X-ray Imaging of Magnetic Nanostructures and their Dynamics

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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
2003
1895
1993
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Andreas Bauer1,2
Jan Lüning2
Andreas Scholl1
Howard A. Padmore1
Yves Acremann2
Aaron Lindenberg3
Andrew Doran1
Sug-Bong Choe1
Hendrik Ohldag2
Squaw Valley, April 2003
1 Advanced Light Source 2 Stanford Synchrotron
Radiation Laboratory 3 UC Berkeley
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Fe metal L edge
Kortright and Kim, Phys. Rev. B 62, 12216 (2000)
Soft X-Rays are best for magnetism!
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Imaging by Coherent X-Ray Scattering
Phase problem can be solved by oversampling
speckle image
? 5 ?m (different areas)
S. Eisebitt, M. Lörgen, J. Lüning, J. Stöhr, W.
Eberhardt, E. Fullerton (unpublished)
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Magnetic Spectroscopy and Microscopy
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Spectromicroscopy of Ferromagnets and
Antiferromagnets
AFM domain structure at surface of NiO substrate
FM domain structure inthin Co film on NiO
substrate
NiO XMLD
Co XMCD
H. Ohldag, A. Scholl et al., Phys. Rev. Lett.
86(13), 2878 (2001).
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Magnetic characterization of interfacial spins
Co/NiO
Co/IrMn
Co
NiO
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)
loop of interfacial spins - only 4 are pinned
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Exchange Bias Model from X-Rays
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Present limitations of magnetic recording

• Present method of magnetic switching is
  unfavorable
• present recording time 1 ns
• unfavorable torque and dependent on thermal
  activation

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Fast Magnetization Dynamics is governed by
Landau-Lifschitz-Gilbert equation
Angular momentum change
Precession torque
Gilbert damping torque
Typically a ltlt 1, 100 ps
1 Tesla field 90o rotation in 10 ps
We want to understand a on atomic level a
controls switching time, a1 optimal
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Time Resolved X-Ray Microscopy
Laser pump x-ray probe synchronization
lt 1 ps
excitation laser pulse
lt 100 ps
observation x-ray pulse
?t
328 ns
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Production of Magnetic Field Pulses
Photoconductive switch
H 200 Oe
Conducting wire
50 ? gt I 200 mA, 10 V bias
Magnetic Cells
Current
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Sample and Magnetic Field Pulse
20 nm Co90Fe10 films with in-plane anisotropy (1
?m) x (1-3 ?m) rectangles
Current
magnetic field
Magnetic Field Pulse 150 Oe at Maximum lt 50
ps rising time gt 300 ps decaying time
with some reflection
M
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Observation of Vortex Motion
H
1 mm x 1 mm
2 mm x 1 mm
1.5 mm x 1 mm

• Vortices rotate oppositely
• vortex cores point in opposite directions

Vortex speed 100 m/s
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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

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The End
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Vortex Structure And Vortex Motion
Elevation view
torque
H
Plane view
Motion antiparallel to field!
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
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Vortex Precession
Under a field pulse, the vortex moves from the
center.
M
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
Magnetostatic field is always perpendicular to
the vortex deviation
Vortex will precess forever if there is no
damping.
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Incoherent vs. Coherent X-Ray Scattering
Small Angle Scattering Coherence length larger
than domains, but smaller than illuminated area
information about domain statistics
Speckle Coherence length larger than illuminated
area
true information about domain structure
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Pulse Structure
Possible solutions - gated detector, pulse
picker - pump at 500 MHz