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H. C. Siegmann, C. Stamm, I. Tudosa, Y. Acremann ( Stanford )

On the Ultimate Speed of Magnetic Switching Joachim Stöhr Stanford Synchrotron Radiation Laboratory. Collaborators:. H. C. Siegmann, C. Stamm, I. Tudosa, Y. Acremann ( Stanford ). A. Vaterlaus (ETH Z. ü. rich). magnetic imaging. A. Kashuba (Landau Inst. Moscow) ; A. Dobin (Seagate).

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H. C. Siegmann, C. Stamm, I. Tudosa, Y. Acremann ( Stanford )

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  1. On the Ultimate Speed of Magnetic Switching Joachim Stöhr Stanford Synchrotron Radiation Laboratory Collaborators: H. C. Siegmann, C. Stamm, I. Tudosa, Y. Acremann ( Stanford ) A. Vaterlaus (ETH Z ü rich) magnetic imaging A. Kashuba (Landau Inst. Moscow) ; A. Dobin (Seagate) theory D. Weller, G. Ju, B.Lu (Seagate Technologies) samples G. Woltersdorf, B. Heinrich (S.F.U. Vancouver)

  2. The Technology Problem: Smaller and Faster The ultrafast technology gap want to reliably switch small magnetic “bits”

  3. 186 years of “Oersted switching”…. How can we switch faster ?

  4. Faster than 100 ps…. Mechanisms of ultrafast transfer of energy and angular momentum Optical pulse Shockwave IR or THz pulse t~ 1 ps Electrons Phonons t= ? t ~ 100 ps Spin Most direct way: Oersted switching Precessional or ballistic switching Exchange switching (spin injection)

  5. Precessional or ballistic switching: Discovery

  6. Creation of large, ultrafast magnetic fields Conventional method - too slow Ultrafast pulse – use electron accelerator C. H. Back et al., Science285, 864 (1999)

  7. Torques on in-plane magnetization by beam field Initial magnetization of sample Max. torque Min. torque Fast switching occurs when HM ┴

  8. Precessional or ballistic switching: 1999 Patent issued December 21, 2000: R. Allenspach, Ch. Back and H. C. Siegmann

  9. Precessional switching case 1: Perpendicular anisotropy sample I. Tudosa, C. Stamm, A.B. Kashuba, F. King, H.C. Siegmann, J. Stöhr, G. Ju, B. Lu, and D. Weller Nature 428, 831 (2004)

  10. M End of field pulse The simplest case: perpendicular magnetic medium

  11. Pattern of perpendicular anisotropy sample CoCrPt perpendicular recording media (Seagate)

  12. Multiple shot switching of perpendicular sample CoCrPt recording film Light areas mean M Dark areas mean M Tudosa et al., Nature 428, 831 (2004)

  13. Data analysis Intensity profiles thru images Landau-Lifshitz- Gilbert theory Experiment curve = M1 1 = white 1 shot 0 = gray Landau-Lifshitz- Gilbert theory -1 = dark Experiment curve = (M1)2 curve = (M1)3 3 shots 2 shots curve = (M1)4 curve = (M1)5 5 shots 4 shots curve = (M1)6 curve = (M1)7 6 shots 7 shots Multiplicative probabilities are signature of a random variable. Analysis reveals a memory-less process.

  14. Magnetization fracture under ultrafast field pulse excitation Non-deterministic region partly due to fractured magnetization

  15. Magnetization fracture under ultrafast field pulse excitation Macro-spin approximation uniform precession Magnetization fracture moment de-phasing Breakdown of the macro-spin approximation Tudosa et al., Nature 428, 831 (2004)

  16. Precessional switching case 2: In-plane anisotropy sample C. Stamm, I. Tudosa, H.C. Siegmann, J. J. Stöhr, A. Yu. Dobin, G. Woltersdorf, B. Heinrich and A. Vaterlaus Phys. Rev. Lett. 94, 197603 (2005)

  17. In-Plane Magnetization: Pattern development • Magnetic field intensity is large • Precisely known field size Rotation angles: 540o 180o 720o 360o

  18. Origin of observed switching pattern 15 layers of Fe/GaAs(110) H increases g • In macrospin approximation, line positions depend on: • angle g = B t • in-plane anisotropy Ku • out-of-plane anisotropy K┴ • LLG damping parameter a from FMR data

  19. Breakdown of the Macrospin Approximation H increases With increasing field, deposited energy far exceeds macrospin approximation this energy is due to increased dissipation or spin wave excitation

  20. Breakdown of the macrospin approximation: why? Experiments reveal breakdown for short pulse lengthtand large B peak power deposition ~ B2 / t = a B / t2 Breakdown appears to be caused by peak power induced fracture of magnetization – non-linear excitations of spin system

  21. Conclusions • The breakdown of the macrospin approximation for • fast field pulses limits the reliability of magnetic switching • Breakdown is believed to arise from energy & angular momentum • transfer within the spin system – excitation of higher spin wave modes • Details are not well understood…. For more, see:http://www-ssrl.slac.stanford.edu/stohr and J. StöhrandH. C. Siegmann Magnetism: From Fundamentals to Nanoscale Dynamics 800+ page textbook ( Springer, to be published in Spring 2006 )

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