Post on 21-Apr-2023
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Paul Scherrer Institut, Swiss Light Source
Frithjof NoltingX-ray absorption spectroscopySurface Science, FS 2011
F. Nolting Surface Science FS 2011
Surface Science lecture
Di, 22.02.2010 Fixing Dates, Intro to Vacuum Technology, Labvisits Basel (Thomas Jung, Roland Steiner) Di, 01.03.2010 Introduction, Concepts Samples and Structure (Thomas Jung) Di, 08.03.2010 Adsorption / Desorption (Thomas Jung) Di, 15.03.2010 Fasnacht Di, 22.03.2010 Electronic Properties and Surface Electron Spectroscopies: XPS/UPS, Auger, ARPES (Nirmalya Ballav) Di, 29.03.2010 Electron Diffraction Methods, in particular RHEED, LEED (Bert Müller) Di, 05.04.2010 Diffusion and Growth (Thomas Jung) Di, 12.04.2010 X-ray Absorption Spectroscopy (Frithjof Nolting) Di, 19.04.2010 Surface Magnetism XMCD / PEEM (Frithjof Nolting) Di, 26.04.2010 Local Probes and Experiments I, STM, Inelastic tunneling and STS (Silvia Schintke and Thomas Jung) Di, 03.05.2010 Local Probes and Experiments II, AFM FIM (Thomas Jung) Di, 10.05.2010 Surface Optics, Kelvin Probe (Thilo Glatzel) Di, 17.05.2010 Applications of Surface Science in Industry (M. de Wild) Di, 24.05.2010 Schlussprüfung (Christian Wäckerlin, Thomas Jung) Di, 31.06.2010 Excursion (Thomas Jung)
Di, 12.04. 2011 X-ray Absorption Spectroscopy (F. Nolting) Di, 19.04. 2011 PEEM and X-ray Microscopy (F. Nolting)Both with an emphasis of magnetism
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Repetition IV
• Winkelaufgeloeste PhotoelektronenspektroskopieARPES / Fermi Surface Mapping
• Diffusion / Random Walk in 2D
• Fick‘s Laws
• Atomistic Diffusion Mechanisms / Defects / Steps
• Mass Transfer / Anisotropy
• Cluster Diffusion
F. Nolting Surface Science FS 2011
Surface Science – Interface science
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F. Nolting Surface Science FS 2011
CoCoO
NiO
Ni
Co
NiO
Surface Science – Interface science
F. Nolting Surface Science FS 2011
X-ray absorption spectroscopy (XAS)Absorption processTotal electron yield modeExamples
X-ray Magnetic Circular Dichroism (XMCD)BasicsExample: Magnetocrystalline Anisotropy
Closer look at the absorption processMulitplet effectsExample: Interface effect in Exchange Bias system
X-ray Magnetic Linear Dichroism (XMLD)Basics
Outline
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F. Nolting Surface Science FS 2011
Absorption - Teflon
http://henke.lbl.gov/optical_constants/
F. Nolting Surface Science FS 2011
Extended X-ray Absorption Fine Structurereflects spatial location of neighboring atoms
Near Edge X-ray Absorption Fine Structurereflects density of unoccupied statesAlso called XANES
Absorption of Photons in the Soft X-ray Range
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F. Nolting Surface Science FS 2011
F. Nolting Surface Science FS 2011
EF energy
d states
s,p states
~~
2p 3/22p1/2
core level valence band
photon energy
absorption
Fermi’s golden rulein dipole approximationwavelength is large compared to charge1000 eV corresponds to 1.2 nm2p core radius is about 0.01 nm
Final state Initial state
Transition matrixDensity of final statesAbsorption ~
E)(ipef2
Interaction of electromagnetic wave with charge
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F. Nolting Surface Science FS 2011
7 0 0 7 2 0 7 4 0 7 6 0 7 8 0 8 0 0 8 2 0 8 4 0 8 6 0 8 8 0 9 0 00
1 0
2 0
3 0
4 0
5 0
6 0
7 0
N i
C oF e
Inte
nsi
ty (
a.u
.)
