Transcript Soft X-Ray Science From Photon Drought to Free Electron Lasers Joachim Stöhr Stanford Synchrotron Radiation Laboratory.

Soft X-Ray Science From Photon Drought to Free Electron Lasers
Joachim Stöhr
Stanford Synchrotron Radiation Laboratory
What are Soft X-Rays ?
Soft X-Rays
VUV
Hard X-Rays
30 eV
100 eV
~ 10 nm
1000 eV
~1 nm
3000 eV
Large angle
optics
grazing incidence optics
vacuum
air
Closing the Soft X-ray Gap > 285 eV: 1975 - 77
Hamburg 1976
Flipper monochromator
Stanford 1977
Grasshopper monochromator
SEXAFS
Oxidized Al
12/ 5/1977
500 eV
800 eV
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
5. Wavelength is of nanometer scale (4 – 1 nm) – nanoscale imaging
real space imaging – lenses & reciprocal space “imaging” - lensless
Tunable soft x- rays offer large interaction cross sections
optical
light
electrons
Photoemission
neutrons
Spectroscopy
1980s & 1990s
Tunable soft x-rays offer elemental specificity
Transitions provide information on valence charge and spin
Tunable x-rays offer chemical specificity
Rich “multiplet structure” reveals local bonding
Polarized x-rays offer orientation sensitivity
Orientational order
Antiferromagnetic
order
Directional Chirality
Ferromagnetic order
Magnetic Circular Dichroism
Spectro – Microscopy
1990s
Nanoscale Devices in Computers
Focusing of x-rays offers nanoscale resolution
Polarization Dependent Imaging with X-Rays
Coupling of ferromagnetic and antiferromagnetic domains
Electron Yield
Co edge – use circular polarization – ferromagnetic domains
8
Co
XMCD
4
s
0
776
780
778
Photon Energy (eV)
Ni edge – use linear polarization – antiferromagnetic domains
Electron Yield
15
NiO
XMLD
10
5
[010]s
0
H. Ohldag et al., PRL 86, 2878 (2001)
870
874
Photon Energy(eV)
2mm
Images of the Ferromagnet-Antiferromagnet Interface
Ohldag et al., PRL 87, 247201 (2001)
Lensless Imaging - Scattering
2000s
Fe L-edges
Kortright and Kim, Phys. Rev. B 62, 12216 (2000)
Lensless imaging by scattering
But can one solve the phase problem and invert the image?
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)
Nanoscale Dynamics
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”
Spin currents: a new way of magnetic switching:
traditional switching
torque on
by “Oersted field”
new idea
torque on
by “spin current”
sensor
layer
reference
layer
Weak, long range
sensor
layer
Strong, short range
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
Detector
leads for
current pulses
Y. Acremann et al., Phys. Rev. Lett. 96, 217202 (2006)
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 (< 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
Sample 100nm x 200nm, 2nm CoFe free layer
current
pulse
switch
switch back
Switching best described by movement of vortex across the sample!
The Future
Information density
Average brightness
Peak brightness
From the past into the future
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 ????