Analysis of the influence of atom-substitution in selected CMR-compounds by x-ray spectroscopic methods

Master thesis
Analysis of the influence of atomsubstitution in selected CMRcompounds by x-ray spectroscopic
methods
by Christian Taubitz
presented to the
Department of Physics
University of Osnabrück
Osnabrück
August 4, 2006
Thesis advisor:
apl. Prof. Dr. Manfred Neumann
”All truths are easy to understand
once they are discovered; the point is
to discover them.”
Galileo Galilei (1564-1642)
Italian astronomer, philosopher, and physicist
Contents
1 Introduction
7
2 Basics of x-ray spectroscopic methods
2.1 X-ray Photoelectron Spectroscopy (XPS) . . .
2.1.1 Characteristics of the XPS-Spectra . .
2.1.1.1 Chemical shift . . . . . . . .
2.1.1.2 Spin orbit coupling . . . . . .
2.1.1.3 Multiplet splitting . . . . . .
2.1.1.4 Satellites . . . . . . . . . . .
2.1.1.5 Auger electrons . . . . . . . .
2.1.1.6 Inelastic background . . . . .
2.1.2 XPS in theory . . . . . . . . . . . . . .
2.1.2.1 Three-step model . . . . . . .
2.1.2.2 One-step model . . . . . . . .
2.2 X-ray Absorption Spectroscopy(XAS) . . . . .
2.3 Experimental equipment . . . . . . . . . . . .
2.3.1 Photoelectron spectrometer PHI 5600ci
2.3.2 Bessy k storage ring . . . . . . . . . .
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3 The CMR-effect
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28
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33
4 Preparation of Sr2 FeReO6
37
4.1 XPS measurements . . . . . . . . . . . . . . . . . . . . . . . . 39
5 The Spinel Fe1−x Cux Cr2 S4
5.1 XPS . . . . . . . . . . . . . . . . . . .
5.1.1 Cu 2p and 3s core level spectra
5.1.2 Cr 2p core level spectra . . . .
5.1.3 Fe 2p core level spectra . . . . .
5.1.4 S 2p core level spectra . . . . .
5.2 XAS . . . . . . . . . . . . . . . . . . .
5
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45
47
47
49
50
54
55
6
CONTENTS
5.2.1
5.2.2
Cr 2p XAS spectra . . . . . . . . . . . . . . . . . . . . 55
Fe 2p XAS spectra . . . . . . . . . . . . . . . . . . . . 57
6 Conclusion and outlook
67
7 Zusammenfassung und Ausblick
71
8 Acknowledgement / Danksagung
75
Chapter 1
Introduction
Compounds that show a huge change in the electrical resistance induced
by an applied magnetic field are called CMR (colossal magneto resistance)compounds. The fact that the CMR-effect changes the electrical resistance in
an amount of several 100% or 1000% and was also reported at room temperature, makes the colossal resistance very interesting for industrial applications.
The discovery of this effect in many different materials like manganites, double perovskites or spinels led to intense studies in order to understand the
electronic and magnetic properties of these materials. But in spite of all
investigations the origin of the CMR-effect is still in question and not understood thoroughly.
A few years ago colossal resistance was found in the double perovskite
Sr2 FeMoO6 [48][49]. This material is of special interest due to a high Curie
temperature (∼ 420K) and a rather large magneto resistance effect already
present at room temperature. Recently in Sr2 CrReO6 , which is a double
perovskite with an even higher Curie temperature (∼ 635K), a CMR-effect
was reported [43]. In order to get more information about the properties of
these compounds during this work a Sr2 FeReO6 double perovskite was produced. Here XPS measurements of the first in Osnabrück produced samples
are presented and discussed with regard to the sample quality.
Spinels are of high interest as well, since they show a CMR-effect despite
their complete different characteristics compared to double perovskites or
7
CHAPTER 1. INTRODUCTION
manganites. Because of this the colossal resistance has to be explained in a
different way. A few years ago a CMR-effect close to room temperature was
reported in the spinel chalcogenide Fe1−x Cux Cr2 S4 [22]. In these compounds
not only the origin of the magneto resistance also the valence state of the
ions is still in discussion. Two models developed by Goodenough [25] and
Lotgering [24] give different descriptions of the valences. In this work the
spinels Fe1−x Cux Cr2 S4 with x=0.2 and x=0.6 are investigated by different
x-ray spectroscopic methods. The results are discussed with attention to the
valence state of the ions in the compounds.
The work has the following structure:
• In Chapter 2 the reader is briefly introduced into the experimental
techniques used, namely x-ray photoelectron spectroscopy (XPS) and
x-ray absorption spectroscopy (XAS). In addition the used experimental equipment is described.
• Chapter 3 contains a brief description of the CMR-effect. Two models
are introduced in order to explain the CMR-effect.
• XPS measurements of four Sr2 FeReO6 samples, produced under different conditions, are presented in Chapter 4. After a brief description
of the sample production, the O 1s and Re 4f core level spectra are
discussed with attention to the quality of the samples. The H2 flow,
under which the samples are reduced, is found to have a big influence
on the sample quality.