P h o to n e n e rg y (e V )
Element specific
F. Nolting Surface Science FS 2011
Initial excited fluorescence Auger
Soft X-ray range ~ 5% ~ 95%
Photoemission
Secondary electrons
Decay channels
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F. Nolting Surface Science FS 2011
vacuum
sample
sample surface
secondary electronsEscape depth ~ 5 nm
h
FluorescenceEscape depth ~ 50 - 100 nm
Auger electronsEscape depth ~ 0.5 nm
X-ray penetration depth~ 50 - 100 nm
Sampling depth
F. Nolting Surface Science FS 2011
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F. Nolting Surface Science FS 2011
xeI)x,E(I )E(0
I(nA)
h
x100
kinetic energy
Ele
ctro
n yi
ed Secondary electrons
Auger peak
Total electron yield (TEY)
F. Nolting Surface Science FS 2011
e-hn
Probability that electron escapes
edgePre edge
Sample surface
Penetration depth depends on the angle of incidence
Why is TEY proportional to absorption coefficient
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F. Nolting Surface Science FS 2011
J. Lüning et al, PRB 67, 214433 (2003)
Saturation Effects in TEY Detection
F. Nolting Surface Science FS 2011
Courtesy J. Stöhr
“Dismantle” a Multilayer
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F. Nolting Surface Science FS 2011
Charge sum rule
Integrated intensity is proportional to number of empty valance states
Courtesy J. Stöhr
Determine number of 3d holes
F. Nolting Surface Science FS 2011
X-ray absorption spectroscopy (XAS)Absorption processTotal electron yield modeExamples
X-ray Magnetic Circular Dichroism (XMCD)Crash class nanomagnetism probed with X-raysBasicsExample: Magnetocrystalline Anisotropy
Closer look at the absorption processMulitplet effectsExample: Interface effect in Exchange Bias system
X-ray Magnetic Linear Dichroism (XMLD)Basics
Outline
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F. Nolting Surface Science FS 2011
Crash class nanomagnetism probed with X-rays
F. Nolting Surface Science FS 2011
Source of magnetism (atomic)
Spin moment ~ 1.5 μB / atom isotropic
Orbital moment ~ 0.1 μB / atom isotropic/anisotropic
They interact via the spin-orbit coupling L · S
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F. Nolting Surface Science FS 2011
Switching on the interaction
Atoms have an magnetic moment
Without interaction (and no applied magnetic field) they point in random directions and no macroscopic magnetic field is created
F. Nolting Surface Science FS 2011
Switching on the interaction
Atoms have an magnetic moment
With interaction they can align to each other and can create macroscopic magnetic field
Domain 1 Domain 2Domainwall
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F. Nolting Surface Science FS 2011
Hysteresis loop
Responses of a material to an applied magnetic field is described by the hysteresis loop
Remnant magnetization
Coercive field
Saturation magnetization
External applied magnetic field
Magnetization of the material
F. Nolting Surface Science FS 2011
Different magnetic interactions
Direct exchangeOverlapping wavefunctions
superexchangeOverlapping wavefunctions via non-magnetic atoms
Indirect exchangeExchange via delocalised electrons
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F. Nolting Surface Science FS 2011
Angular momentum conservation
Angular moment of orbiting mass Photon angular moment
Polarized Photons
F. Nolting Surface Science FS 2011
Photon energy
I
Photon energy
diff
Photon energy
I
Polarized Photons
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F. Nolting Surface Science FS 2011
Spin moment
Origin of XMCD
F. Nolting Surface Science FS 2011
EF
2p1/2
2p3/2
3d
4sp
Dipole selection rules:
ΔL = -1, +1
Δ J = -1, 0, +1
Δ MJ = -1, 0, +1
Polarization q +1, 0, -1 (s+, p, s-)
L2: 2p1/2�4s, 3d3/2
L3: 2p3/2�4s, 3d3/2, 3d5/2
n transitiodipole ,)E(ipef 2
:rulegoldensFermi'
Dipole selection rules
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F. Nolting Surface Science FS 2011
1. StepCirc. Pol. X-rays generate spin-polarized electrons from inner shell
2. StepSpin-split 3d bands act as spin analyser
Spin is conserved in transition
L3
L2
2p3/22p1/2
Two-step Model of XMCD
F. Nolting Surface Science FS 2011
Sum rules
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F. Nolting Surface Science FS 2011
Electron occupation
magnetic dipole moment
effective spin magnetic moment
<Tz> is the expectation value of the intra-atomic magnetic dipole operator, accounting for a possible asphericity of the spin density distribution.
