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Optimization of tunneling magnetotransport and thermal properties in magnetic tunnel junctions by rapid thermal anneal
Microelectronic Engineering 69 (2003) 305–308 www.elsevier.com / locate / mee
Optimization of tunneling magnetotransport and thermal properties in magnetic tunnel junctions by rapid thermal anneal a a a b c d, a K.I. Lee , K.H. Chae , J.H. Lee , J.G. Ha , K. Rhie , W.Y. Lee *, K.H. Shin a
Nano Device Research Center, Korea Institute of Science and Technology, P.O. Box 131, Seoul 136 -792, South Korea b Department of Electronic Materials Engineering, Kwangwoon University, Seoul 139 -050, South Korea c Department of Physics, Korea University, Seochang, Chungnam 339 -700, South Korea d Department of Materials Science and Engineering, Yonsei University, 134 Shinchon-Dong, Seoul 120 -749, South Korea
Abstract We report on a systematic investigation of rapid thermal anneal (RTA) effects on the properties of FeMn exchange-biased magnetic tunnel junctions (MTJs). The tunneling magnetoresistance (TMR) in an as-grown MTJ is found to be |27%, whereas the TMR in MTJs annealed by RTA increases with annealing temperature up to 300 8C, reaching |46%. A significant change in the effective barrier thickness (t eff ) and height (Feff ) occurs within 10 s during RTA. Transmission electron microscopy and X-ray reflectivity studies demonstrate that the interface of the alumina tunnel barrier for the MTJ annealed by RTA became sharper and clearer, giving rise to the enhanced TMR. 2003 Elsevier B.V. All rights reserved. Keywords: Magnetic tunnel junctions (MTJs); Magnetic random access memory (MRAM); Rapid thermal anneal (RTA)
1. Introduction In recent years, a new paradigm of electronics, ‘‘spintronics’’ has rapidly emerged, which combines standard microelectronics with the spin degree of freedom of the carrier, and offers novel functionalities to carry signals and process information [1]. Magnetic random access memory (MRAM) based on magnetic tunnel junctions (MTJs) is recognized to be a promising candidate for application of spintronics as the next revolutionary memory technology, potentially replacing DRAM, SRAM and flash memory. The combination of magnetic technology and highdensity semiconductor memory is expected to provide the commercial availability of products embedded with 256 Mb MRAM technology as early as *Corresponding author. E-mail address: [email protected] (W.Y. Lee).
2004. The manufacturing technology, however, remains still lacking. Significant efforts [2–7] have focused on studies of the anneal effect on the magnetic, magnetotransport and thermal properties in MTJs, which continue to be a central issue for MRAM applications. It has been demonstrated that an anneal process is required to improve the properties of MTJs, leading to higher tunneling magnetoresistance (TMR) and exchange bias field (Hex ) [2–7]. In particular, the thermal stability of the MTJs is of great importance for complementary metal-oxidesemiconductor (CMOS) compatibility, since a hightemperature anneal is indispensable in order to eliminate transistor damage due to plasma processing [7]. To date, however, studies have largely centered on MTJs annealed by conventional thermal anneal (CTA), i.e., furnace annealing based on a long-time
0167-9317 / 03 / $ – see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00313-7
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K.I. Lee et al. / Microelectronic Engineering 69 (2003) 305–308
process including slow ramp-up and cool-down. In this paper, we present a systematic investigation of rapid thermal anneal (RTA) effects on the properties of FeMn exchange-biased MTJs, leading to a remarkable enhancement in the tunneling magnetoresistance, exchange bias field, and thermal stability. The detailed mechanisms for the RTA-induced enhancement in the properties of FeMn exchangebiased MTJs are addressed.
