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Spin-transfer pulse switching in all perpendicular spin-valve nanopillars

Spin-transfer pulse switching in all perpendicular spin-valve nanopillars. NYU. H. Liu 1* , D. Bedau 1 , J. A. Katine 2 , E. E. Fullerton 3 , S. Mangin 4 , J. Z. Sun 5 and A. D. Kent 1 Department of Physics, New York University, 4 Washington Place, New York, New York 10003, USA

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Spin-transfer pulse switching in all perpendicular spin-valve nanopillars

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  1. Spin-transfer pulse switching in all perpendicular spin-valve nanopillars NYU H. Liu1*, D. Bedau1, J. A. Katine2, E. E. Fullerton3, S. Mangin4 , J. Z. Sun5 and A. D. Kent1 Department of Physics, New York University, 4 Washington Place, New York, New York 10003, USA San Jose Research Center, Hitachi-GST, San Jose, California 95135, USA CMRR, University of California, San Diego, La Jolla, California 92093-0401, USA IJL, Nancy-Université, UMR CNRS 7198, F-54042 VandoeuvreCedex, France IBM T. J. Watson Research Center, P.O. Box 218, Yorktown Heights, New York 10598, USA *presenter 56th MMM Scottsdale, DA-02

  2. Outline • Introduction • Spin-transfer switching • All perpendicular spin-valves • Modeling • A macrospin model analytical results • Direct comparison to experiments • Switching probability experiments • Quasi-static measurements • Switching by current pulses (50 ps – 1 s) • Double pulse experiments • Method and results • Beyond LLG – the Fokker-Planck simulation NYU 56th MMM Scottsdale, DA-02

  3. NYU Spin-transfer switching damping magnetizationrotation Spin transfer torque (STT): spin transfer torque Introduction Introduction Modeling Switching Double Pulse Landau-Lifshitz-Gilbert (LLG) equation + STT: 56th MMM Scottsdale, DA-02

  4. NYU All perpendicular spin valves • High energy barrier • Low critical current • High symmetry Introduction Introduction Modeling Switching Double Pulse 56th MMM Scottsdale, DA-02

  5. NYU Sample structure • [Co/Ni] free layer (FL), 1.6 nm thick • [Co/Ni]|[Co/Pt] reference layer (RL) • high coercive field (> 0.5 T) • Measured > 20 samples • Present results on: • 100 nm x 100 nm • 50 nm x 50 nm samples Introduction Introduction Modeling Switching Double Pulse Mangin et al., Nat. Mat. (2006) Mangin et al., APL (2009) Bedau et al., APL (2010) Bedau et al., APL (2010) 56th MMM Scottsdale, DA-02

  6. Modeling Is there an analytic prediction for experiments? NYU 56th MMM Scottsdale, DA-02

  7. NYU A macrospin model Current Pulse: In general, both STTand thermal fluctuation influence the switching process. Modeling Introduction Modeling Switching Double Pulse • θ0: Initial magnetization angle when current pulse is applied, a distributed quantity whose probability is determined by the Boltzmann distribution. • θτ: Final magnetization angle when current pulse stops, determines whether or not a switching event would result. 56th MMM Scottsdale, DA-02

  8. NYU STT Thermal fluctuation Initial state Final state Large current – short time limit: STT > Thermal fluctuation Small current – long time limit: STT < Thermal fluctuation Pulse simplify simplify • Thermally distributed initial states and deterministic switching process (only STT). • Thermal activation over an energy barrier which is modified by STT. Modeling Introduction Modeling Switching Double Pulse deterministic 56th MMM Scottsdale, DA-02

  9. NYU Analytic solutions • Short time: • Linear boundary: • Long time: • Exponential boundary: Modeling Introduction Modeling Switching Double Pulse 56th MMM Scottsdale, DA-02

  10. Experiments on Switching How does the switching probability P depend on the pulse amplitude I and duration τ? NYU 56th MMM Scottsdale, DA-02

  11. NYU Experimental setup Switching Introduction Modeling Switching Double Pulse 56th MMM Scottsdale, DA-02

  12. NYU Measurement circuit Bias-tee: τ< 10 ns Switch: τ > 5 ns μ0Happ Switching Introduction Modeling Switching Double Pulse e- 56th MMM Scottsdale, DA-02

