Cheiron School 2014

Soft X-ray Absorption Spectroscopy

Kenta Amemiya (KEK-PF)

1

Absorption Edges in the Soft X-ray Region

Atomic Number

M edgeL edgeK edge.
Li
Absorption-edge Energy (eV)2Studies using Soft X-ray
Soft X-ray Beamlines~15/50 at Photon Factory (2.5 & 6.5 GeV)
~5/55 at Spring-8 (8 GeV)
Experimental Techniques
X-ray Absorption Spectroscopy (XAS)
Photoemission Spectroscopy (PES)
Resonant X-ray Scattering (RXS)
Applications:
Organic Molecules & Polymers (C, N, O…)Magnetic Materials (Fe, Co, Ni, …)
Surface & Thin Film
3

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
4

X-ray Absorption Spectroscopy (XAS)
Partial Elec tron Yield (arb. uni ts)- Transition from core to unoccupied states -1. Element selectivity

C4H4S (thiophene) molecule

12

C K edge
10



h

8

(C: 290 eV, O: 530 eV, Fe: 710 eV, Ni: 850 eV…)



64<- Core-hole excitation (1s, 2p…)

 C- S

2. Information on chemical species

 = 15oo = 55o = 90<- Characteristic spectral features (π*，σ*…)3. Structural information (bond length, etc.)

C -C

2

EXAFS (Extended X-ray Absorption Fine Structure)

4. Information on anisotropy

0
280

290

3 00

Photon Energy (eV)In the Soft X-ray region,

310<- Linear polarization(molecular orientation, lattice anisotropy)

5. Magnetic information<- Circular polarization

1. Vacuum environment is normally required. (NOT ultra-high vacuum)Special sample cell is available for ambient pressure.
2. Surface sensitivel (probing depth):
several nm for electron yield, ~0.1 μm for fluorescence yield

5

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)

Electron yield XAS

0.005

Total electron yield (TEY) l~3 nm
0700 710 720 730 740 750

Photon Energy (eV)Partial electron yield (PEY) l~1-2 nm
cf. Fluorescence yield (FY) l~100 nm
6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.
7

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

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

Chemical species
Structural information (orientation)

9

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.
10

Determination of Geometric Structure
Extended X-ray Absorption Fine Structure (EXAFS)S (E) •••Coordination number (N)Inter-atomic distance (r)DW factor (σ2)

EXAFS oscillation

0 ( k )   -  S  /  0
2 N i Fi ( k i ) - 2 ki2 i2 - 2 ri / li ( k )  S0eesin(2ki ri  i (ki ))
2
ki ri
i

Fe K-edge XAFS spectrum μ(E) of FeO
11

Determination of Geometric 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
Higher energy -> Shorter bondAngle (θ) dependence -> molecular orientation

12

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

2. Determination of spin and orbital
magnetic moments<- Sum rules
3. High sensitivity

0-0.005

1. Element selectivity<- resonant absorption (2p->3d…)

μ↑↓ －μ↑↓Element-specific vector average ofmagnetic moment over the wholeinvestigation area

-0.01
700 710 720 730 740 750

Photon Energy (eV)Fe↑ Gd↓ Fe↑ Gd↓ → Fe and Gd XMCDFe↑ Gd↑ Fe↑ Gd↓ → no Gd XMCD 13

Principle of XMCD
No 3d orbital moment

Finite 3d orbital moment
Majority

Helicity+1-1

Dipole transition probability

μ↑↑= 3+6+6=15μ↑↓= 18+6+1=25μ↑↑< 15μ↑↓> 25μ↑↑= 3+12=15μ↑↓= 3+2=5μ↑↑> 15μ↑↓< 514

XMCD Sum Rules
Orbital moment

Spin moment

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

15

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

P. Gambardella, Nature 416, 301 (2002)

Larger orbital moment, ml
due to lower dimension
16

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)

10

20

30

17

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

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

840

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

perpendicular Information on anisotropy of orbital moment-> estimation of magnetic anisotropy
in plane

Abe et al., J. Magn. Magn. Mater. 206 (2006) 86.
Sakamaki and Amemiya, Appl. Phys. Express 4 (2011) 073002.
Sakamaki and Amemiya, Phys. Rev. B 87 (2013) 014428.
18

Angle Dependence of XMCD
(2) High magnetic field measurements
Angle-dependent XMCD
> Magnetic anisotropy
Direct determination of ms and mT
mT : dipole magnetic moment-B+B