2. Experimental The MTJs were deposited on a thermally oxidized Si(100) substrate in a d.c. magnetron sputtering system with a base pressure of 4310 28 Torr (1 Torr5133.322 Pa). The generic structure of the MTJs was Si(100) / Ta(50) / Ni 81 Fe 19 (60) / Fe 50 Mn 50 (80) / Co 84 Fe 16 (40) /Al 2 O 3 / Co 84 Fe 16 (20) / ˚ Ni 81 Fe 19 (100) / Ta(20) (in A). The Al 2 O 3 tunnel barrier was formed by plasma oxidation in an O 2 ˚ atmosphere after growing 16-A-thick Al film. A combination of photolithography, ion milling, and a lift-off process was utilized to fabricate MTJs in the range 50350–10310 mm 2 . Both CTA and RTA were performed under vacuum conditions (,10 26 Torr) at various temperatures with a magnetic field of 1 kOe. A radiating heating furnace (ULVAC, RHL-E) with an infrared lamp was used for RTA. In the present work, it should be noted that RTA (,2 min) is much faster than CTA (.2.5 h).
3. Results and discussion Fig. 1 presents representative TMR curves and M–H loops for the MTJs (a) as-grown (sample A), (b) annealed at 300 8C by RTA (sample B), (c) annealed by CTA (sample C), and (d) by CTA after RTA (sample D), respectively. The TMR and exchange bias field (Hex ) for sample A are found to be |27% and 180 Oe, respectively. The TMR and Hex for sample B are |46% and 230 Oe, whereas those for sample C are |14% and 104 Oe, respectively. The enhanced TMR for sample B is believed to result from the oxygen redistribution and homogenization in the Al oxide barrier during annealing process [4,6]. On the other hand, the drastic reduc-
Fig. 1. Representative TMR curves and M–H loops for the MTJs (a) as-grown (sample A), (b) annealed at 300 8C by RTA (sample B), (c) annealed by CTA (sample C), and (d) by CTA after RTA (sample D), respectively,.
tion of the TMR and Hex for sample C is attributable to either interdiffusion of Mn at the interface of CoFe and FeMn [6] or diffusion of Mn to the oxide barrier [7] during CTA. Interestingly, the TMR and Hex for sample D exhibiting |25% and 180 Oe are similar to those for sample A, and much higher than those for sample C. These results demonstrate that RTA enhances not only TMR and exchange-bias field in the MTJs, but also thermal stability to subsequent thermal treatment. The detailed reason for this will be discussed later. Fig. 2 shows the variation of (a) the effective barrier thickness (t eff ) and (b) height (Feff ) with annealing time for sample B (annealed at 300 8C by RTA), obtained from the I–V curve analysis by fitting to the Simmons’ model. Surprisingly, a significant change in the effective barrier thickness (t eff : ˚ and height (Feff : from 2.35 to from 13.0 to 12.5 A) 2.55 eV) occurs within 10 s during RTA. After 10 s, the barrier parameters vary gradually with annealing time. Our results indicate that the structural transformation in the oxide barrier, i.e., oxygen redistribution and homogenization [3,5], takes place abruptly at the initial step of the thermal anneal. The microstructural properties for (a) sample A and (b) sample B have been investigated using highresolution transmission electron microscopy (HRTEM) in order to clarify the RTA effect on the morphology of the junction, especially, the interfaces
K.I. Lee et al. / Microelectronic Engineering 69 (2003) 305–308
Fig. 2. The variation of (a) the effective barrier thickness (t eff ) and (b) height (Feff ) versus annealing time for sample B (annealed by RTA at 300 8C), obtained from the I–V curve analysis by fitting to the Simmons’ model.
