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Mössbauer Spectroscopy and Nanomagnetism -
a Materials Scientist’s View
Institute for Nanotechnology
Forschungszentrum Karlsruhe
Joint Research Laboratory
Nanomaterials
Technische Universität Darmstadt
Horst Hahn
International Conference on Hyperfine Interactions
Iguassu Falls, 7.8.2007
Iguassu Falls, 7. August 2007 2
Constitutional and Thermal Defects in Intermetallics
How does a materials scientist get in contact with nuclear probe techniques?
Problems on diffusion and point defects
H. G. Müller, H. Hahn, Phil. Mag., 1984. A50, p. 71.
Some intermetallic B2 (CsCl)-compounds, such as PdIn, exhibit very
large concentrations of constitutional and thermal point defects, i.e.
vacancies and antistructure atoms
Probe atom: 111In 111Cd
Not an impurity
Constitutional defects
Pd50In50: no defects
Pd-rich compounds: PdIn
In-rich compounds: VPd
Thermal defects
•For 49at%<cPd<50.5at%: single
vacancies in each sublattice (VIn
remain invisible for PAC)
•Reaction with PdIn antistructure
defectsPd-rich sideIn-rich side
Iguassu Falls, 7. August 2007 5
Contents
Introduction
DCEMS: a highly sensitive method with monolayer resolution
Metallic Nanostructures
Synthesis methods
Nanoparticles
Thin films and multilayers
Laterally structured thin films
Summary 5 0 n m5 0 n m
Iguassu Falls, 7. August 2007 6
Interface and finite size effects
Some examples of properties
Magnetic properties
Chemical properties
Optical properties
Biofunctionality
Multifunctionality
Iguassu Falls, 7. August 2007 7
Metallic nanostructures come in different morphologies
5 nm
Embedded nanocrystal
Rapid
quenching 20 nm
Self-assembling
Co-nanoparticles
Chemical
Synthesis
100 nm
Metallic Nano-rods
Pressure Infiltration
NanostructuredMaterials
Nanocrystalline Metals
Cold-rolling
Laterally structured metal dots
Anodic Oxidation 500 nm
Thin films
Molecular Beam Epitaxy
Fe
Fe
Al2O3
Ta
Iguassu Falls, 7. August 2007 8
Morphology and applications
5 nm
Multilayers Nano-Dots Nano-Particles
XMR-sensors Data storagematerials
N S
Biotechnology, ferrofluids
5 0 n m5 0 n m
GMR-read heads
Iguassu Falls, 7. August 2007 11
Depth Sensitivity
UHV Orange spectrometer
T = 40 % von 2
E/E ≈ 1%
typically 5 nm
SURFACE sensitivity
and
DEPTH sensitivity
due to electron energy loss
Iguassu Falls, 7. August 2007 12
Depth sensitive Conversion Electron Mössbauer Spectrometer
1 m
Mössbauer-AbsorptionElectronEmission
Electron Detector
UHV Orange Spectrometer
Electron Source
MagnetCoils
UHV-Orange-Spektrometer
• high sensitivity
• high energy resolution
• short measuring time !
The UHV-Orange-Mössbauer-Spektrometer
Iguassu Falls, 7. August 2007 13
Karslruhe Integrated Synthesis and Characterization System
Total view of open spectrometer showing
the current leads to the magnet coils
Mössbauer-source
Special sample holder
with cooling
Iguassu Falls, 7. August 2007 15
Characteristics of DCEMS Orange Spectrometer
Transmission
Conversion
electrons
almost 2 collection
high sensitivity
temperature range from 10 to 450 K
extreme surface sensitivity
monolayer resolution with special samples
in-situ sample preparation
UHV (p 10-9 mbar)
no transmission geometry
Iguassu Falls, 7. August 2007 16
How to obtain monolayer resolution
Typically, the depth resolution of DCEMS is approx. 5 nm (assuming
natural Fe is used)
The depth resolution can be improved by using the following sample
geometry:
Cover layer, i.e. oxide (for
TMR) or another metal (Pt)
One or two monolayers of
Fe57 (Mössbauer active)
Layer without any Fe or
pure Fe56 (Mössbauer
inactive)
Substrate
Mössbauer signal
originates from this
layer only, i.e.
