Development of the magnetic tunnel junction MRAM at IBM: From first junctions to a 16-Mb MRAM demonstrator chip

IBM Journal of Research and Development

Volumn 50, Issue 1, 2006, Pages 5-23

  Gallagher, William J ^a   Parkin, Stuart S P ^b  

^a IBM T J WATSON RESEARCH CENTER   (United States)

^b IBM ALMADEN RESEARCH CENTER   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

DEMONSTRATIONS; MAGNETIC STORAGE; NONVOLATILE STORAGE; PRODUCT DESIGN; TUNNEL JUNCTIONS;

DEMONSTRATOR CHIP; MAGNETIC RANDOM ACCESS MEMORY (MRAM); MAGNETIC TUNNEL JUNCTION; STORAGE DEVICE; STORAGE SENSING DEVICE;

RANDOM ACCESS STORAGE;

EID: 32944481005     PISSN: 00188646     EISSN: 00188646     Source Type: Journal    
DOI: 10.1147/rd.501.0005     Document Type: Review

References (87)

1. E. W. Pugh, R. A. Henle, D. L. Critchlow, and L. A. Russell, "Solid-State Memory Development in IBM," IBM J. Res.&Dev. 25, No. 5, 585-602 (1981).
2. T. S. Crutcher and J. A. Sartell, "Plated Wire Memory: Its Evolution for Aerospace Utilization," Honeywell Computer J. 6, 15-22 (1972).
3. J. M. Daughton, "Magnetoresistive Memory Technology," Thin Solid Films 216, No. 1, 162-168 (1992).
4. B. Dieny, V. S. Speriosu, S. Metin, S. S. P. Parkin, B. A. Gurney, P. Baumgart, and D. R. Wilhoit, "Magnetotransport Properties of Magnetically Soft Spin-Valve Structures," J. Appl Phys. 69, No. 8, 4774-4779 (1991);
5. S. S. P. Parkin, N. More, and K. P. Roche, "Oscillations in Exchange Coupling and Magnetoresistance in Metallic Superlattice Structures: Co/ Ru, Co/Cr, and Fe/Cr," Phys. Rev. Lett. 64, 2304-2307 (1990);
6. S. S. P. Parkin, Z. G. Li, and D. J. Smith, "Giant Magnetoresistance in Antiferromagnetic Co/Cu Multilayers," Appl. Phys. Lett. 58, 2710 (1991);
7. S. S. P. Parkin, R. Bhadra, and K. P. Roche, "Oscillatory Magnetic Exchange Coupling Through Thin Copper Layers," Phys. Rev. Lett. 66, 2152 (1991).
8. For reviews of the development of spin-valve devices, see S. S. P. Parkin, "Giant Magnetoresistance in Magnetic Nanostructures," Annual Review of Materials Science, Vol. 25, B. W. Wessels, Ed., Annual Reviews, Inc., Palo Alto, CA, 1995, pp. 357-388;
9. and S. S. P. Parkin, X. Jiang, C. Kaiser, A. Panchula, K. Roche, and M. Samant, "Magnetically Engineered Spintronic Sensors and Memory," Proc. IEEE 91, 661-680 (2003).
10. D. D. Tang, P. K. Wang, V. S. Speriosu, S. Le, and K. K. Kung, "Spin-Valve RAM Cell," IEEE Trans. Magn. 31, 3206-3208 (1995).
11. R. R. Katti, "Giant Magnetoresistive Random-Access Memories Based on Current-in-Plane Devices," Proc. IEEE 91, No. 5, 687-702 (2003).
12. I. Giaever, "Energy Gap in Superconductors Measured by Electron Tunneling," Phys. Rev. Lett. 5, 147 (1960).
13. P. M. Tedrow and R. Meservey, "Spin-Dependent Tunneling into Ferromagnetic Nickel," Phys. Rev. Lett. 26, No. 4, 192-195 (1971).
14. R. Meservey and P. M. Tedrow, "Spin-Polarized Electron Tunneling," Phys. Rep. 238, No. 4, 173-243 (1994).
15. M. Julliere, "Tunneling Between Ferromagnetic Films," Phys. Lett. A 54, No. 3, 225-226 (1975).
