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Stanford - SSRL : J. Lüning W. Schlotter H. C. Siegmann Y. Acremann ...students

Probing Magnetization Dynamics with Soft X-Rays Joachim Stöhr Stanford Synchrotron Radiation Laboratory. Collaborators:. Stanford - SSRL : J. Lüning W. Schlotter H. C. Siegmann Y. Acremann ...students. Berlin - BESSY : S. Eisebitt M. Lörgen O. Hellwig W. Eberhardt.

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Stanford - SSRL : J. Lüning W. Schlotter H. C. Siegmann Y. Acremann ...students

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  1. Probing Magnetization Dynamics with Soft X-Rays Joachim Stöhr Stanford Synchrotron Radiation Laboratory Collaborators: Stanford - SSRL: J. Lüning W. Schlotter H. C. Siegmann Y. Acremann ...students Berlin - BESSY: S. Eisebitt M. Lörgen O. Hellwig W. Eberhardt Berkeley - ALS: A. Scholl magnetization dynamics

  2. Today Lasers X-rays Ultrafast

  3. Present: Pump/Probe Experiments laser pump pulse x-ray probe pulse after  sample …over and over… • Can produce various pump pulses: • heat electrons (photon pulse) • kick magnetization (field pulse) • heat lattice (pressure pulse) Process has to be reversible: Not enough intensity for single shot experiments

  4. Non-Equilibrium Magnetization Dynamics transfer of energy and angular momentum tl= 1 ps te < 0.1ps tel = 1 ps Optical excitation Pressure Electrons Lattice tes tsl = 100 ps ts Spin Magnetic field (Oersted switching) Spin injection (Spin-torque switching) Science: Mechanisms of energy and angular momentum transfer Technology: How fast can we switch reliably?

  5. Optical excitation: Ni metal MOKE: pump-probe E. Baurepaire, J. C. Merle, A. Daunois, and J. Y. BigotPRL 76,4250 (1996)

  6. Optical excitation studies benefit from x-ray probe • spin moments, • orbital moments • nanoscale resolution But – need sub- 50-ps time resolution: streak camera, slicing, XFELs

  7. Magnetic Excitation - Today: Imaging of Nanoscale Magnetization Dynamics Pump / probe requires reversibility of excitation process - not enough intensity to obtain single shot images Cannot presently study irreversible processes: this would require single shot pictures

  8. Magnetic Patterns in 20 nm Co90Fe10 films on waveguide M 3mm Field pulse x-ray "spin" S.-B. Choe, Y. Acremann, A. Scholl, A. Bauer, A. Doran, J. Stöhr, H.A. Padmore, Science 304, 430 (2004)

  9. Nanaoscale Magnetization Dynamics - Smaller and Faster

  10. STXM image of spin injection structure STXM image of device 1. Cu lead delivers current to device • Pt lead guides current to bottom of spin injection pillar 3. Current flows through pillar, switching second ferromagnet 4. Current extracted through second Cu lead Challenge is nanoscale sample production – pillar diameter is < 100 nm

  11. Why are irreversible or random processes important? • Two examples: • Technology: • Reversible processes are required for technology • but • Irreversible processes determine technological limit ! • Science: • Equilibrium magnetization dynamics at Curie temperature • Critical magnetizattion fluctuations are random Such studies require single snapshot images !

  12. Technology:Exploring the ultimate speed of magnetic recording In R&D labs we can only produce 100 ps pulses – things still work fine Unique method for faster switching 100 fs – 10 ps Relativity allows packing 1010electrons into “bunch” → Ultra-short and high field pulses (up to 5 T) Tudosa et al. Nature 428, 831 (2004)

  13. The Ultimate Speed of Magnetic Switching tpulse= 3 ps tpulse= 100 fs 90 mm 90 mm Deterministic switching Chaotic switching Under ultrafast excitation the magnetization fractures ! Tudosa et al. Nature 428, 831 (2004) and unpublished

  14. Size of spin blocks? Fluctuation time? Science:Equilibrium Magnetization Dynamics critical fluctuations Tc Magnetization Temperature ?

  15. Toward femtosecond magnetic motion pictures …..

  16. not enough intensity – need to repeat over and over

  17. Lensless Imaging by Coherent X-Ray Scattering or “Speckle“ 5 mm coherent x-rays Eisebitt et al. (BESSY) With present sources it takes minutes to take an image Challenge: Inversion from reciprocal to real space image

  18. Transmission X-ray Microscopy Reconstruction from Speckle Intensities Loss of x-ray phase complicates image reconstruction In principle: phase problem can be solved by “oversampling” speckle image  5 m (different areas)

  19. Toward single-shot imaging: soft x-rayspectro-holography coherent x-ray beam S. Eisebitt, J. Lüning, W. F. Schlotter, M. Lörgen, O. Hellwig, W. Eberhardt, J. Stöhr, Nature (in press)

  20. Digital Image Reconstruction FFT (Difference) Difference (RCP – LCP) saturated lin. scale Convolution theorem applied to diffraction: FT(diffraction) = Autocorrelation (Object)

  21. * * W * B Is it real? FT Hologram STXM W. F. Schlotter Y. Acremann Reference hole  100 nm Resolution 30 - 40 nm

  22. Motion pictures with multiple pulses mirror mirror Pulse 1 Pulse 1 CCD 2 CCD 2 XFEL XFEL CCD 1 CCD 1 beam splitter beam splitter Pulse 2 Pulse 2 sample sample delay - change of optical path length Present detectors not fast enough for multiple images

  23. For more, see: http://www-ssrl.slac.stanford.edu/stohr/index.htm and J. StöhrandH. C. Siegmann Magnetism: From Fundamentals to Nanoscale Dynamics Springer 2005 (to be published)

  24. Lake Tahoe, April 2004

  25. The end

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