Probing Magnetism with X-ray Techniques
Transcript of Probing Magnetism with X-ray Techniques
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Probing Magnetism with X-ray Techniques
Jan Lüning
Sorbonne University, Paris (France) and Helmholtz-Zentrum Berlin (Germany)[email protected]
Lecture topics:
1) A brief introduction to X-rays- The basics of the interaction of X-rays with matter- Origin and properties of synchrotron radiation
2) X-ray based techniques- X-ray absorption spectroscopy- Types of magnetic dichroism - XMCD and Sum Rules - XMLD
- Resonant magnetic scattering
3) X-ray microscopy- STXM- XPEEM- Lensless microscopy
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Reference for synchrotron radiation
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Discovery of X-rays
8 November 1895
(Würzburg, Germany)
1901 : Nobel price in Physics
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First applications
1914
A chest X-ray in progress at Professor Menard's radiology department at theCochin hospital, Paris, 1914. (Jacques Boyer/Roger Viollet—Getty Images)
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Why are X-rays so useful
J. Stöhr
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Electromagnetic Waves
J. Stöhr
Only difference: the wavelength!
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3d TM: strong magnetic L-edge resonances
C, N, OK-edges
Energy
Soft X Hard Xnano atomicWavelength
Three ‘common’ definitions:
Soft: Grating UHV electronic structure
Hard: Crystal 'Air' crystal structure
Soft & Hard X-rays as part of the electromagnetic spectrum
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Emission angle Θ = 1/γ rad ~ 100 μrad
100 m
10 mm
Small angle: tan (Θ [rad]) = Θ [rad]
Origin of synchrotron radiation
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Note: The coherence of a source is proportional to its brilliance!
(Liouville : A∙Ω = const)
Brilliance = Photons/second
Source size x Divergence x ΔE
UNE grandeur caractéristique pour évaluerla qualité d’une source est la luminance.
Brilliance of X-ray sources
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Bend magnet
Wiggler
Undulator
Bending magnet and insertion device radiation
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50+ SR sources world-wide
13 SR sources
in Europe
… and 4
XUV/X-FELs
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Photon – Matter Interaction
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GoldCarbon
101
103
105
107
109
1011
1013
Total
Absorption
Coherent
Incoherent
Pair
Triplet
Photon Energy (eV)
K
L
M
N
10-4
10-2
100
102
104
106
108
101
103
105
107
109
1011
1013
Total
Absorption
Coherent
Incoherent
Pair
Triplet
Ph
oto
n C
ross
Sec
tio
n (
ba
rn)
Photon Energy (eV)
K
Absorption → dominates in X-ray photon energy range
Coherent = Thomson scattering → X-ray scattering/diffractionIncoherent = Compton scattering
Bin
din
g E
ner
gy
empty
Valence
States
Core
Level
~~C 1s
Band gap
filled
Vacuum
level
Photon – Matter Interactions
Photon Absorption Cross Section: =X
# of absorbed photons per second
# of incident photons per second per area
[] = barn = 12 cm-24 2
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GoldCarbon
101
103
105
107
109
1011
1013
Total
Absorption
Coherent
Incoherent
Pair
Triplet
Photon Energy (eV)
K
L
M
N
10-4
10-2
100
102
104
106
108
101
103
105
107
109
1011
1013
Total
Absorption
Coherent
Incoherent
Pair
Triplet
Ph
oto
n C
ross
Sec
tio
n (
ba
rn)
Photon Energy (eV)
K
Bin
din
g E
ner
gy
K (n = 1)
L (n = 2)
M (n = 3)
N (n = 4)
Bin
din
g E
ner
gy
empty
Valence
States
Core
Level
~~C 1s
Band gap
filled
Vacuum
level
Photon Matter Interactions
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Photoemission
Diffusion
Sectio
n e
fficace
(barn
/ ato
me
)
Neutrons
ElectronsVisible light(Metals)
Visible light(Insulators)
Charged probe
No charge,but a spin
Interaction strengths of different probes
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Mean free path of electrons in materials
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Absorption : hν < EB [+ ET.d.Sortie]L’électron excité ne sortpas de l’atome/du solide.