16. J. C. Slonczewski, "Magnetic Bubble Tunnel Detector," IBM Tech. Disclosure Bull. 19, No. 6, 2328-2330 (1976).
17. J. C. Slonczewski, "Magnetic-Field Tunnel-Sensor," IBM Tech. Disclosure Bull. 19, No. 6, 2331-2332 (1976).
18. J. C. Slonczewski, "Magnetic-Barrier Current Amplifier," IBM Tech. Disclosure Bull. 19, No. 6, 2333-2336 (1976).
19. S. Maekawa and U. Gafvert, "Electron Tunneling Between Ferromagnetic Films," IEEE Trans. Magn. MAG-18, 707-708 (1982).
20. J. C. Slonczewski, "Conductance and Exchange Coupling of Two Ferromagnets Separated by a Tunneling Barrier," Phys. Rev. B (Condens. Matter) 39, No. 10, 6995-7002 (1989).
21. J. Matisoo, "Tunneling Cryotron Using the Josephson Effect," IEEE Trans. Magn. MAG-4, No. 3, 324 (1968).
22. W. Anacker, "Josephson Computer Technology: An IBM Research Project," IBM J. Res.&Dev. 24, No. 2, 107-112 (1980); see also the other papers in this issue.
23. M. B. Ketchen, D. J. Herrell, and C. J. Anderson, "Josephson Cross-Sectional Model Experiment," J. Appl. Phys. 57, 2550-2573 (1985).
24. J. M. Rowell, M. Gurvitch, and J. Geerk, "Modification of Tunneling Barriers on Nb by a Few Monolayers of Al," Phys. Rev. B (Condens. Matter) 24, No. 4, 2278-2281 (1981).
25. M. Gurvitch, J. M. Rowell, H. A. Huggins, M. A. Washington, and T. A. Fulton, "Nb Josephson Tunnel Junctions with Thin Layers of Al Near the Barrier," IEDM Tech. Digest, pp. 115-117 (1981).
26. M. B. Ketchen, D. Pearson, A. W. Kleinsasser, C. K. Hu, M. Smyth, J. Logan, K. Stawiasz, E. Baran, M. Jaso, T. Ross, K. Petrillo, M. Manny, S. Basavaiah, S. Brodsky, S. B. Kaplan, W. J. Gallagher, and M. Bhushan, "Sub-μ, Planarized, Nb-AlO-Nb Josephson Process for 125 mm Wafers Developed in Partnership with Si Technology," Appl. Phys. Lett. 59, No. 20, 2609-2611 (1991).
27. 3/Co Magnetic Tunneling Junction," J. Magn. Magn. Mater. 98, No. 1-2, L7-L9 (1991).
28. 3/Co Junction - Dependence of the Tunneling Conductance on the Angle Between the Magnetizations of Two Ferromagnetic Layers," IEEE Trans. J. Magn. Jpn. 8, No. 8, 498-504 (1993).
29. J. Nowak and J. Rauluszhkiewicz, "Spin Dependent Electron Tunneling Between Ferromagnetic Films," J. Magn. Magn. Mater. 109, 79-90 (1992).
30. 3/Fe Junction," J. Magn. Magn. Mater. 139, No. 3, L231-L234 (1995).
31. 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. 74, No. 16, 3273-3276 (1995).
32. S. S. P. Parkin, "Giant Magnetoresistance and Oscillatory Interlayer Coupling in Polycrystalline Transition Metal Multilayers," Ultrathin Magnetic Structures, Vol. II, B. Heinrich and J. A. C. Bland, Eds., Springer-Verlag, Berlin, 1994, p. 148.
33. W. J. Gallagher, "Three-Terminal Superconducting Devices," IEEE Trans. Magn. MAG-21, No. 2, 709-716 (1985).
34. R. L. Sandstrom, A. W. Kleinsasser, W. J. Gallagher, and S. I. Raider, "Josephson Integrated Circuit Process for Scientific Applications," IEEE Trans. Magn. MAG-23, No. 2, 1484-1488 (1987).
35. W. P. Pratt, Jr., S.-F. Lee, J. Slaughter, R. Loloee, P. A. Schroeder, and J. Bass, "Perpendicular Giant Magnetoresistance of Ag/Co Multilayers," Phys. Rev. Lett. 66, 3060 (1991).
