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Investigation of nanostructures Investigation of nanostructures with large scale Xwith large scale X--rays facilitiesrays facilities

J.L. GALLANIJ.L. GALLANIS. CHERIFIS. CHERIFI

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TheThe synchrotron assynchrotron as seenseen by...by...

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the Korean studentthe Korean student........

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First synchrotron light at General Electric, 1947

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ESRF ESRF -- GrenobleGrenobleDESY

White beam

Pohang Accelerator Laboratory

APS Argonne (USA)

BESSY

SLS SwitzerlandSOLEIL - France

NSLS - Brookhaven

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LINACLINAC

BoosterBooster

beam

line

beam

line

undulatorundulatorwigglerwiggler

bending magnetbending magnet

quadrupolequadrupoleopticsopticssamplesample

insertioninsertion devicedevice

hutchhutch

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charged particlecharged particle

γγ

bremstrahlungbremstrahlung

bending magnetbending magnet -- SLSSLS

««sweepingsweeping» light» light

BB

bending magnetbending magnet

needneed more flux? more flux?

put moreput more magnetsmagnets!!

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SC SC -- 7T7T wigglerwiggler(BESSY)(BESSY)

wigglerwiggler

undulatorundulator

broad beambroad beam ofofincoherentincoherent lightlight

narrow beamnarrow beam of of semisemi--coherentcoherent lightlight

TunableTunable insertioninsertion devicesdevices :: wigglerswigglers anan undulatorsundulators

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xx--rays are EM radiationsrays are EM radiationsthey can be polarizedthey can be polarized

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monochromatormonochromator SiSi monocrystalsmonocrystals

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44--23 MBI23 MBI--BESSYBESSY beamlinebeamlinephotonphoton induced process atinduced process at surfacessurfaces

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So what can weSo what can we dodo withwith a synchrotron?a synchrotron?

nanolithographynanolithographythinthin filmsfilms magnetometrymagnetometry : XMCD: XMCDxx--raysrays spectrospectro--microscopymicroscopy

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AA firstfirst application :application : lithographylithography

ITRSITRS roadmaproadmap

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RayleighRayleigh criteriacriteria : :

Current wavelengthCurrent wavelength : 193nm (: 193nm (DeepDeep UV)UV)Next stepNext step : 13.5nm (: 13.5nm (ExtremeExtreme UV)UV)

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many problemsmany problems !!

-- everything absorbs ateverything absorbs at 13.5nm13.5nm-- nono diffractive opticsdiffractive optics-- fabrication offabrication of the maskthe mask-- source source

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zone plateszone plates

binarybinary sinusoidalsinusoidal

resolutionresolution :: focal distance :focal distance :

zone plate operatingzone plate operating atat λλ=45nm=45nm

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masklessmaskless nanonano--lithographylithography

H.I. Smith, MITH.I. Smith, MIT

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compact synchrotron (Strathclyde Uni.)compact synchrotron (Strathclyde Uni.)

fabrication offabrication of thethe zone plateszone plateslight sourcelight source at high energies at high energies

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A secondA second exampleexample : XMCD: XMCD

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XMCD : X-ray Magnetic Circular Dichroism

2 transition selection rules : Δl = 1 and Δm = ±1

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Fermi’s «golden rule» gives the absorption coefficients µ+ and µ-:

)()(~)( 20 ρσργ Δ±±=±

zc EMµµEµ

Absorption is spin dependent:

fifif Mhρπλ

22= ∫= υψψ dVM ifif

*where :

L2 absorption of RCP photon

At the L2 edge, the atom preferentially (3/4) emits spin-down electrons

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L3 absorption of RCP photon

The photoelectron ejected in the absorption process acquires a spin σz and an orbital polarization lz

At the L3 edge, the atom preferentially (5/8) emits spin-up electrons

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μs = (nh↓ − nh

↑ )μB

2p3/2

σ+σ–

2p3/2

2p1/2

3d

3dEF

exchange

spin–orbit

2p

3d

σ+ σ–

62.5%

25% 75%

37.5%

« spin-polarized source »

« spin-sensitive detector »

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Sample in cryostat

X-rays

fluorescence photo-electrons

Differences in light absorption cause differences inthe photocurrent → dichroic signal

H→

λ

XMCD :

- is very sensitive (0.001 monolayers)

- is element specific (also gives oxydation state)

- can provide information on spin and orbital magnetizations separately(magneto-optical sum rules)