Photoémission : hν > EB [+ ETdS]L’électron excité sort de l’atome/
du solide. EKin = hν - EB [- ETdS]
Bin
din
g E
ner
gy
empty
Valence
States
Core
Level
~~
C 1s
Band gap
filled
Vacuum
level
Atome Solide
Absorption and Photoemission in atoms and solides
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X-ray absorption spectroscopy
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h
Bin
din
g E
nerg
y
empty
filled
ValenceStates
CoreLevels
~~Fe 2p
~~Co 2p
~~Ni 2p
Chemical Sensitivity
Elemental Specificity
700 800 900
Photon Energy (eV)
CoNiFe
X-ray absorption resonances
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“Photons lost”
“Electrons generated”
X-ray absorption: Transmission versus Yield detection
Surface
(1 – 2 nanometer)
Volume
(< 1 - 10 microns)
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Fe
Total electron yield detection to render
X-ray absorption spectroscopy surface sensitive
Studying complex materials layer by layer
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L2,3
edge absorption spectroscopy on 3d transition metals
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L32 ‘white line’ intensity of transition metals (3d)
J. Stohr et al.
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X-ray magnetic circular dichroism (XMCD)
in X-ray absorption spectroscopy
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J. Stöhr, NEXAFS SPECTROSCOPY, Springer Series in Surface Sciences 25, Springer, Heidelberg, 1992.
J. Stöhr and H. C. Siegmann MAGNETISM: FROM FUNDAMENTALS
TO NANOSCALE DYNAMICS, Springer Series in Solid State Sciences 152,Springer, Heidelberg, 2006
More about X-rays and magnetism
Many (!) transparencies are taken from Jo Stohr’s 2007presentation on ‘X-rays and magnetism’
www-ssrl.slac.stanford.edu/stohr
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Magneto-Optical effects in the visible and X-ray regime
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Origin of the XMCD effect
Relative transition amplitudesare given by the respectiveClebsch Gordon coefficients
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XMCD spectra of ferromagnetic 3d metals Fe, Co and Ni
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XAS / XMCD sum rules
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Magnetic moment of 3d transition metals Fe, Co and Ni
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Spin polarization of valence band states
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XMCD of transition metal M2,3
and L2,3
edges
L2,3
R. Nakajima et al., Phys. Rev. B 59, 6421 (1999)
2p1/2, 3/2
→ 3dM2,3
S. Valencia et al., New J. Phys. 8, 254 (2006)
3p1/2, 3/2
→ 3d
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X-ray magnetic LINEAR dichroism (XMLD)
in X-ray absorption spectroscopy
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X-ray Magnetic Circular and Linear Dichroism
XMLD :
XMCD :
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XMLD: Linear dichroism and presence of AFM order
Below Néel temperature
Strong linear dichroism
Warnings:
Crystal fields can cause linear dichroism
Relationship between orientation ofAFM axis and dichroic ratio can dependon crystal orientation
Above Néel temperature
No linear dichroism
La0.6Sr0.4FeO3 / SrTiO3 (110)
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Orientation of AFM axis in a thin film of LaFeO3 / SrTiO3 (110)
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Adding spatial resolution to x-ray spectroscopy
X-ray spectro-microscopy
Application: Co / NiO
The interface between a ferromagnetic metaland an antiferromagnetic metal oxide
The next step:
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CondenserLens
Quantitative imaging with sensitivity toelemental and chemical distribution and charge/spin ordering
X-ray spectromicroscopy techniques
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Fresnel Zone Plate Lenses
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Fresnel Zone Plate Principle
Spherical Grating with varying line density
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Principle of a diffraction grating
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Focussing with a Fresnel zone plate
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Spatial resolution: resolving two point sources
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Scanning (STXM) vs fulf field (TXM) X-ray microscopy
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CondenserLens
Quantitative imaging with sensitivity toelemental and chemical distribution and charge/spin ordering
X-ray spectromicroscopy techniques
Techniques ideally suited to study phenomenaoccurring on the nanometer length scale
• Thin film and surface sensitivity
• Spectroscopic contrast mechanism
• Individual parts of complex structures accessible
• Spatial Resolution 20 nm – 50 nm routinely- 10 nm and better demonstrated- volume zone plate needed for further improvement
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CondenserLens
Quantitative imaging with sensitivity toelemental and chemical distribution and charge/spin ordering
X-ray spectromicroscopy techniques
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Topographical ContrastSchematic layout of the PEEM
Photoemission Electron Microscopy
Elemental Contrast
Ti
La
Co
Fe
Magnetic lenses
e-
16°
analyzer
20 kV
Chemical Sensitivity
Elemental Specificity
700 800 900
Photon Energy (eV)
CoNiFe
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Topographical Contrast
Photoemission Electron Microscopy
Elemental Contrast
Ti
La
Co
Fe
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XMCD as contrast mechanism in X-ray spectroscopy
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X-ray microscopy for local X-ray spectroscopy
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Magnetic domain structure in:
ferromagnetic metal
interface
antiferromagnetic oxide
Resolving the spin structure of a magnetic multilayer
“Exchange bias” magnetic multilayer
Spectroscopic Identification and Direct Imaging of Interfacial Magnetic SpinsH. Ohldag et al., Phys. Rev. Lett 87, 247201 (2001).