information from the
first and second
monolayer at the
interface
i.e. Ta/56Fe/2ML 57Fe/Al2O3/56Fe
or Ta/56Fe/2ML 57Fe/5ML Pt
Iguassu Falls, 7. August 2007 17
Proof of monolayer resolution
Fe-Pt Interface
•magnetic properties of planar interfaces
Determine local magnetic properties
DCEMS – Mössbauer-spectroscopy as local probe method
Preparation of smooth interface
Ta – buffer layer
Iguassu Falls, 7. August 2007 18
Fe-Pt interface
Two components clearly separable :
different temperature behaviour
different hyperfine fields
Assignment :
sub-interfacial layer [Fe(I-1)]
direct interface [Fe(I)]
Fe(I)Fe(I-1)
5 ML Pt
33 ML Fe56
1 nm Ta
Si/SiO2
= 0,39(7) mm/s; = 0,035(2) mm/s
= 0,13(3) mm/s; = 0,018(7) mm/s
well defined
Iguassu Falls, 7. August 2007 19
Temperature dependence
Fe(I)Fe(I-1)
5 ML Pt
33 ML Fe56
1 nm Ta
Si/SiO2
Interfacial layer [Fe(I)]:
reduced ground state
hyperfine field
reduced Curie-Temperatur TC
Sub-interfacial layer [Fe(I-1)]:
almost bulk, BUT:
enhanced ground state
hyperfine field
Iguassu Falls, 7. August 2007 20
Contents
Introduction
DCEMS: a highly sensitive method
Metallic Nanostructures
Synthesis methods
Nanoparticles
Thin films and multilayers
Laterally structured thin films
Summary
Iguassu Falls, 7. August 2007 22
Magnetic nanoparticles of different alloys
5 nm
FePt
5
nm
Fe45Co55 MS KA
< D >= 5.66 0.76 nm
100 nm
Fe25Co25Pt50
Iguassu Falls, 7. August 2007 23
Synthesis of functionalized metallic nanoparticles
FePt nanoparticles
Pt(acac)2 Fe(CO)5
reductionthermal
decomposition
particle-ligand
TEM
4.1 nm
Challenges:
• Class of material
• Synthesis method
• Particle size
• Morphology
• Defects
• Functionalization
• Self organisation
• Structuring
Iguassu Falls, 7. August 2007 24
Lokale Eigenschaften lokale Methoden
C O
O
C
OO
C
OO
CO
O
COO
?
Local structure ?
Local magnetic properties ?
Influence of surface ?
Mössbauer spectroscopy
EXAFS
Local methods needed
J. Ellrich, TU Darmstadt
Iguassu Falls, 7. August 2007 25
Properties of FePt nanoparticles
-3 -2 -1 0 1 2 3
0,65
0,66
185 K
1,98
1,98
220 K
1,39
1,39
230 K
240 K
Co
un
ts /x10
6
3,70
3,70
0,69
0,69
250 K
-12 -8 -4 0 4 8 12
1,31
1,32
Velocity/mm s-1
10 K
33.3 T
46.5 T
4 nm
fast
slow
frozen
Brownian
motion
Particle moment
superpara-
magnetic
blocked
FePt particles
protected with
oleic acid
Brownian
motion of
entire particle
Magnetisation
of entire
particle
Ordering
within the
particles
Iguassu Falls, 7. August 2007 26
Mössbauer-Spektroskopie
TB=55 K
Brownian motion of coupled particles
large Hyperfine fields :
oxides ?
surface ?
TB
2.4 nm Fe
F. Bodker, S. Morup, S. Linderoth, PRL 1994
-10 10
Iguassu Falls, 7. August 2007 27
EXAFS Modellierung
Method :
generating atom positions
relaxation with EAM potentials
Simulation with FEFF 8.10
Fit of paths
Results :
Pt rich core (20%Fe)
Particle surface covered by
two monolayers Fe
Termination of surface by O
bond of Oleic acid via O to Fe
Iguassu Falls, 7. August 2007 32
Contents
Introduction
DCEMS: a highly sensitive method
Metallic Nanostructures
Synthesis methods
Nanoparticles
Thin films and multilayers
Laterally structured thin films
Summary
5 0 n m5 0 n m
Iguassu Falls, 7. August 2007 33
Synthesis methods – Thin films and multilayers
PVD: Physical Vapor Deposition
Molecular Beam Epitaxy
(layer-by-layer growth)
Sputtering
Thermal evaporation
CVD: Chemical Vapor Deposition
ALD: Atomic layer deposition
(self-limiting, sequential surface chemistry for
monolayer-monolayer growth)
Iguassu Falls, 7. August 2007 34
Motivation: understanding interfaces in TMR systems
How is the barrier interface affected by
=> the preparation of the barrier?
=> annealing of the sample?
Characterization: DCEMS, TEM, TMR,
XRR, and XPS
Fe
SiO2
2.8 (2) nm Al2O3
Fe = 20 nmFe
Ta
H. Schmitt, et al., APL 2006. 88: 122505, 1-3.TEM-image as prepared
10nm
Fe
Fe
Al2O3
Ta
Iguassu Falls, 7. August 2007 35
Structural changes at the interfaces
Fe3+
Fe2+ (spinel)
Fe3+ (spinel)
Fe
SiO2
2.8 nm Al2O3
Fe = 20 nmFe
Taafter
oxidation
co
unts
/100
0
Velocity [mm/sec] H. Schmitt, et al., APL 2006. 88: 122505, 1-3.