• The next Chapter 5 deals with an investigation of the spinel chalcogenide Fe1−x Cux Cr2 S4 (x=0.2, 0.6) by x-ray photoelectron spectroscopy
(XPS) and x-ray absorption spectroscopy (XAS). The measurements
are compared to configuration-interaction calculations [30] and discussed with regard to the valence state of the ions. To determine the
ion valency also multiplet calculations are performed. The results seem
to confirm the Lotgering valency model [24]. In addition recently presented XAS measurements of Fe0.5 Cu0.5 Cr2 S4 [31] are compared to our
8
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 1. INTRODUCTION
measurements. The line shapes of the spectra are found to differ from
our measurements. Possible reasons for this are discussed.
• Finally, in Chapter 6 and 7 in english and german the main results
achieved in the present work are summed up and an outlook is given.
These chapters are followed by my acknowledgment and a list containing the bibliographic references.
9
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 1. INTRODUCTION
10
(p) 2006, Christian Taubitz, University of Osnabrück
Chapter 2
Basics of x-ray spectroscopic
methods
In this chapter the experimental methods used in this work are reviewed. At
first the x-ray photoelectron spectroscopy (XPS) is described. Than a brief
introduction in the x-ray absorption spectroscopy (XAS) is given. Finally
the used experimental equipment is presented.
2.1
X-ray Photoelectron Spectroscopy (XPS)
If an atom is irradiated with light, it is possible that electrons, the so called
photoelectrons, are emitted. This photoelectric effect was discovered and
described in 1887 by Hertz [1] and Hallwachs [2]. Later in 1905 Albert
Einstein explained this process with his quantum light hypothesis [3]. His
thesis no longer describes light as a wave, but as a flow of particles, the
photons, which hold a specific quantised amount of energy proportional to
the Planck constant h and the frequency ν. For this work Albert Einstein
was awarded with the Nobel price in 1921.
11
CHAPTER 2. BASICS OF X-RAY SPECTROSCOPIC METHODS
_
D
S
hν
+
p
ψ
ϑ
e- , Ekin
Figure 2.1: Principle of XPS (adapted from [5][32]).
According to the quantum light hypothesis the maximum kinetic energy
of an emitted electron is given by
Ekin = hν − Φsolid .
(2.1)
Here hν denotes the energy of the exciting photon and Φsolid the work function of the solid. This material-specific function describes the energy an
electron needs to leave the atom.
But the equation (2.1) only describes the photoemission process for valence
electrons close to or at the Fermi level. Stronger bonded core level electrons
also have to overcome their binding energy in order to leave the atom, which
leads to
Ekin = hν − EB,ef f − Φsolid
(2.2)
where EB,ef f is the effective binding energy of the emitted electron.
Rewriting this equation
EB,ef f = hν − Ekin − Φsolid
12
(p) 2006, Christian Taubitz, University of Osnabrück
(2.3)
CHAPTER 2. BASICS OF X-RAY SPECTROSCOPIC METHODS
reveals that it is possible to determine the binding energy of an emitted electron by measuring its kinetic energy, if the photon energy and work function
is known. Since the reference of the binding energy is by definition the Fermi
energy EF the spectra must be calibrated by Φsolid .
But the work function Φsolid is a specific characteristic of the material, usually unknown and difficult to measure. Therefore conductive samples are
connected to the spectrometer. Thus, the Fermi levels adjust and the spectrometer can be considered as an electron supplier or vice versa. In this
case the kinetic energy Ekin of the photoelectron is modified by the electric
field arising from the difference of the work functions of the solid and the
spectrometer
∆Φ = Φsolid − Φspectrometer
(2.4)
0
and the measured kinetic energy Ekin is given by
0
Ekin = Ekin + ∆Φ
= Ekin + (Φsolid − Φspectrometer )
= (hν − EB − Φsolid ) + (Φsolid − Φspectrometer )
0
⇒ Ekin = hν − EB − Φspectrometer
Φsolid vanishes and the binding energy of an emitted electron can be determined by the following equation.
0
EB = hν − Ekin − Φspectrometer
(2.5)
The work function of the spectrometer Φspectrometer is usually well known.
As shown in the Figures below the connection of a sample to the spectrometer
can lead to two different situations. Depending on whether Φspectrometer or
Φsolid is higher the spectrometer or the sample limits the measurable binding
energy EBmax .
13
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 2. BASICS OF X-RAY SPECTROSCOPIC METHODS
1 .C a s e : T h e s p e c tr o m e te r lim its !
0
h v
E
= > .
E
S p ec
m ax
.
2 .C a s e : T h e s a m p le lim its !
S o lid
0
m ax
h v
m in
k in
]
= 0
E
.
.
S p ec
.
S o lid
m ax
k in
S o lid
E
m in
k in
h v
.
S p ec
E
F
m ax
E
B
F
m ax
B
d e e p e s t v is ib le
c o re -le v e l!
d e e p e s t v is ib le c o re -le v e l !
in v is ib le , s in c e th e
w o rk fu n c tio n o f th e
s o lid lim its !
in v is ib le , s in c e th e w o rk fu n c tio n
o f th e s p e c tro m e te r lim its !
S a m p le
.
B
S o lid
E
<
S p ec
E
h v
h v
.
m ax
h v
E
= >
k in
[ E
B
>
S p e c tr o m e te r
S a m p le
S p e c tr o m e te r
Figure 2.2: The Fermi level adjustment of sample and spectrometer.
In practice the electrons are excited by X-rays or UV-radiation. In the
first case the method is called X-ray Photoelectron Spectroscopy (XPS),
if UV-radiation is used it is called Ultraviolet Photoelectron Spectroscopy
(UPS).
