Cheiron School 2012

Absorption Edges in the Soft X-ray Region

Soft X-ray Absorption Spectroscopy

Atomic Number

M edgeL edgeKenta Amemiya (KEK-PF)

K edge.
Li
Absorption-edge Energy (eV)Studies using Soft X-ray
Soft X-ray Beamlines~14/53 at Photon Factory (2.5 & 6.5 GeV)
~5/50 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

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

1. El1
Elementt selectivity㻌
l ti it

12



h

8

(C: 290 eV, O: 530 eV, Fe: 710 eV, Ni: 850 eV…)6  C- S

4

2. Information on chemical species

 = 15o = 55o = 90oCharacteristic spectral features (π*䠈σ*…)

3. Structural information (bond length, etc.)

C -C

2

EXAFS (Extended X-ray Absorption Fine Structure)

4. Information on anisotropy

0280

How can we measure
X-ray absorption spectrum ?

3 ML Fe / Cu(100) Fe L-edge XAS

Core-hole excitation (1s, 2p…)

C K edge
10

XAS Measurement in the Soft X-ray Region

290

3 00

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

310

Linear polarization㻌 㻌(molecular orientation, lattice anisotropy)

5. Magnetic information
Circular polarization

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

0.025

Absorption Intensity (arb. units)

Partial Elec tron Yield (arb. uni ts)

X-ray Absorption Spectroscopy (XAS)

2p3/2→3d(LIII)

0.02

X-raysCore
hole

Auger electrons

A

0.015

0.01

~nm

~mm

(core hole relaxation)

2p1/2→3d(LII)

Detector(Retarding voltage)

0.005

Electron yield XAS
Total electron yield (TEY)㻌 ~3 nm

0700 710 720 730 740 750

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

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.
㻌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 Spectroscopy

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

Thiophene (C4H4S) molecule on Au(111)

DeterminationDeteerminarmination of Chemical Species
rmin
Initial oxidation process of Si
Dipped in thiophene
solution

Molecular O2

Oxidation

C6H13SH

Exposed to thiophene
gas in vacuum

Chemical species
Structural information (orientation)

Determination of Atomic Structure

Different chemical species depending
on preparation processes

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

Sako et al., Chem. Phys. Lett. 413, (2005) 267.

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

Extended X-ray Absorption Fine Structure (EXAFS)Application to surface molecule (CH3O)
CH3O/Cu(111)S (E) •••Coordination number (N)Inter-atomic distance (r)DW factor (σ2)

CH3O/Cu(111)EXAFS

NEXAFS
EXAFS oscillation

0 ( k )    S  /  0
N F (k ) 2 2
  S 02 i i 2 i e 2 ki i e 2 ri / i ( k ) sin(2ki ri  i (ki ))ki riiFe K-edge XAFS spectrum μ(E) of FeO

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

Magnetic structures studied by XMCD
3 ML Fe / Cu(100)
Fe L-edge XMCD

Surface-EXAFS
SurfaceEXAFFSo = 152

Absorption Intensity (arb. units)

1
0

-2

o

 = 902

k (k)

-1

2
1
0
-1
-2
3

4

5

6

7

8

9

10

11

Fourier Transform [arb. units]

k [1/A]

C6H13S/Cu(100)

S-Cu1

3.0

o

2.5

90

2.0

15

o

1.5

S-S1
S-C

1.0

S-Cu2

S-Cu3

0.5
0.0

0.025
0.02
0.015
0.01

X-ray Magnetic Circular Dichroism (XMCD)Helicity Spinμ↑↑ (μ+)μ↑↓ (μ䠉)

Difference in absorption intensities between
right- and left-hand circular polarizations

2p3/2→3d(LIII) 2p →3d
1/2
(LII)

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

0.005
0

-0.005

3. High sensitivityμ↑↓ 䠉μ↑↓

-0.01
0

1

2

3
4
Distance R [A]5

6

700 710 720 730 740 750

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

0.03

Finite 3d orbital moment

XMCD Sum Rules
Orbital moment

Helicity+1

Spin moment1

Dipole transition probability

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

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

Magnetism of Thin Films Studied by XMCD

Utilization of Element Selectivity of XMCD㻡P. Gambardella, Nature 416, 301 (2002)㻵㼚㼠㼑㼚㼟㼕㼠㼥㻌㻔㼍㼞㼎㻚㻌㼡㼚㼕㼠㼟㻕

Co L-edge XMCD spectra㻠Fe L-edgeFe㻟Co L-edge

Co

㻞
㻝
㻜
㻢㻤㻜

㻣㻜㻜

㻣㻞㻜

㻣㻠㻜

㻣㻢㻜

㻣㻤㻜

㻤㻜㻜

㻤㻞㻜

㻤㻠㻜

㻼㼔㼛㼠㼛㼚㻌㻱㼚㼑㼞㼓㼥㻌㻔㼑㼂㻕

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)

XMCD sum rules

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

㻌

840

860

880

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

840

0.0

Grazing
Normal-0.5

㻌Grazing
Normal
-0.5

㻌

0.0

-1.0

-1.0

20

30

-1.5

-30

-20

-10

0

10

20

30

H (Oe)

05
0.5

XMCD intensity [a.u.]㻌

XMCD intensity [a.u.]

