Investigation of nanostructures with large scale X-rays...
Transcript of Investigation of nanostructures with large scale X-rays...
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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)