of Al 2 O 3 layer with the two electrodes (see Fig. 3). The pinned layer of the CoFe exhibits stronger lattice fringes due to the k111l texture originating from the Ta layer after RTA as seen in Fig. 3b, giving rise to the enhanced Hex from 180 to 230 Oe. In particular, the alumina tunnel barrier for sample B is found to remarkably differ from that for sample A. The interfaces of the Al oxide layer with the pinned and free layers for sample A look blurred and wavy due to excessive plasma oxidation with high power, resulting in abnormal temperature dependence of TMR [8]. The interface of the alumina barrier for sample B is found to change into much clearer morphology after RTA. The random appearance corresponding to the amorphous structures in the interfaces of the tunnel barrier was transformed into clear lattice fringes due to oxygen redistribution and homogenization in the Al oxide barrier during RTA [4,6]. The oxygen redistribution and homogenization are believed to lead to the enhancement of tunneling magnetoresistance, which is susceptible to spin polarization of 1–2 monolayers in the magnetic layers. Fig. 4 shows the X-ray reflectivity scans for samples A, B, C, and D. The X-ray reflectivity
307
Fig. 3. Cross-sectional TEM micrographs for (a) sample A (as grown) and (b) sample B (annealed by RTA at 300 8C).
measurements are well known to provide information on layer thickness, and electron density as well as interface widths in multiplayer films, having two components, i.e., intermixing and topographical roughness [9]. In the present work, the X-ray reflectivity scans were utilized to probe the MTJ struc-
Fig. 4. The X-ray reflectivity scans for (a) sample A (as grown), (b) sample B (annealed by RTA), (c) sample C (by CTA), and (d) sample D (by CTA after RTA).
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K.I. Lee et al. / Microelectronic Engineering 69 (2003) 305–308
tures, although a simple specular scan is unable to separate the two components and offer detailed information on each layer. In particular, the interfaces of the tunnel barrier are expected to dominate over the MTJ structure with respect to the intermixing and topographical roughness. Below 2u 568, the oscillations of the intensity for all the samples seem almost identical. Above 2u 568, however, the features of the oscillations appear to be various for the samples. The oscillations for Sample A are seen to die out irregularly with increasing angle, as compared to that for sample B. These results support that view that the interfaces of Al oxide layer with the two electrodes become sharper after RTA due to oxygen redistribution and homogenization. The oscillation and periodicity for sample C greatly vary with angle, indicating severe intermixing and topographical roughness resulting from either interdiffusion of Mn at the interface of CoFe and FeMn [6] or diffusion of Mn to the oxide barrier [7]. However, the features of the oscillations for sample D are analogous to those for sample B, illustrating that RTA provides structural robustness for MTJs, giving rise to thermal stability, as seen in Fig. 1. It is believed to result from an additional RTA effect to reduce structural defects in the CoFe pinned layer, preventing the interdiffusion of Mn in the FeMn pinning layer along the grain boundaries of the pinned layer to the oxide barrier.
4. Conclusion The effects of RTA on the TMR, exchange bias field, and thermal stability of FeMn exchange-biased MTJs have been investigated. The TMR and exchange bias field (Hex ) for an as-grown MTJ were found to be 27% and 180 Oe, respectively, whereas those for the MTJ annealed at 300 8C by RTA were found to increase to 46% and 230 Oe, respectively. The variation of barrier parameters due to the structural transformation in the tunnel barrier, i.e., oxygen redistribution and homogenization, were also found to take place abruptly at the initial step of the rapid thermal anneal. The interface of the alumina tunnel barrier was observed to become sharper and clearer after RTA, leading to the enhanced TMR.
More importantly, it was found that RTA prevents the interdiffusion of Mn in the FeMn pinning layer by reducing structural defects in the pinned layer, leading to thermal stability of the MTJs.
Acknowledgements This work was supported by the National Program for Tera-level Nanodevices of the Ministry of Science and Technology. One of the authors (J.G.H.) thanks Kwangwoon University for a research grant in 2001.
