Switching and Equilibration in All-Perpendicular Spin Valves Subject to Short Current Pulses

1. Switching and Equilibration in All-Perpendicular Spin Valves Subject to Short Current Pulses D. Bedau1, H. Liu1*, 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 March Meeting 2011, Dallas TX

2. Outline March Meeting 2011, Dallas TX

3. Motivation • Why study relaxation dynamics? • Always existsafter each switching attempt. • Important information for application. • Not fully explored by experiments. • Which experimental method to use? • Traditional pump – probe: ignore all relaxation information. • Direct time – resolve: fail for small MR or relaxation near equilibrium. • Pump – probe + double pulses: resolve 50 psupward for < 1% MR. • Why all perpendicular spin valves? • Larger current overdrive (I / Ic0) than MTJs. • Simple dynamics– separation of switching and precession. • Slower(switching) motion – timescale suitable for the method. March Meeting 2011, Dallas TX

4. Sample structure A typical 100 nm x 100 nm sample Small MR = 0.3% CoNi easy axis Cu CoNi/CoPt March Meeting 2011, Dallas TX

5. Basic idea Use the second pulse to probe the magnetization dynamics excited by the first pulse delay I1 I2 I1 I2 t1 t2 t1 t2 • Apply double pulses • Measure switching probability (SP) • Compare with single pulses SP March Meeting 2011, Dallas TX

6. Double pulses SP (switching probability) delay I1 I1 delay I2 t1 t1 t2 t2 • If the delay is long enough: • Otherwise, the delay is within the relaxation time of the first pulse. I2 March Meeting 2011, Dallas TX

7. Experiment – Relaxation to the initial state long delay: AP AP 1 2 t1 = 1 ns I1 = 8.5 mA First pulse Second pulse AP AP or P t2 = 0~5 ns I2 = 8.5 mA March Meeting 2011, Dallas TX

8. Analyze – collapse SP distributions delay delay I I I I I t2 t1 t1 True for all t2 March Meeting 2011, Dallas TX δt δt + t2

9. Exponential fitting to the data delay I I t1 March Meeting 2011, Dallas TX δt

10. delay a a a delay Angular momentum– the only variable t1 a t2 t2 + δt a t1 δt • Sample takes angular momentum linearly with time during the pulse t1 • Switching probability in short time regime only depends on the net angular momentum (L) transferred from the spin polarized current[1], [2] • The sample dissipates angular momentum exponentially with time during the delay • Bedau et al., Appl. Phys. Lett. 96 022514 (2010) • Bedau et al., Appl. Phys. Lett. 97 262502 (2010) March Meeting 2011, Dallas TX

11. Modeling L: angular momentum in the sample L2 L0 L1 delay first pulse equal L0 single pulse L2 From fitting Good agreement within experimental accuracy Fixed input March Meeting 2011, Dallas TX

12. Conclusion • Method • Fast – resolve dynamics with timescale of 50 psupward • Sensitive – Change of readout signal < 1% of total signal • Experiments and analysis • 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 • Modeling • Sample gains angular momentum linearly with time from I • Sample dissipates angular momentum exponentially with time during the delay March Meeting 2011, Dallas TX

13. Thank you! March Meeting 2011, Dallas TX

14. Fokker – Planck simulation • FP equation for single domain with uniaxial symmetry • Notations: probability density of the magnetization zero temperature critical current easy axis dimensionless energy barrier Intrinsic dynamic time March Meeting 2011, Dallas TX

15. Fokker – Planck simulation • Initial condition – Boltzmann distribution for one potential well: • Parameter values used in the simulation[1],[2]: normalization constant • Bedau et al., Appl. Phys. Lett. 96 022514 (2010) • Bedau et al., Appl. Phys. Lett. 97 262502 (2010) March Meeting 2011, Dallas TX

16. Probability density change An indicator of the angular momentum taken transferred into the system by the current. March Meeting 2011, Dallas TX