36. M. A. M. Gijs, S. K. J. Lenczowski, and J. B. Giesbers, "Perpendicular Giant Magnetoresistance of Microstructured Fe/Cr Magnetic Multilayers from 4.2 to 300 K," Phys. Rev. Lett. 70, 3343 (1993).
37. W. J. Gallagher, S. S. P. Parkin, L. Yu, X. P. Bian, A. Marley, R. A. Altman, S. A. Rishton, K. P. Roche, C. Jahnes, T. M. Shaw, and X. Gang, "Microstructured Magnetic Tunnel Junctions," J. Appl. Phys. 81, No. 8, 3741-3746 (1997);
38. S. S. P. Parkin, R. E. Fontana, and A. C. Marley, "Low Field Magnetoresistance in Magnetic Tunnel Junctions Prepared by Contact Masks and Lithography: 25% Magnetoresistance at 295 K in Mega Ohm Micron Sized Junctions," J. Appl. Phys. 81, 5521 (1997).
39. S. S. P. Parkin, K. P. Roche, M. G. Samant, P. M. Rice, R. B. Beyers, R. E. Scheuerlein, E. J. O'Sullivan, S. L. Brown, J. Bucchignano, D. W. Abraham, Y. Lu, M. Rooks, P. L. Trouilloud, R. A. Wanner, and W. J. Gallagher, "Exchange-Biased Magnetic Tunnel Junctions and Application to Nonvolatile Magnetic Random Access Memory," J. Appl. Phys. 85, No. 8, 5828-5833 (1999).
40. S. A. Rishton, Y. Lu, R. A. Altman, A. C. Marley, X. P. Bian, C. Jahnes, R. Viswanathan, G. Xiao, W. J. Gallagher, and S. S. P. Parkin, "Magnetic Tunnel Junctions Fabricated at Tenth-Micron Dimensions by Electron Beam Lithography," Microelectron. Eng. 35, No. 1-4, 249-252 (1997).
41. S. S. P. Parkin, K. S. Moon, K. E. Pettit, D. J. Smith, R. E. Dunin-Borkowski, and M. R. McCartney, "Magnetic Tunnel Junctions Thermally Stable to Above 300 Degrees C," Appl. Phys. Lett. 75, No. 4, 543-545 (1999).
42. S. Gider, B. U. Runge, A. C. Marley, and S. S. P. Parkin, "The Magnetic Stability of Spin-Dependent Tunneling Devices," Science 281, No. 5378, 797-799 (1998).
43. S. S. P. Parkin, N. More, and K. P. Roche, "Oscillations in Exchange Coupling and Magnetoresistance in Metallic Superlattice Structures: Co/ Ru, Co/Cr and Fe/Cr," Phys. Rev. Lett. 64, 2304-2307 (1990).
44. M. G. Samant, J. A. Luning, J. Stohr, and S. S. P. Parkin, "Thermal Stability of IrMn and MnFe Exchange-Biased Magnetic Tunnel Junctions," Appl. Phys. Lett. 76, No. 21, 3097-3099 (2000).
45. W. J. Gallagher and S. S. P. Parkin, "High-Speed 128Kbit MRAM Core in a 0.18μm CMOS Technology Utilizing PtMn-Based Magnetic Tunnel Junctions," Proceedings of the Nonvolatile Memory Technology Symposium, November 2003, p. 10-1; available as Jet Propulsion Laboratory Publication 03-18.
46. H. Kang, K. Bessho, Y. Higo, K. Ohba, M. Hashimoto, T. Mizuguchi, and M. Hosomi, "MRAM with Improved Magnetic Tunnel Junction Material," Digest of Technical Papers, IEEE INTERMAG Europe Magnetics Conference, 2002, p. BB4.
47. D. Wang, C. Nordman, J. M. Daughton, Z. Qian, and J. Fink, "70% TMR at Room Temperature for SDT Sandwich Junctions with CoFeB as Free and Reference Layers," IEEE Trans. Magn. 40, No. 4, 2269-2271 (2004).
48. K. Tsunekawa, D. D. Djayaprawira, H. Maehara, Y. Nagamine, M. Nagai, and N. Watanabe, presentation at the 9th Joint Magnetism and Magnetic Materials-Intermag Conference, Jacksonville, FL, 2004.