AFM
FM
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Exchange Bias Phenomenon
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Spin structure in a magnetic multilayer
NiO XMLD
XMLD for imaging ofantiferromagnetic spinorder in NiO substrate
Co XMCD
XMCD for imaging offerromagnetic spin orderin Co film
NiO
Co
Parallel alignment of spins onboth sides of the FM – AFM interface
Elemental Specificity
700 800 900
Photon Energy (eV)
CoNiFe
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Upon deposition of
2 ML of Co on NiO
2 ML CoO (Co oxidized)
2 ML Ni (Ni reduced)
linear combination of metaland oxide spectra possible
776 778 780
Co CoO Co/NiO Model
L3-edge
Co
Photon Energy (eV)
868 870 872 874
L2-edge
BA
Ni Ni NiO Co/NiO Model
Interface chemistry
M MxOy
Chemical environmentinfluences NEXAFS
X-ray absorption spectroscopy
NiO
CoCoNiO
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NiO
Co
NiO
CoCoNiO
Spin structure in a magnetic multilayer
NiO XMLD Co XMCD
XMLD for imaging ofantiferromagnetic spinorder in NiO substrate
XMCD for imaging offerromagnetic spin orderin Co film
Interfacial uncompensatedchemical → ferromagnetic Ni spinsreaction at interface
Ni(O) XMCD
XMCD for imaging offerromagnetic spin orderin NiO substrate
868 870 872 874
L2-edge BANi
Ni NiO Co/NiO Model
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Adding time resolution to X-ray spectro-microscopy
The next step:
Example: Magnetization switching by spin injection
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CondenserLens
Quantitative imaging with sensitivity toelemental and chemical distribution and charge/spin ordering
X-ray spectromicroscopy techniques
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current current
cell to be switched
better: switching by current in wireswitching by Oersted field around wire
Motivation: Switching of magnetic memory cells (MRAM)
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Co.9Fe.1 4 nm
Cu 3.5 nm
Co.9Fe.1 2 nm
prepared by Jordan Katine, Hitachi Global Storage
Challenge:
measuring thin 4 nm magnetic layer buried in 250 nm of metals !
to be switched back and forth
polarizes spins
current
Oersted field
100nm
Sample realization for spin-injection experiment
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100 x 300 nm
Detector
leads for current pulses
4 nm magnetic layer
buried in 250 nm of metals
cu
rrent
~100 nm
Y. Acremann et al., Phys. Rev. Lett. 96, 217202 (2006)
STXM image of spin injection structure
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Sensitivity to buried thin layer (4 nm)
Cross section just right - can see signal from thin layer X-rays can distinguish layers, tune energy to Fe, Co, Ni or Cu L edges
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~50 ps x-ray pulses
Soft x-ray spectro-microscopy at its best
Fast detector forX-ray pulse selection
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Static images of the burried layer’s magnetization
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Limitation:
Process has to be repeatable
sample
repeat over and over…
X-ray probe
Studying dynamics by pump – probe cycles
Problem: Today not enough intensity for single shot experiments with nanometer spatial and picosecond time resolution
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Storage ring is filled with electron bunches → emission of X-ray pulses
Bunch spacing 2 ns
Bunch width ~ 50 ps
pulsed x-rays
Pulse structure of synchrotron radiation
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Switching best described by movement of vortex across the sample!
switch back
current pulse switch
Magnetization reversal dynamics by spin injection
0 ns 6 ns
1.8 ns 2.2 ns
12 ns
2.0 ns 2.4 ns
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Magnetic switching by interplay of charge and spin current
= 950 Oersted
for 150x100nm,
j = 2x108 A/cm2
CHARGE CURRENT:
creates vortex state
SPIN CURRENT:
drives vortex across sample
Y. Acremann et al., Phys. Rev. Lett. 96, 217202 (2006)
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General requirements: Technique requirements:
distinguish components elemental (chemical) specificity
study thin films and interfaces large cross section for “signal”
look below the surface depth sensitivity
see the invisible nanoscale spatial resolution
resolve dynamic motions time resolution < 1 nanosecond
separate spin and orbital contributions sensitive to s-o couplingx-rays cover them all
Requirements for exploring modern magnetic materials
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• x-ray cross section and flux
• x-ray tunability: resonances
• x-ray polarization
What makes x-rays so unique
But:
Never ignore the power of other experimental techniques, because:
- Good argument: Each technique has specific strengths
- Good but dangerous argument: More readily accessible for you
• sum rules
• x-ray spatial resolution
• x-ray temporal resolution