magnetometer (VSM). It was obtained that the magnetic properties of the sandwiches were isotropic in their planes. Magnetoresistance curves were examined automatically using a computer-controlled four-probe method with 1 mA current and ^60 Oe sweeping field applied. Minor MR loops were measured by first applying magnetic field to 2100 Oe, followed by sweeping the field less than ^ 30 Oe, depending on different MR ratios. The interlayer coupling between the two magnetic layers and the effective coercivity were obtained from the minor loops, described in the following context. In this paper, saturation field is defined as a field at which the MR ratio is decreased to 0.1% of the corresponding maximum value. MR sensitivity is designated as maximum MR slope obtained by differentiation with respect to MR curves. All the measurements were made at room temperature.
Fig. 1. Change of coercivity for the Co layers deposited on Ta, Cu and Cr.
thicknesses in the parentheses is nanometer. Fig. 2 shows the MR curve for the trilayer with the structure of (c) Ta(2)Co(2)Cu(1)NiFe(2), along with those of (a) Co(2)Cu(1)NiFe(2) and (b) Co(2)Cu(1)NiFe(2)Ta(1) for comparison. For the trilayer deposited on Ta buffer layer, as can be seen in Fig. 2, in addition to the increase in the MR value, saturation field has been largely reduced from 43 to 9.3 Oe, and the mganetoresistance sensitivity has been increased by a factor of more than 6 and 1.1%/Oe. On the other hand, we obtained that coercive forces for the Ta(2)Co(2)Cu(1)NiFe(2), Co(2)Cu(1)NiFe(2) and Co(2)Cu(1)NiFe(2)Ta(1) sandwiches were 7.8, 30, and 30 Oe, respectively. It can be obtained that the large decrease in coercivity for the Ta(2)Co(2)Cu(1)NiFe(2) sandwich is attributed to the presence of the Ta buffer layer. Since the ratios of the magnetization components between the Co layer and the NiFe layer for each sandwich
3. Results and discussion We have first investigated the change of coercivity for Co layers deposited on Ta, Cu and Cr buffer layers. As shown in Fig. 1, the coercivity was reduced for the Co layers when deposited on Ta and Cu layers. On the contrary, the coercivity was enhanced when deposited on Cr buffer layer. X-ray diffraction analysis demonstrated that the Co layer was grown in a somehow epitaxial way on the Cr buffer layer with k111l texture. It was considered that the Co layers grown on the Cr buffer layer would have large stress, induced by lattice mismatch [17], compared to those on the other buffer layers, such as Ta or Cu. Based on the results shown in Fig. 1, the Co(2)Cu(1)NiFe(2) was deposited on Ta buffer layer in an attempt to decrease the coercivity of the Co hard layer. The unit of the
Fig. 2. Magnetoresistance curves for (a) the Co(2)/Cu(1)/NiFe(2) sandwich, (b) Co(2)/Cu(1)/NiFe/Ta(1) (the sandwich with the Ta cap layer), and (c) Ta(2)/Co(2)/Cu(1)/NiFe (the sandwich deposited on the Ta buffer layer).
X. Bi et al. / Solid State Communications 131 (2004) 459–462
are kept constant, we believe that the change in coercivity for the sandwich is approximately equivalent to the coercivity of the Co layer. Hence, it can be inferred that the reduction of coercivity for Co(2)Cu(1)NiFe(2) deposited on Ta buffer layer arises from the reduction of coercivity for the Co layer. This is consistent with the result, as exhibited in Fig. 1, that the coercivity for Co layers can be reduced using Ta as buffer layer. It is suggested that the reduction of saturation field be associated with the softening of the Co hard layers. The argument is supported by the results as shown in Fig. 3. Fig. 3 is the change of coercivity for Ta(t)Co(2)Cu(1)NiFe(2) sandwiches with different Ta buffer layer thicknesses. With the increase of the Ta thicknesses, the coercivity first decreases to 7.8 Oe at tTa ¼ 2 nm and then increases with further increasing the Ta thicknesses to 25.7 Oe at tTa ¼ 4 nm: We believe that the effect of the Ta thicknesses on the coercivity of the Co layers is related mainly with microstructures such as grain size and surface morphology of the Ta buffer layers. Microstructural observations are needed for further discussion. On the other hand, it can be seen, from the inserted MR curves, that Ta(4)Co(2)Cu(1)NiFe(2) exhibits not only a much smaller MR but also a lower MR sensitivity and a larger saturation field than those of Ta(2)Co(2)Cu(1)NiFe(2). Furthermore, to probe for the reason for the role of the softening of the Co layer in the enhancement of the MR sensitivity, we have also investigated the change of the effective coercivity for the NiFe soft layer and interlayer coupling. Fig. 4 demonstrates the minor loops for the samples. It can be obtained that the effective coercivity of the NiFe soft layer is reduced from 11.4 Oe for the Co(2)Cu(1)NiFe(2) sandwich to 8.2 Oe when using Ta as cap layer and even to 4.7 Oe when using Ta as buffer layer. Obviously the enhancement of the MR sensitivity of the trilayer resulting from the softening of the Co hard layer, is
Fig. 3. Dependence of coercivity on the Ta buffer layer thickness for the Ta(t)/Co(2)/Cu(1)/NiFe(2) sandwiches. MR curves for the sandwiches at Ta thickness 2 and 4 nm are inserted, respectively.