References [1] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. von Molnar, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Spintronics: a spin-based electronics vision for the future, Science 294 (2001) 1488–1495. [2] S.S.P. Parkin, K.-S. Moon, K.E. Pettit, D.J. Smith, R.E. Dunin-Borkowski, M.R. McCartney, Magnetic tunnel junctions thermally stable to above 300 8C, Appl. Phys. Lett. 75 (1999) 543–545. [3] R.E. Dunin-Borkowski, M.R. McCartney, D.J. Smith, S. Gider, B.-U. Runge, S.S.P. Parkin, Microstructural and micromagnetic characterization of thin film magnetic tunnel junctions, J. Appl. Phys. 85 (1999) 4815–4817. [4] R.C. Sousa, J.J. Sun, V. Soares, P.P. Freitas, A. Kling, M.F. da Silva, J.C. Soares, Temperature dependence and annealing effects on spin dependent tunnel junctions, J. Appl. Phys. 85 (1999) 5258–5260. ¨ [5] J. Schmalhorst, H. Bruckl, G. Reiss, M. Vieth, G. Gieres, J. Wecker, Temperature stability of Co /Al 2 O 3 / Co junctions, J. Appl. Phys. 87 (2000) 5191–5193. [6] S. Cardoso, P.P. Freitas, C. de Jesus, P. Wei, J.C. Soares, Spin-tunnel-junction thermal stability and interface interdiffusion above 300 8C, Appl. Phys. Lett. 76 (2000) 610– 612. ¨ ¨ [7] M.G. Samant, J. Luning, J. Stohr, S.S.P. Parkin, Thermal stability of IrMn and MnFe exchange-biased magnetic tunnel junctions, Appl. Phys. Lett. 76 (2000) 3097–3099. [8] K.I. Lee, J.H. Lee, W.Y. Lee, K.H. Shin, Y.B. Sung, H.G. Ha, K. Rhie, B.C. Lee, Temperature dependence of magnetoresistance for tunnel junctions with high-power plazmaoxdized barrier: effects of annealing, J. Appl. Phys. 91 (2002) 7959–7961. [9] W.F. Egelhoff Jr., P.J. Chen, R.D. McMichael, C.J. Powell, R.D. Deslattes, F.G. Serpa, R.D. Gomez, Surface oxidation as a diffusion barrier for Al deposited on ferromagnetic metals, J. Appl. Phys. 89 (2001) 5209–5214.
Optimization of tunneling magnetotransport and thermal properties in magnetic tunnel junctions by rapid thermal anneal a a a b c d, a K.I. Lee , K.H. Chae , J.H. Lee , J.G. Ha , K. Rhie , W.Y. Lee *, K.H. Shin a
Nano Device Research Center, Korea Institute of Science and Technology, P.O. Box 131, Seoul 136 -792, South Korea b Department of Electronic Materials Engineering, Kwangwoon University, Seoul 139 -050, South Korea c Department of Physics, Korea University, Seochang, Chungnam 339 -700, South Korea d Department of Materials Science and Engineering, Yonsei University, 134 Shinchon-Dong, Seoul 120 -749, South Korea
Abstract We report on a systematic investigation of rapid thermal anneal (RTA) effects on the properties of FeMn exchange-biased magnetic tunnel junctions (MTJs). The tunneling magnetoresistance (TMR) in an as-grown MTJ is found to be |27%, whereas the TMR in MTJs annealed by RTA increases with annealing temperature up to 300 8C, reaching |46%. A significant change in the effective barrier thickness (t eff ) and height (Feff ) occurs within 10 s during RTA. Transmission electron microscopy and X-ray reflectivity studies demonstrate that the interface of the alumina tunnel barrier for the MTJ annealed by RTA became sharper and clearer, giving rise to the enhanced TMR. 2003 Elsevier B.V. All rights reserved. Keywords: Magnetic tunnel junctions (MTJs); Magnetic random access memory (MRAM); Rapid thermal anneal (RTA)
1. Introduction In recent years, a new paradigm of electronics, ‘‘spintronics’’ has rapidly emerged, which combines standard microelectronics with the spin degree of freedom of the carrier, and offers novel functionalities to carry signals and process information [1]. Magnetic random access memory (MRAM) based on magnetic tunnel junctions (MTJs) is recognized to be a promising candidate for application of spintronics as the next revolutionary memory technology, potentially replacing DRAM, SRAM and flash memory. The combination of magnetic technology and highdensity semiconductor memory is expected to provide the commercial availability of products embedded with 256 Mb MRAM technology as early as *Corresponding author. E-mail address: [email protected] (W.Y. Lee).
