1. X-ray Imaging of Magnetic Nanostructures and their Dynamics Joachim Stöhr Stanford Synchrotron Radiation Laboratory 18951993 X-Rays have come a long way…… 2003 1  m

2. Sug-Bong Choe 1 Yves Acremann 2 Andreas Bauer 1,2 Andreas Scholl 1 Andrew Doran 1 Aaron Lindenberg 3 Howard A. Padmore 1 1 Advanced Light Source 2 Stanford Synchrotron Radiation Laboratory 3 UC Berkeley Hendrik Ohldag 2 Squaw Valley, April 2003 Jan Lüning 2

5. Kortright and Kim, Phys. Rev. B 62, 12216 (2000) Fe metal – L edge Soft X-Rays are best for magnetism!

6. Transmission X-ray Microscope Reconstruction from Speckle Intensities  5  m (different areas) Imaging by Coherent X-Ray Scattering Phase problem can be solved by “oversampling” speckle image S. Eisebitt, M. Lörgen, J. Lüning, J. Stöhr, W. Eberhardt, E. Fullerton (unpublished)

7. Magnetic Spectroscopy and Microscopy

9.  m [010]   NiO XMLD Co XMCD Spectromicroscopy of Ferromagnets and Antiferromagnets AFM domain structure at surface of NiO substrate FM domain structure in thin Co film on NiO substrate H. Ohldag, A. Scholl et al., Phys. Rev. Lett. 86(13), 2878 (2001).

10. Co/NiO Magnetic characterization of interfacial spins loop of interfacial spins - only 4% are pinned Co NiO Stöhr et al., Phys. Rev. Lett. 83, 1862 (1999) Thomas et al., Phys. Rev. Lett. 84, 3462 (2000) Scholl et al., Science 287, 1014 (2000) Nolting et al., Nature 405, 707 (2000) Regan et al. Phys. Rev. B 64, 214422 (2001) Ohldag et al., Phys. Rev. Lett. 86 2878 (2001) Ohldag et al., Phys. Rev. Lett. 87, 247201 (2001) Ohldag et al., Phys. Rev. Lett. 91, 017203 (2003) Publications: Co/IrMn

11. Exchange Bias Model from X-Rays ideal AFMpoly AFM

12. Present limitations of magnetic recording Present method of magnetic switching is unfavorable: –present recording time ~1 ns –unfavorable torque and dependent on thermal activation

13. Fast Magnetization Dynamics is governed by Landau-Lifschitz-Gilbert equation: Precession torqueGilbert damping torque Angular momentum change Typically  100 ps We want to understand  on atomic level  controls switching time,  ~1 optimal 1 Tesla field: 90 o rotation in 10 ps

14. Time Resolved X-Ray Microscopy Laser pump – x-ray probe synchronization < 1 ps < 100 ps 328 ns tt excitation laser pulse observation x-ray pulse

15. Production of Magnetic Field Pulses 100  m 2  m Photoconductive switch Current Conducting wire Magnetic Cells 10  m H ~ 200 Oe 50  => I = 200 mA, 10 V bias

16. Current magnetic field Sample and Magnetic Field Pulse M Magnetic Field Pulse ~ 150 Oe at Maximum < 50 ps rising time > 300 ps decaying time with some reflection 20 nm Co 90 Fe 10 films with in-plane anisotropy (1  m) x (1-3  m) rectangles

17. Observation of Vortex Motion 1  m x 1  m 2  m x 1  m H Vortex speed ~ 100 m/s 1.5  m x 1  m Vortices rotate oppositely - vortex cores point in opposite directions

18. Conclusions The challenge of the future is to control the magnetization on the nanometer length scale and picosecond/femtosecond time scale Our current capabilities are: image the magnetization with 50 nm spatial resolution, image the response of the magnetization with 100 ps time- and 100 nm spatial resolution Outlook into the future: 5 nm spatial resolution – PEEM3, under construction 100 fs time resolution: pump-probe excitations single snapshots of equilibrium dynamics Modern x-ray sources offer unique opportunities for studies of the ultrafast magnetic nanoworld

19. The End

20. Vortex Structure And Vortex Motion H Plane view Elevation view Motion antiparallel to field! torque Landau-Lifshitz equation: (neglect damping) The field acts like a screw driver. Depending on the orientation of the thread pitch, the screw (vortex) will move either forward or backward

21. Magnetostatic field is always perpendicular to the vortex deviation Vortex Precession Under a field pulse, the vortex moves from the center. H applied H magnetostatic M After the field pulse, the vortex continues to move radially due to the magnetostatic energy. Induced magnetostatic field is always perpendicular to the vortex motion. Vortex will precess forever if there is no damping. H dH x dx

22. Small Angle Scattering Coherence length larger than domains, but smaller than illuminated area Speckle Coherence length larger than illuminated area Incoherent vs. Coherent X-Ray Scattering -40-2002040 -40 -20 0 20 40 scattering vector q (  m -1 ) -40-2002040 -40 -20 0 20 40 scattering vector q (  m -1 ) information about domain statistics true information about domain structure

23. Pulse Structure Possible solutions: - gated detector, pulse picker - pump at 500 MHz