A users viewpoint: absorption spectroscopy at a synchrotron Frithjof Nolting.
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Transcript of A users viewpoint: absorption spectroscopy at a synchrotron Frithjof Nolting.
A users viewpoint: absorption spectroscopy at a synchrotron
Frithjof Nolting
Bottom layer “pinned”Hard layer orExchange coupling
Top layer “free” can be switched
Ferromagnet
Ferromagnet
Antiferromagnet
(1) (0)
MRAMHard disk head
Magnetic data storage and recording
Low resistance high resistance
GMR Element
FM
AFM
Interface
- Different models for AFM/FM coupling exist.
- Different assumptions on AFM structure lead to complete different results.
- Spin arrangement at the interface is important
•High Spatial Resolution
•Elemental Sensitivity
•Ferromagnetic Contrast
•Antiferromagnetic Contrast
•Surface/Interface Sensitivity
How?
X-ray absorption spectroscopy (XAS) with spatial resolution
Photoemission Electron Microscope (PEEM)
Photoemission Electron Microscope
I(nA)
h
Energy
EF
~~
h(x-ray)
core level
valence band
2p 3/2
2p1/2
Magnetic lenses e-
16°
analyzer
20 kVX-rays
Sensitive to:• elemental composition• chemistry• structural parameters• electronic structure• magnetic properties
ELMITEC GmbH
Magnetic microscopy
S
E
XMCD (X-ray Magnetic Circular Dichroism)
XMLD (X-ray Magnetic Linear Dichroism) LaFeO3
775 780 785 790 795 800
L2
L3
Photon energy (eV)
TE
Y (
a.u.
)
Co
5 µm
5 µm
J. Stöhr et al. Science 1993 A. Scholl et al. Science 2000 F. Nolting et al. Nature 2000
840 850 860 870 880 890
0.6
0.9
1.2
1.5
left polarized right polarized
Tot
al Y
ield
(a.
u.)
Photon Energy(eV)
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Asymmetry smoothed
Asy
mm
etry
(%)
Exchange biased Co/NiO multilayer
EPU beamline 4 at ALS
.
.
3 nm Co50 nm NiO
1.5 nm Ru
3 nm Co 30°
Magnetic field
Left and right circularly polarized light
XMCD
Hysteresis of Co and NiO
XMCD spectra – switching polarization
-3000 -2000 -1000 0 1000 2000 3000-20
-15
-10
-5
0
5
10
15
20
XM
CD
(%
)
Applied Field (Oe)
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Co Ni
Pinned Moments ?
H. Ohldag, A. Scholl, F. Nolting, E. Arenholz, S. Maat, A.T. Young, M. Carey, and J. Stöhr, Phys. Rev. Lett. 91(1), 017203/1-4 (2003).
Ratio of A/B
Antoine Barbier et alUser experiment at SLS, July 2004
SIM Beamline Layout
UndulatorT. Schmidt
• 2 Elliptical undulators
• Pure permanent magnet
• 95eV < h < 2000eV• >1019 photons/s/mrad2/mm2/400mA
• 100 % circular polarization[125 - 900 eV]reduced on higher harmonics
• Hor. & vert. linear polar.
OpticsU. Flechsig
• Plane grating monochromator
• E/E > 8000
• <5% 2nd order light
• Switch helicity
• Focus 30x100m2
EndstationC. Quitmann & F. Nolting
• SLS endstation:
– PEEM & LEEMx~25 - 50nm spatialE~150meV energy
– Sample Prep chamber
– User endstation
Front end
Prepchamber
ID1 ID2 Monochromator Chopper
Refocusingoptics
MicroscopeUser
endstation
705 710 715 720 725
0.44
0.46
0.48
0.50
0.52
0.54
0.56
0.58
0.60
Inte
nsi
ty (
a.u
.)
Photon energy (eV)
reference signal sample signal
705 710 715 720 7250.78
0.80
0.82
0.84
0.86
0.88
Inte
nsi
ty (
a.u
.)
Photon energy (eV)
normalized signal
TEY
Reference signal Sample signal
Magnet
Sample = Norm
Reference
Moving gap and monochromator, stop, measure (1s – 1minute), moving …
How do we measure
710 720-0.0020.0000.0020.0040.006
Diff
. [a
.u.]
Photon Energy [eV]
difference0.78
0.80
0.82
0.84
0.86
0.88
1 %
Inte
nsi
ty [a
.u.]
circular plus circuls minus
How do we measure
•Change polarization and repeat
•Take difference of spectra
Absorption spectrum requires frequent moving of gap and shift
must not effect other beamlinestransparent!
switching by moving phase
switching by moving the gap
tuned detuned
move up 56 mm
move 2 mm
move 0 mm
detuned tuned
circ plus circ minus
chopper
120 s
1s - 8 s
100 Hz
Modes for switching the polarization
switching by using a chopper
Alignment of IDs - Horizontal1. Horizontal overlap at Frontend
ID1
ID2
ID2Asymmetric bump
2D Frontend scan
Closed bump using chicane magnets
2. Horizontal overlap at focus of experiment ID2In
tens
ity
Horizontal position
Alignment of IDs - Vertical
3. Vertical (energy) overlap at focus of experiment
ID1
ID2
not yet finished !
Photon energy
Inte
nsity
Inte
nsity
Inte
nsity ID2
Absorption spectrumschematic
778.3 eV
source mono Exit slit
Beam variation - Noise
horizontal movement intensity variationXAS normalization reduces it by a factor of 10-100PEEM no normalization
days no problemhours no problemsec - minutes badmili seconds ok
vertical movement energy variationno normalization!
days no problemhours badseconds badmili seconds ok
10 μm about 2%
10 μm about 10 meV
Energy shift
Energy shift of 10 meVIdentical spectra
1 %
Orbit feedback
Fast orbit feedback slow orbit feedback
beam positionin ID(Bergoz)
X-ray positionin beamline
Shift and gap
Orbit correctors?
705 710 715 720 725
-0.001
0.000
0.001
0.002
0.003
0.004
0.005
0.006
Inte
nsi
ty (
a.u.
)
photon energy (eV)
with fast orbit feedback with slow orbit feedback with no orbit feedback
705 710 715 720 725
0.80
0.82
0.84
0.86
0.88
No
rm s
ign
al (
a.u
.)
Photon energy (eV)
with fast orbit feedback with slow orbit feedback
Orbit feedback – effect on measurement
Normalized signal, circular plus Difference circular plus and minus
Increased noise!!!
Slow Orbit feedback
Circular plus Circular minus
beam positionin ID(Bergoz)
X-ray positionin beamline
Shift and gap
Orbit correctors?
705 710 715 720 725
0.55
0.56
0.57
0.58
0.59
0.60
raw signal
Inte
nsi
ty (
a.u
.)
photon energy (eV)
measurement one measurement two
Slow Orbit feedback
Not transparent !
10 μm
690 700 710 720 730 740-0.004
0.000
0.004
0.0081%
Diff
. (a
.u.)
Photon Energy [eV]
diff
1.15
1.20
1.25
1.30
1.35
1.40
Inte
nsi
ty (
a.u
.)
norm1 norm2
Summary
We can do great measurements at SLS
Transparent IDs are essential! Very difficult to make an EPU transparent. Have to rely on Orbit feedback
For the measurement “no” difference between slow and no Orbit feedback
Critical time scale second – hour (10 Hz – 0.0001 Hz)Intensity variation 0.1% 0.5 μmenergy variation 1meV 1 μm
Slow Orbit feedback is not sufficient
Fast Orbit feedback is great