X-ray Imaging of Magnetic Nanostructures and their Dynamics Joachim Stöhr Stanford Synchrotron Radiation Laboratory X-Rays have come a long way…… 1 mm.
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X-ray Imaging of Magnetic Nanostructures and their Dynamics Joachim Stöhr Stanford Synchrotron Radiation Laboratory X-Rays have come a long way…… 1 mm 1895 1993 2003 Andreas Bauer1,2 Andreas Scholl1 Jan Lüning2 Howard A. Padmore1 Andrew Doran1 2 Aaron Lindenberg3 Yves Acremann Sug-Bong Choe1 Hendrik Ohldag2 Squaw Valley, April 2003 1 Advanced Light Source 2 Stanford Synchrotron Radiation Laboratory 3 UC Berkeley Fe metal – L edge Kortright and Kim, Phys. Rev. B 62, 12216 (2000) Soft X-Rays are best for magnetism! Imaging by Coherent X-Ray Scattering Phase problem can be solved by “oversampling” speckle image Transmission X-ray Microscope Reconstruction from Speckle Intensities 5 mm (different areas) S. Eisebitt, M. Lörgen, J. Lüning, J. Stöhr, W. Eberhardt, E. Fullerton (unpublished) Magnetic Spectroscopy and Microscopy Spectromicroscopy of Ferromagnets and Antiferromagnets AFM domain structure at surface of NiO substrate s [010] s 2mm NiO XMLD 0.10 8 TEY (a.u.) TEY (a.u.) 0.15 0.05 Co XMCD 4 0 0.00 868 870 872 874 Photon Energy(eV) 777 778 779 Photon Energy (eV) H. Ohldag, A. Scholl et al., Phys. Rev. Lett. 86(13), 2878 (2001). FM domain structure in thin Co film on NiO substrate Magnetic characterization of interfacial spins Co/NiO Co/IrMn Co 15 Co 10 XMCD Asymmetry (%) 5 NiO 0 -5 -10 -15 0.3 Mn 0.2 0.1 0.0 -0.1 Publications: 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) -0.2 -0.3 -3k -2k -1k 0 1k 2k 3k Applied Field (Oe) loop of interfacial spins only 4% are pinned Exchange Bias Model from X-Rays ideal AFM poly AFM Present limitations of magnetic recording • Present method of magnetic switching is unfavorable: – present recording time ~1 ns – unfavorable torque and dependent on thermal activation Fast Magnetization Dynamics is governed by Landau-Lifschitz-Gilbert equation: 1 dM - M H dt Angular momentum change 1 M Precession torque 1 Tesla field: 90o rotation in 10 ps M dM dt Gilbert damping torque Typically << 1, 100 ps We want to understand on atomic level controls switching time, ~1 optimal Time Resolved X-Ray Microscopy Laser pump – x-ray probe synchronization excitation laser pulse t observation x-ray pulse 328 ns < 1 ps < 100 ps Production of Magnetic Field Pulses Photoconductive switch 100 mm 100 mm 2 mm 2 mm H ~ 200 Oe Conducting wire 50 => I = 200 mA, 10 V bias Magnetic Cells Current 10 mm Sample and Magnetic Field Pulse 20 nm Co90Fe10 films with in-plane anisotropy (1 mm) x (1-3 mm) rectangles Current magnetic field 1.06 Magnetic Field Pulse ~ 150 Oe at Maximum < 50 ps rising time > 300 ps decaying time with some reflection Deflection ratio M 1.04 150 100 1.02 50 1.00 0 0 2000 4000 Delay (ps) 6000 8000 Magnetic field (Oe) 200 Observation of Vortex Motion H 2 mm x 1 mm 1.5 mm x 1 mm 400 100 y displacement (nm) y displacement (nm) 1 mm x 1 mm 50 0 -50 -50 0 50 x displacement (nm) 100 200 Vortices rotate oppositely - vortex cores point in opposite directions 0 -200 0 200 400 600 x displacement (nm) Vortex speed ~ 100 m/s 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 The End Vortex Structure And Vortex Elevation view Motion torque H Plane view d m m H eff dt Landau-Lifshitz equation: (neglect damping) Motion antiparallel to field! The field acts like a screw driver. Depending on the orientation of the thread pitch, the screw (vortex) will move either forward or backward Vortex Precession M Under a field pulse, the vortex moves from the center. Happlied 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. Hmagnetostatic x dx H dH Magnetostatic field is always perpendicular to the vortex deviation Vortex will precess forever if there is no damping. Incoherent vs. Coherent X-Ray Scattering Small Angle Scattering -40 40 -20 0 20 scattering vector q (mm-1) Coherence length larger than domains, but smaller than illuminated area information about domain statistics 20 0 -20 -40 40 log (intensity) -40 -20 0 20 40 scattering vector q (mm-1) Coherence length larger than illuminated area -40 40 -20 0 20 scattering vector q (mm-1) Speckle true information about domain structure 20 0 -20 -40 40 log (intensity) -40 -20 0 20 scattering vector q (mm-1) 40 Pulse Structure Possible solutions: - gated detector, pulse picker - pump at 500 MHz