461
Fig. 4. Minor loops for (a) the Co(2)/Cu(1)/NiFe(2) sandwich, (b) Co(2)/Cu(1)/NiFe/Ta(1) (the sandwich with the Ta cap layer), and (c) Ta(2)/Co(2)/Cu(1)/NiFe (the sandwich deposited on the Ta buffer layer), respectively.
associated with the decrease in the effective coercivity of the NiFe layer. The argument is in agreement with what has been previously reported that a decrease in coercivity of the NiFe layer leads to low Barkhausen noise and hence can improve the MR response [18]. On the other hand, the interlayer coupling fields ðHint Þ between the NiFe soft layer and the Co hard layer were obtained from the field that the minor loops are shifted. It has been observed that the interlayer coupling field is decreased slightly for the Co(2)Cu(1)NiFe(2)Ta(1) while largely for the Ta(2)Co(2)Cu(1)NiFe(2), compared to the Co(2)Cu(1)NiFe(2). The Hint is 7.62 Oe for the Co(2)Cu(1)NiFe(2), 6 Oe for the Co(2)Cu(1)NiFe(2)Ta(1), and 2.8 Oe for the Ta(2)Co(2)Cu(1)NiFe(2)Ta(1) trilayers, respectively. From the result that the interlayer coupling was weakened with the decrease of the coercivity for the Co layers, it is suggested that the decrease in the effective coercivity of the NiFe layer due to the softening of the Co layers be associated basically with NiFe layer and interface structures. The related work on the structure characterizations is currently being performed.
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4. Conclusion We have found that the coercivity for the Co layers can be decreased when deposited on Ta and Cu layer, while it increases when deposited on Cr buffer layer. On the basis of the results, we have prepared the Co/Cu/NiFe trilayer and found that its MR sensitivity was enhanced and saturation field was reduced to a large extent when using Ta as the buffer layer. Experimental results has demonstrated that the coercivity of the Co hard layer in the sandwich decreases by using the Ta buffer layer, which finally leads to the decrease in the effective coercivity of the NiFe soft layer. It is believed that the former results in the reduction of the saturation field and the latter in the improvement of the MR sensitivity. The interlayer coupling between the Co and the NiFe layer was also decreased with the softening of the Co layer.
Acknowledgements This research is sponsored by National Natural Science Foundation of China (No. 60371001).
References [1] M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Etienne, G. Creuzet, A. Friederich, J. Chazealas, Phys. Rev. Lett. 66 (1988) 2472.