  13. NYU Switching at DC Switching Introduction Modeling Switching Double Pulse Room temperature coercive field: 100 mT Hysteresis loop shift: 25 mT Switching currents: -7 mA, 4 mA P AP 56th MMM Scottsdale, DA-02

  14. NYU Switching at DC Switching Introduction Modeling Switching Double Pulse Room temperature coercive field: 90 mT Hysteresis loop shift: -40 mT Switching currents: 0.4 mA, -1 mA P AP 56th MMM Scottsdale, DA-02

  15. NYU Switching at DC Switching Introduction Modeling Switching Double Pulse • Ratio of size: 4 • Ratio of resistance: ~ 1 / 3 • Ratio of MR ~ 1 • Ratio of coercive field: ~ 1 • Ratio of critical current: ~ 8 56th MMM Scottsdale, DA-02

  16. NYU Pulse switching measurement μ0Happ = 0.2 T 3 1 2 4 μ0Happ = 0 T Apply measurement field and current Saturate Check if switched Apply pulse Switching Introduction Modeling Switching Double Pulse If switched go here If NOT switched go here Apply the same pulse 100 – 10,000 times 56th MMM Scottsdale, DA-02

  17. NYU 50% switching boundary A: dynamic parameter, the slope. – Efficiency Ic: zero temperature critical current, the intercept at x-axis. – Threshold Intrinsic damping Icτ Switching Introduction Modeling Switching Double Pulse Net charge (I – Ic)τ 56th MMM Scottsdale, DA-02

  18. NYU 50% switching boundary Switching Introduction Modeling Switching Double Pulse 56th MMM Scottsdale, DA-02

  19. NYU 50% switching boundary Switching Introduction Modeling Switching Double Pulse 56th MMM Scottsdale, DA-02

  20. NYU Field dependence of A andIc Initial state Final state Pulse Switching Introduction Modeling Switching Double Pulse • Possible reasons causing the discrepancy: • Switching process is not uniform • Thermal fluctuations influence the switching process as well 56th MMM Scottsdale, DA-02

  21. NYU 50% switching boundary Short time regime Long time regime Switching Introduction Modeling Switching Double Pulse Cross over regime • Measure with pulse durations over 10 order of magnitude in time • Model yields correct forms at both short and long time limits • Three regimes can be distinguished 56th MMM Scottsdale, DA-02

  22. NYU Probability ~ pulse duration Switching Introduction Modeling Switching Double Pulse short time regime: long time regime: 56th MMM Scottsdale, DA-02

  23. NYU Probability ~ pulse amplitude Switching Introduction Modeling Switching Double Pulse short time regime: long time regime: 56th MMM Scottsdale, DA-02

  24. NYU Probability ~ angular momentum • Probability only depends on the net charge in short time regime • Angular momentum conservation • Refine short time regime net charge: Switching Introduction Modeling Switching Double Pulse 56th MMM Scottsdale, DA-02

  25. Experiments on Relaxation How long does it take for magnetization to relax? NYU 56th MMM Scottsdale, DA-02

  26. NYU Motivation • Why study relaxation dynamics? • Always exists after each switching attempt. • Important information for application. • Not fully explored by experiments. • How to resolve relaxation dynamics? • Fast pulse slow readout: ignore all relaxation information. • Direct time–resolve: fail for small MR or relaxation near equilibrium. • Double pulses: resolve 50 psupward for < 1% MR. Double Pulse Introduction Modeling Switching Relaxation 56th MMM Scottsdale, DA-02

  27. NYU Basic idea Use the second pulse to probe the magnetization dynamics excited by the first pulse tdelay I1 I2 I1 I2 t1 t2 • Apply double pulses • Measure switching probability (SP) • Compare with single pulses SP t1 t2 Double Pulse Introduction Modeling Switching Relaxation 56th MMM Scottsdale, DA-02

  28. NYU Double pulses SP (switching probability) tdelay I1 I1 tdelay I2 t1 • If tdelay is long enough: • Otherwise, tdelay is within the relaxation time of the first pulse. t1 t2 t2 I2 Double Pulse Introduction Modeling Switching Relaxation 56th MMM Scottsdale, DA-02

  29. NYU Experiment – Relaxation to the initial state long delay: short delay: AP AP 1 2 Double Pulse t1 = 1 ns I1 = 8.5 mA Introduction Modeling Switching Relaxation First pulse Second pulse AP AP or P t2 = 0~5 ns I2 = 8.5 mA 56th MMM Scottsdale, DA-02

