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Magnetization dynamics with picosecond magnetic field pulses

Magnetization dynamics with picosecond magnetic field pulses. 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)

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Magnetization dynamics with picosecond magnetic field pulses

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  1. Magnetization dynamics with picosecond magnetic field pulses 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)

  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 mm peak field of ~7 Tesla 10 mm from center, falling off with 1/R FWHM = 5 ps

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

  5. After Magnetic Field Pulse perpendicularmagnetization CoCrPt granular media Image of M: Polar Kerr Microscopy (size 150 mm) 50 mm

  6. Multiple Field Pulses 1 pulse 3 pulses 5 pulses 7 pulses 50 mm 2 pulses 4 pulses 6 pulses

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

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

  9. 2 Field Pulses • static distribution isdeterministic2 pulses should reverse • not observed • dynamic distribution is stochasticindependent switching probability for each pulse • YES 50 mm

  10. Stochastic Switching Independent stochastic events: calculate switching by successive multiplication M2 = M1 · M1 M3 = M2 · M1 : Mn = (M1)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 mm M0 Au 10 layers Fe 10 or 15 layers GaAs (001) 50 mm

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

  14. Images of Fe / GaAs SEMPA images of M (SEM with Polarization Analysis) one magnetic field pulse 10 ML Fe / GaAs (001) M0 50 mm 50 mm 50 mm

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

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

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

  18. Enhanced Damping Precessing spins in ferromagnet: Tserkovnyak, Brataas, BauerPhys 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 q1º in FMR, but q 110º in our experiment

  19. Effective Field H 3 components of H act on M HEexternally applied field HD = -MS demagnetizing field M HE HK = 2K/m0MS crystalline anisotropy HK HD

  20. Magnetic Field Strength 1010 electrons: B * r = 50 Tesla * mm duration of magnetic field pulse: 5 ps

  21. Perpendicular Magnetization Time evolution perpendicular anisotropy M0=(0, 0, -MS) 5 ps field pulse2.6 Tesla precession and relaxation towards (0, 0, +MS)

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

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

  24. In-Plane Magnetization Time evolution of M switching by precession around demagnetizing field after excitation by 5 ps field pulse0.27 Tesla(finished at *) (uniaxial in-plane)

  25. Precessional Torque: MxH in-plane magnetized sample: figure-8 pattern M circular in-plane magnetic field H 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 reversethe magnetization: initiate precession around magnetic field patented by IBM H M0 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. Previously: Strong Coupling 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

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