Soft X-Ray Studies of
Surfaces, Interfaces and Thin Films:
From Spectroscopy to Ultrafast Nanoscale Movies
Joachim Stöhr
SLAC, Stanford University
http://www-ssrl.slac.stanford.edu/stohr
Work supported by the DOE Office of Basic Energy Science
Overview of my talk
The Power of Soft X-rays
Polarized X-Ray Absorption Spectroscopy
 Liquid crystal alignment on surfaces
X-Ray Spectro-Microscopy
 Ferromagnetic alignment on an antiferromagnetic surface
Time Dependent X-Ray Spectro-Microscopy
 Switching of magnetic nano-structures with spin currents
A Glimpse of the Future
What are soft x-rays anyway?
Soft X-Rays
VUV
Hard X-Rays
30 eV
100 eV
~ 10 nm
1000 eV
~1 nm
3000 eV
Opening the soft x-ray region – late 1970s
Stanford Synchrotron Radiation Lab
Grasshopper
monochromator
Photon flux
Oxygen SEXAFS
oxidized Al surface
12/ 5/1977
500 eV
800 eV
O K-edge
Spectroscopy in the important region
280 – 1000 eV became possible
Photon energy (eV)
Tunable x-rays offer atom specific valence shell
information through guided transitions
polymer
magnetic multilayer
Element specificity, Chemical specificity, Valence properties
Polarized x-ray absorption determines charge and spin orientation
Orientational order
of bonds
Antiferromagnetic
order
Ferromagnetic order
Use of soft x-rays to solve a 100 year old puzzle
Liquid crystal alignment on rubbed polymer surfaces
…discovered in 1907…
Note LC “pretilt” out of plane
Alignment is basis of liquid crystal displays
A $30 billion world-wide business
Conventional models of alignment mechanism
 Oldest model assumes micro grooves in polymer surface
 X-ray diffraction on polyimide suggests epitaxy-like nucleation
Models cannot explain LC “pretilt” angle up from plane
A key observation in 1998:
Directional ion beam irradiated polymers also align liquid crystals
Pretilt direction is opposite !
X-ray spectroscopy of ion beam modified polymer surface
reference
sample
• LCs align on a-carbon surface layer, not on polymer substrate
• Is LC alignment due to bond orientation on substrate surface?
Do not need polymers at all !
start with a-Carbon – align with ion beam
• Rubbing and ion beam create molecular level orientational order
• Highest resolution displays today use ion beam aligned carbon films
Nature 411, 56 (2001); Science 292, 2299 (2001)
Polarization Dependent Imaging with X-Rays
Ferromagnetic regions
Oriented molecular regions
Antiferromagnetic regions
Tackling a 50 year old mystery with x-rays: “Exchange bias”
Key modern magnetic building blocks are based
on fixed (“pinned”) ferromagnetic reference layers
turns in external fields: “0” or “1” bits
Reference layer
does not turn in external fields
pinned by antiferromagnet
How can a “neutral“ antiferromanet bias a ferromagnet?
Effect remained a puzzle ever since its discovery in 1956
Conventional techniques could not study the all-important interface
X-Rays reveal interfacial coupling of FM and AFM domains
Electron Yield
Co edge – use circular polarization – ferromagnetic domains
2nm
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
X-Rays-in / Electrons-out: A way to study thin film interfaces
pure
Co/NiO
pure
pure
Co/NiO
pure
Interface is mixed CoNiOx layer - is it magnetic?
Images of the Ferromagnet-Antiferromagnet Interface
Interface layer contains ferromagnetic NiOx - is it coupled to AFM NiO?
Ohldag et al., PRL 87, 247201 (2001)
Exchange bias model
•
•
•
•
•
A thin interfacial diffusion layer (1–2 layers) of CoNiOx is formed
Interface layer contains ferromagnetic Ni spins from modified NiO
About 95% of interfacial Ni spins rotate with FM (not pinned)
Only < 5% of interfacial Ni spins are pinned to bulk NiO
This tiny fraction is the origin of exchange bias
Ohldag et al PRL 91, 017203 (2003)
What have we learned so far ?
• Interface effects play import role in modern nanoscale materials
• Suble interface properties can lead to important phenomena
• Soft x-rays are powerful tool to reveal interface-specific effects
 elemental specificity
 chemical specificity
 magnetic specificity
 orientational specificity
 nanoscale spatial resolution
The new frontier: dynamics or “the need for speed”
Drivers of Modern Magnetism Research: Smaller and Faster
Operational
Timescales
The ultrafast
technology
gap
The goal
Fundamental
Timescales
Time Resolution:
Pulsed X-Rays from Electron Storage Ring
Bunch width ~ 50 ps
Bunch spacing 2 ns
beam line
pulsed 50 ps x-rays
State-of-the art ultrafast electronics :
Y. Acremann et al., Rev. Sci. Instr. 78, 14702 (2007).
J. P. Strachan et al., Rev. Sci. Instr. 78, 54703 (2007).
From reading to writing information
Suggested by J. Slonczewski & L. Berger in 1996
“spin torque switching” – no external magnetic field !
free
fixed
Verified by:
F.J. Albert, J.A. Katine, R.A. Buhrman, D. Ralph, Appl. Phys. Lett. 77, 3809 (2000)
Time-Resolved Scanning Transmission X-Ray Microscopy
X-ray image
2 nm magnetic layer
buried in 250nm of metals
100nm
~100 nm
current
5mm
Detector
leads for
current pulses
Y. Acremann et al., Phys. Rev. Lett. 96, 217202 (2006)
Spin Torque Switching:
180nm x 110nm x 2 nm nanostructure of CoFe
t=0
+
_
switch
current
pulse
switch back
100 nm
200ps
400ps
600ps
800ps
Y. Acremann et al., Phys. Rev. Lett. 96, 217202 (2006)
J. P. Strachan et al., Phys. Rev. Lett. 100, 247201 (2008)
Vortices are important on all length scales
Hurricane
Nano-element
100 km
Milky Way
~ 50nm
100,000 light years
= 1018 km
A Glimpse of the Future
X-ray snap shots on the fundamental time
scales of motion of
atoms, electrons and spins
….femtoseconds and faster….
The Light Fantastic
Birth of the X-Ray Laser
…..and a New Era of Science
The End