2004. The manufacturing technology, however, remains still lacking. Significant efforts [2–7] have focused on studies of the anneal effect on the magnetic, magnetotransport and thermal properties in MTJs, which continue to be a central issue for MRAM applications. It has been demonstrated that an anneal process is required to improve the properties of MTJs, leading to higher tunneling magnetoresistance (TMR) and exchange bias field (Hex ) [2–7]. In particular, the thermal stability of the MTJs is of great importance for complementary metal-oxidesemiconductor (CMOS) compatibility, since a hightemperature anneal is indispensable in order to eliminate transistor damage due to plasma processing [7]. To date, however, studies have largely centered on MTJs annealed by conventional thermal anneal (CTA), i.e., furnace annealing based on a long-time
0167-9317 / 03 / $ – see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00313-7
306
K.I. Lee et al. / Microelectronic Engineering 69 (2003) 305–308
process including slow ramp-up and cool-down. In this paper, we present a systematic investigation of rapid thermal anneal (RTA) effects on the properties of FeMn exchange-biased MTJs, leading to a remarkable enhancement in the tunneling magnetoresistance, exchange bias field, and thermal stability. The detailed mechanisms for the RTA-induced enhancement in the properties of FeMn exchangebiased MTJs are addressed.
2. Experimental The MTJs were deposited on a thermally oxidized Si(100) substrate in a d.c. magnetron sputtering system with a base pressure of 4310 28 Torr (1 Torr5133.322 Pa). The generic structure of the MTJs was Si(100) / Ta(50) / Ni 81 Fe 19 (60) / Fe 50 Mn 50 (80) / Co 84 Fe 16 (40) /Al 2 O 3 / Co 84 Fe 16 (20) / ˚ Ni 81 Fe 19 (100) / Ta(20) (in A). The Al 2 O 3 tunnel barrier was formed by plasma oxidation in an O 2 ˚ atmosphere after growing 16-A-thick Al film. A combination of photolithography, ion milling, and a lift-off process was utilized to fabricate MTJs in the range 50350–10310 mm 2 . Both CTA and RTA were performed under vacuum conditions (,10 26 Torr) at various temperatures with a magnetic field of 1 kOe. A radiating heating furnace (ULVAC, RHL-E) with an infrared lamp was used for RTA. In the present work, it should be noted that RTA (,2 min) is much faster than CTA (.2.5 h).
3. Results and discussion Fig. 1 presents representative TMR curves and M–H loops for the MTJs (a) as-grown (sample A), (b) annealed at 300 8C by RTA (sample B), (c) annealed by CTA (sample C), and (d) by CTA after RTA (sample D), respectively. The TMR and exchange bias field (Hex ) for sample A are found to be |27% and 180 Oe, respectively. The TMR and Hex for sample B are |46% and 230 Oe, whereas those for sample C are |14% and 104 Oe, respectively. The enhanced TMR for sample B is believed to result from the oxygen redistribution and homogenization in the Al oxide barrier during annealing process [4,6]. On the other hand, the drastic reduc-
Fig. 1. Representative TMR curves and M–H loops for the MTJs (a) as-grown (sample A), (b) annealed at 300 8C by RTA (sample B), (c) annealed by CTA (sample C), and (d) by CTA after RTA (sample D), respectively,.
