1. Christian Stamm Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center I. Tudosa, H.-C. Siegmann, J. Stöhr (SLAC/SSRL) A. Vaterlaus (ETH Zürich) A. Kashuba (Landau Inst. Moscow) D. Weller, G. Ju (Seagate Technologies) G. Woltersdorf, B. Heinrich (S.F.U. Vancouver) Magnetization dynamics with picosecond magnetic field pulses

2. Why Magnetization Dynamics? constant current alignment parallel to field pulsed current (5 ps) precessional switching

3. Magnetic Field Pulse Relativistic electron bunches from the Stanford Linear Accelerator are focused to ~10 m peak field of ~7 Tesla 10 m from center, falling off with 1/R FWHM = 5 ps

4. Precession torque Gilbert damping torque change in angular momentum Direction of torques Motion of M for constant H Dynamic equation for M Landau- Lifshitz- Gilbert

5. CoCrPt granular media Image of M: Polar Kerr Microscopy (size 150 m) After Magnetic Field Pulse 50 m perpendicular magnetization

6. 1 pulse 3 pulses5 pulses 2 pulses 7 pulses 4 pulses6 pulses Multiple Field Pulses 50 m

7. Transition Region Observed: wide transition region Calculated:sharp transitions Model assuming distribution of initial direction for M

8. Initial Distributions of M Look identical at one point in time Differences appear with multiple pulses Static: angle of anisotropy axes x-ray diffraction: ±4º Dynamic: thermal motion, random fields 10º V=(6.5 nm) 3

9. 2 Field Pulses static distribution is deterministic 2 pulses should reverse not observed dynamic distribution is stochastic independent switching probability for each pulse YES 50 m

10. Stochastic Switching Independent stochastic events: calculate switching by successive multiplication M 2 = M 1 · M 1 M 3 = M 2 · M 1 : M n = (M 1 ) n

11. Conclusions A picosecond fast magnetic field pulse causes the magnetization to precess and - if strong enough - switch its direction In granular perpendicular magnetic media, switching on the ps time scale is influenced by stochastic processes Possible cause is the excitation of the spin system due to inhomogeneous precession in the large applied field

12. Epitaxial Fe / GaAs SEMPA images of M (SEM with Polarization Analysis) one magnetic field pulse 50 m M0M0 GaAs (001) Fe 10 or 15 layers Au 10 layers

13. Epitaxial Fe layer GaAs (001) Fe 10 or 15 layers Au 10 layers Fe / GaAs (001) FMR characterization: damping = 0.004 and anisotropies (G. Woltersdorf, B. Heinrich) Kerr hysteresis loop H C = 12 Oe

14. Images of Fe / GaAs SEMPA images of M (SEM with Polarization Analysis) one magnetic field pulse 10 ML Fe / GaAs (001) 50 m M0M0

15. Thermal Stability Important aspect in recording media Néel-Brown model (uniform rotation) Probability that grain has not switched: withand for long-term stability:

16. Comparison of Patterns Observed (SEMPA) Calculated (fit using LLG) Anisitropies same as FMR Damping  = 0.017 4x larger than FMR WHY? 100 m

17. Energy Dissipation After field pulse: Damping causes dissipation of energy during precession (energy barrier for switching: K U )

18. Enhanced Damping Precessing spins in ferromagnet: Tserkovnyak, Brataas, Bauer Phys Rev Lett 88, 117601 (2002) Phys Rev B 66, 060404 (2002) source of spin current pumped across interface into paramagnet causes additional damping spin accumulation 1º in FMR, but  110º in our experiment

19. Effective Field H 3 components of H act on M H D = -M S demagnetizing field H K = 2K/ 0 M S crystalline anisotropy H E externally applied field M HEHE HDHD HKHK

20. Magnetic Field Strength 10 10 electrons: B * r = 50 Tesla * m duration of magnetic field pulse: 5 ps

21. Perpendicular Magnetization perpendicular anisotropy M 0 =(0, 0, -M S ) 5 ps field pulse 2.6 Tesla precession and relaxation towards (0, 0, +M S ) Time evolution

22. Granular CoCrPt Sample Size of grains  8.5 nm Paramag. envelope  1 nm 1 bit  100 grains TEM of magnetic grains

23. Radial Dependence of M Perpendicular magnetized sample (CoCrPt alloy)

24. In-Plane Magnetization switching by precession around demagnetizing field after excitation by 5 ps field pulse 0.27 Tesla (finished at *) (uniaxial in-plane) Time evolution of M

25. Precessional Torque: MxH in-plane magnetized sample: figure-8 pattern circular in-plane magnetic field H M lines of constant (initial) torque MxH

26. Magnetization Reversal Magnetization is Angular Momentum Applying torque changes its direction immediate response to field Fastest way to reverse the magnetization: initiate precession around magnetic field patented by IBM H M0M0 M(t)

27. Picosecond Field Pulse Generated by electron bunch from the Stanford Linear Accelerator data from: C.H. Back et al. Science 285, 864 (1999)

28. Outline Magnetization Dynamics: What is precessional switching? How do we generate a picosecond magnetic field pulse? Magnetization reversal in granular perpendicular media Enhanced Gilbert damping in epitaxial Fe / GaAs films

29. Co/Pt multilayer magnetized perpendicular Domain pattern after field pulse from: C.H. Back et al., PRL 81, 3251 (1998): MOKE – line scan through center switching at 2.6 Tesla Previously: Strong Coupling