[2] S.S.P. Parkin, Z.G. Li, D.J. Smith, Appl. Phys. Lett. 58 (1991) 2710. [3] S.S.P. Parkin, Appl. Phys. Lett. 61 (1992) 1358. [4] K. Bouziane, J. Ben Youssef, M. El Harfaoui, O. Koshkina, H. Le Gall, J.M. Desvignes, M. El Yamani, A. Fert, J. Magn. Magn. Mater. 165 (1997) 284. [5] Y. Saito, S. Hashimoto, K. Inomata, Appl. Phys. Lett. 60 (1992) 2436. [6] H. Yamamoto, T. Okuyama, H. Dohnomae, T. Shinjo, J. Magn. Magn. Mater. 99 (1991) 243. [7] T. Lin, D. Mauri, N. Staud, S. Hwang, J.K. Howard, G.L. Gorman, Appl. Phys. Lett. 65 (1994) 1183. [8] D. Miyauchi, S. Araki, Y. Honda, K. Noguchi, T. Chou, O. Shinoura, Y. Narumiya, J. Mag. Soc. Jpn. 18 (1994) 3327. [9] M. Mao, C. Cerjan, B. Law, F. Grabner, L. Miloslavsky, C. Chien, IEEE Trans. Magn. 36 (2000) 2866. [10] F.J. Castano, S. Haratani, Y. Yao, C.A. Ross, H.I. Smith, Appl. Phys. Lett. 81 (2002) 2809. [11] H. Takashima, O. Kitakami, Y. Shimada, J. Mag. Soc. Jpn. 19 (1995) 381. [12] D.T. Margulies, M.E. Schabes, W. McChesney, E.E. Fullerton, Appl. Phys. Lett. 80 (2002) 91. [13] Y.H. Wu, K.B. Li, J.J. Qiu, Z.B. Guo, G.C. Han, Appl. Phys. Lett. 80 (2002) 4413. [14] T.L. Hylton, K.R. Coffey, M.A. Parker, J.K. Howard, Science 261 (1993) 1021. [15] S. Gangopadhyay, S. Hossain, J. Yang, J.A. Barnard, M.T. Kief, H. Fujiwara, M.R. Parker, J. Appl. Phys. 76 (1994) 6522. [16] X.F. Bi, L.Q. Gan, X.Y. Ma, S.K. Gong, H.B. Xu, J. Magn. Magn. Mater. 268 (2004) 321. [17] T. Kobayashi, Bull. Jpn. Inst. Metals 31 (1992) 41. [18] M.A. Akhter, D.J. Mapps, Y.Q. Ma Tan, A.K. Petford-Long, R. Doole, J. Appl. Phys. 81 (1997) 4122.
Enhancement of magnetoresistance sensitivity in Co/Cu/NiFe trilayers by softening the Co hard layer Xiaofang Bi*, Bai Yang, Liqing Gan, Shengkai Gong, Huibin Xu School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Xueyuan Road, Beijing 100083, China Received 17 March 2004; accepted 4 June 2004 by A. Zawadowski Available online 19 June 2004
Abstract Co/Cu/NiFe trilayers were prepared by sputtering without magnetic field applied. We have found that the Co(2 nm)Cu(1 nm)NiFe(2 nm) trilayer using Ta as buffer layer exhibits an enhanced magnetoresistance (MR) sensitivity by a factor of more than 6 and a low saturation field of 9.3 Oe. Experimental results have demonstrated that the low saturation field is attributed to the softening of the Co layer by depositing the Co(2 nm)Cu(1 nm)NiFe(2 nm) sandwich on Ta layer. The decrease of the coercivity of the Co layer also plays an important role in the enhancement of MR sensitivity by reducing the effective coercivity of the NiFe layer, which is discussed in terms of the change in interlayer coupling. q 2004 Elsevier Ltd. All rights reserved. PACS: 75.47.De Keywords: D. Trilayer; D. Magnetoresistance; D. Sensitivity; D. Coercivity; D. Interlayer coupling
1. Introduction Since the discovery of the giant magnetoresistance (GMR) in Fe/Cr multilayers [1], many other systems such as Co/Cu [2] and NiFe/Cu multilayers [3] that exhibit the GMR effect have been found. In those multilayer systems, the GMR effect is attributed to the antiferromagnetic coupling between the adjacent magnetic layers across the nonmagnetic layers. However, it is the antiferromagnetic coupling that results in a high saturation field for the multilayers and hence a low magnetoresistance (MR) sensitivity [4]. This limits their applications for use in MR sensors and high-density read-out heads [5]. On the other hand, a different type of GMR in hard/Cu/soft [6 – 10] multilayers has been of great interest both from scientific and practical points of view, because they exhibit a much lower saturation field, for example, a saturation field of 500 Oe for Cu/Co/Cu/NiFe multilayers, than those in Fe/Cr and Co/Cu multilayers. They are composed of the weakly * Corresponding author. Tel.: þ86-108-231-5999; fax: þ 86-108231-4871. E-mail address: [email protected] (X. Bi). 0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2004.06.005