14
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 2. BASICS OF X-RAY SPECTROSCOPIC METHODS
h v (U P S )
C B
V B
h v (X P S )
c o re
le v e ls
Figure 2.3: Principle of XPS and UPS.
Although this method is based upon the photoelectric process, the intensities in a photoelectric spectrum can not be explained completely by this.
Various effects beside the photoelectric process highly affect the measured
spectra. Some of them just modify the kinetic energy of the emitted electrons, like the Chemical shift or the Spin-orbit coupling, others compete with
the photoelectric process by emitting additional electrons, like Satellites or
the Auger-effect. All this effects have to be completely understood in order
to get correct information. This seems to be a disadvantage, but the various
side effects in the PES are the big advantage of this method. Since they
are highly influenced by the chemical environment and the valence state of
an atom, they reveal a lot of information about the chemical and electronic
structure of the measured material.
15
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 2. BASICS OF X-RAY SPECTROSCOPIC METHODS
In addition the peak intensities correspond to the elements contained
in the sample, therefore this method can also give information about the
stoichiometry of the sample.
In contrast to other x-ray spectroscopic methods the XPS gives the total
density of states(tDOS).
2.1.1
Characteristics of the XPS-Spectra
In the following a short summery of the basic side effects appearing in a
photoelectric spectrum is given.
2.1.1.1
Chemical shift
Although valence electrons are involved in chemical bondings, core level electrons are affected by them. Due to a change of the electric environment, the
electric potential changes and with it the binding energy of the core level
electrons. For instance, if in a bonding the valence electrons of an atom migrate, like for the Fe atom in FeO, the core electrons feel a stronger Coulomb
interaction with the nucleus. Therefore Fe 2p electrons in FeO have a higher
binding energy than in pure Fe (Figure 2.4 ).
The magnitude of the energy shift depends on the type of binding and the
neighboring atoms. By comparing the binding energy shift of core level electrons, the so called Chemical shift, with reference measurements, one gets
information about the bonding and the chemical environment of an atom in
a sample.
The theoretical approach of the Chemical shift is very difficult, because the
influence of several factors can not be determined and calculated correctly.
In general the equation 2.5 is modified in order to describe the changes of
the effective binding energy in a chemical bonding.
EB,ef f = EB (atom) + ∆(Echem + EM ad )
(2.6)
∆Echem = KqA denotes the chemical shift in atom A relating to a reference.
qA describes the valency difference to the reference and K the interaction of
16
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 2. BASICS OF X-RAY SPECTROSCOPIC METHODS
valence level electrons with the core level electrons. The latter is an empirical
parameter. The Madelung term ∆EM ad considers the influence of the other
atoms in a molecule or bulk. It is the sum of the effective charge qB divided
by the distance rAB of every surrounding atom B to the atom A where A 6= B.
With this the effective binding energy can be described as
EB,ef f = EB (atom) + KqA +
X
(
B6=A
qB
)
rAB
(2.7)
It has to be mentioned that in equation 2.7 only electrostatic considerations
are taken into account.
2p3/2 :707.0 eV
Intensity (arb. units)
2p1/2 :720.3 eV
Fe metal
709.5 eV
723.3 eV
satellite
satellite
FeO
740
735
730
725
720
715
710
705
700
Binding Energy (eV)
Figure 2.4: Chemical shift of the Fe 2p XPS lines of FeO. The Spin orbit
coupling and the satellites will be explained in the following (adapted from
[21]).
17
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 2. BASICS OF X-RAY SPECTROSCOPIC METHODS
2.1.1.2
Spin orbit coupling
To describe the way an electron is bound to an atom quantum numbers are
used. The main quantum number n [n = 1, 2, ...] for example denotes the
atomic shell where the electron is located. The angular momentum of the
electron is given by l [l = 0, 1, 2, ..., n − 1] and the spin by s [s = 21 , − 12 ].
Instead of l=0,1,2... the angular momentum is often expressed with s,p,d.
The total angular momentum j is the sum of l and s [here j = l ± 12 ].
To describe the state of an electron in an atom the expression nlj is used. For
instance, 2p 3 denotes an electron in the second atomic shell with an angular
2
momentum of l=1 and the spin s=+ 21 , which results in j= 32 . The electron
2p 1 is located at the same atomic shell and has the same angular momentum
2
as 2p 3 . The only difference is the coupling of its angular momentum l and
2
its spin s. Here s=− 12 , which results in j= 12 . This coupling is called the Spin
orbit coupling. Within an atom it results in states with different binding
energies. Therefore every core level line in PES is a doublet like for example
the Cr 2p lines (Figure 2.5) or the Fe 2p lines (Figure 2.4). Only for levels
with l=0 there is a singulet because j can not be negative.
The relative intensities of the two levels of a doublet are given by:
I(l+ 1 )
2
I(l− 1 )
2
=
l+1
l
(2.8)
For instance, for the d levels (l=2) the relative intensities are I5/2 /I3/2 =3/2.
The doublet splitting increases with increasing atomic number for fixed main
quantum number n and total angular momentum j.