tion of the TMR and Hex for sample C is attributable to either interdiffusion of Mn at the interface of CoFe and FeMn [6] or diffusion of Mn to the oxide barrier [7] during CTA. Interestingly, the TMR and Hex for sample D exhibiting |25% and 180 Oe are similar to those for sample A, and much higher than those for sample C. These results demonstrate that RTA enhances not only TMR and exchange-bias field in the MTJs, but also thermal stability to subsequent thermal treatment. The detailed reason for this will be discussed later. Fig. 2 shows the variation of (a) the effective barrier thickness (t eff ) and (b) height (Feff ) with annealing time for sample B (annealed at 300 8C by RTA), obtained from the I–V curve analysis by fitting to the Simmons’ model. Surprisingly, a significant change in the effective barrier thickness (t eff : ˚ and height (Feff : from 2.35 to from 13.0 to 12.5 A) 2.55 eV) occurs within 10 s during RTA. After 10 s, the barrier parameters vary gradually with annealing time. Our results indicate that the structural transformation in the oxide barrier, i.e., oxygen redistribution and homogenization [3,5], takes place abruptly at the initial step of the thermal anneal. The microstructural properties for (a) sample A and (b) sample B have been investigated using highresolution transmission electron microscopy (HRTEM) in order to clarify the RTA effect on the morphology of the junction, especially, the interfaces
K.I. Lee et al. / Microelectronic Engineering 69 (2003) 305–308
Fig. 2. The variation of (a) the effective barrier thickness (t eff ) and (b) height (Feff ) versus annealing time for sample B (annealed by RTA at 300 8C), obtained from the I–V curve analysis by fitting to the Simmons’ model.
of Al 2 O 3 layer with the two electrodes (see Fig. 3). The pinned layer of the CoFe exhibits stronger lattice fringes due to the k111l texture originating from the Ta layer after RTA as seen in Fig. 3b, giving rise to the enhanced Hex from 180 to 230 Oe. In particular, the alumina tunnel barrier for sample B is found to remarkably differ from that for sample A. The interfaces of the Al oxide layer with the pinned and free layers for sample A look blurred and wavy due to excessive plasma oxidation with high power, resulting in abnormal temperature dependence of TMR [8]. The interface of the alumina barrier for sample B is found to change into much clearer morphology after RTA. The random appearance corresponding to the amorphous structures in the interfaces of the tunnel barrier was transformed into clear lattice fringes due to oxygen redistribution and homogenization in the Al oxide barrier during RTA [4,6]. The oxygen redistribution and homogenization are believed to lead to the enhancement of tunneling magnetoresistance, which is susceptible to spin polarization of 1–2 monolayers in the magnetic layers. Fig. 4 shows the X-ray reflectivity scans for samples A, B, C, and D. The X-ray reflectivity
307
Fig. 3. Cross-sectional TEM micrographs for (a) sample A (as grown) and (b) sample B (annealed by RTA at 300 8C).
measurements are well known to provide information on layer thickness, and electron density as well as interface widths in multiplayer films, having two components, i.e., intermixing and topographical roughness [9]. In the present work, the X-ray reflectivity scans were utilized to probe the MTJ struc-
Fig. 4. The X-ray reflectivity scans for (a) sample A (as grown), (b) sample B (annealed by RTA), (c) sample C (by CTA), and (d) sample D (by CTA after RTA).
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K.I. Lee et al. / Microelectronic Engineering 69 (2003) 305–308
tures, although a simple specular scan is unable to separate the two components and offer detailed information on each layer. In particular, the interfaces of the tunnel barrier are expected to dominate over the MTJ structure with respect to the intermixing and topographical roughness. Below 2u 568, the oscillations of the intensity for all the samples seem almost identical. Above 2u 568, however, the features of the oscillations appear to be various for the samples. The oscillations for Sample A are seen to die out irregularly with increasing angle, as compared to that for sample B. These results support that view that the interfaces of Al oxide layer with the two electrodes become sharper after RTA due to oxygen redistribution and homogenization. The oscillation and periodicity for sample C greatly vary with angle, indicating severe intermixing and topographical roughness resulting from either interdiffusion of Mn at the interface of CoFe and FeMn [6] or diffusion of Mn to the oxide barrier [7]. However, the features of the oscillations for sample D are analogous to those for sample B, illustrating that RTA provides structural robustness for MTJs, giving rise to thermal stability, as seen in Fig. 1. It is believed to result from an additional RTA effect to reduce structural defects in the CoFe pinned layer, preventing the interdiffusion of Mn in the FeMn pinning layer along the grain boundaries of the pinned layer to the oxide barrier.
