Cheiron School 2011

Soft X-ray Absorption Spectroscopy

Kenta Amemiya (KEK-PF)

Absorption Edges in the Soft X-ray Region

Atomic Number

M edgeL edgeK edge.
Li

Absorption-edge Energy (eV)Soft X-ray Absorption Spectroscopy
1. Advantages and Disadvantages of Soft X-ray
Absorption Spectroscopy (SXAS)
2. SXAS studies on Surface and Thin films
3. Novel SXAS Techniques
3-1. Depth-resolved XAS
3-2. Wavelength-dispersive XAS

Soft X-ray Absorption Spectroscopy (~100-4000 eV)
Partial Elec tron Yield (arb. uni ts)

1. Element selectivity<- Core-hole excitation (1s, 2p…)

12

C K edge
10



h

8

(C: 290 eV, O: 530 eV, Fe: 710 eV, Ni: 850 eV…)64 C- S

2. Information on chemical species

 = 15oo = 55o = 90<- Characteristic spectral features (π*，σ*…)3. Structural information (bond length, etc.)
EXAFS (Extended X-ray Absorption Fine Structure)C -C2

4. Information on anisotropy

0
280

290

3 00

Photon Energy (eV)310<-Linear polarization(molecular orientation, lattice anisotropy)

5. Magnetic information<- Circular polarization

In the Soft X-ray region,

6. High sensitivity

1. Vacuum condition is normally required. (NOT ultra-high vacuum)Special sample cell or He atmosphere is available for ambient pressure.
2. Surface sensitivel = several nm for electron yield, ~0.1 μm for fluorescence yield

XAS Measurement in the Soft X-ray Region
3 ML Fe / Cu(100) Fe L-edge XAS

Absorption Intensity (arb. units)

0.025

How can we measure
X-ray absorption spectrum ?

2p3/2→3d(LIII)~mm0.02

X-rays

Core
hole

A

0.015

0.01

~nm

2p1/2→3d(LII)

Auger electrons(core hole relaxation)

Detector(Retarding voltage)

0.005

Electron yield XAS
Total electron yield (TEY)0700 710 720 730 740 750

Partial electron yield (PEY)

Photon Energy (eV)cf. Fluorescence yield (FY)

Advantages and Disadvantages of SXAS
Short Penetration Length
Transmission mode can be available only for a very thin sample on avery thin or without substrate.
Electron yield mode is usually adopted because of high efficiency.
Special care is necessary for insulators (powders might be OK).
Fluorescence yield efficiency is very small for light elements.
<1 % for C, N, O
Be careful for the self absorption (saturation) effect.
Samples should be usually kept in vacuum (NOT ultra-high vacuum).
Some attempts have been made to realize ambient-pressure
or liquid-state measurements.

Surface Sensitive
Sub-monolayer samples can be investigated.
Bulk information is hardly obtained, especially in the electron yield mode.
Sensitive to Electronic and Magnetic States of light elements
Valence electrons can be directly investigated by 1s->2p excitation of
C, N, O,… and 2p->3d excitation of 3d transition metals.

1. Advantages and Disadvantages ofSoft X-ray Absorption Spectroscopy (SXAS)
2. SXAS studies on Surface and Thin films
3. Novel SXAS Techniques
3-1. Depth-resolved XAS
3-2. Wavelength-dispersive XAS

Near-edge Spectroscopy
Near-edge X-ray Absorption Fine Structure (NEXAFS)
X-ray Absorption Near-edge Structure (XANES)

Chemical species
Structural information (orientation)

Near-edge Spectroscopy

Thiophene (C4H4S) molecule on Au(111)

Determination of Chemical Species
Initial oxidation process of Si
Dipped in thiophene
solution

Molecular O2

Oxidation

C6H13SH

Exposed to thiophene
gas in vacuum

Existence of molecular oxygenin the initial stage of Si oxidation
Matsui et al., Phys. Rev. Lett. 85, (2000) 630.

Different chemical species depending
on preparation processes
Sako et al., Chem. Phys. Lett. 413, (2005) 267.