exchange coupled magnetic layers with different coercive forces and a nonmagnetic spacer. An antiparallel alignment for the magnetization in the two magnetic layers can be realized at a moderate field by the spin rotation of only the soft layers with lower coercivity. Many researches have been done for the hard/nonmagnetic/soft sandwich systems mainly regarding their MR ratios [9,11] and their applications into synthetic AFM layers to replace conventional AFM materials such as NiO and FeMn [12,13]. In addition to this, however, MR sensitivity is of significant importance in terms of the applications in sensors, high-density read-out heads and other magnetic storage technology. Hence, much interest has been focused on the improvement of sensitivity for the multilayers [4,14,15]. Among those researches, it has been reported that the MR sensitivity for the Co/Cu multilayers can be improved by replacing the Co layer with a soft magnetic layer, such as CoZr/Cu [4] and NiFeCo/ Cu multilayers [15], which stimulated us to study the change of the MR sensitivity in hard/Cu/soft trilayers by changing the coercivity of the hard layer. In this paper, we report our recent results on the improvement of MR sensitivity by softening the Co hard layer in the Co/Cu/NiFe trilayers. We have found that for the
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trilayers, the MR sensitivity can be largely enhanced by a factor of more than 6 and saturation field be reduced from 43 Oe to less than 10 Oe when deposited on Ta buffer layer. Experimental results have shown that the reduction of saturation field is attributed to the magnetic softening of the Co hard layer by depositing the Co(2 nm)Cu(1 nm)NiFe(2 nm) trilayer on Ta layer. The decrease of the coercivity of the Co layer also leads to the enhancement of MR sensitivity by reducing the effective coercivity of the NiFe soft layer.
2. Experimental All the films were deposited continuously in a DC magnetron sputtering system without interruption of vacuum. No magnetic field was applied during the deposition process. Other detail deposition conditions for both magnetic layers and nonmagnetic layers have been described elsewhere [16]. Coercivities for the trilayers were obtained from their hysteresis loops measured by LDJ9600 vibrating sample magnetometer (VSM). It was obtained that the magnetic properties of the sandwiches were isotropic in their planes. Magnetoresistance curves were examined automatically using a computer-controlled four-probe method with 1 mA current and ^60 Oe sweeping field applied. Minor MR loops were measured by first applying magnetic field to 2100 Oe, followed by sweeping the field less than ^ 30 Oe, depending on different MR ratios. The interlayer coupling between the two magnetic layers and the effective coercivity were obtained from the minor loops, described in the following context. In this paper, saturation field is defined as a field at which the MR ratio is decreased to 0.1% of the corresponding maximum value. MR sensitivity is designated as maximum MR slope obtained by differentiation with respect to MR curves. All the measurements were made at room temperature.
Fig. 1. Change of coercivity for the Co layers deposited on Ta, Cu and Cr.
thicknesses in the parentheses is nanometer. Fig. 2 shows the MR curve for the trilayer with the structure of (c) Ta(2)Co(2)Cu(1)NiFe(2), along with those of (a) Co(2)Cu(1)NiFe(2) and (b) Co(2)Cu(1)NiFe(2)Ta(1) for comparison. For the trilayer deposited on Ta buffer layer, as can be seen in Fig. 2, in addition to the increase in the MR value, saturation field has been largely reduced from 43 to 9.3 Oe, and the mganetoresistance sensitivity has been increased by a factor of more than 6 and 1.1%/Oe. On the other hand, we obtained that coercive forces for the Ta(2)Co(2)Cu(1)NiFe(2), Co(2)Cu(1)NiFe(2) and Co(2)Cu(1)NiFe(2)Ta(1) sandwiches were 7.8, 30, and 30 Oe, respectively. It can be obtained that the large decrease in coercivity for the Ta(2)Co(2)Cu(1)NiFe(2) sandwich is attributed to the presence of the Ta buffer layer. Since the ratios of the magnetization components between the Co layer and the NiFe layer for each sandwich
3. Results and discussion We have first investigated the change of coercivity for Co layers deposited on Ta, Cu and Cr buffer layers. As shown in Fig. 1, the coercivity was reduced for the Co layers when deposited on Ta and Cu layers. On the contrary, the coercivity was enhanced when deposited on Cr buffer layer. X-ray diffraction analysis demonstrated that the Co layer was grown in a somehow epitaxial way on the Cr buffer layer with k111l texture. It was considered that the Co layers grown on the Cr buffer layer would have large stress, induced by lattice mismatch [17], compared to those on the other buffer layers, such as Ta or Cu. Based on the results shown in Fig. 1, the Co(2)Cu(1)NiFe(2) was deposited on Ta buffer layer in an attempt to decrease the coercivity of the Co hard layer. The unit of the
Fig. 2. Magnetoresistance curves for (a) the Co(2)/Cu(1)/NiFe(2) sandwich, (b) Co(2)/Cu(1)/NiFe/Ta(1) (the sandwich with the Ta cap layer), and (c) Ta(2)/Co(2)/Cu(1)/NiFe (the sandwich deposited on the Ta buffer layer).