2.1.1.3
Multiplet splitting
If core level electrons are emitted out of systems with unpaired electrons in
the valence levels, multiplet (exchange) splitting of the core level lines can
occur. In contrast to the spin orbit coupling this splitting originates from
a spin to spin coupling. In case of transition-metal compounds for example
the spin s=1/2 of a 3s core hole created during the photoemission can couple
parallel or antiparallel to the total spin of the valence electrons (S). The two
18
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 2. BASICS OF X-RAY SPECTROSCOPIC METHODS
types of coupling release a different amount of energy that is absorbed by
the emitted electron. This causes a spitting of the core line and the exchange
splitting (∆Es ) can be written according to the van Vleck theorem [8] :
∆Es =
2S + 1 2
G (3s, 3d)
2l + 1
(2.9)
G2 (3s, 3d) is the Slater exchange integral and l the orbital quantum number
(l=2). The binding energy of the state with (S+1/2) is lower than the binding
energy corresponding to (S-1/2). This creates a doublet in the spectrum and
the intensity ratio of the two peaks is given by:
IS+1/2
S+1
=
IS−1/2
S
(2.10)
In 1970 it was found that there are measurements for which the van Vleck
theorem was not fulfilled. The multiplet intensities ratio was higher than that
predicted by the equation 2.10 and the value of the splitting was about two
times smaller than expected [9][10]. This discrepancy has been associated
to intra-atomic ”near-degeneracy” correlation effects [11]. Nonetheless the
above equations can be used as a valuable ”diagnostic” tool for the analysis
of the magnetic ground state. Nowadays the treatment of the 3s multiplet
splitting is based upon full multiplet calculations [12].
The multiplet splitting is even more complicated for the other core levels
(l 6= 0), because in addition spin orbit splitting occurs in the spectra. The
Figure 2.5 shows the Cr 2p lines with both splitting types.
19
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CHAPTER 2. BASICS OF X-RAY SPECTROSCOPIC METHODS
spin orbit coupling
multiplet splitting
Intenstiy (arb. units)
Cr 2p3/2
Cr 2p-XPS
610
600
Cr 2p1/2
590
580
Binding energy (eV)
570
Figure 2.5: Spin orbit coupling and multiplet splitting of the Cr 2p lines (of
the Spinel Fe0.4 Cu0.6 Cr2 S4 ).
2.1.1.4
Satellites
A photoelectron that is emitted during a photoelectric process can interact
with the (N-1 electron) excited state of the atom. This leads to additional
lines, the so called satellites, beside the main lines in the spectra (Figure
2.4). Satellites are separated in two classes, the extrinsic and the intrinsic
satellites. The first are due to inter-atomic excitations, the second occur
because of intra-atomic relaxations.
During a photoemission process it is possible that a second electron is excited.
The necessary energy is supplied from the kinetic energy of the primary
photoelectron. Because of the energy loss the primary electron will appear
with a higher binding energy (lower kinetic energy) on the spectrum. If the
second electron is transferred to a higher energy orbit, this line is called
shake-up satellite, if it is completely removed, it is called shake-off satellite
[13].
20
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 2. BASICS OF X-RAY SPECTROSCOPIC METHODS
In transition metals yet another type of satellite, the charge transfer satellite, can occur. A transfer of one electron from the ligand 2p (L) to the metal
3d shell is involved in the origin of this satellite: 3dn L → 3dn+1 L−1 . This
extrinsic charge transfer process requires the energy (∆):
∆ = E(3dn+1 L−1 ) − E(3dn L)
2.1.1.5
(2.11)
Auger electrons
If a photoelectron has been emitted from the sample the remaining hole is
filled with an electron of a higher energy level. This process releases energy,
which is either radiated in the form of a photon or absorbed by an electron.
This so called Auger electron is excited into the continuum and appears at
low kinetic energies that means high binding energies in the spectrum [7].
e-
Φ
eEVacuum
EFermi
EVacuum
Φ
Φ
EFermi
hv
1. Excitation and emission
of a photoelectron by
radiation.
2. The hole is filled by an
electron of a higher level
and the released energy
leads to the emission of an
auger electron.
3. The final state
Figure 2.6: Principle of the emission of an Auger electron.
21
(p) 2006, Christian Taubitz, University of Osnabrück
EVacuum
EFermi
CHAPTER 2. BASICS OF X-RAY SPECTROSCOPIC METHODS
The labelling of Auger electrons is based upon the shell the first photoelectron is emitted, the shell the filling electron falls down from and the shell
from which the Auger electron is excited. For instance, an Auger electron
is called KL1 L23 when an electron of the K shell (1s level) was removed, an
electron of the L1 (2s) level recombines with the hole in the K shell and the
resultant photon excites an electron of the L23 (2p1/2 or 2p3/2 ) level.
2.1.1.6
Inelastic background
Photoelectrons excited during a photoemission process and moving through
the sample to the surface of the solid can be scattered either elastic or inelastic. In the first case the electron energy remains the same. But inelastic
scattered electrons lose energy and appear at lower kinetic energy in the spectra. Because of this there is a redistribution of the intensities in the spectra.
The intensities of inelastic scattered electrons is called inelastic background.
The higher the binding energies the more these intensities overlay the spectrum.
2.1.2
XPS in theory
In theory the photoemission process can be approached full quantum mechanically. Two wave functions Ψi and Ψf describe a system comprising
N electrons. They correspond to the initial and final state of the system
before and after the photoemission, respectively. The transition probability
dominates the photocurrent intensity and fulfills Fermi’s Golden rule,
ω ∼ |hΨf |H ∗ |Ψi i|2 δ(Ef − Ei − hv)
(2.12)
Here it is assumed that the perturbation H ∗ to the N-1 electron system is
small. The δ function ensures the energy conservation.