17. Similar results as experiments March Meeting 2011, Dallas TX

18. Measure switching probability — the pump–probe method • Information we cannot resolve: • Relaxation to the final state of the sample after the pulse if it has switched. • Or relaxation back to the initial state if it has not switched. Measure Rac before pulse Pulse (~ ns) Measure Rac after pulse ~ -100 ms 0 ~ 100 ms March Meeting 2011, Dallas TX

19. Results • Good agreement • δt exponentially decreases with delay in both experiments and modeling • From the fitting: • In the measurement: • Obtain the damping coefficient • Assume: • From other experiments[1], [2]: • Reasonable damping coefficient: • Bedau et.al. Appl. Phys. Lett. 96 022514 (2010) • Bedau et.al. Appl. Phys. Lett. 97 262502 (2010) March Meeting 2011, Dallas TX

20. Conclusion • We can resolve intrinsic time scales of magnetic dynamics(50 psupward) even when the change of the readout signal is only several thousandth of the total signal. • A single pulse followed by a delay leaves the sample in an identical state as a single pulse with a modified duration δt would. δt depends exponentially on the delay. • The sample relaxes to equilibrium by dissipating angular momentum exponentially with time, and the lifetime of the angular momentum dissipation is 0.28 ns. March Meeting 2011, Dallas TX

21. Relax to the final state – design Second pulse First pulse long delay: t1 = 0.9 ns I1 = 14.1 mA P P P AP t2 = 5 ns I2 = -11.3 mA March Meeting 2011, Dallas TX

22. Relax to the final state – results • We can resolve relaxation dynamics within ~ 100 ps • The relaxation time after the first pulse is ~ 0.5 ns March Meeting 2011, Dallas TX

23. Relax to the initial state – results • Switching probability (SP) decreases with longer delay • SP distributions collapse when delay is 0.5 ns or larger March Meeting 2011, Dallas TX

24. Analyze – collapse SP distributions delay I I I t2 t1 Red curve: P depends on the duration of both pulses (d1, d2) and delay Blue curve: universal for delay > 0.5 ns, only depends on d2 March Meeting 2011, Dallas TX δt + t2

25. Double pulses – to resolve relaxation time delay I1 I2 t1 t2 First pulse delay second pulse March Meeting 2011, Dallas TX

26. Fokker – Planck simulation • FP Equation: • Initial condition: is the angle between the magnetization and the easy axis, is the probability density of the magnetization at the angle and a time . is the zero temperature critical current. is the dimensionless energy barrier , and , where is the energy barrier, is the saturation magnetization, is the volume, is the Boltzmann constant, is the temperature, is the damping coefficient, is the gyromagnetic ratio, is the magnetic constant and is the zero temperature coercive field. Where is a normalization constant so that March Meeting 2011, Dallas TX

27. Types of measurements Relaxation to the final state if the sample has switched Relaxation to the initial state if the sample has not switched March Meeting 2011, Dallas TX

28. Modeling How does δt depend on delay ? LLG equation Angular momentum accumulation and dissipation March Meeting 2011, Dallas TX

29. From LLG equation • LLG equation with spin torques: • In our experiment, no applied field and initially the two magnetic layers are in AP state. • Solve the LLG equation within a macro-spin frame and linearize the result. March Meeting 2011, Dallas TX

30. From LLG equation II delay First pulse equal Small angle single pulse March Meeting 2011, Dallas TX

31. Apply DC Current Another way to change the initial condition March Meeting 2011, Dallas TX

32. Duration scans for different Idcs March Meeting 2011, Dallas TX

33. Analysis March Meeting 2011, Dallas TX

34. Switching in short time regime Universal behavior when duration is less than about 5 ns Switching Probability distribution only depends on the net charge March Meeting 2011, Dallas TX

35. P vs delay for different DC currents March Meeting 2011, Dallas TX

36. Fitting parameters March Meeting 2011, Dallas TX

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38. Free Layer Free Layer Free Layer Hard Layer Hard Layer Hard Layer March Meeting 2011, Dallas TX