49. W. H. Butler, X. G. Zhang, T. C. Schulthess, and J. M. MacLaren, "Spin-Dependent Tunneling Conductance of Fe MgO Fe Sandwiches," Phys. Rev. B (Condens. Matter) 63, No. 5, 1-12 (2001).
50. J. Mathon and A. Umerski, "Theory of Tunneling Magnetoresistance of an Epitaxial Fe/MgO/Fe(001) Junction," Phys. Rev. B (Condens. Matter & Mater. Phys.) 63, No. 22, 1-4 (2001).
51. S. S. P. Parkin, C. Kaiser, A. Panchula, P. M. Rice, B. Hughes, M. Samant, and S. H. Yang, "Giant Tunneling Magnetoresistance at Room Temperature with MgO(100) Tunnel Barriers," Nature Mater. 3, 862-867 (2004).
52. D. C. Worledge and P. L. Trouilloud, "Magnetoresistance Measurement of Unpatterned Magnetic Tunnel Junction Wafers by Current-in-Plane Tunneling," Appl. Phys. Lett. 83, No. 1, 84-86 (2003).
53. D. W. Abraham, P. L. Trouilloud, and D. C. Worledge, "Rapid-Turnaround Characterization Methods for MRAM Development," IBM J. Res.&Dev. 50, No. 1, 55-67 (2006, this issue).
54. J. Faure-Vincent, C. Tiusan, E. Jouguelet, F. Canet, M. Sajieddine, C. Bellouard, E. Popova, M. Hehn, F. Montaigne, and A. Schuhl, "High Tunnel Magnetoresistance in Epitaxial Fe/MgO/Fe Tunnel Junctions," Appl. Phys. Lett. 82, No. 25, 4507-4509 (2003).
55. S. Yuasa, A. Fukushima, T. Nagahama, K. Ando, and Y. Suzuki, "High Tunnel Magnetoresistance at Room Temperature in Fully Epitaxial Fe/MgO/ Fe Tunnel Junctions Due to Coherent Spin-Polarized Tunneling," Jpn. J. Appl. Phys. 43, No. 4B, L588-L590 (2004).
56. S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, and K. Ando, "Giant Room-Temperature Magnetoresistance in Single-Crystal Fe/MgO/Fe Magnetic Tunnel Junctions," Nature Mater. 3, 868-871 (2004).
57. D. D. Djayaprawira, K. Tsunekawa, M. Nagai, H. Maehara, S. Yamagata, N. Watanabe, S. Yuasa, Y. Suzuki, and K. Ando, "230% Room-Temperature Magnetoresistance in CoFeB/MgO/CoFeB Magnetic Tunnel Junctions," Appl. Phys. Lett. 86, No. 9, 092502 (2005).
58. D. D. Djayaprawira, K. Tsunekawa, M. Nagai, H. Maehara, S. Yamagata, N. Watanabe, S. Yuasa, and K. Ando, "Huge Room Temperature Magnetoresistance in CoFeB/MgO/CoFeB Magnetic Tunnel Junctions," presented at the IEEE International Magnetics Conference (Intermag), Nagoya, Japan, 2005, Abstract CA-1.
59. R. E. Scheuerlein, "Magneto-Resistive IC Memory Limitations and Architecture Implications," Proceedings of the Seventh Biennial IEEE International Nonvolatile Memory Technology Conference (IEEE Cat. No. 98EX141), 1998, pp. 47-50.
60. W. J. Gallagher and R. E. Scheuerlein, "Voltage Biasing for Magnetic RAM with Magnetic Tunnel Memory Cells," U.S. Patent 5,991,193, 1999.
61. F. Z. Wang, "Diode-Free Magnetic Random Access Memory Using Spin Dependent Tunnel Effect," Appl. Phys. Lett. 77, 2036-2038 (2000).
62. W. Reohr, H. Honigschmid, R. Robertazzi, D. Gogl, F. Pesavento, S. Lammers, K. Lewis, C. Arndt, L. Yu, H. Viehmann, R. Scheuerlein, W. Li-Kong, P. Trouilloud, S. Parkin, W. Gallagher, and G. Muller, "Memories of Tomorrow," IEEE Circuits&Devices Mag. 18, No. 5, 17-27 (2002).