4. Conclusion The effects of RTA on the TMR, exchange bias field, and thermal stability of FeMn exchange-biased MTJs have been investigated. The TMR and exchange bias field (Hex ) for an as-grown MTJ were found to be 27% and 180 Oe, respectively, whereas those for the MTJ annealed at 300 8C by RTA were found to increase to 46% and 230 Oe, respectively. The variation of barrier parameters due to the structural transformation in the tunnel barrier, i.e., oxygen redistribution and homogenization, were also found to take place abruptly at the initial step of the rapid thermal anneal. The interface of the alumina tunnel barrier was observed to become sharper and clearer after RTA, leading to the enhanced TMR.
More importantly, it was found that RTA prevents the interdiffusion of Mn in the FeMn pinning layer by reducing structural defects in the pinned layer, leading to thermal stability of the MTJs.
Acknowledgements This work was supported by the National Program for Tera-level Nanodevices of the Ministry of Science and Technology. One of the authors (J.G.H.) thanks Kwangwoon University for a research grant in 2001.
References [1] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. von Molnar, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Spintronics: a spin-based electronics vision for the future, Science 294 (2001) 1488–1495. [2] S.S.P. Parkin, K.-S. Moon, K.E. Pettit, D.J. Smith, R.E. Dunin-Borkowski, M.R. McCartney, Magnetic tunnel junctions thermally stable to above 300 8C, Appl. Phys. Lett. 75 (1999) 543–545. [3] R.E. Dunin-Borkowski, M.R. McCartney, D.J. Smith, S. Gider, B.-U. Runge, S.S.P. Parkin, Microstructural and micromagnetic characterization of thin film magnetic tunnel junctions, J. Appl. Phys. 85 (1999) 4815–4817. [4] R.C. Sousa, J.J. Sun, V. Soares, P.P. Freitas, A. Kling, M.F. da Silva, J.C. Soares, Temperature dependence and annealing effects on spin dependent tunnel junctions, J. Appl. Phys. 85 (1999) 5258–5260. ¨ [5] J. Schmalhorst, H. Bruckl, G. Reiss, M. Vieth, G. Gieres, J. Wecker, Temperature stability of Co /Al 2 O 3 / Co junctions, J. Appl. Phys. 87 (2000) 5191–5193. [6] S. Cardoso, P.P. Freitas, C. de Jesus, P. Wei, J.C. Soares, Spin-tunnel-junction thermal stability and interface interdiffusion above 300 8C, Appl. Phys. Lett. 76 (2000) 610– 612. ¨ ¨ [7] M.G. Samant, J. Luning, J. Stohr, S.S.P. Parkin, Thermal stability of IrMn and MnFe exchange-biased magnetic tunnel junctions, Appl. Phys. Lett. 76 (2000) 3097–3099. [8] K.I. Lee, J.H. Lee, W.Y. Lee, K.H. Shin, Y.B. Sung, H.G. Ha, K. Rhie, B.C. Lee, Temperature dependence of magnetoresistance for tunnel junctions with high-power plazmaoxdized barrier: effects of annealing, J. Appl. Phys. 91 (2002) 7959–7961. [9] W.F. Egelhoff Jr., P.J. Chen, R.D. McMichael, C.J. Powell, R.D. Deslattes, F.G. Serpa, R.D. Gomez, Surface oxidation as a diffusion barrier for Al deposited on ferromagnetic metals, J. Appl. Phys. 89 (2001) 5209–5214.