Determination of Atomic Structure
Extended X-ray Absorption Fine Structure (EXAFS)•••

Coordination number (N)Inter-atomic distance (r)DW factor (σ2)

EXAFS oscillation

 (k)   S02i

N i Fi (ki ) -2ki2 i2 -2ri / li (k)eesin(2ki ri  i (ki ))
2
ki ri



Fe K-edge XAFS spectrum μ(E) of FeO

Determination of Atomic Structure
Amemiya et al., Phys. Rev. B 59, (1999) 2307.

Application to surface molecule (CH3O)
CH3O/Cu(111)
CH3O/Cu(111)EXAFS

NEXAFS

CH3O/Ni(111)

CH3OH (solid)

Oscillation period -> O-Cu bond length
Angle (θ) dependence -> bond angle
adsorption site

Peak B (1s→σ*CO) -> C-O bond length
Angle (θ) dependence -> molecular orientation

Determination of Atomic Structure
S K-edge EXAFS

Surface-EXAFS
 = 15

2

o

1
0

-2

 = 90o2k (k)

-1

2
1
0
-1
-2
3

4

5

6

7

8

9

10

11

C6H13S/Cu(100)Fourier Transform [arb. units]

k [1/A]
S-Cu1

3.0
2.5

90

o

2.0

15

o

1.5

S-S1
S-C

1.0

S-Cu2

S-Cu3

0.5
0.0

0

1

2

3
4
Distance R [A]5

6

Magnetic structures studied by XMCD
Absorption Intensity (arb. units)

3 ML Fe / Cu(100)
Fe L-edge XMCD
0.03
0.025
0.02
0.015
0.01

Helicity Spinμ↑↑ (μ+)μ↑↓ (μ－)X-ray Magnetic Circular Dichroism (XMCD)
Difference in absorption intensities between
right- and left-hand circular polarizations

2p3/2→3d(LIII) 2p →3d
1/2
(LII)

0.005
0
-0.005

1. Element selectivity<- resonant absorption (2p->3d…)
2. Determination of spin and orbital
magnetic moments<- Sum rules

3. High sensitivityμ↑↓ －μ↑↓-0.01
700 710 720 730 740 750

Photon Energy (eV)Principle of XMCD
No 3d orbital moment

Finite 3d orbital moment

Helicity+1μ↑↑= 3+6+6=15μ↑↓= 18+6+1=25-1μ↑↑< 15μ↑↓> 25μ↑↑= 3+12=15μ↑↓= 3+2=5μ↑↑> 15μ↑↓< 5XMCD Sum Rules
Orbital moment

Spin moment

B.T. Thole et al., PRL 68, 1943 (1992).
P. Carra et al., PRL 70, 694 (1993).

Magnetism of Thin Films Studied by XMCD
Co L-edge XMCD spectra

P. Gambardella, Nature 416, 301 (2002)

XMCD sum rules

C.T. Chen et al., PRL 75, 152 (1995).

Utilization of Element Selectivity of XMCDIntensity (arb. units)

54

Fe L-edgeFe

3

Co L-edge

Co

2
1
0
680

700

720

740

760

780

800

820

840

Photon Energy (eV)

Magnetic-field dependence of XMCD at Fe and Co L edges
Fe(6 ML)/Co(2 ML)/Cu(001)

Fe(5 ML)/Co(2 ML)/Cu(001)
1.0

0.5

0.0

-0.5

Fe
Co

Intensity (arb. units)

Intensity (arb. units)

1.0

0.5

0.0

-0.5

Fe
Co

-1.0

-1.0
-30

-20

-10

0

H (Oe)

10

20

30

-1.5

-30

-20

-10

0

H (Oe)

1020

30

Angle Dependence of XMCD
(1) weak magnetic field or remanent measurements
0.5

Grazing
Normal

-0.5

0.5

0.0

Grazing
Normal
-0.5

-1.0

-1.0
840

860

880

840

Photon Energy [eV]Ni (7.5 ML)
Cu(100)in plane

perpendicularin plane

XMCD intensity [a.u.]

