X-ray Imaging of Magnetic Nanostructures and their Dynamics
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Transcript of 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
1895 1993
X-Rays have come a long way……
2003
1 m
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Sug-Bong Choe1 Yves Acremann2
Andreas Bauer1,2 Andreas Scholl1
Andrew Doran1 Aaron Lindenberg3
Howard A. Padmore1
1 Advanced Light Source2 Stanford Synchrotron Radiation Laboratory 3 UC Berkeley
Hendrik Ohldag2
Squaw Valley, April 2003
Jan Lüning2
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Kortright and Kim, Phys. Rev. B 62, 12216 (2000)
Fe metal – L edge
Soft X-Rays are best for magnetism!
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TransmissionX-ray
Microscope
Reconstructionfrom
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S. Eisebitt, M. Lörgen, J. Lüning, J. Stöhr, W. Eberhardt, E. Fullerton (unpublished)
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Magnetic Spectroscopy and Microscopy
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868 870 872 874
0.00
0.05
0.10
0.15
TE
Y (
a.u.
)
Photon Energy(eV)
777 778 779
0
4
8
TE
Y (
a.u.
)
Photon Energy (eV)
m
[010]
NiOXMLD
CoXMCD
Spectromicroscopy of Ferromagnets and Antiferromagnets
AFM domainstructure at surface of NiOsubstrate
FM domainstructure inthin Co film onNiO substrate
H. Ohldag, A. Scholl et al., Phys. Rev. Lett. 86(13), 2878 (2001).
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-3k -2k -1k 0 1k 2k 3k
-0.3
-0.2
-0.1
0.0
0.1
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0.3
Applied Field (Oe)
-15
-10
-5
0
5
10
15
Mn
Co
XM
CD A
sym
met
ry (%
)
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
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Exchange Bias Model from X-Rays
ideal AFM poly AFM
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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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dt
MdM
M
HM
dt
Md
1
- 1
Fast Magnetization Dynamics is governed by Landau-Lifschitz-Gilbert equation:
Precession torque Gilbert damping torqueAngular momentum change
Typically 100 ps
We want to understand on atomic level controls switching time, ~1 optimal
1 Tesla field: 90o rotation in 10 ps
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Time Resolved X-Ray Microscopy
Laser pump – x-ray probesynchronization
< 1 ps
< 100 ps
328 nst
excitationlaser pulse
observationx-ray pulse
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Production of Magnetic Field Pulses
100 m
100 m
2 m 2 m
Photoconductive switch
Current
Conducting wire
Magnetic Cells
10 m
H ~ 200 Oe
50 => I = 200 mA, 10 V bias
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Current
magnetic field
0 2000 4000 6000 8000
1.00
1.02
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1.06
De
flect
ion
ra
tio
Delay (ps)
0
50
100
150
200
Ma
gn
etic
fie
ld (
Oe)
Sample and Magnetic Field Pulse
MMagnetic Field Pulse ~ 150 Oe at Maximum < 50 ps rising time > 300 ps decaying time with some reflection
20 nm Co90Fe10 films with in-plane anisotropy (1 m) x (1-3 m) rectangles
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Observation of Vortex Motion
-50 0 50 100
-50
0
50
100
y di
spla
cem
ent (
nm)
x displacement (nm)
0 200 400 600
-200
0
200
400
y di
spla
cem
ent (
nm)
x displacement (nm)
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
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ConclusionsThe 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
HPlane 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
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Magnetostatic field is always
perpendicular to the vortex deviation
Vortex Precession
Under a field pulse,the vortex moves from the center.
Happlied
Hmagnetostatic
M
After the field pulse,the vortex continues to move radiallydue 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
xdx
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Small Angle ScatteringCoherence length larger than domains,but smaller than illuminated area
SpeckleCoherence lengthlarger than illuminated area
Incoherent vs. Coherent X-Ray Scattering
log (intensity)40
-20
-40
0
20
-40 -20 0 20 40
-40
-20
0
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scattering vector q (m-1)
scat
teri
ng v
ecto
r q
(m
-1)
log (intensity)40
-20
-40
0
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-40 -20 0 20 40
-40
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scattering vector q (m-1)
scat
teri
ng v
ecto
r q
(m
-1)
informationabout
domainstatistics
trueinformation
about domainstructure
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Pulse Structure
Possible solutions: - gated detector, pulse picker
- pump at 500 MHz