The interaction Hamiltonian H ∗ can be expressed by the following equation.
H∗ =
e2
e
· A · P − eΦ +
|A|2
mc
2mc2
22
(p) 2006, Christian Taubitz, University of Osnabrück
(2.13)
CHAPTER 2. BASICS OF X-RAY SPECTROSCOPIC METHODS
e is the charge and m the mass of an electron. As usual c denotes the
speed of light. A and Φ refer to the vector and the scalar potential of the
incident radiation. Finally P is the momentum operator of the electron.
Different kinds of approximations are done to describe the photoemission
process easier, but several important effects are not taken into account by
them [21].
2.1.2.1
Three-step model
Based upon the theories of Berglund and Spicer [14], in this model the photoemission process is described as consisting of three separated steps:
1. The local excitation of an electron by absorbing a photon.
2. The propagation of the photoelectron through the sample to the surface. During their movement some of the excited photoelectrons lose
energy mainly due to electron-electron interaction if high energies are
used. For low energies this scattering process is dominated by electronphonon interaction. Here the so called mean free path λ is a very important parameter. It reflects the mean distance between two inelastic
impacts of an electron propagating through the sample [15].
λ(E) =
E
Eplas βln(γE)
(2.14)
β and γ are parameters, E denotes the energy of the excited electron
and finally Eplas the plasmon energy of a free electron gas. In the
soft x-ray energy range (∼ 100 − 1000 eV) the mean free path may be
approximated by λ ∝ E p , p ranging from 0.6 to 0.8 [16].
3. The penetration of the photoelectron through the surface and the emission of those electrons into the vacuum which have enough kinetic energy normal to the surface to overcome the potential barrier. The other
electrons are reflected back.
23
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 2. BASICS OF X-RAY SPECTROSCOPIC METHODS
2.1.2.2
One-step model
Although the three-step model is very descriptive and illustrative, it turned
out not to be a good approach for computational simulation of PES spectra.
Here models, which consider the whole photoemission process as a single
step, are much more useful. Many different so called one-step models have
been developed, most recently a relativistic one-step approach [17]. If one
uses characteristic crystal potentials as input data one-step models are an
appropriate tool for the simulation of XPS spectra [18].
2.2
X-ray Absorption Spectroscopy(XAS)
In contrast to XPS in the x-ray absorption spectroscopy (XAS) an electron
is not emitted out of the sample but excited into an unoccupied state of
the conduction band (Figure 2.7). Thus, this method probes the partial
density of states (pDOS) of the empty states in the conduction band. XAS
is site specific because each element has individual excitation energies. The
required energy Eexc for the excitation is given by:
Eexc = hν = Ef inal + EB,ef f
(2.15)
Here EB,ef f is the effective binding energy of the electron before the excitation. Ef inal is the energy of the final state in the conduction band.
24
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 2. BASICS OF X-RAY SPECTROSCOPIC METHODS
C B
V B
h v (X A S )
c o re
le v e ls
Figure 2.7: Principle of XAS.
Due to dipole selection rules only specific transitions are allowed. Thus in
XAS only excitations which change the angular momentum quantum number l of the electron by one occur in the process (∆l = ±1). In addition the
spin s has to be conserved (∆s = 0), while the z-component of the orbital
momentum m can also change by one (∆m = ±1, 0). In particular for left
hand circularly polarized light it has to be ∆m = +1 and for right hand
circularly polarized light ∆m = −1.
A tunable source, e.g. the radiation of a synchrotron, is necessary to determine different states in the conduction band. The transition intensity can
only be determined indirectly. One way is to measure the transmission or
reflection of the radiation and calculate the absorption. But this is only possible for thin samples. For metals it is possible to measure the drain current
from the sample which is proportional to the XAS signal. This method is
called total electron yield (TEY). In case of insulators one can measure the
intensity of radiant recombination (PFY- or TFY-mode).
25
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 2. BASICS OF X-RAY SPECTROSCOPIC METHODS
The XAS can be divided into two spectral regions. One is the so called
near edge x-ray absorption fine structure (NEXAFS) which reflects excitations of the photoelectron into the unoccupied states. The other one is the
extended x-ray absorption fine structure (EXAFS) where the photoelectron
is excited into the continuum and scatters with the environment. The superposition of this scattering leads to characteristic features in the XAS. This
region is usually at photon energies well above the corresponding NEXAFS
threshold.
For a better comparison of the spectra we made a background correction of
the XAS measurements in this work. Therefore we subtracted an exponential
background and a step-function from the measured data. A reason for the
exponential background can be the monochromator, which is influenced by
the incoming radiation and its wavelength. The step-function is subtracted in
order to subtract the transitions into continuum states [55]. A step-function
aligned at the maxima of the L3 and L2 edges with relative heights of 2 : 1,
which is the expected intensity ratio for transitions into the two continua,
was used. To get a better result the function was smoothed. In Figure 2.8
an example is given. On the top of the Figure the original measured XAS
Fe 2p core level spectrum of the spinel Fe0.4 Cu0.6 Cr2 S4 together with the
exponential background is shown. In the middle one can see the spectrum
from which the background was subtracted together with the step-function.