0.0

XMCD intensity [a.u.]

XMCD intensity [a.u.]

0.5

0.0

Grazing
Normal

-0.5

-1.0
860

880

Photon Energy [eV]Fe (0.7 ML)
Ni (7.5 ML)
Cu(100)

840

860

880

Photon Energy [eV]Fe (4 ML)Ni (7.5 ML)
Cu(100)XMCD reflects magnetic component which isparallel to X-ray beam.
->determination of easy axis of magnetization
Information on orbital moment-> estimation of magnetic anisotropy
Abe et al., J. Magn. Magn. Mater. 206 (2006) 86.

Angle Dependence of XMCD
(2) High magnetic field measurements
XMCD (X-ray Magnetic Circular Dichroism)
Element selectivity
Quantitative determination of spin & orbital
magnetic moments by using the sum rules-B+B

X raysAngle-dependent XMCD
> Magnetic anisotropy
Separation of ms from mT

T. Koide et al., Rev. Sci. Instrum. 63, 1462 (1992).

Angle-dependent XMCD Sum Rules
Orbital sum rule-[I L3  I L2 ]
I L3  I L2

3  ml4nh   B

Spin sum rule-[I L3 - 2  I L2 ]
-I L3  I L2

(ms  7  mT )2nh   B

B.T. Thole et al., PRL 68, 1943 (1992).
P. Carra et al., PRL 70, 694 (1993).

-

ml = ml⊥cos2 + ml∥sin2
mT = mT⊥cos2 + mT∥sin2

Investigation of Interface Magnetism
T. Koide et al., Phys. Rev. Lett. 87, 257201 (2001).

Au/Co(2 ML)/Au(111)
Self-assembled Co islands
due to a reconstruction ofAu surface
All Co atoms are regarded to“interface” because of 2 ML
thickness⇒ Direct observation ofinterface magnetism

Angle-dependent XMCD
⇒ Direct determination of
ms, ml//, ml⊥, mT//, mT⊥

Angle-dependent XMCD Measurements
T. Koide et al., Phys. Rev. Lett. 87, 257201 (2001).

PF BL-11A

Angle dependence in XMCD<- Anisotropy in ml, mT
mj = mj⊥cos2 + mj∥sin2
(j = l or T)
mT⊥+ 2 mT∥= 0⇒ Determination of all
moments including
their anisotropy

Determined Magnetic Moments
T. Koide et al., Phys. Rev. Lett. 87, 257201 (2001).

Cluster-size dependent
phase transition

1. Advantages and Disadvantages ofSoft X-ray Absorption Spectroscopy (SXAS)

2. SXAS studies on Surface and Thin films
3. Novel SXAS Techniques

3-1. Depth-resolved XAS
3-2. Wavelength-dispersive XAS

Introduction: Exploring Magnetic Depth Profile
Surface

Interface effect: TMR

Thin Film
Interface(Substrate)
Surface and interface show different
magnetism from inner layers
Surface and Interface sometimes
affect magnetism of whole film

Surface effect: Gas adsorption
H
Ni(8 ML)
Cu
Vollmer, et al., Phys. Rev. B 60 (1999) 6277.

Adsorption-induced change inmagnetic easy axis
⇒ What happens at surface?

Yuasa, et al., Jpn. J. Appl. Phys. 43 (2004) L588.

Chemical and magnetic statesat interface affect MR ratio

Co/Cu(100)- Surface & interface orbital moment Tischer et al., Phys. Rev. Lett.
75 (1995) 1602.

[Pt/Co(111)] multilayer - Interface orbital moment Nakajima et al., Phys. Rev. Lett.
81 (1998) 5231.

Conventional Technique for Depth Profiling
SQUID, VSM, MOKE, XMCD…

Total Magnetic Moment

Gives averaged information over the whole sample.
⇒ also averaged in depth

3
2
1
0
0

1

2

3

4

5

6

7

Film Thickness

Based on an assumptionthat magnetic structure of surface and interface
dose not change upon layer growth
Direct technique for depth profiling

XAS Measurement in the Soft X-ray Region
3 ML Fe / Cu(100) Fe L-edge XAS

Absorption Intensity (arb. units)

0.025

How can we measure
X-ray absorption spectrum ?

