Statistical and Time Resolved Studies of Switching in Orthogonal Spin Transfer MRAMs

1. NYU Statistical and Time Resolved Studies of Switching inOrthogonal Spin Transfer MRAMs D. Bedau,1 H. Liu,1 D. Backes,1J. Langer2, P. Manandhar4, J. A. Katine,3 and A. D. Kent1,4 1. Department of Physics, New York University, New York, NY 2. Singulus Technologies AG, 63796 Kahl am Main, Germany 3.San Jose Research Center, Hitachi-GST, San Jose, CA4. Spin Transfer Technologies, Boston, MA MMM Phoenix AF-02

2. NYU Spin-Transfer MRAM Spin Torque on Bloch Sphere • Some states switch only slowly Time to switch stagnation zone Initial Boltzmanndistribution large torque Low bias Switching time / s High bias Initial angle / rad small torque • Fastest magnetization motion (precession) does not contribute to switching J. L. Beaujour et al., Spintronics II, Proc. SPIE 7398, 73980D (2009) O. J. Lee et al., APL 95, 12506 (2009) C. Papusoi et al., APL 95, 72506 (2009)

3. NYU Switching with a Perpendicular Polarizer large initial torque • Large initial torque. • Demag field makes switching fast. • Short switching trajectory. • Needs readout layer. • Bipolar switching. A.D. Kent et al., Appl. Phys. Lett. 84, 3897 (2004)

4. NYU Capping Characterization of Device Layer Stacks PtMn CoFe Ru CoFeB MgO CoFeB Cu Co/Ni ML Co/Pd ML Electrode • Perpendicular Polarizer: • Ms = 550 emu/cc • Hc = 265 Oe • HK = 7100 Oe(from FMR measurements) Reference Layer/ SAF • High TMR ~100% • Low RA ~5 Wmm2 Tunnel barrier Free Layer See AF-05for more information Spacer Layer Perpendicular Polarizer VSM CIPT, D. C. Worledge et al., APL 83, 84 (2003).

5. NYU Hysteresis of a Device 60 nm x 180 nm hexagon xh=0 = 163 kT t0 = 15 ps • 40 nm x 80 nm up to 80 nm x 240 nm. • Stable hysteresis and high energy barrier.

6. NYU Pos. Potential: Fast Switching Results e- Neg. Potential: e- m0H=10mT Free layer field switching Short Pulse Current Induced Switching Switching 1000/1000 events with pulses of 500 ps duration that are greater than 0.6 V in amplitude 60 nm x 180 nm hexagon, MR: 107% • 100% switching probability for less than 500 ps pulse duration • Required energy for 100% switching is less than 450 fJ • No incubation delay of several nanoseconds H. Liu, D. Bedau, D. Backes, J. A. Katine, J. Langer and A. D. Kent, Applied Physics Letters 97, 242510 (2010) Recent confirmation of OST-RAM characteristics: G. E. Rowlands et al., APL 98, 102509 (2011) Micromagnetic modeling: Nikonov et al., JAP 107, 113910 (2010)

7. NYU Fast Switching at zero field P -> AP 50 nm x 150 nm hexagon • 103% MR. • Hc = 17 mT. • Switching at zero effective field on free layer. • 498 switches out of 500 events @ 1 ns duration.

8. NYU Bipolar switching Pulse duration 700 ps • Switching for both polarities observed • After certain current threshold the switching probability increases • Increase is either monotonic or non-monotonic depending on the polarity H. Liu, D. Bedau, D. Backes, J. A. Katine, J. Langer and A. D. Kent, Applied Physics Letters 97, 242510 (2010)

9. NYU Time-resolved Measurements AP t start -0.62V Pulse P t switch 50 nm x 115 nm Device, 4 kW, 104% MR • Individual switching events can be time resolved, showing the intrinsic timescales of the switching process. • Switching starts after delay. • Transition very fast, < 500 ps.

10. NYU Switching Statistics Start time Transition time Heff=0 50 nm x 115 nm 4 kW, 104% MR P to AP switching • Switching process starts at about 2 ns. • Switching time < 500 ps.

11. NYU Precessional Switch P to P AP 0.62V Pulse P 50 nm x 115 nm device • Sample precesses and switches back to the initial state (P).

12. NYU Summary AF-05 • Fabricated OST-MRAM devices that incorporate magnetic tunnel junctions • Achieved high TMR (>100%) with low RA~ 5 W mm2 and perpendicular polarizer with excellent characteristics: high spin-polarization and large perpendicular magnetic anisotropy • Demonstrated 100% switching probability in thermally stable elements with 500 ps duration pulses (0.7 V), requiring just 450 fJ. • Can time-resolve individual switching events showing fast and precessional switching. H. Liu, D. Bedau, D. Backes, J. A. Katine, J. Langer and A. D. Kent, Applied Physics Letters 97, 242510 (2010) www.spintransfer.com

13. NYU Switching Times and Variances at High Bias τstart Tstart τswitch τswitch • Switching process starts after 500 ps to 2 ns. • Switching duration < 500 ps. • Higher amplitude decreases switching time and variance

14. Sample Characteristics 50 nm x 115 nm Ellipse • 0mT, 0.62V, 2k-4k, 100% MR

15. NYU Switching at High Bias SW2140 • 60 nm x 180 nm hexagon • P to AP switching • Individual switching events can be time resolved, showing the intrinsic timescales of the switching process. • Switching starts deterministically.