Finally this step-function is subtracted from the spectrum as well and the
resulting spectrum is shown at the bottom of Figure 2.8.
26
(p) 2006, Christian Taubitz, University of Osnabrück
Intensity (arb. units)
CHAPTER 2. BASICS OF X-RAY SPECTROSCOPIC METHODS
Fe04Cu06Cr2S4
exponential background
730
720
710
700
Photon energy (eV)
Intensity (arb. units)
Fe04Cu06Cr2S4
step-function
730
720
710
700
Photon energy (eV)
Intensity (arb. units)
Fe04Cu06Cr2S4
730
720
710
700
Photon energy (eV)
Figure 2.8: Background correction of the XAS Fe 2p core level measurement
of Fe0.4 Cu0.6 Cr2 S4 .
27
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 2. BASICS OF X-RAY SPECTROSCOPIC METHODS
2.3
2.3.1
Experimental equipment
Photoelectron spectrometer PHI 5600ci
The XPS measurements presented in this work were performed with a PHI
5600ci multitechnique spectrometer produced by the Perkin Elmer Coorperation [19].
Figure 2.9: The PHI 5600ci multitechnique spectrometer [20].
In order to make in situ experiments a preparation chamber was added
to the spectrometer. This was done by the fine mechanical workshop of the
department of physics. The chamber is equipped with a diamond file and
a pincer to rasp or to cleave the sample in vacuum. Thereby it is possible
to perform measurements on very clean surfaces, which is essential for the
rather surface sensitive XPS.
In addition the surface of a sample can be cleaned in the mainchamber by
sputtering with an ion gun. Argon ions are accelerated with a maximal
voltage of 4kV and hit the surface of the sample. But not every sample
can be cleaned this way. Whereas metallic compounds can be sputtered
successfully, other samples like oxides can be damaged by the ions.
28
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 2. BASICS OF X-RAY SPECTROSCOPIC METHODS
To prevent the sample from being contaminated during a measurement and
the photoelectrons from being scattered by gas-molecules on their way from
the sample to the analyser, UHV (Ultra High Vacuum) is needed. This is
achieved by the use of several types of vacuum-pumps. Rotation pumps can
reach up to a pressure in that turbomolecular pumps can work. With these
the pressure is about 1 × 10−8 mbar. An ion getter pump and a titanium
sublimation pump can then be activated to achieve an even better pressure of
about 1 × 10−9 mbar. In this condition XPS-measurements can be performed
for hours without taking care of sample-contamination or scattering processes
of the photoelectrons.
The PHI 5600ci is equipped with two x-ray sources. One is a dual Mg/Al
x-ray anode and the other one a monochromatised Al anode. The radiation
energies of the dual anode are 1486.6 eV for the Al Kα with a half-width of
0.85 eV and 1253.6 eV for the Mg Kα with a 0.7 eV half-width. The Kα
radiation is caused by a transition of an electron from the L shell to a hole
in the K shell, which was created because of a photoemission process. The
x-rays of the dual anode are unmonochromatised in contrast to the radiation
of the single Al anode. This source is usually used for measurements. Based
upon the Bragg equation nλ = 2d · sin(θ) the Al Kα is monochromatised by
a quartz crystal to a half-width of 0.3 eV.
In order to analyse the excited photoelectrons an 11 inches hemispherical
analyser is used, in which at first the electrons are focused by a lens system.
After that their kinetic energy is reduced to a certain pass energy Ep to ensure
a constant absolute resolution for the hole spectrum. In the so called constant
analyser transition (CAT) mode only electrons with an energy Ep ± ∆E may
pass the analyser, ∆E denotes the absolute energy resolution. Reducing the
pass energy leads to a higher energy resolution of the recorded spectra but a
smaller overall intensity of the XPS signal.
29
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 2. BASICS OF X-RAY SPECTROSCOPIC METHODS
2.3.2
Bessy k storage ring
Synchrotron radiation is necessary to perform measurements like XAS, that
need high intensities and a tunable energy. This radiation occurs when
charged particles, e.g. electrons, travel close to the speed of light and are deflected by a magnet. Such magnets are the so called bending magnets which
force the charged particles on the circular path of the storage ring. Wigglers
and undulators are deflecting magnets as well, though they do not change
the direction of the particle beam. These magnets just make the particles
oscillate so that they are radiating. Since synchrotron radiation is always
emitted in the forward direction, beamlines are arranged tangential with respect to the storage ring. Due to the high intensity and weak divergence of
the synchrotron radiation measurements with a high resolution are possible.
Figure 2.10: Bessy storage ring and facility.
30
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 2. BASICS OF X-RAY SPECTROSCOPIC METHODS
The XAS measurements in this thesis were performed at Bessy k in Berlin.
We worked at the Russian-German Beamline PM-RD-BL, which provides
radiation energies between 100 - 1500 eV. The spectrometer description and
its specifications are available on the internet [36]. The measurements where
done with linear polarized light at room temperature. The total electron
yield (TEY) was measured.
Figure 2.11: Left: Beamlines at Bessy k. Right: The Russian-German Beamline PM-RD-BL.
31
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 2. BASICS OF X-RAY SPECTROSCOPIC METHODS
32
(p) 2006, Christian Taubitz, University of Osnabrück
Chapter 3
The CMR-effect
The so called magneto resistance (MR) is the change in the electrical resistance of a conductor by an applied magnetic field (H). The MR is determined
by the following equation.