2p3/2→3d(LIII)~mm0.02

X-rays

Core
hole

A

0.015

0.01

~nm

2p1/2→3d(LII)

Auger electrons(core hole relaxation)

Detector(Retarding voltage)

0.005

Electron yield XAS
Total electron yield (TEY)0700 710 720 730 740 750

Partial electron yield (PEY)

Photon Energy (eV)cf. Fluorescence yield (FY)Principle of Depth-resolved XAS
Surface

X-rays

X-raysSamplee-

Microchannel platedeAuger electrons
XAS data

Phosphor screen

eCCD camerae- Surface sensitive
Electron yield XAS measurements at different detection angles
A set of XAS data with different probing depths

Probing Depth (effective escape depth)：leNumber of detected electrons emitted at depth z : I = I0exp(-z/le)
I0：Original number of emitted electrons

Proportion of detected electrons

1

Small leλe = 5 nm
10 nm

0.8

⇒ Large contribution from surface

100 nmXAS:
Averaged information per atom

0.6

0.4

Depth-resolved XAS：

exp(-z/le)-weighted average

0.2

0
0

2

4

6

8

Depth z (nm)10Feasibility Study: Depth-resolved XMCD of Fe/Cu(100)Amemiya et al., Appl. Phys. Lett. 84 (2004) 936.

Normal Incidence, 130 K

3 ML Fe

7 ML Feoo

Fe

Uniform

Fe

Magnetization
Cu(100)Surface
Magnetization

Cu(100)Effective Spin Moment ms+7mT (B)

Interpretation of depth-resolved XMCD dataFe

2.5
Cu(100)2

3 ML (130 K)

1.5
Fe

1

7 ML (200 K)
Cu(100)0.5

7 ML (130 K)

0
6

8

10

12

14

Probing Depth

16

18

20

FM

o

(A)

SDW
q=2/2.6d

Fe/Cu(100)>200 K<200 K
Surface (FM)Fe

Cu(100)Inner layers (AFM or SDW)
No (little) magnetic interaction between

Cu and interface (bottom) Fe

Fe/Ni/Cu(100)Fe

Ni
Cu(100)

Any magnetic interaction
among surface, inner layers
and interface ?

Fe(x ML)/Ni( 6 ML)/Cu(100)

Fe L-edge Depth-resolved XMCD
Grazing Incidence (200 K)

Fe

Fe

Ni

Ni

Fe
Ni

Curve fitting with a three-region model
9 ML Fe
Effective Spin Moment ms+7mT (B)

Fe/Ni(6 ML)/Cu(100)1.1 B

200 K

0.6-0.1 B

0.4

1.5 B
9 ML Fe

0.2
0

5 ML Fe-0.2

5 ML Fe

2 ML
Fe

-0.4

1 ML
7

8

9

10

Probing Depth

11

12
o

(A)

13

14

Ni

-0.7 B
0.0 B
2.1 BFe thickness dependence at 200 K

Ni

1
0

Ni

-1

Oscillatory
surface magnetization-2-3

Interface magnetization
Minterface(B)

Fe

Surface magnetization
Msurface(B)

Fe

Positiveinterface magnetization

4
3
2
1
0

4

5

6

7
8
9 10 11Fe thickness (ML)12

Oscillatory
magnetic coupling
betweensurface and interface

Amemiya et al., Phys. Rev. B 70 (2004) 195405.

Atomic structure of Ru/Co/Ru(0001) thin films
Fluorescence-yield EXAFS (Co K edge) : average over the whole film
Miyawaki et al., Phys. Rev.
B 80 (2009) 020408(R).

Bare Coin planeout of plane

Interface Co layer iscommensurate to Ru
Rapid relaxation uponfurther Co deposition

Effects of Ru cappingin planeout of planein planeout of planein planeout of plane

Is that true?

