EE 201 C Winter 2012 Introduction to Spintronics

EE 201 C: Winter 2012 Introduction to Spintronics: Modeling and Circuit Design Richard Dorrance Yuta Toriyama

Outline • Spintronics Primer – 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 2

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

That’s Nice! But why should I care? • Spintronics has existed since the mid-1930 s! • 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 4

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

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 6

MTJ Characteristics 7

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

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

Modified Landau–Lifshitz–Gilbert Equation Direction of Mag. of the Free Layer % Spin-Polarization in the p 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 10

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

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

Thermal Stability 13

Write Bit Error Rate 14

Write Threshold 15

Read Disturbance 16

Breakdown Voltages 17

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

Spin-Transfer Torque Writing 19

Spin-Transfer Torque Reading 20

Selection of Read Direction 21

Reducing Read Disturbance 22

Reading with Reduced Bitline Voltages 23

Chip Summary 24

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

Field Induced Magnetic Switching 26

Writing: Current Steering Circuit 27

Reading: Dual-MTJ Latch 28

Recovery from a Cosmic Particle Strike 29

References 1. J. C. Slonczewski, “Current-Driven Excitation of Magnetic Multilayers, ” J. Magn. Mater. , vol. 159, pp. L 1 – L 7, 1996. 2. 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. 3. 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. 4. K. J. Hass, et al. , “Magnetic Flip Flops for Space Applications, ” IEEE Trans. Magn. , vol. 42, no. 10, pp. 2751 -2753, Oct. 2006. 30

Questions? 31