… more complex … of course
F. Nolting Surface Science FS 2011
Taken from PhD Thesis Armin Kleibert, 2005
Only contribution to the resonance absorption should be considered
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F. Nolting Surface Science FS 2011
L3 and L2 must be separated
… more complex … of course
F. Nolting Surface Science FS 2011C.T. Chen et al. PRL 75(1), 152 (1995)
Normalize SpectraCorrected for
incident anglePolarization degreesaturation effects
MCDdifferenceintegrate
XASsum spectracorrect backgroundnon resonant
Calculate/neglect<TZ><Sz>
calculating
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F. Nolting Surface Science FS 2011
L
775 780 785 790 795 800 805
L2
L3
Photon energy (eV)
TE
Y (
a.u
.)
XMCD ~ <M> cos(M,L)
L
Photonsno massno spinangular momentum
Magnetism 3d metalssmall orbital momentlarge spin moment
X-ray Magnetic Circular Dichroism (XMCD)
F. Nolting Surface Science FS 2011
J. Stöhr, JMMM 200 (1999) 470 – 497Reiko Nakajima PhD Thesis 1998
easy hard
Magnetic Anisotropypreferential magnetization along axeseasy / hard axis
(magneto-crystalline anisotropy)
The magneto-crystalline anisotropy is the energy that it takes to rotate themagnetization from the “easy” direction into the “hard” direction
Study Magneto-crystalline anisotropy
XMCD in action
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F. Nolting Surface Science FS 2011
Hysteresis loop – anisotropy
hard axis
easy axis
F. Nolting Surface Science FS 2011
Spin moment ~ 1.5 μB / atom isotropic
Orbital moment ~ 0.1 μB / atom isotropic/anisotropic
They interact via the spin-orbit coupling L · S
Magneto-crystalline anisotropy
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F. Nolting Surface Science FS 2011
Anisotropy
F. Nolting Surface Science FS 2011
The Bruno model states that the orbital moment is larger along the easy magnetization direction, and that the difference between the orbital moments along the easy and hard directions is proportional to the magneto-crystalline anisotropy
P. Bruno, PRB 39, 865 (1989)
Bruno model
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F. Nolting Surface Science FS 2011
705 710 715 720 725
0.44
0.46
0.48
0.50
0.52
0.54
0.56
0.58
0.60
Inte
nsi
ty (
a.u
.)
Photon energy (eV)
reference signal sample signal
705 710 715 720 7250.78
0.80
0.82
0.84
0.86
0.88
Inte
nsi
ty (
a.u
.)
Photon energy (eV)
normalized signal
TEY
Reference signal Sample signal
Magnet
Sample= Norm
Reference
How do we measure
F. Nolting Surface Science FS 2011
How do we measure
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F. Nolting Surface Science FS 2011
Test system
F. Nolting Surface Science FS 2011
Thin filmOrbital moment is anisotropic and larger out-of-plane
Thick filmOrbital moment is isotropicshape anisotropy is dominating
Results
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F. Nolting Surface Science FS 2011
Mulitlayer with stronger out-of-plane bonding out-of-plane moment
in-plane orbits are quenched out-of-plane orbits are less perturbed
Free monolayer in-plane moment
Simple picture – Ligand fields
F. Nolting Surface Science FS 2011
P. Gambardella et al. Nature 416, 301 (2002)
Ferromagnetism in one-dimensional monatomic metal chains
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F. Nolting Surface Science FS 2011
P. Gambardella et al. Nature 416, 301 (2002)
ml ~ 0.68 μB / atom ml ~ 0.31 μB / atom ml ~ 0.14 μB / atom
Increased orbital moment in Co chains
F. Nolting Surface Science FS 2011
A. Scheybal, T. Ramsvik, R. Bertschinger, M. Putero, F. Nolting, and T.A. Jung, Chemical Physics Letters 411, 214 (2005)
Induced magnetic ordering in a molecular monolayer
