Soft X-Ray Studies of Surfaces, Interfaces and Thin Films: From Spectroscopy to Ultrafast Nanoscale Movies Joachim St - PowerPoint PPT Presentation

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Soft X-Ray Studies of Surfaces, Interfaces and Thin Films: From Spectroscopy to Ultrafast Nanoscale Movies Joachim St

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... Ferromagnetic alignment on an antiferromagnetic surface Time Dependent X-Ray Spectro-Microscopy Switching of magnetic nano ... 2 nm magnetic layer buried in ... – PowerPoint PPT presentation

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Title: Soft X-Ray Studies of Surfaces, Interfaces and Thin Films: From Spectroscopy to Ultrafast Nanoscale Movies Joachim St


1
Soft X-Ray Studies of Surfaces, Interfaces and
Thin Films From Spectroscopy to Ultrafast
Nanoscale MoviesJoachim StöhrSLAC, Stanford
University http//www-ssrl.slac.stanford.edu/stoh
r
Work supported by the DOE Office of Basic Energy
Science
2
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

3
What are soft x-rays anyway?
Soft X-Rays
VUV
Hard X-Rays
100 eV 10 nm
1000 eV 1 nm
30 eV
3000 eV
4
Opening the soft x-ray region late 1970s
Stanford Synchrotron Radiation Lab
Grasshopper monochromator
Oxygen SEXAFS oxidized Al surface
12/ 5/1977
Photon flux
500 eV
800 eV
O K-edge
Spectroscopy in the important region 280
1000 eV became possible
Photon energy (eV)
5
Tunable x-rays offer atom specific valence shell
information through guided transitions
magnetic multilayer
polymer
Element specificity, Chemical specificity,
Valence properties
6
Polarized x-ray absorption determines charge and
spin orientation
Antiferromagnetic order
Orientational order of bonds
Ferromagnetic order
7
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
8
Alignment is basis of liquid crystal displays
A 30 billion world-wide business
9
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
10
A key observation in 1998 Directional ion
beam irradiated polymers also align liquid
crystals
Pretilt direction is opposite !
11
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?

12
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)
13
Polarization Dependent Imaging with X-Rays
Ferromagnetic regions
Oriented molecular regions
Antiferromagnetic regions
14
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
15
X-Rays reveal interfacial coupling of FM and AFM
domains
Co edge use circular polarization
ferromagnetic domains

2nm
Ni edge use linear polarization
antiferromagnetic domains

H. Ohldag et al., PRL 86, 2878 (2001)
16
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?
17
Images of the Ferromagnet-Antiferromagnet
Interface
Interface layer contains ferromagnetic NiOx - is
it coupled to AFM NiO?
Ohldag et al., PRL 87, 247201 (2001)
18
Exchange bias model
  • A thin interfacial diffusion layer (12 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 lt 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)
19
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
20
Drivers of Modern Magnetism Research Smaller and
Faster
Operational Timescales
The ultrafast technology gap
Fundamental Timescales
The goal
21
Time Resolution Pulsed X-Rays from Electron
Storage Ring
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).
22
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)
23
Time-Resolved Scanning Transmission X-Ray
Microscopy
X-ray image
2 nm magnetic layer buried in 250nm of metals
100nm
100 nm
current
5mm
leads for current pulses
Detector
Y. Acremann et al., Phys. Rev. Lett. 96, 217202
(2006)
24
Spin Torque Switching 180nm x 110nm x 2 nm
nanostructure of CoFe

t0
_
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)
25
Vortices are important on all length scales
Hurricane
Nano-element
100 km
Milky Way
50nm
100,000 light years 1018 km
26
A Glimpse of the Future
X-ray snap shots on the fundamental time
scales of motion of
atoms, electrons and spins
.femtoseconds and faster.
27
The Light Fantastic Birth of the X-Ray
Laser ..and a New Era of Science
28
The End
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