  30. NYU “Similar” distributions tdelay tdelay I I I I I t2 t1 t1 Double Pulse Introduction Modeling Switching Relaxation True for all t2 56th MMM Scottsdale, DA-02 δt δt + t2

  31. NYU δt ~tdelay tdelay I I t1 LLG: Time scale of the relaxation dynamics Double Pulse Introduction Modeling Switching Relaxation 56th MMM Scottsdale, DA-02 δt

  32. NYU Fokker – Planck simulation • LLG equation fails to explain the data: • δt is linear with tdelayfrom LLG, but exponential from experiments. • Magnetization following LLG equation will relax to θ = 0 instead of a Boltzmann distribution. • Fokker – Planck Equation with uniaxial symmetry: • Thermal fluctuation is important for relaxation dynamics. • Track the whole probability instead of just an average magnetization angle. Double Pulse Introduction Modeling Switching Relaxation Probability to find the magnetization at angle θ at time τ 56th MMM Scottsdale, DA-02

  33. NYU Tracking the probabilities End of the delay Double Pulse Introduction Modeling Switching Relaxation End of the 1st pulse 56th MMM Scottsdale, DA-02

  34. NYU Simulation results Double Pulse Introduction Modeling Switching Relaxation 56th MMM Scottsdale, DA-02

  35. Summary Switching Double Pulse • Experiments with pulse durations from 50 ps to 1 s s. • Short time regime: switching probability only depends on the net charge, a result of angular momentum conservation. • Long time regime: thermal activation over an energy barrier. • Analytic results agrees with experiments at both limits. • Resolve relaxation down to 50 ps with MR ~ 0.3%. • A single pulse + delay gives an identical state as a single pulse with a modified duration δt. • δt exponentially decays with the delay and the lifetime of the decay is 0.28 ns. • Thermal noise needs to be considered for relaxation. 56th MMM Scottsdale, DA-02

  36. Thank you! 56th MMM Scottsdale, DA-02

  37. NYU LLG explanation fails delay First pulse equal Double Pulse Small angle Introduction Modeling Switching Relaxation single pulse with reduced duration 56th MMM Scottsdale, DA-02

  38. NYU Short time limit • Landau-Lifshitz-Gilbert (LLG) equation + STT: • Apply uniaxial symmetry: Modeling Introduction Modeling Switching Relaxation 56th MMM Scottsdale, DA-02

  39. NYU Short time limit • Room Temperature T • Applied field μ0Happ Initial state Final state θτ θ0 current pulse: Modeling θτ > θc: switch θτ < θc: not switch Introduction Boltzmann distribution Modeling Switching Double Pulse 56th MMM Scottsdale, DA-02

  40. NYU LLG solution and approximations Modeling Introduction Modeling Switching Relaxation 56th MMM Scottsdale, DA-02

  41. J. Sun’s approximation vs. ours • Both approximations ignore the second term, which yields a largerτ. • Sun’s approximation also change tan(θ/2) to θ/2 • tan(x) < x for all 0 < x < π / 2 • the difference between tan(x) and x increases as x increases. Which will reduce the value of τ. • Need to compare the increase of τ(both) with the decrease of τ(Sun). τ Ours tan(x) –>x Sun’s no 2nd term Exact solution Approximations tan(x) –>x 56th MMM Scottsdale, DA-02 Sun’s

  42. NYU Results and Agreements Modeling Introduction Modeling Switching Relaxation 56th MMM Scottsdale, DA-02

  43. NYU Results and Agreements Modeling Introduction Modeling Switching Relaxation 56th MMM Scottsdale, DA-02

  44. Introduction damping magnetizationrotation spin torque Introduction Switching Model Relaxation 56th MMM Scottsdale, DA-02

  45. Initial state Final state Pulse 56th MMM Scottsdale, DA-02

  46. NYU NYU Intrinsic damping Ic0τ NYU Introduction Introduction Switching Model Relaxation Net charge (I – Ic0)τ 56th MMM Scottsdale, DA-02

  47. 100 nm Label 56th MMM Scottsdale, DA-02

  48. 100 nm No Label 56th MMM Scottsdale, DA-02

  49. 50 nm label 56th MMM Scottsdale, DA-02

  50. 50 nm No label 56th MMM Scottsdale, DA-02

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