A good review : T. Funk. et al., Coord. Chem. Rev. 249, 3-30, 2005More details : H. Wende, Rep. Prog. Phys. 67 (2004) 2105–2181

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cryostat

beamline(LURE)

chamber lockincoming photons

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∗ 1 cryostat, two setups : -liquid 4He, down to 1.5K-3He dilution, down to 300mK

∗ Field up to 7T∗ Ultra-high vacuum (10-10mbar)

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920 930 940 950 960 970 980

-0,04

-0,02

0,00

0,02

1,0

1,1

1,2

Cu L3,2 edges

CuTb-hfac

T~340mK & H=3T

Photon Energy (eV)

XM

CD

(arb

. uni

ts)

XAS spectra for 2 polarizations

Dichroism

A real example

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Dic

hroi

smus

nur μS

L3

L2

sz+1/2–1/2

lz –2 –1 0 1 2

3d

spin moment only

sz+1/2–1/2

lz –2 –1 0 1 2

orbital moment only

3d

dich

rois

m

only μL

Sum rules

dich

rois

m

only μS

Dic

hroi

smus

nur μL

L3

L2

L 3 L 3

L 2

L 2

Dic

hroi

sm

Dic

hroi

sm

Energy

dich

rois

m

μS and μLμS

und μL

Dic

hroi

smus

L3

L2

L 3

L 2Dich

rois

m

Energy

Energy

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1170 1185 1200 1215 1230 1245

-0,2

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

MIV

MV

Gd M edge

MIV: 3d3/2 → 4fMV: 3d5/2 → 4f

GdN 10K/6T

TEY

Photon energy (eV)

Measured signals

Dichroic signals

pqq

1170 1180 1190 1200 1210 1220 1230 12400,0

0,5

1,0

1,5

2,0

2,5

Sum

of a

bsor

ptio

n si

gnal

s

E (eV)

0

2

4

6

8

10

12

r

Integrated signals770 775 780 785 790 795 800 805 810

-0.2

-0.1

0.0

1.4

1.5

1.6

1.7

Co edge

LII: 2p1/2 → 3dLIII : 2p3/2 → 3d

LII

LIII

Co particles on K surface

TEY

Photon Energy (eV)

Sum rules for 3d metals:

rnqpatomm

rnqatomm

dBspin

dBorb

)10)(46(]/[

)10(4]/[

3

3

−−−≈

−−=

μ

μ

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abso

rptio

n

900880860840820800780760

photon energy (eV)

CoNi

magnetization parallel to x-rays

magnetization antiparallel to x-rays

L3

L2L3

L2

σ– circular polarization6 ML Co/5 ML Cu/15 ML Ni/Cu(001)

CoCuNi

Element selectivity

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A few recent examples :

- Metal nanoparticles

- Chromium on gold

- Cobalt on platinum

- Molecular magnetism

- Single Molecule Magnets

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Why metal nanoparticles?

- fundamental reasons and questions:

magnetism in low D, quantum effects, crossover 2D-3D,

reduced coordination, new effects or behaviours (e.g. AF→ FM)

- practical reasons : applications, applications, applications…

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Chrome on gold

C. Boeglin et al., Phys. Stat. Sol. B, 242, 9, 1775

Possibility of ferromagnetism for Cr atoms in hexagonal symmetry

0.07 ML 0.5 ML 0.75 ML

0.004 monolayer

- adatoms are atomic Cr- M = 4.5µB (paramagnetic)- clusters are AF with M = 0

when size increases.

7T / 10K

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MAE of Cobalt on platinum

Magnetic Anisotropy Energy determines the stabilityof the magnetization in the bulk and in nanoparticles.

85 Å

Co adatoms / Pt 0.01 Monolayer

5K / 7T

mL(meas.) = 1.1 µB/atommS(calc.) = 2.14µB mtot. = 5µB/atom

P. Gambardella et al., Science, 300, 1130, 2003

Quenching of Lwith cluster size

Bulk Co 2D Co 1D Co chainsLmes. = 0.15 0.29 0.68 µB

«Survival» of the atomic characters for 3d metal atoms on surfaces because of the reduced coordination

And broken symmetry.