XMCD intensity [a.u.]Grazing
Normal

-0.5

10

㻌

0.5
05

0.0

0

Angle Dependence of XMCD
(2) High magnetic field measurements

Angle Dependence of XMCD
(1) weak magnetic field or remanent measurements㻌-10

H (Oe)

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

0.5

-20

Angle-dependent XMCD Magnetic anisotropy
Separation of ms from mT

-1.0
860

880

Photon Energy [eV]

840

860

880

Photon Energy [eV]Fe (0.7 ML)
Ni (7.5 ML)
Cu(100)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 orbital moment estimation of magnetic anisotropy-B+B

X raysAbe et al., J. Magn. Magn. Mater. 206 (2006) 86.
Sakamaki and Amemiya, Appl. Phys. Express 4 (2011) 073002.

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

Investigation of Interface Magnetism

Angle-dependent XMCD Sum Rules
Orbital sum rule-[I L3  I L2 ]
I L3  I L2

Au/Co(2 ML)/Au(111)

3  ml
4nh   B

(ms  7  mT )2nh   B

Spin sum rule[I L3-2  I L2 ]

I L3  I L2-

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

ml = mlԋcos2 + mlҋsin2
mT = mTԋcos2 + mTҋsin2
ms does not depend on -

T. Koide et alTal., Phy
Phys.
ys Re
ys
Rev
Rev. Lett. 87, 257201 (2001).

Self-assembled Co islands
due to a reconstruction ofAu surface
All Co atoms are regarded to“interface” because of 2 ML
thickness㻌 э Direct observation ofinterface magnetism
Determination of
ms, ml//, mlԋ, mT//, mTԋ㻌 э Direct determination of
ms, ml//, mlԋ, mT//, mTԋ

Angle-dependent XMCD Measurements
T. Koide et al., Phys.
Ph
Rev. Lett.. 8
87, 257201 (2001).

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

PF BL-11A

Angle dependence in XMCD
Anisotropy in ml, mT㻌mj = mjԋcos2 + mjҋsin2㻌 㻌 (j = l or T)

mTԋ+ 2 mTҋ= 0

э Determination of all
moments including㻌 their anisotropy

Cluster-size dependent
phase transition

Introduction: Exploring Magnetic Depth Profile
Surface

Interface effect: TMR

Thin Film
Interface

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

Surface effect: Gas adsorptionH㻌 㻌 Ni(8 ML)
㻌 㻌 CuVollmer, et al., Phys. Rev. B 60 (1999) 6277.

Adsorption-induced change inmagnetic easy axisэ㻌 What happens at surface?

- Surface & interface orbital moment -Chemical and magnetic statesat interface affect MR ratio

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

Tischer et al., Phys. Rev. Lett.
75 (1995) 1602.

Gives averaged information over the whole sample.
э㻌 also averaged in depth

Total Magnetic Moment

Co/Cu(100)

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

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

How can we measure
X-ray absorption spectrum ?

0.025

Absorption Intensity (arb. units)

Principle of Depth-resolved XAS

2p3/2→3d(LIII)~mm

0.02

SampleX-rays(core hole relaxation)

2p1/2→3d(LII)

Detector(Retarding voltage)

0.005

e-

Auger electrons
XAS data

Total electron yield (TEY)
700 710 720 730 740 750

Partial electron yield (PEY)

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

Probing Depth (effective escape depth)䠖 e
Number of detected electrons emitted at depth z : I = I0exp( z/ e)
I0䠖Original number of emitted electrons

Phosphor screen

eCCD camerae- Surface sensitive

Electron yield XAS0dMicrochannel platee-Auger electrons

A

0.01

X-rays

~nm
Core
hole

0.015

Surface

X-raysElectron yield XAS measurements at different detection angles
A set of XAS data with different probing depths

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 Fe

Proportion of detected electrons

1

Smaller e㻌 э Larger contribution from surface

λe = 5 nm
10 nm

0.8

100 nmXAS:
㻌 Averaged information per atom

0.6

o
o

0.4

Depth-resolved XAS䠖㻌 exp( z/ e)-weighted average

0.2

Fe

Uniform

Fe

Magnetization

0

0

2

4

6

8

Depth z (nm)10

Cu(100)Surface
Magnetization

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

Interpretation of depth-resolved XMCD data

Fe/Cu(100)>200 K<200 KFe

Surface (FM)2.5
Cu(100)Fe

2

3 ML (130 K)

1.5

Cu(100)Fe

1

7 ML (200 K)
Cu(100)0.5

Inner layers (AFM or SDW)
No (little) magnetic interaction between
Cu and interface (bottom) Fe