X. Bi et al. / Solid State Communications 131 (2004) 459–462
are kept constant, we believe that the change in coercivity for the sandwich is approximately equivalent to the coercivity of the Co layer. Hence, it can be inferred that the reduction of coercivity for Co(2)Cu(1)NiFe(2) deposited on Ta buffer layer arises from the reduction of coercivity for the Co layer. This is consistent with the result, as exhibited in Fig. 1, that the coercivity for Co layers can be reduced using Ta as buffer layer. It is suggested that the reduction of saturation field be associated with the softening of the Co hard layers. The argument is supported by the results as shown in Fig. 3. Fig. 3 is the change of coercivity for Ta(t)Co(2)Cu(1)NiFe(2) sandwiches with different Ta buffer layer thicknesses. With the increase of the Ta thicknesses, the coercivity first decreases to 7.8 Oe at tTa ¼ 2 nm and then increases with further increasing the Ta thicknesses to 25.7 Oe at tTa ¼ 4 nm: We believe that the effect of the Ta thicknesses on the coercivity of the Co layers is related mainly with microstructures such as grain size and surface morphology of the Ta buffer layers. Microstructural observations are needed for further discussion. On the other hand, it can be seen, from the inserted MR curves, that Ta(4)Co(2)Cu(1)NiFe(2) exhibits not only a much smaller MR but also a lower MR sensitivity and a larger saturation field than those of Ta(2)Co(2)Cu(1)NiFe(2). Furthermore, to probe for the reason for the role of the softening of the Co layer in the enhancement of the MR sensitivity, we have also investigated the change of the effective coercivity for the NiFe soft layer and interlayer coupling. Fig. 4 demonstrates the minor loops for the samples. It can be obtained that the effective coercivity of the NiFe soft layer is reduced from 11.4 Oe for the Co(2)Cu(1)NiFe(2) sandwich to 8.2 Oe when using Ta as cap layer and even to 4.7 Oe when using Ta as buffer layer. Obviously the enhancement of the MR sensitivity of the trilayer resulting from the softening of the Co hard layer, is
Fig. 3. Dependence of coercivity on the Ta buffer layer thickness for the Ta(t)/Co(2)/Cu(1)/NiFe(2) sandwiches. MR curves for the sandwiches at Ta thickness 2 and 4 nm are inserted, respectively.
461
Fig. 4. Minor loops for (a) the Co(2)/Cu(1)/NiFe(2) sandwich, (b) Co(2)/Cu(1)/NiFe/Ta(1) (the sandwich with the Ta cap layer), and (c) Ta(2)/Co(2)/Cu(1)/NiFe (the sandwich deposited on the Ta buffer layer), respectively.
associated with the decrease in the effective coercivity of the NiFe layer. The argument is in agreement with what has been previously reported that a decrease in coercivity of the NiFe layer leads to low Barkhausen noise and hence can improve the MR response [18]. On the other hand, the interlayer coupling fields ðHint Þ between the NiFe soft layer and the Co hard layer were obtained from the field that the minor loops are shifted. It has been observed that the interlayer coupling field is decreased slightly for the Co(2)Cu(1)NiFe(2)Ta(1) while largely for the Ta(2)Co(2)Cu(1)NiFe(2), compared to the Co(2)Cu(1)NiFe(2). The Hint is 7.62 Oe for the Co(2)Cu(1)NiFe(2), 6 Oe for the Co(2)Cu(1)NiFe(2)Ta(1), and 2.8 Oe for the Ta(2)Co(2)Cu(1)NiFe(2)Ta(1) trilayers, respectively. From the result that the interlayer coupling was weakened with the decrease of the coercivity for the Co layers, it is suggested that the decrease in the effective coercivity of the NiFe layer due to the softening of the Co layers be associated basically with NiFe layer and interface structures. The related work on the structure characterizations is currently being performed.