MR = −
R(H) − R(0)
R(0)
In nonmagnetic conductors the MR is relatively small and due to the Lorentz
force that a magnetic field exerts on moving electrons. But magnetic materials often show a large MR, sometimes even in low magnetic fields. The large
spin polarisation of the electrons giving rise to additional contributions could
be the reason for this. There are many different kinds of MR effects [39], but
the so called Colossal magneto resistance (CMR) effect is the biggest one. It
can reach up to several 100% or 1000%. In 1993 the effect was discovered in
manganese perovskites [52]. It has been associated with half metallic (HM)
ferromagnetism (FM) and was explained with a so called double-exchange
interaction. It is shown in Figure 3.1 for La1−x Cax MnO3 . In this compound
the presents of two Mn valences (Mn3+ /Mn4+ ) is assumed. Because of the
Hund coupling only in a ferromagnetic state an (itinerant) electron can move
from one Mn3+ -ion to empty states of the Mn4+ -ion. In an not ferromagnetic
state the jump-process needs too much energy. An applied magnetic field can
enhance a ferromagnetic state and therefore change the electrical resistance.
33
CHAPTER 3. THE CMR-EFFECT
Mn
3+
O
2−
e−
Mn
4+
Mn
e−
1−
4+
O
4+
O
Mn
Mn
2−
3+
O
2−
e−
Mn
Mn
4+
Mn
4+
Mn
4+
e−
O
1−
Mn
4+
3+
ferromagnetic
paramagnetic
Figure 3.1:
Schematic plot of the double-exchange interaction in
La1−x Cax MnO3 adapted from [21]. Left panel: ferromagnetic spin state,
right panel: canted spin structure.
Besides the double exchange model there are also other effects (e.g. magnonphonon interaction [53]) that are assumed to be involved in the CMR behavior. Nevertheless many facts like the remarkable rich phase diagram of these
compounds or the metal to insulator transition have yet not been understood
thoroughly.
During the last years huge MR effects have been reported also for other
types of materials like double perovskites (e.g. Sr2 FeMoO6 [48] [49]) or
the magnetic chalcogenides Fe1−x Cux Cr2 S4 [47][46]. Especially for the latter compounds the origin of the CMR-effect is very interesting. Since these
compounds have complete different characteristics compared to manganites,
the CMR-effect can not be explained in the same way. For example in
Fe1−x Cux Cr2 S4 the ions occupy both tetrahedral and octahedral site, whereas
in manganites they only occupy octahedral sites. Because of this the double
exchange model has to be modified for the chalcogenide. Palmer and Greaves
proposed the so called triple-exchange model [50]. An illustration is given
in Figure 3.2. The Fe2+ ions have six 3d electrons. The sixth electron is
34
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 3. THE CMR-EFFECT
located in the eg band with the spin antiparallel to the spins of the other
five electrons, but parallel to the Cr moments, which define the direction of
the magnetization. The single electron in the spin-up eg band hops with an
exchange mechanism, similar to the double exchange, via a p orbital of the
sulphur to Cr. This leads to an intermediated Cr2+ state. From there it
proceeds via the second S to the Fe3+ ion, changing its valence to Fe2+ .
Figure 3.2: Illustration of the triple-exchange between Fe2+ and Fe3+ via S
and Cr (adapted from [51]). The mobile electrons and the empty states, into
which they are hopping, are circled.
Recently it was assumed that in Fe1−x Cux Cr2 S4 the conductivity is due to
triple-exchange mechanisms for the concentration range x<0.5 and doubleexchange mechanisms for x≥0.5 [51]. Since our measurements of the spinel
with x=0.6 did not show two Cr valences, which are essential for doubleexchange, this has to be further investigated. In addition many facts concerning the temperature and pressure depending behavior of these compounds are
still not understood [51].
35
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 3. THE CMR-EFFECT
36
(p) 2006, Christian Taubitz, University of Osnabrück
Chapter 4
Preparation of Sr2FeReO6
A lot of transition metal oxides have attracted much attention due to their
rich phase diagrams and transport properties since the discovery of colossal
magneto resistance at room temperature by Kobayashi et al. [40]. Especially
ordered double perovskites like Sr2 FeMoO6 (SFMO) have been studied a lot
(e.g. [38], [39]). These compounds have the general structure A2 BB’O6 .
The B and the B’ sites are usually occupied by transition metals (e.g. Fe
and Mo), which are in the center of an oxygen octahedron. In a perfectly
ordered double perovskite the BO6 and the B’O6 octahedrons are alternating
[41] (Figure 4.1). SFMO is ferrimagnetic because the magnetic moments of
iron and molybdenum are antiparallel and the magnetic moment of iron is
much larger than the magnetic moment of molybdenum. The compound is
in a half metallic antiferromagnetic state with a Curie temperature of about
400K [42].
37
CHAPTER 4. PREPARATION OF SR2 FEREO6
Fe
Oxygen
octahedron
Mo
Figure 4.1: Ordered double perovskite structure of Sr2 FeMoO6 .
Sr-ions are not illustrated.
A few years ago a double perovskite with an even higher Curie temperature was presented [43]. By choosing Cr for the B and Re for the B’ site an
CMR-compound was created (Sr2 CrReO6 ) with TC = 635K and the characteristics of a metallic ferromagnet.
