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

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

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

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

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

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

MTJ Characteristics

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

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

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

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

TMR vs. RP Variation of RP and TMR approximately Gaussian

Thermal Stability

Write Bit Error Rate

Write Threshold

Read Disturbance

Breakdown Voltages

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

Spin-Transfer Torque Writing

Spin-Transfer Torque Reading

Selection of Read Direction

Reducing Read Disturbance

Reading with Reduced Bitline Voltages

Chip Summary

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

Field Induced Magnetic Switching

Writing: Current Steering Circuit

Reading: Dual-MTJ Latch

Recovery from a Cosmic Particle Strike

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. 2008. 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. 480-481, 617, Feb. 2007. K.J. Hass, et al., “Magnetic Flip Flops for Space Applications,” IEEE Trans. Magn., vol. 42, no. 10, pp. 2751-2753, Oct. 2006.

Questions?