Magnetism and X-Rays:

1
Magnetism and X-Rays Past, Present, and A
Vision of the Future
X-rays have come a long way magnetic
imaging, nanoscale, ultrafast
Joachim Stöhr Stanford Synchrotron Radiation
Laboratory Stanford University
Static image
Femtosecond single shot image
Pump-probe 100 picoseconds dynamics
1993
2003
200X
1895
http//www-ssrl.slac.stanford.edu/stohr/index.htm
2
Collaborators
SSRL Stanford H. C. Siegmann H. Ohldag Y.
Acremann J. Lüning W. Schlotter S. Andrews J. P.
Strachan V. Chembrolu
Berlin - BESSY S. Eisebitt M. Lörgen O.
Hellwig W. Eberhardt
Berkeley - ALS A. Scholl S. B. Choe A. Bauer
(FU Berlin)
3
Magnetic Structure Neutron Scattering
Press release by the Royal Swedish Academy of
Sciences, Nobel Prize in Physics B. N.
Brockhouse and C. G. Shull 1994 Neutrons are
small magnets (that) can be used to study the
relative orientations of the small atomic
magnets. .. the X-ray method has been powerless
and in this field of application neutron
diffraction has since assumed an entirely
dominant position. It is hard to imagine modern
research into magnetism without this aid."
Antiferromagnet NiO
4
The New Paradigm Smaller and Faster
Examples Magnetic Devices in Computers
Present Size gt 100 nm, Speed gt 1 nsec Future
Size lt 100 nm, Speed lt 1 nsec
Ultrafast Nanoscale Dynamics
5
What does it take to explore
advanced magnetic materials and phenomena ?
Need techniques that can see the invisible
spatial
resolution lt 300 nm study thin films and
interfaces large cross section for
signal look below the surface
depth sensitivity distinguish
components elemental
(chemical) specificity resolve dynamic motions
time resolution lt 1
nanosecond
X-rays can do all
that ! - so today, one may say It is hard to
imagine modern research into magnetism without
the aid of x-rays!
6
Capabilities of X-Rays
Experimental X-Ray Methods
7
Experimental X-Ray Absorption Techniques
8
(No Transcript)
9
Magnetic Information through Polarization
Dichroism

a few hundred eV
a few eV
10
Magnetic Spectroscopy and Microscopy
x-ray "spin"
Soft X-Rays are best for magnetism!
11
Fe metal L edge
Kortright and Kim, Phys. Rev. B 62, 12216 (2000)
12
Tunable x- rays offer variable interaction cross
sections
electrons
optical light
Photoemission
neutrons
13
Focusing of x-rays offers nanoscale
resolution
14

• Experimental Results 1
• The Puzzle of Exchange Bias

15
(No Transcript)
16
The Problem Exchange Bias
M
M
H
H
M of blue layer is pinned or exchange
biased
17
Example The Spin Valve Sensor
FM1
Cu
FM2
AFM
Giant magnetoresistance effect
18
X-Rays-in / Electrons-out - A way to study
Interfaces
FM Co tune to Co edge circular
polarization AFM NiO tune to Ni edge
linear polarization FM Ni(O) tune to Ni edge
circular polarization
19
Spectro-Microscopy of Ferromagnets on
Antiferromagnets
Tune to Co edge use circular polarization
ferromagnetic domains

Tune to Ni edge use linear polarization
antiferromagnetic domains

H. Ohldag et al., PRL 86, 2878 (2001).
20
(No Transcript)
21
X-Ray Picture of Exchange Bias
Co/IrMn
Co/NiO
Co
NiO
Element specific FM loops
AFM "loop" rotation
Imaging

• The interface is not sharp
• Parallel coupling
• ? AFM axis is rotated at interface

• Free spins 96 of ML - coercivity
• Pinned Spins 4 of ML - bias

• AFM spring is formed
• anchored in bulk

22

• Experimental Results 2
• Time resolved imaging of magnetic structures

23
x-ray pulse
electron storage ring
24
Imaging of Motion of Nanoscale Domains (100 ps)
real image
differential image
25
Two pattern with same static structure, but ..
Field response
Field response
Opposite rotation is caused by direction of
vortex core magnetization, i.e. chirality
26

• Experimental Results 3
• Beyond 186 years of Oersted switching.
• Magnetic switching by spin currents

27
A new way of magnetic switching spin injection
spin current acts like an Exchange field
electric current creates magnetic field
Oersted field
Weak, long range
Strong, short range
28
Switching of magnetic memory cells (MRAM)
cell to be switched
better switching by current in wire
switching by Oersted field around wire
current
current
29
A new way of magnetic switching spin injection
current becomes spin-polarized
30
Samples for spin-injection studies e-beam
lithography
prepared by Jordan Katine, Hitachi Global Storage
100nm
Au 100 Å Cu 1600 Å Co.86Fe.14 40 Å Cu 35
Å Co.86Fe.14 20 Å Ru 8 Å Co.86Fe.14 18 Å PtMn
175 Å Ta 25 Å Cu 200 Å Ta 50 Å
Oersted field
to be switched
polarizes spins
Challenge see thin 4 nm magnetic layer buried
in 250nm of metals!
current
31
(No Transcript)
32
STXM image of spin injection structure
100 x 300 nm
x
y
leads for ns current pulses
Detector
33
Images before and after spin injection pulse
magnetization has complicated C
state both electrical current and spin current
contribute
34
A Vision of the Future..
Ultrafast pictures -
single shot images with x-ray lasers (XFEL)
35
not enough intensity cannot do
microscopy
36
Coming to Stanford in 2008 The first x-ray
laser - LINAC COHERENT LIGHT SOURCE (LCLS)
0 Km
2 Km
3 Km
37
A new way to take fast images no lenses !
coherence length smaller than illuminated
area larger than domains
coherence length larger than illuminated area
Can the reciprocal space pattern be inverted ?
38
Towards single shot imaging
39
True images! - soft x-ray spectro-holography
coherent x-ray beam
Eisebitt, Lüning, Schlotter, Lörgen, Hellwig,
Eberhardt and Stöhr, Nature 432, 885 (2004)
40
Is it real?
FT Hologram
STXM
W. F. Schlotter Y. Acremann
Reference hole ? 100 nm
Resolution 30 - 40 nm
41
Conclusions

• X-rays have become an important probe of magnetic
  materials and phenomena
• X-rays offer elemental, chemical and magnetic
  specificity with nanoscale spatial resolution
• Transmission experiments probe bulk, electron
  yield experiments probe surfaces and interfaces
• X-rays allow time-dependent studies, paving the
  way for picosecond nanoscale technology
• Future x-ray sources, new techniques and
  instrumentation will allow the complete
  exploration of magnetic phenomena in space and
  time

For more, see http//www-ssrl.slac.stanford.edu/
stohr
and new textbook (820 pages)
J. Stöhr and H. C. Siegmann Magnetism From
Fundamentals to Nanoscale Dynamics Springer
Verlag, February/March 2006