Department of Electronics

1. Advanced Information Storage 14 Atsufumi Hirohata Department of Electronics 17:00 18/November/2013 Monday (AEW 105)

2. Quick Review over the Last Lecture 6T-SRAM (static random access memory) operation : EEPROM (electrically erasable read only memory) : * http://allthingsvlsi.wordpress.com/tag/6t-sram-operation/; * http://www.answers.com/topic/eeprom

3. 14 Magnetic Random Access Memory • Non-volatility • Read-out operation • Spin-transfer torque • Coherent tunnelling • Perpendicular magnetisation • Content addressable memory

4. Memory Types Dynamic Rewritable Volatile DRAM SRAM Static Non-volatile Static MRAM FeRAM PRAM Read only Non-volatile PROM Static Mask ROM Read majority (Writable) Flash Non-volatile Static EPROM * http://www.semiconductorjapan.net/serial/lesson/12.html

5. Advantages of MRAM * After K. Inomata, J. Magn. Soc. Jpn. 23, 1826 (1999).

6. Magnetic Random Access Memory Basic operation of magnetic random access memory (MRAM) : * S. S. P. Parkin, 1st Int'l Sch. on Spintronics and Quantum Info. Tech., May 13-15, 201 (Maui, HI, USA).

7. MRAM Cell MRAM cell structure : MRAM read-out : Bit line Sensing current Magnetic free layer Magnetic tunnel / spin-valve junctions Insulator / nonmagnet Magnetic pin layer Word line Selection transistor (MOSFET) Parallel magnetisation ↓ Low resistant state “0” Antiparallel magnetisation ↓ High resistant state “1” * http://www.wikipedia.org/

8. MRAM Products Freescale (now EverSpin Technologies) 4 Mbit MRAM : * http://www.freescale.com/; ** http://www.chipworks.com/blogs.aspx?id=2514

9. Improved MRAM Operation Required writing currents for several techniques dependent upon cell size: Ampère-field-induced magnetisation reversal with a ferromagnetic overlayer (Current technology) Ampère-field-induced magnetisation reversal without a ferromagnetic overlayer (Current technology) Current-induced magnetisation reversal JC ~ 10 7 A / cm 2 (Current technology) Write current (mA) Current-induced magnetisation reversal JC ~ 10 6 A / cm 2 Current-induced magnetisation reversal JC ~ 5  10 5 A / cm 2 MRAM cell size (µm) * S. Nakamura, Y. Saito and H. Morise, Toshiba Rev. 61, 40 (2006).

10. Current-Induced Magnetisation Reversal Anti-parallel (AP)  parallel (P) reversal in a GMR / TMR junction : Spin-transfer torque (STT) ** * M. Oogane and T. Miyazaki, “Magnetic Random Access Memory,” in Epitaxial Ferromagnetic Films and Spintronic Applications, A. Hirohata and Y.Otani (Eds.)(Research Signpost, Kerala, 2009) p. 335. ** J. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996); L. Berger, Phys. Rev. B 54, 9353 (1996).

11. STT-MRAM Products In 2012, EverSpin Technologies introduced 64 Mbit MRAM : * http://www.everspin.com/

12. STT-MRAM Advantages 1 * http://www.everspin.com/

13. STT-MRAM Advantages 2 * http://www.everspin.com/

14. Spin-Dependent Electron Tunneling Atom 1 Barrier Atom 2 r(r ) 3d 4p 4s r Jullière's model : FM / insulator / FM junctions * * M. Jullière., Phys. Rep. 54A, 225 (1975).

15. TMR for Device Applications NOT following Jullière's model : ** TMR =2P1P2/(1-P1P2) ** S. S. P. Parkin, 1st Int'l Sch. on Spintronics and Quantum Info. Tech., May 13-15, 2001 (Maui, HI, USA). Recent progress in TMR ratios : > 400 % (604 % in 2008) TMR ratio has been achieved !  > Gbit MRAM can be realised. * M. Jullière., Phys. Rep. 54A, 225 (1975);

16. Improved Tunnel Barriers Epitaxial (oriented) barriers : * 1 2 , 5 1 2 , 5 • Disorder at the interface : • FM over-oxidation • lattice defects Defects in the barrier • Disorder at the interface : • FM over-oxidation • lattice defects • island growth of the barrier Conventional amorphous barriers : * * After S. Yuasa et al., 28th Annual Conference on Magnetics, Sep. 21-24, 2004 (Okinawa, Japan).

17. Perpendicular MTJ In 2007, Toshiba demonstrated STT operation with perpendicular magnetisation : * * http://www.toshiba.co.jp/

18. Advantages of Perpendicular MTJ Energy barrier can be lowered using perpendicular magnetisation : * Magnetisation reversal by spin-transfer torque Magnetisation reversal by spin-transfer torque Easy axis Easy axis Magnetisation reversal by thermal fluctuation Magnetisation reversal by thermal fluctuation Energy Energy 50-nm perpendicular MTJ In-plane magnetisation Perpendicular magnetisation Resistance Voltage [V] * http://www.toshiba.co.jp/

19. Content Addressable Memory (CAM) In 2011, NEC and Tohoku University announced a new memory concept : * • Fast latency : 5 ns • Low power consumption : 9.4 mW • 50 % area reduction by sharing transistors * http://www.csis.tohoku.ac.jp/

20. Thermally Assisted (TA)-MRAM Crocus demonstrated 1-Mbit MRAM with thermally assisted STT operation : * * I. L. Prejbeanuet al., J. Phys. D: Appl. Phys.46, 074002 (2013).

21. Reduced Energy Consumption 3-orders of reduction in energy consumption was demonstrated by UCLA team : * Voltage-induced magnetisation reversal was used. * http://newsroom.ucla.edu/portal/ucla/ucla-engineers-have-developed-241538.aspx

22. Comparison between Next-Generation Memories * http://techon.nikkeibp.co.jp/article/HONSHI/20070926/139715/