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F. Nolting Surface Science FS 2011
X-ray absorption spectroscopy (XAS)Absorption processTotal electron yield modeExamples
X-ray Magnetic Circular Dichroism (XMCD)BasicsExample: Magnetocrystalline Anisotropy
Closer look at the absorption processMulitplet effectsExample: Interface effect in Exchange Bias system
X-ray Magnetic Linear Dichroism (XMLD)Basics
Outline
F. Nolting Surface Science FS 2011
Initial excited
Single electron
Coulomb, ExchangeElectron – electron interaction
Electron – hole interaction
Valence – Valence interaction : many body effects
Valence – Core interaction : multiplet effects
Hybridization between ground state and final state leads to a multiplet structure of the spectrum
Interactions
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F. Nolting Surface Science FS 2011
+ 2p spin orbit coupling
+ interaction 2p and 3d(Slater-Condon parameter)
+ 3d spin orbit coupling
F. de Groot
2p
3d
2p
3d
2p
3d
Interactions – NiO (Ni 2+ )
F. Nolting Surface Science FS 2011
F. de Groot et al. PRB 42, 5459 (1990)
Further parameter is charge transfer
Crystal field
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F. Nolting Surface Science FS 2011
Ni metal vs NiO
F. Nolting Surface Science FS 2011
Chemical sensitivity
Complex but good for us
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F. Nolting Surface Science FS 2011
Footprint of complex bindings
Courtesy Harald Ade
F. Nolting Surface Science FS 2011
Absorption spectroscopy library
Photon Energy / eV285 290
C O
O
73O
C
O
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vectra
CH3
CH3
O C
O
O n
PC
PAR
CH3
CH3
O C
O C O
O
n
CC
O O
N
H
N
Hn
Kevlar™
CC
O
OO
O (CH2)4 n
PBT
PNIC
O
O
CH2 CH3
CH3
CH3
CH2 *CO
O*
n
CCO
OO
O (CH2)2 n
PET
Photon Energy / eV285 290
PS
PBrS
SAN
n
78
Br
22
n
*
C N
m
CH2 NH C
O
NHNH ** n
MDI polyurea
CH2 NH C
O
ONHC
O
OCH2 4* *n
MDI polyurethane
CH3
N
H
N
H
C
O
n
TDI polyurea
CH3
N
H
N
H
C
O
OC
O*
O
(CH2)4 *n
TDI polyurethane
Photon Energy / eV285 290
CH2 C
C
CH3
O
OCH3(CH2)3
n PBMA
CH2 C
C
CH3
O
OCH3
n PMMA
*N
H
C*
O
n
On PEO
Nylon-6
EPRCH3
* n
PP *
CH3
n
PE * n
Dhez, Ade, and Urquhart J. Electron Spectrosc. 128, 85 (2003)
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F. Nolting Surface Science FS 2011
Effect of exchange bias is still poorly understood
Ideal Interface Model
FM
AF
Spin Flop ModelFM
AF
FM
AF
Domain Size Model
Uncompen-sated spins at AF domain boundaries
Unidirectional anisotropy in FM adjacent to AFMdiscovered 1956 by Meiklejohn and Bean
Exchange-bias field(size depends on many parameters)
H
M
Reviews: A.E. Berkowitz and K. Takano, J. Magn. Magn. Mater. 200(1-3), 552 (1999).J. Nogues and I.K. Schuller, J. Magn. Magn. Mater. 192, 203 (1999)
Exchange Bias
F. Nolting Surface Science FS 2011
CoCoO
NiO
Ni
Tom Regan et al, Phys. Rev. B 64, 214422 (2001)
Oxidation/reduction at the interface is responsible for increased coercivity
Co
NiO
Exchange biased Co/NiO multilayer
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F. Nolting Surface Science FS 2011
840 850 860 870 880 890
0.6
0.9
1.2
1.5
left polarized right polarized
Tot
al Y
ield
(a.
u.)
Photon Energy(eV)
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Asymmetry smoothed
Asy
mm
etry
(%)
EPU beamline 4 at ALS, Elke Arenholz, Tony Young3 nm Co
50 nm NiO
1.5 nm Ru
.
.
3 nm Co 30°
Magnetic field
Left and right circularlypolarized light
XMCD
Hysteresis of Co and NiO
XMCD spectra – switching polarization
-3000 -2000 -1000 0 1000 2000 3000-20
-15
-10
-5
0
5
10
15
20
XM
CD
(%
)
Applied Field (Oe)
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Co Ni
Pinned Moments ?
