Joachim%20St

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Soft X-Ray Science - From Photon Drought to Free
Electron Lasers
Joachim Stöhr Stanford Synchrotron Radiation
Laboratory
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What are Soft X-Rays ?
Soft X-Rays
VUV
Hard X-Rays
100 eV 10 nm
1000 eV 1 nm
30 eV
3000 eV
Large angle optics
grazing incidence optics
air
vacuum
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Closing the Soft X-ray Gap gt 285 eV 1975 - 77
Stanford 1977
Hamburg 1976
Flipper monochromator
Grasshopper monochromator
SEXAFS Oxidized Al 12/ 5/1977
500 eV
800 eV
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Why are Soft X-Rays so Useful?

• 1. X-ray absorption cross section is large
  sensitive to small atoms
• surface science, interfaces, thin films,
  nanostructures
• 2. Lifetime width is narrow spectroscopy of
  electronic structure
• valence states information through
  core-to-valence transitions
• 3. Spectral range contains important absorption
  edges (elements)
• C, N, O chemistry/biology Fe, Co, Ni
  magnetism
• 4. Large resonance effects and polarization
  effects dichroism
• NEXAFS - charge magnetic dichroism spin /
  orbital moments
• Wavelength is of nanometer scale (4 1 nm)
  nanoscale imaging
• real space imaging lenses reciprocal space
  imaging - lensless

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Tunable soft x- rays offer large interaction
cross sections
electrons
optical light
Photoemission
neutrons
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Spectroscopy 1980s 1990s
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Tunable soft x-rays offer elemental specificity
Transitions provide information on valence charge
and spin
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Tunable x-rays offer chemical specificity
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Rich multiplet structure reveals local bonding
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Polarized x-rays offer orientation sensitivity
Antiferromagnetic order
Orientational order
Directional Chirality
Ferromagnetic order
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Magnetic Circular Dichroism
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Spectro Microscopy 1990s
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Nanoscale Devices in Computers
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Focusing of x-rays offers nanoscale
resolution
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Polarization Dependent Imaging with X-Rays
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Coupling of ferromagnetic and antiferromagnetic
domains
Co edge use circular polarization
ferromagnetic domains

Ni edge use linear polarization
antiferromagnetic domains

H. Ohldag et al., PRL 86, 2878 (2001)
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Images of the Ferromagnet-Antiferromagnet
Interface
Ohldag et al., PRL 87, 247201 (2001)
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Lensless Imaging - Scattering
2000s
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Fe L-edges
Kortright and Kim, Phys. Rev. B 62, 12216 (2000)
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Lensless imaging by scattering
But can one solve the phase problem and invert
the image?
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Development of coherent imaging with low
intensity synchrotron radiation
soft x-ray spectro holography
coherent x-ray beam
Eisebitt, Lüning, Schlotter, Lörgen, Hellwig,
Eberhardt and Stöhr, Nature 432, 885 (2004)
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Nanoscale Dynamics
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The Technology Problem Smaller and Faster
The ultrafast technology gap want to
reliably switch small magnetic bits
X-rays combine nanometer spatial with picosecond
time resolution
seeing the ultrafast nanoworld
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Spin currents a new way of magnetic switching
new idea torque on by spin
current
traditional switching torque on by
Oersted field
sensor layer
current
current
referencelayer
sensor layer
Weak, long range
Strong, short range
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Scanning Transmission X-Ray Microscopy image of
spin injection structure
100 x 300 nm
2 nm magnetic layer buried in 250nm of metals
100 nm
current
leads for current pulses
Detector
Y. Acremann et al., Phys. Rev. Lett. 96, 217202
(2006)
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Soft x-rays at their best..

• Sensitivity to buried thin layer (2nm)
• Cross section just right - can see signal
  from thin layer
• X-rays can distinguish layers, tune
  energy to Fe, Co, Ni, Cu
• Resolving nanoscale details (lt 100 nm)
• Spatial resolution, x-ray spot size 30
  nm
• Magnetic contrast
• Polarized x-rays provide magnetic contrast
  (XMCD)
• Sub-nanosecond timing
• Synchronize spin current pulses with 100 ps
  x-ray pulses

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Sample 100nm x 200nm, 2nm CoFe free layer
current pulse
switch
switch back
Switching best described by movement of vortex
across the sample!
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The Future
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From the past into the future
Peak brightness
Average brightness
Information density
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Summary

• Soft x-rays offer complementary capabilities to
  hard x-rays
• Spectroscopic studies reveal atom-projected
• charge and spin properties of valence
  electrons
• Microscopic studies reveal charge and spin
  distributions on nanoscale
• Time dependent studies reveal nanoscale dynamics
  down to tens of picoseconds
• The future femtosecond snapshots ????