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field

x-rayssample

θ

T=5.5K

isolated adatoms 4 atoms 16 atoms

Extremely large MAE

Adatoms Co in SmCo5 Co atomic chains

MAE = 9.3 1.8 2.0 meV

P. Gambardella et al., Science, 300, 1130, 2003

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µ Bµ B

µ B

MAE

MAE

MAE

MAE

0.0

0.05

0.9

9.3

meV

Mag

neti

zati

on

Magnetic field (T) 10

10

1

10

Mag

neti

zati

onM

agne

tiza

tion

Mag

neti

zati

onMagnetic field (T)

Magnetic field (T)

Magnetic field (T)

in plane

out of plane

3.0 3.0

LS

µ B

2.11.1

LS

2.1

0.9

LS

1.6

0.15LS

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Reduced coordination of atoms on a flat surface favors survival of atomic-likecharacter.

Stable ferromagnetic particles can be made smaller by artificially reducing the coordination.

Magnetic anisotropy can be so high that a 7T field does not switch the moment of an adatom.

Single Cobalt atoms on platinum

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Ferromagnetism in Co chains on Pt

P. Gambardella et al., Nature, 416, 301, 2002

Easy direction

80° away from easy direction

Chains of ~80 atoms

1D superparamagnetic ferro – TB ~ 15K

~15 spins blocks in ~80 atoms chains

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Single Molecule Magnets : SMMs

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Traditionally, ferromagnetism is a collective phenomenon :

Tc

But some molecules individually behave as small magnets :

Single Molecule Magnets or SMM

Fe8 Mn12Mn84Mn84

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[Mn12O12(CH3COO)16(H2O)4] has S=10[V15As6O42(H2O)]- has S=1/2[Fe8O2(OH)12(tcan)6]8+ has S=10[Ni21(cit)12(OH)10(H2O)10]16- has S=3

MnIVS=3/2

MnIIIS=4/2

Magnetization in Mn12

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Measurement of a monolayer of Mn12

1,2

1,4

L2L3

H = 3TT = 2K

RHCPL LHCPL degraded Mn12

Photon Energy [eV]

XA

S In

tens

ity [a

rb. u

nits

]

Mn edges

640 650 660-0,05

0,00

0,05

XMCD monolayer XMCD bulk

XM

CD

[arb

. uni

ts]

-3 -2 -1 0 1 2 3

-1,0

-0,5

0,0

0,5

1,0[Mn12(L3,4,5-CB)16]

2.0K

XMCD data

Mag

netic

mom

ent o

f Mn

[arb

. uni

ts]

Applied field [T]

-1,0

-0,5

0,0

0,5

1,0

Mag

n. m

omen

t of m

olec

ule

[arb

. uni

ts]

SQUID on bulk sample

M

H

EurEur. Phys. J. B., 73, 103. Phys. J. B., 73, 103--108, 108, 2010. 2010.

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OO

Cu

N

N

O

Tb

O

O

F3C

CF3

O

O

CF3

CF3

OO

O Cu N

N

Tb

O

O

O

OF3C

CF3

O

O

CF3

F3C

S=8.5 - TB=1.2K

S. Osa, et al., J. Am. Chem. Soc., 2004, 126, 420-421.

A 3d-4f SMM

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1230 1240 1250 1260 1270 1280 1290-0.14-0.12-0.10-0.08-0.06-0.04-0.020.00

1.0

1.1

1.2

Tb M5,4 edge

CuTb-hfac

T~280mK XMCD (x10) at H=0T

(before applying any field) XMCD at H=5T

Photon Energy (eV)

XM

CD

(arb

. uni

ts)

920 930 940 950 960 970 980

-0.04

-0.02

0.00

0.02

1.0

1.1

1.2

Cu L3,2 edges

CuTb-hfac

T~340mK & H=3T

Photon Energy (eV)

XM

CD

(arb

. uni

ts)

-6 -4 -2 0 2 4 6

-0,8

-0,6

-0,4

-0,2

0,0

0,2

0,4

0,6

0,8

Dy3Cu6Dy M5 edgeT=350mK

XM

CD

/ a

.u.

Applied field / T

T. Hamamatsu, et al., Inorg. Chem., 2007, 46, 4458.

« butterfly » magnetization curve :spin-phonons interactions

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Magnetic polarization on N and O atoms

395 400 405 410 415-0.0020

-0.0015

-0.0010

-0.0005

0.0000

0.0005

1.00

1.05

1.10

XMCD ±6T1% edge jump effect

CuTb_hfac

T=600mKH=±6T

Photon Energy (eV)

XM

CD (a

rb. u

nits)

N K edge O K edge

528 530 532 534

-1

0

1

2

3

CuIITbIII

NiIITbIII

CuIIDyIIIXAS

(arb

. uni

ts)

Energy (eV)

0.0

0.2

0.4

XM

CD

(arb. units)