Relaxation of Co distortion upon Ru cappingDepth profile of atomic structure
Normal incidence: dominated by in-plane distancein planeout of plane

Layer-resolved EXAFS
Co(3 ML)/Ru(0001) Normal Incidence

6

Intensity (arb. units)

1.3
1.2

4

1.1
1.0
900

2

1000

1100

1200

It might be true…

1st layer (surface)
2nd
3rd

0
800

900

1000

1100

1200

Photon Energy (eV)

Surface shows longer oscillation period: shorter bond lengthDepth-resolved EXAFS at grazing incidenceLonger out-of-plane bond length at surface?

Grazing Incidence
Intensity (arb. units)

3
1st (surface)
2nd
3rd (interface)
2

Ru

1

800

900

1000

Photon Energy (eV)1100

Preliminary analyses by Bayes-Turchin method
In plane

Out of plane

0.251 nm0.260 nm0.255 nm0.252 nm0.255 nm0.252 nmRu

Effects of Ru capping
Normal incidence: in-plane bond lengthin planeout of plane

Layer-resolved EXAFS

EXAFS Intensity (arb. units)

1.0

Ru(4 ML)/Co(3 ML)/Ru(0001) Normal Incidence

0.5

0.0

-0.5

-1.0

1st layer (surface)
2nd
3rd
900

1000

1100

Relaxation
1200

Photon Energy (eV)

Little difference in the bond length1. Advantages and Disadvantages ofSoft X-ray Absorption Spectroscopy (SXAS)
2. SXAS studies on Surface and Thin films

3. Novel SXAS Techniques
3-1. Depth-resolved XAS

3-2. Wavelength-dispersive XAS

Development of Wavelength-dispersive XAS
XAS: Element selectivity, Chemical species determination,
Structural information,…
Takes long time (~5 min/spectrum) for a measurement.
Possibility of “One shot” measurement.

S2

Wavelength

Position-selective
electron detection

Experimental setup for wavelength-dispersive XAS
Wavelength-dispersed X rays + Position-sensitive electron detector

Test Measurement

Amemiya et al., Jpn. J. Appl. Phys. 40, (2001) L718.

Comparison withconventional XAS~100 times faster!

θ= 90°Partial Electron Yield (arb. units)

Example：C4H4S/Au(111)12

C K edge
10

h



8



 = 15 o = 55 oo = 906
4C-S
C-C

2

“flat-lying”

0
2 80

290

300

310

Photon Energy ( eV)

Orientation changewith increasing coverageChemical Reaction：NO/N/Rh(111)
Nakai et al., J. Phys. Chem. B 110 (2006) 25578.

T=250 K

T=120 KNO
N+NO→N2O↑

atomic N

405

405
0

400

1000

2000

2000

4000

400
6000

0.7
0.20

0.20

0.5

0.16

0.2

0.04
0.00

0

NO

500

1000

Time [s]

1500

2000

N

0.6
0.5
0.4

NO [ML]0.3
0.08

N [ML]

0.4

0.12

NO [ML]N [ML]

0.6

NO

N

0.16

0.7

0.12

0.3
0.08

0.1

0.04

0.0

0.00

0.2
0.1
0

1000 2000 3000 4000 5000 6000

Time [s]Induction period: reaction does not start immediately
Faster reaction at lower temperature

0.0Lower Temperature
New species

T=70 K, PNO=5x10-9 Torr

Nakai et al., J. Phys. Chem.
B 126 (2007) 044704.

Coverage [arb. units]

1.0

NO
N
405

0.8
0.6

Atomic NNO
New species

0.4
0.2
0.0

400
50

100

150

200

500

1000

1500

2000

2500

3000

Time [sec]New species

Intensity [arb. units]

0Appearance of new species⇒ NO dimer

NO
N“New species” might be precursor.
396

398

400

402

404

406

Further Development
Undulator beamline (BL-16A)
Video rate (~30 Hz)
CO + O reaction on Ir(111)

Amemiya et al., Appl. Phys. Lett. 99 (2011) 074104.