16. NYU Free Layer Energy Barrier Fitting to measurement results in: Change field sweep rate: 60 nm x 180 nm hexagon: 107% MR μ0Hc(T=300K) =14mT μ0Hbias=-2mT Resulting energy barrier:

17. Bloch Sphere for OST-MRAM

18. NYU Switching Statistics AP to P Start time Transition time Heff=0 50 nm x 115 nm 4 kW, 104% MR • Switching process starts at about 2 ns. • Switching time < 500 ps.

19. Initialdistribution NYU Spin-Transfer MRAM • Initial misalignment not a solution: • WER probability follows same statistics for given overdrive (I/Ic0), R. Heindl et al., PRB 2011 & JAP 2011 • Critical current increases. Spin Torque on Bloch Sphere • Some states switch only slowly stagnation zone Initialdistribution Time to switch large torque Switching time / s Initial angle / rad small torque • Fastest magnetization motion (precession) does not contribute to switching

20. NYU Capping OST-MRAM Devices Layer Stacks PtMn CoFe Ru CoFeB MgO CoFeB Cu Co/Ni ML Co/Pd ML Electrode Advantages: • Large spin torques • No incubation delay • Fast Switching • Low Energy Reference Layer/ SAF TIMARIS PVD System, Singulus Technologies, Kahl am Main, Germany Tunnel barrier Free Layer • Device requirements: • Perpendicular polarizer • Large perpendicular anisotropy • High spin polarization • Compensated MTJ with low RA on top. Spacer Layer Perpendicular Polarizer

21. NYU Spin-Transfer MRAM • Possible solution: initial misalignment of the polarizer and free layer stagnation zone |Spin-torque| large torque small torque • Initial misalignment not a solution: • WER probability follows same statistics for given overdrive (I/Ic0), R. Heindl et al., PRB 2011 & JAP 2011 • Critical current increases. • Critical current increases

22. NYU Intermediate Long times Short Spin-Transfer MRAM • Switching probability in all perpendicular spin valves • Ballistic to the thermally assisted ST regime [Co/Ni]x2/Co/Pt Cu easy axis [Co/Pt]x4/Co/[Ni/Co]x2 Bedau, Liu, Sun, Katine, Fullerton, Mangin & Kent, APL 97, 262502 (2010) & APL 96, 022514 (2010)

23. Spin Transfer and Magnetic Anisotropy All-perpend.-MRAM OST-MRAM ST-MRAM Kent et al., APL (2004) Ebels et al., Nat. Mat (2007) Lee et al., APL (2009) Papusoi et al., APL(2009) Beaujour et al., SPIE(2009) H. Liu et al., APL (2010) Mangin et al., Nat. Mater (2006) Mangin et al. APL (2009) Bedau et al. 2x APL(2010) Sun, PRB (2000)

24. NYU Spin-Transfer MRAM • MRAM • Unlimited read/write capability • Non-volatile • Fast • Radiation hard Free Layer Cu or MgO Pinned Layer • Spin-Transfer MRAM • Spin currents write information • Scalable to nanometer dimensions • Combines the speed of SRAM and DRAM with the non-volatility of FLASH memories Potential Universal Memory Technology Challenge: Fast, Reliable and Low Power Spin-Transfer Switching of Nanometer Scale Magnetic Elements

25. NYU Switching of an In-Plane Magnetized MRAM Device Stochastic incubation delay Incident and transmitted voltage pulses when a device switches from P to AP. Devolder et al., PRL 100, 057206 (2008) Initial torque is low • Small initial torque (arrows) causes incubation delay • Magnetization follows a complicated trajectory • Switching speed is limited by the low precession frequency

26. NYU Orthogonal Spin Transfer Devices • Advantages: • Large spin torques • No incubation delay • Fast Switching • Low Energy Concept demonstrated in all metallic spin-valves: J. L. Beaujour et al., Spintronics II, Proc. SPIE 7398, 73980D (2009) O. J. Lee et al., APL 95, 12506 (2009) C. Papusoi et al., APL 95, 72506 (2009) but devices have a low impedance (~5 Ohms) and low MR (<5%) Magnetic integration of magnetic tunnel junction provides larger readout signal.

27. NYU Outline • Introduction • Magnetic RAM • Orthogonal Spin Transfer Devices (OST-RAM) • Device Layer Stacks • High Speed Current Induced Switching • Time Resolved Studies • Summary