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

19

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
ms does not depend on -

⇒ Direct determination of
ms, ml//, ml⊥, mT//, mT⊥

20

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
Determination of
ms, ml//, ml⊥, mT//, mT⊥
21

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
22

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

Cluster-size dependent
phase transition

FM: ferromagnetic
SPM: super-paramagnetic

23

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

24

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
25

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

26

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)27

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, dA set of XAS data with different probing depths
28

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

Smaller le

λe = 5 nm
10 nm

0.8

⇒ Larger 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)1029

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)30

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

SDW: Spin density wave
31

Application 1: Ferromagnetic/Antiferromagnetic interface
AFM

FM

Exchange bias
Uncompensated spin

Atomically sharp interface

Intermixing

is desirable

Roughness…

Idea of our studyO

NiO (AFM)

Single-layer NiO ?

Ni (FM)

Ni

Ideal FM-AFM interface
Cu(100)Sample preparation(1) Oxygen adsorption
5X10-4 Pa, 300 s @ 500 K

Cu(100)R. Nunthel et al., Surf. Sci. 531, 53 (2003).

(2) Ni deposition (in UHV)OEB heating of a Ni rod

O (< 1 ML)room temperatureNi

Cu(100)in situ RHEED

Cu(100)32

O
Ni (5.5 ML)

Depth-resolved XAFS for O/Ni/Cu(100)Cu(100)O/Ni(5.5 ML)/Cu(001) Ni L-edge XAS (X-ray Absorption Spectrum)6

12 (a)

le = 0.6 nm

8
6
4
2

Intensity (arb. units)

Intensity (arb. units)

5

Surface sensitive

10

(b)

le0.6 nm

Increase of L3 peakle0.8 nm

4le1.0 nm

1.5

3Growth ofshoulder

1.0

2

868

870

872

1
le = 1.0 nm

0
850

860

870

880

Photon Energy (eV)890

Decrease of Plateau

0
850

860

870

880

890

Photon Energy (eV)33

Layer-resolved X-ray absorption spectra
O/Ni(5.5 ML)/Cu(001)
Ni L-edge XAS (E//x) Extracted Spectra
1st
2nd

35 Large L3 peakIntensity (arb. units)

30O

1st Layer(Surface)

Split of L2 peak

5th

1st
2nd

Ni (5.5 ML)
5th
6th

Cu(100)

25

2nd
20

3rd

XAS for the 1st layer is different fromthose in the underlying layers

4th=> suggests single-layer oxidation

15
10

5th
5

=> 2-region model is adopted.

6th
0
850

860

870

880

Photon Energy (eV)89034

Extracted X-ray absorption Spectra

K. Amemiya and M. Sakamaki,
Appl. Phys. Lett. 98 (2011) 012501.

O

Intensity (arb. units)

Intensity (arb. units)

O/Ni(5.5 ML)/Cu(001)
Extracted Ni L-edge XAS

Ni (5.5 ML)

10

Surface8

Cu(100)

2

6

1

4

0

XAS spectra at surface
868

870

872

shows NiO-like features2
0
4

Inner Layers

3
2

NiO thin film

1
0
850

860

870

880

Photon Energy (eV)890
Haverkort et al., Phys. Rev. B 69, 020408(R).
35

Intensity (arb. units)

Extracted XMCD spectra
8

Ni L-edge XMCD

Surface64--O-Ni (5.5 ML)
Cu(100)2

Intensity (arb. units)

0
4

Surface layer shows small

Inner Layers

negative magnetization.

3
2

Uncompensated spin

1

at the interface?

0

K. Amemiya and M. Sakamaki,

-1
840

850

860

870

880

Photon Energy (eV)890

Appl. Phys. Lett. 98 (2011) 012501.
36

Application 2: 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 capping37

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
K.Amemiya et al., J. Phys.: Conf. Ser. 190 (2009) 012108.

38

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

1100

Photon Energy (eV)K.Amemiya et al., J. Phys.: Conf. Ser. 190 (2009) 012108.

39

Preliminary analyses

40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
K.Amemiya et al., J. Phys.: Conf. Ser. 190 (2009) 012108.

41

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

42

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

44Comparison with conventional XAS
Amemiya et al., Jpn. J. Appl. Phys. 40 (2001) L718.
Amemiya，放射光，25 (2012) 269.