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4. Conclusion We have found that the coercivity for the Co layers can be decreased when deposited on Ta and Cu layer, while it increases when deposited on Cr buffer layer. On the basis of the results, we have prepared the Co/Cu/NiFe trilayer and found that its MR sensitivity was enhanced and saturation field was reduced to a large extent when using Ta as the buffer layer. Experimental results has demonstrated that the coercivity of the Co hard layer in the sandwich decreases by using the Ta buffer layer, which finally leads to the decrease in the effective coercivity of the NiFe soft layer. It is believed that the former results in the reduction of the saturation field and the latter in the improvement of the MR sensitivity. The interlayer coupling between the Co and the NiFe layer was also decreased with the softening of the Co layer.
Acknowledgements This research is sponsored by National Natural Science Foundation of China (No. 60371001).
References [1] M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Etienne, G. Creuzet, A. Friederich, J. Chazealas, Phys. Rev. Lett. 66 (1988) 2472.
[2] S.S.P. Parkin, Z.G. Li, D.J. Smith, Appl. Phys. Lett. 58 (1991) 2710. [3] S.S.P. Parkin, Appl. Phys. Lett. 61 (1992) 1358. [4] K. Bouziane, J. Ben Youssef, M. El Harfaoui, O. Koshkina, H. Le Gall, J.M. Desvignes, M. El Yamani, A. Fert, J. Magn. Magn. Mater. 165 (1997) 284. [5] Y. Saito, S. Hashimoto, K. Inomata, Appl. Phys. Lett. 60 (1992) 2436. [6] H. Yamamoto, T. Okuyama, H. Dohnomae, T. Shinjo, J. Magn. Magn. Mater. 99 (1991) 243. [7] T. Lin, D. Mauri, N. Staud, S. Hwang, J.K. Howard, G.L. Gorman, Appl. Phys. Lett. 65 (1994) 1183. [8] D. Miyauchi, S. Araki, Y. Honda, K. Noguchi, T. Chou, O. Shinoura, Y. Narumiya, J. Mag. Soc. Jpn. 18 (1994) 3327. [9] M. Mao, C. Cerjan, B. Law, F. Grabner, L. Miloslavsky, C. Chien, IEEE Trans. Magn. 36 (2000) 2866. [10] F.J. Castano, S. Haratani, Y. Yao, C.A. Ross, H.I. Smith, Appl. Phys. Lett. 81 (2002) 2809. [11] H. Takashima, O. Kitakami, Y. Shimada, J. Mag. Soc. Jpn. 19 (1995) 381. [12] D.T. Margulies, M.E. Schabes, W. McChesney, E.E. Fullerton, Appl. Phys. Lett. 80 (2002) 91. [13] Y.H. Wu, K.B. Li, J.J. Qiu, Z.B. Guo, G.C. Han, Appl. Phys. Lett. 80 (2002) 4413. [14] T.L. Hylton, K.R. Coffey, M.A. Parker, J.K. Howard, Science 261 (1993) 1021. [15] S. Gangopadhyay, S. Hossain, J. Yang, J.A. Barnard, M.T. Kief, H. Fujiwara, M.R. Parker, J. Appl. Phys. 76 (1994) 6522. [16] X.F. Bi, L.Q. Gan, X.Y. Ma, S.K. Gong, H.B. Xu, J. Magn. Magn. Mater. 268 (2004) 321. [17] T. Kobayashi, Bull. Jpn. Inst. Metals 31 (1992) 41. [18] M.A. Akhter, D.J. Mapps, Y.Q. Ma Tan, A.K. Petford-Long, R. Doole, J. Appl. Phys. 81 (1997) 4122.