Exchange biased Co/NiO multilayer
F. Nolting Surface Science FS 2011
-3000 -1500 0 1500
-0.4
-0.2
0.0
0.2
0.4
Applied field (Oe)
HBias
positiveHBias
negative
Asy
mm
etry
(%
)
-1500 0 1500 3000
-0.4
-0.2
0.0
0.2
0.4
biasedunbiased
Co/IrMn multilayerField grown, Hc = 600 Oe
Matt Carey at IBM, Almaden, USA
Pinned Moments
H. Ohldag et al, PRL 91, 017203 (2003)
Pinned Moments
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F. Nolting Surface Science FS 2011
X-ray absorption spectroscopy (XAS)Absorption processTotal electron yield modeExamples
X-ray Magnetic Circular Dichroism (XMCD)BasicsExample: Magnetocrystalline Anisotropy
Closer look at the absorption processMulitplet effectsExample: Interface effect in Exchange Bias system
X-ray Magnetic Linear Dichroism (XMLD)Basics
Outline
F. Nolting Surface Science FS 2011
Cu
O
X-ray Natural linear dichroism“search light effect”
C.T. Chen et al PRL 68, 2543 (1998)
No absorption absorption
Excitation into 3d band
Interaction with linear light - charge
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F. Nolting Surface Science FS 2011
F. Nolting Surface Science FS 2011
Electron charge density is isotropicno linear dichroism
Preferred spin axisspin orbit coupling changes charge density
linear dichroism
XMLD X-ray Magnetic Linear Dichroism
Interaction with linear light - magnetic
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F. Nolting Surface Science FS 2011
700 705 710 715 720 725 730
-0.5
0.0
0.5
Diff
eren
ce
Photon Energy (eV)
0.81.01.21.41.61.82.02.22.42.62.83.03.2
T
EY
(a.
u.)
linear vertical linear horizontal
700 705 710 715 720 725 730
-0.5
0.0
0.50.00.51.01.52.02.53.03.54.0
D
iffer
ence
Photon Energy [eV]
linear horizontal linear vertical
In
tens
ity [a
.u.]
Fe Metal Fe oxide
XMLD enhanced by mulitplet
Enables to measure antiferromagnetic systems!
Ferromagnet (FM)Net magnetic moment
Antiferromagnet (AFM)No net magnetic moment
XMCD ~ M cos(M,S) XMLD ~ <M2> cos2(M,E)
XMLD
F. Nolting Surface Science FS 2011
Strong magnetic dichroism predicted in the M4,5 X-ray absorption spectra of magnetic rare-earth materials, B.T. Thole, G. van der Laan, and G.A. Sawatzky, Phys. Rev. Lett. 55, 2086 (1985).
Experimental proof of magnetic x-ray dichroism, G. van der Laan, B.T. Thole, G.A. Sawatzky, J.B. Goekoop, J.C. Fuggle, J.-M. Esteva, R. Karnatak, J.P. Remeika, and H.A. Dabkowska, Phys. Rev. B 34, 6529 (1986).
Absorption of Circularly Polarized X-rays in Ion, G. Schütz, W. Wagner, W. Wilhelm, P. Kienle, R. Zeller, R. Frahm, G. Materlik, Phys. Rev. Lett. 58, 737 (1987).
Soft X-ray magnetic circular dichroism at the L2,3 edges of nickel, C.T. Chen, F. Sette, Y. Ma, and S. Modesti, Phys. Rev. B 42, 7262 (1990).
X-ray circular dichroism as a probe of orbital magnetization, B.T. Thole, P. Carra, F. Sette, and G. van der Laan, Phys. Rev. Lett, 1943 (1992).
Magnetic X-ray dichroism – general features of dipolar and quadrupolar spectra, P. Carra, H. König, B.T. Thole, and M. Altarelli, Physica B 192, 182 (1993).
Determination of Spin- and Orbital-Moment Anisotropies in Transition Metals by Angle-Dependent X-Ray Magnetic Circular Dichroism,J. Stöhr, H. König, Phys. Rev. Lett. 75, 3748 (1995)
Some pioneering papers on XMCD and XMLD
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F. Nolting Surface Science FS 2011
F. Nolting Surface Science FS 2011
is sensitive toelemental compositionchemical bondsstructural parameterselectronic structuremagnetic properties
-3 0 0 0 -2 0 0 0 -1 0 0 0 0 1 0 0 0 2 0 0 0 3 0 0 0-2 0
-1 5
-1 0
-5
0
5
1 0
1 5
2 0
XM
CD
(%
)
A p p lie d F ie ld (O e )
-0 .8
-0 .6
-0 .4
-0 .2
0 .0
0 .2
0 .4
0 .6
0 .8
1 .0
C o N i
Polarization depend soft X-ray absorption spectroscopy is a powerful tool tostudy elemental resolved ferromagnetic and anitferromagnetic thin films and interfaces
Non-spatially resolved spatially resolved …next time
has a very broad range of applicationMagnetic multilayersDiluted systemsNanoparticlesMolecular magnets…
The technique
Conclusions