Data acquisition time: 1/10,000 !
45

Observation of Chemical Reaction
Undulator beamline (BL-16A)
Video rate (30 Hz: 33 ms resolution)
CO + O reaction on Ir(111): CO + O → CO2↑O K edge

K.Amemiya et al., Appl. Phys. Lett. 99 (2011) 074104.
46

Adsorption of C4H4S on Au(111)θ= 90°

Partial Electron Yield (arb. units)

Determination of Molecular Orientation
12

C K edge
10



h

8



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

C-C

2

0
2 80

290

300

310

Photon Energy ( eV)

Orientation changewith increasing coverage

K.Amemiya et al., J. Electron Spectrosc. Relat. Phenom. 124 (2002) 151.

47

Development of Real-time Observation of Orientation
Combination of dispersive XAFS and linear polarization switching
between horizontal and vertical polarizations
Polarization dependence

Dispersive XAFS

θ= 90°

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

K.Amemiya et al., J. Electron Spectrosc. Relat.
Phenom. 124 (2002) 151.

Polarization switching

to BL
Undulator I
Kicker magnets

Undulator II

Fast
Switching

T. Muro et al., AIP Conf. Proc. 705, 1051 (2004); 879, 571 (2007).
T. Muro et al., J. Electron Spectrosc. Relat. Phenom. 144-147, 1101 (2005).
K. Amemiya et al., J. Phys.: Conf. Ser. 425, 152015 (2013).

48

Combination of Dispersive XAFS and Polarization Switching
Adsorption of NO molecule on Ir(111)NO/Ir(111)2.5

Intensity (arb. units)

1 Hz switching between horizontal andvertical linear polarizations

Vertical Polarization
Horizontal Polarization

2.0

1.5

1.0

400

405

410

Photon Energy (eV)

10 Hz switching is now available(measurement: 1000 Hz)

49

Real-time Observation of Molecular Orientation
Amemiya et al., Appl. Phys. Lett., 101 (2012) 161601.
1 Hz switching

Intensity (arb. units)

N K edge

400.8 eV

140
120
100
80

Horizontal (Eh)Vertical (Ev)

60
40
20

NO exposure start

20

22

24

26

28

30

32

Time (s)80

Intensity (arb. units)(b)

Horizontal (Eh)60
Vertical (Ev)

40

20

20.0

20.5

Time (s)21.050

Polarization Dependence at Saturation Coverage
Amemiya et al., Appl. Phys. Lett., 101 (2012) 161601.

N K edge(a) NO/Ir(111)
Vertical Polarization
Horizontal Polarization
N 1s -> NO *

3

20(+15/-20)

o

2
1
0

400

405

410

Photon Energy (eV)N 1s -> N2O *

Intensity (arb. units)

Intensity (arb. units)4(b) N2O/Ir(111)2

1
45 (+/-10)

0

395

400

405

o

410

Photon Energy (eV)51

Molecular Orientation Change during NO Adsorption
Amemiya et al., Appl. Phys. Lett., 101 (2012) 161601.

Molecular adsorption of NO at 140 K
80

Intensity (arb. units)(a)60

Horizontal (Eh)Vertical (Ev)

40

20

20.0

20.5

21.0

Time (s)Simultaneous increase between vertical and horizontal polarizations=> Constant orientation during adsorption
52

Molecular Orientation Change during N2O Adsorption
N2O adsorption at 140 K

1.0

Vertical polarization
Horizontal polarization

2nd

3rd

50
o

35

o

45

40

0.5
1.0
o

40

0.5

30

0.0

0.0
40

50

N2O exposure start
50

100

60 70 80
Orientation angle

150

Orientation Angle (deg)Normalized Peak Intensity

1st

Amemiya et al., Appl. Phys. Lett., 101 (2012) 161601.

Three stages withdifferent adsorption
rates
Averaged orientation
gradually increases

20200Time (s)1st stage: Fast adsorption up to ~2/3 of saturation. Constant orientation at ~35º.
2nd stage: Up to ~90% of saturation. Gradual increase in orientation angle.
⇒ Suggests another adsorption site with 45-50º orientation.
53
3rd stage: Very slow changes towards saturation adsorption state.

Soft X-ray absorption spectroscopy- Suitable for familiar materials
Organic molecules & polymers
Magnetic nano-materials- Surface sensitive
a double-edged swordeSoft X rays

e

h

h
~ nm
~ 0.1 m???
54