63. W. J. Gallagher, D. W. Abraham, S. Assefa, S. L. Brown, J. DeBrosse, M. Gaidis, E. Galligan, E. Gow, B. Hughes, J. Hummel, S. Kanakasabapathy, C. Kaiser, M. Lamorey, T. Maffitt, K. Milkove, Y. Lu, J. Nowak, P. Rice, M. Samant, E. O'Sullivan, S. S. P. Parkin, R. Robertazzi, P. Trouilloud, D. Worledge, G. Wright, and S.-H. Yang, "Recent Advances in MRAM Technology," Proceedings of the IEEE International Symposium on VLSI Technology (VLSI TSA TECH), 2005, pp. 72-73.
64. N. D. Rizzo, M. DeHerrera, J. Janesky, B. Engel, J. Slaughter, and S. Tehrani, "Thermally Activated Magnetization Reversal in Submicron Magnetic Tunnel Junctions for Magnetoresistive Random Access Memory," Appl. Phys. Lett. 80, No. 13, 2335-2337 (2002).
65. M. Durlam, D. Addie, J. Akerman, B. Butcher, P. Brown, J. Chan, M. DeHerrera, B. N. Engel, B. Feil, G. Grynkewich, J. Janesky, M. Johnson, K. Kyler, J. Molla, J. Martin, K. Nagel, J. Ren, N. D. Rizzo, T. Rodriguez, L. Savtchenko, J. Salter, J. M. Slaughter, K. Smith, J. J. Sun, M. Lien, K. Papworth, P. Shah, W. Qin, R. Williams, L. Wise, and S. Tehrani, "A 0.18 μm 4Mb Toggling MRAM," IEDM Tech. Digest, pp. 34.6.1-34.6.3 (2003).
66. D. C. Worledge, "Single-Domain Model for Toggle MRAM," IBM J. Res. & Dev. 50, No. 1, 69-79 (2006, this issue).
67. S. A. Wolf, A. Y. Chtchelkanova, and D. M. Treger, "Spintronics - A Retrospective and Perspective," IBM J. Res.&Dev. 50, No. 1, 101-110 (2006, this issue).
68. R. Scheuerlein, W. Gallagher, S. Parkin, A. Lee, S. Ray, R. Robertazzi, and W. Reohr, "A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in Each Cell," Digest of Technical Papers, IEEE International Solid-State Circuits Conference (IEEE Cat. No. 00CH37056), 2000, pp. 128-129.
69. A. R. Sitaram, D. W. Abraham, C. Alof, D. Braun, S. Brown, G. Costrini, F. Findeis, M. Gaidis, E. Galligan, W. Glashauser, A. Gupta, H. Hoenigschmid, J. Hummel, S. Kanakasabapathy, I. Kasko, W. Kim, U. Klostermann, G. Y. Lee, R. Leuschner, K. S. Low, Y. Lu, J. Nutzel, E. O'Sullivan, C. Park, W. Raberg, R. Robertazzi, C. Sarma, J. Schmid, P. L. Trouilloud, D. Worledge, G. Wright, W. J. Gallagher, and G. Muller, "A 0.18 μm Logic-Based MRAM Technology for High Performance Nonvolatile Memory Applications," Digest of Technical Papers, Symposium on VLSI Technology (IEEE Cat. No. 03CH37407), 2003, pp. 15-16.
70. J. DeBrosse, D. Gogl, A. Bette, H. Hoenigschmid, R. Robertazzi, C. Arndt, D. Braun, D. Casarotto, R. Havreluk, S. Lammers, W. Obermaier, W. R. Reohr, H. Viehmann, W. J. Gallagher, and G. Muller, "A High-Speed 128-Kb MRAM Core for Future Universal Memory Applications," IEEE J. Solid-State Circuits 39, No. 4, 678-683 (2004).
71. M. C. Gaidis, E. J. O'Sullivan, J. J. Nowak, Y. Lu, S. Kanakasabapathy, P. L. Trouilloud, D. C. Worledge, S. Assefa, K. R. Milkove, G. P. Wright, and W. J. Gallagher, "Two-Level BEOL Processing for Rapid Iteration in MRAM Development," IBM J. Res.&Dev. 50, No. 1, 41-54 (2006, this issue).
72. J. Z. Sun, "Spin Angular Momentum Transfer in Current-Perpendicular Nanomagnetic Junctions," IBM J. Res.&Dev. 50, No. 1, 81-100 (2006, this issue).
