1. EE201C: Winter 2012
Introduction to Spintronics: Modeling and Circuit Design Richard Dorrance Yuta Toriyama

2. Outline Spintronics Primer Magnetic Tunnel Junctions (MTJs)
What is it?
Why should I care?
How does it work?
Magnetic Tunnel Junctions (MTJs)
Modeling
Statistical Variation
Circuit Design with MTJs
Magnetic Random Access Memories
Magnetic Flip-Flops

3. Spintronics? What’s that?
Electrons:
Mass
Charge
Velocity
Spin
Most modern electronics exploit charge
Spintronics exploits electron spin

4. That’s Nice! But why should I care?
Spintronics has existed since the mid-1930s!
You use it every single day is these devices:
Hard Drives
Radiation Hardened Memories
Polarized LEDs
Next-Generation Devices
Spin-FETs
Universal Memories
Terahertz Lasers
Energy-Efficient LEDs

5. Spintronic Operation Spin Injector Spin Detector
Ferromagnetic layers tend to spin-polarize a current
Spin Detector
Ferromagnetic layers tend to scatter anti-parallel currents

6. Spin Valves and Magnetic Tunnel Junctions
A Spin Valve combines a spin injector and a detector
Practical Spin Valve: Magnetic Tunnel Junction
Two ferromagnetic layers separated by a thin insulator
Parallel
Antiparallel

7. MTJ Characteristics

8. Current-Driven Excitation of Magnetic Multilayers
J. C. Slonczewski
J. Magn. Magn. Mater., 1996

9. Landau–Lifshitz–Gilbert Equation
Describes the precessional motion of magnetization in a solid

10. Modified Landau–Lifshitz–Gilbert Equation
Direction of Mag.of the Free Layer
% Spin-Polarization in the p Direction
Direction of Mag. of the Fixed Layer
Landé Factor of an Electron
“Normalized” Effective Magnetic Field
Current Density
Magnetization Saturation
Absolute Value of Electron Charge
Gilbert Damping Constant
Bhor Magneton
Gyromagnetic Ratio
Thickness of the Free Layer

11. A Statistical Study of Magnetic Tunnel Junctions for High-Density Spin Torque Transfer-MRAM
R. Beach, et al.
IEDM ’08, Dec. 2008

12. TMR vs. RP
Variation of RP and TMR approximately Gaussian

13. Thermal Stability

14. Write Bit Error Rate

15. Write Threshold

16. Read Disturbance

17. Breakdown Voltages

18. T. Kawahara, et al. ISSCC ‘07, Feb. 2007
2 Mb SPRAM with Bit-by-Bit Bi-Directional Current Write and Parallelizing-Direction Current Read
T. Kawahara, et al.
ISSCC ‘07, Feb. 2007

19. Spin-Transfer Torque Writing

20. Spin-Transfer Torque Reading

21. Selection of Read Direction

22. Reducing Read Disturbance

23. Reading with Reduced Bitline Voltages

24. Chip Summary

25. Magnetic Flip Flops for Space Applications
K.J. Hass, et al.
IEEE Trans. Magn., Oct. 2008

26. Field Induced Magnetic Switching

27. Writing: Current Steering Circuit

28. Reading: Dual-MTJ Latch

29. Recovery from a Cosmic Particle Strike

30. References
J. C. Slonczewski, “Current-Driven Excitation of Magnetic Multilayers,” J. Magn. Magn. Mater., vol. 159, pp. L1 – L7, 1996.
R. Beach, et al., “A Statistical Study of Magnetic Tunnel Junctions for High-Density Spin Torque Transfer-MRAM (STT-MRAM),” IEDM 2008, pp. 1-4, Dec
T. Kawahara, et al., “2 Mb SPRAM (SPin-Transfer Torque RAM) With Bit-by-Bit Bi-Directional Current Write and Parallelizing-Direction Current Read,” ISSCC’ 07, pp , 617, Feb
K.J. Hass, et al., “Magnetic Flip Flops for Space Applications,” IEEE Trans. Magn., vol. 42, no. 10, pp , Oct

31. Questions?