Ta–Fe–Al2O3–Fe
Iguassu Falls, 7. August 2007 36
Structural changes at the interfaces
Fraction of
57Fe atoms in
this
environment
in monolayers
(ML)
H. Schmitt, et al., APL 2006. 88: 122505, 1-3.
Ta–Fe–Al2O3–Fe
Iguassu Falls, 7. August 2007 37
Effects of annealing on TMR effect
more homogeneous & thicker barrier
appearance of Fe3+ (over-oxidation)
increase of Al2O3 in the barrier
reduction of Fe3+
appearance of FeAl2O4
smooth interfaces
Observed
changes of
TMR effect
upon
annealing
No other technique
available to detect
changes in the (sub-)
monolayer level in
buried interfaces
H. Schmitt, et al., APL 2006. 88: 122505, 1-3.
Ta–Fe–Al2O3–Fe
Iguassu Falls, 7. August 2007 38
TMR measurements
H. Schmitt, Ph.D. thesis, TU Darmstadt (2005)
Pd–Fe–Al2O3–Fe
Iguassu Falls, 7. August 2007 39
Hyperfine field distribution
Pd
Fe
Fe Al O2 3
60nm
H. Schmitt, et al., JAP (2005) 97:113902-1-113902-5
Pd–Fe–Al2O3–Fe
Iguassu Falls, 7. August 2007 40
Results: 3,5 ML tracer
Free surface
Fe57
bcc Fe
High field
Low field
Paramagnetic Fe
ionic Fe
Al
Oxygen
H. Schmitt, et al., JAP (2005) 97:113902-1-113902-5
Pd–Fe–Al2O3–Fe
Iguassu Falls, 7. August 2007 42
Contents
Introduction
DCEMS: a highly sensitive method
Metallic Nanostructures
Synthesis methods
Nanoparticles
Thin films and multilayers
Laterally structured thin films
Summary
Iguassu Falls, 7. August 2007 43
Fe-Pt system
Fe50Pt50:
K. Watanabe, H. Masumoto, Trans. Jap. Inst. Met. 24, 627 (1983)
Fe
Pt
Fe
Pt
Fe
a
c a = 0,385 nm
c/a = 0,96
L10
• ordered phase: L10-structure (fct)
• high magnetocrystalline anisotropy
Keff ≈ 5 x 107 J m-3
• order-disorder transition
• disordered phase fcc
Iguassu Falls, 7. August 2007 44
FePt nanodots as magnetic recording media
Nanoporous, ultrathin Al2O3 mask
by Lei Yong / INT
MgO/{2ML57Fe/2MLPt}6 → ttotal ≈ 4 nm
Tgrow = 280°C
layer by layer growth → artificial L10 structure
highly ordered, magnetic nanostructures with sizes < 20 nm
perpendicular anisotropy @ RT
J. Ellrich, Lei Yong, H. Hahn,
patent pending
J. Ellrich et al., submitted to
Small (2007)
Iguassu Falls, 7. August 2007 45
Laterally structured thin films or nanodots
Lei Yong, INT-FZK
Alumina
nanoporous
thin film
Y. Lei, et. Al, Chem. Mat. (2005) 17: 580
Iguassu Falls, 7. August 2007 46
Influencing the dot size and shape
Y. Lei, et. Al, Chem. Mat. (2005) 17: 580
Iguassu Falls, 7. August 2007 48
Local magnetic properties
DCEMS:
in-situ measurement
L10 AND fcc Fe-Pt environment
reflects sample structure
L10-Fe-Pt well defined
fcc Fe-Pt rather broad
distribution of sites
magnetization direction:
=12° for 22 % L10-phase
=30° for 78 % fcc-phase
Iguassu Falls, 7. August 2007 49
Conclusions
Nuclear probe techniques provide very useful information on the local
structure and magnetic properties of nanostructures
DCEMS is a highly sensitive detection technique with monolayer resolution for
the characterization of the atomic and magnetic nature of the interfaces
XMR-properties depend strongly on the atomic and magnetic structure at the
interfacial regions in
nanoparticles
thin films and multilayers
5 0 n m5 0 n m
Iguassu Falls, 7. August 2007 50
Acknowledgements
INT M. Ghafari, J. Ellrich, B. Stahl, H. Schmitt, A. Hütten,
R. Kruk, Lei Yong, H. Rösner, G. Wilde, R. Theissmann
TUD M. Winterer, K. Albe, M. Müller
Financial support by:
DFG, CFN, AvH, DAAD, BMBF, Bosch, SusTech Darmstadt