73. X. Jiang, R. Wang, R. M. Shelby, and S. S. P. Parkin, "Highly Efficient Room-Temperature Tunnel Spin Injector Using CoFe/MgO(001, " IBM J. Res. &Dev. 50, No. 1, 111-120 (2006, this issue).
74. J. DeBrosse, C. Arndt, C. Barwin, A. Bette, D. Gogl, E. Gow, H. Hoenigschmid, S. Lammers, M. Lamorey, Y. Lu, T. Maffitt, K. Maloney, W. Obermeyer, A. Sturm, H. Viehma D. Willmott, M. Wood, W. J. Gallagher, G. Mueller, and A. R. Sitaram, "A 16Mb MRAM Featuring Bootstrapped Write Drivers," Digest of Technical Papers, Symposium on VLSI Circuits (IEEE Cat. No. 04CH37525), 2004, pp. 454-457.
75. T. M. Maffitt, J. K. DeBrosse, J. A. Gabric, E. T. Gow, M. C. Lamorey, J. S. Parenteau, D. R. Willmott, M. A. Wood, and W. J. Gallagher, "Design Considerations for MRAM," IBM J. Res.&Dev. 50, No. 1, 25-39 (2006, this issue).
76. S. S. Iyer, J. E. Barth, Jr., P. C. Parries, J. P. Norum, J. P. Rice, L. R. Logan, and D. Hoyniak, "Embedded DRAM: Technology Platform for the Blue Gene/L chip," IBM J. Res.&Dev. 49, No. 2/3, 333-350 (2005).
77. M. Durlam, P. Naji, A. Omair, M. DeHerrera, J. Calder, J. M. Slaughter, B. Engel, N. Rizzo, G. Grynkewich, B. Butcher, C. Tracy, K. Smith, K. Kyler, J. Ren, J. Molla, B. Feil, R. Williams, and S. Tehrani, "A Low Power 1 Mbit MRAM Based on 1T1MTJ Bit Cell Integrated with Copper Interconnects," Digest of Technical Papers, Symposium on VLSI Circuits (IEEE Cat. No. 02CH37302), 2002, pp. 158-161.
78. 2 Cells," Digest of Technical Papers, Symposium on VLSI Technology, 2005, pp. 186-187.
79. T. Tsunoda, M. Kosuda, K. Tsunekawa, and N. Watanabe, "A Novel PVD Tool for Magnetoresistive Random Access Memory (MRAM) Mass Production," Semicond. Manuf. 4, 154-165 (2003).
80. 2," IEEE Trans. Magn. 38, 78 (2002).
81. D. W. Abraham and P. L. Trouilloud, "Thermally Assisted Magnetic Random Access Memory," U. S. Patent 6,385,082, May 7, 2002;
82. I. L. Prejbeanu, W. Kula, K. Oundadjela, R. C. Sousa, O. Redon, B. Dieny, and J.-P. Nozieres, "Thermally Assisted Switching in Exchange-Biased Storage Layer Magnetic Tunnel Junctions," IEEE Trans. Magn. 40, 2625 (2004);
83. O. Redon, M. Kerekes, R. Sousa, L. Prejbeanu, H. Sibuet, F. Ponthennier, A. Persico, and J. P. Nozières, "Thermo Assisted MRAM for Low Power Applications" Proceedings of the First International Conference on Memory Technology and Design (ICMTD-2005), Giens, France, May 21-24, 2005, pp. 113-114.
84. Y. Huai, F. Albert, P. Nguyen, M. Pakala, and T. Valet, "Observation of Spin-Transfer Switching in Deep Submicron-Sized and Low-Resistance Magnetic Tunnel Junctions," Appl. Phys. Lett. 84, 3118 (2004);
85. G. D. Fuchs, N. C. Emley, I. N. Krivorotov, P. M. Braganca, E. M. Ryan, S. I. Kiselev, J. C. Sankey, D. C. Ralph, R. A. Buhrman, and J. A. Katine, "Spin Transfer Effects in Nanoscale Magnetic Tunnel Junctions," Appl. Phys. Lett. 85, 1205 (2004).
86. S. S. Parkin, "Shiftable Magnetic Shift Register and Method of Using the Same," U.S. Patent 6,834,005, 2004;
87. S. S. Parkin, "System and Method for Writing to a Magnetic Shift Register," U.S. Patent 6,898,132, 2005.