Fe/Ni/Cu(100)

7 ML (130 K)
Fe

0
6

8

10

12

14

Probing Depth

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

16

18

FM

20

o

(A)

Ni

SDW
q=2/2.6d

Cu(100)Fe L-edge Depth-resolved XMCD
Grazing Incidence (200 K)

Fe

Fe

Ni

Ni

Fe
Ni

Any magnetic interaction
among surface, inner layers
and interface ?

Fe thickness dependence at 200 K

Curve fitting with a three-region model

Surface magnetization
Msurface(B)Fe

9 ML Fe

200 K

0.6

1.1 BFe

0.1 BNi

1.5 B

0.4

5 ML Fe

5 ML Fe0.7 B0.0 B
2.1 B

2 ML
Fe

-0.4

1 ML
7

8

9

10

Probing Depth

Oscillatory
surface magnetization

-3

0
-0.2

Ni

-2

9 ML Fe

0.2

0

-1

Interface magnetization
Minterface(B)

Effective Spin Moment ms+7mT (B)

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

1

11

12
o

13

14

Ni

(A)

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.

Depth profile of atomic structure

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

Bare Coin planeout of planein planeout of planein planeout of plane

Is that true?

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

6

1.3

Intensity (arb. units)

Effects of Ru cappingin planeout of plane

Layer-resolved EXAFS

Interface Co layer iscommensurate to Ru
Rapid relaxation uponfurther Co depositionin planeout of plane

Normal incidence: dominated by in-plane distance

1.2

4

1.1
1.0
900

2

1000

1100

1200

1st layer (surface)
2nd
3rd

0
800

900

1000

1100

It might be true…
1200

Photon Energy (eV)

Surface shows longer oscillation period: shorter bond length
Relaxation of Co distortion upon Ru cappingDepth-resolved EXAFS at grazing incidence

Effects of Ru capping
Normal incidence: in-plane bond length

Longer out-of-plane bond length at surface?

Layer-resolved EXAFS

Grazing Incidence3

1.0

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

Ru

1

EXAFS Intensity (arb. units)

Intensity (arb. units)

in planeout of planeRu(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)

800

900

1000

1100

Photon Energy (eV)

Little difference in the bond length

Development of Wavelength-dispersive XAS
XAS:㻌 Element selectivity, Chemical species determination,
Structural information,…

1. Advantages and Disadvantages ofTakes long time (~5 min/spectrum) for a measurement.
Possibility of “One shot” measurement.

Soft X-ray Absorption Spectroscopy (SXAS)
2. SXAS studies on Surface and Thin films
S2

3. Novel SXAS Techniques
3-1. Depth-resolved XAS
3-2. Wavelength-dispersive XAS

Wavelength

Position-selective
electron detection

Test Measurement

Experimental setup for wavelength-dispersive XAS

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

Comparison withconventional XAS

zWavelength-dispersed X rays + Position-sensitive electron detector~100 times faster!

Chemical Reaction䠖NO/N/Rh(111)
Nakai et al., J. Phys. Chem. B 110 (2006) 25578.

T=250 K

T=120 K㻌 㻌 NO

Lower Temperature
New species

T=70 K, PNO=5x10-9 Torr㻌

N+NO→N2O↑

1.0

NO

2000

4000

400
6000

405

㻌

㻌

0.12

0.3
0.08

0.2

0.04
0.00

0

500

1000

Time [s]

1500

2000

0.16

N [ML]

0.4

N

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

50

㻌

150

100

200

0.0

Atomic NNO
New species

0.4
0.2

0

500

1000

1500

2000

2500

3000

Time [sec]

0.5
0.4

0.12

400

0.6

New species

Intensity [arb. units]

0.5

NO [ML]N [ML]

NO

N

0.16

NO

0.20

0.6

0.6

0.0

0.7

0.7
0.20

NO

㻌

2000

NO [ML]

1000

N

405

405
400

0.8

㻌

Coverage [arb. units]

atomic N

0

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

Appearance of new species㻌 㻌 э㻌 NO dimer

N“New species” might be precursor.
396

398

400

402

404

Photon Energy [eV]406

U d l t b
Undulator
beamlineli (BL(BL-16A)
16A)
Video rate (~30 Hz)

Utilization of Linear Polarization䠖

Adsorption of C4H4S on Au(111)

CO + O reaction on Ir(111)θ= 90㼻

Partial Electron Yield (arb. units)

Recent Development

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