In order to investigate the property change of these two compounds we decided to create a double perovskite with Fe at the B and Re at the B’ site
(Sr2 FeReO6 (SFRO)). Four samples were produced in the crystal growth facility of the University of Osnabrück. Dr. R. Pankrath is acknowledged for
preparing the samples. To get an idea of the conductivity and magnetic
behavior of the compounds we used a multimeter and a magnet.
38
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 4. PREPARATION OF SR2 FEREO6
The first produced sample showed a high resistance and seemed to be not
magnetic.
The second sample was produced the same way, but afterwards it was reduced in H2 . This was done to lower the amount of oxygen in the sample.
The resistance of this sample decreased indicating an improve in the crystal
structure, but, like for the first sample, a magnetic behavior could not be
seen.
The third sample was reduced again, but with a small H2 gas flow. The gas
flow was only about 1.5 l/h. There seemed to be no change in the resistance,
but a small magnetic behavior could be found.
The same was present for the last sample, which was produced with a reduction gas flow of only 1 l/h. In the following XPS measurements of these
four samples are presented and discussed with attention to the quality of the
compounds.
4.1
XPS measurements
In Figure 4.2 XPS O 1s core level spectra of the four prepared samples are
shown. As one can see the O 1s lines of the first two samples show two
valences. This could indicate a second phase in the crystal and therefore a
deficit in the crystal structure. The spectra of the third sample shows one
broad O 1s line indicating a better structure. The O 1s line of the fourth
sample is the best defined one, which could indicate an even better structure.
39
(p) 2006, Christian Taubitz, University of Osnabrück
Intensity (arg. units)
CHAPTER 4. PREPARATION OF SR2 FEREO6
SFRO_1.sample
536
534
532
530
528
526
Intensity (arb. units)
Binding energy (eV)
SFRO_2.sample
536
534
532
530
528
526
Intensity (arb. units)
Binding energy (eV)
SFRO_3.sample
534
532
530
528
Binding energy (eV)
Intensity (arb. units)
536
526
SFRO4.sample
536
534
532
530
528
526
Binding energy (eV)
Figure 4.2: The XPS O 1s core level spectra of different produced Sr2 FeReO6
samples.
40
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 4. PREPARATION OF SR2 FEREO6
Figure 4.3 shows XPS Re 4f core level spectra of the prepared samples.
The spectra exhibit a doublet structure due to the spin orbit coupling. The
first sample shows two broaden lines indicating most of the Re in a 6+ valence
state. The Re 4f line of the second samples is showing many peaks. This can
be due to many different Re valences present in the compound. The spectra
of the third sample exhibits 3 peaks. This indicates that a mixed Re valence
state (Re5+ /Re6+ ) is present. Since the second peak of the Re5+ doublet and
the first peak of the Re6+ doublet overlap only 3 peaks are present. The
fourth sample shows two peaks again, but in addition a shoulder at lower
binding energies indicating a small amount of Re5+ or Re4+ valences besides
Re6+ -ions.
41
(p) 2006, Christian Taubitz, University of Osnabrück
Intensity (arb. units)
CHAPTER 4. PREPARATION OF SR2 FEREO6
SFRO_1.sample
Intensity (arb. units)
54
50
48
46
44
42
40
48
46
44
42
40
48
46
44
42
40
48
46
44
42
40
Binding energy (eV)
SFRO_2.sample
Intensity (arb. units)
54
52
50
Binding energy (eV)
SFRO_3.sample
54
Intensity (arb.units)
52
52
50
Binding energy (eV)
SFRO_4.sample
54
52
50
Binding energy (eV)
Figure 4.3: The XPS Re 4f core level spectra of different produced Sr2 FeReO6
samples.
42
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 4. PREPARATION OF SR2 FEREO6
In conclusion one can say that the structure of the samples seem to improve by a small H2 reduction. While a high gas flow leads to a lot of
additional valences of O and Re, this does not occur for a small gas flow.
The sample that was reduced with 1.5 l/h is very interesting since the Re
shows clearly two valences. Since in Sr2 FeMoO6 the Mo ion is present also
in two valences, which makes double exchange possible, this could indicate a
well ordered double perovskite structure [21][39].
In the near future XRD measurements will be done to get more information
about the structure and quality of the samples.
43
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 4. PREPARATION OF SR2 FEREO6
44
(p) 2006, Christian Taubitz, University of Osnabrück
Chapter 5
The Spinel Fe1−xCuxCr2S4
Compounds with the structure ACr2 X4 crystallize in the normal spinel closepacked fcc lattice [Fd3m], in which the A-ions occupy tetrahedral and the
B-ions octahedral sites (Figure 5.1). These crystals are simply called spinels.
The chalcogenide Fe1−x Cux Cr2 S4 is a spinel where the A-ions are a mixture
of Fe and Cu.
Figure 5.1: The spinel crystal structure.
45
CHAPTER 5. THE SPINEL FE1−X CUX CR2 S4
Recently a colossal magnetoresistance effect(CMR) was reported in
Fe1−x Cux Cr2 S4 close to room temperature [22]. This renewed an remarkable interest in these compounds, since other mechanisms, besides double
exchange and electron-phonon interactions, seem to be responsible for the
CMR-effect. Because of complete different characteristics compared to manganites (first reported CMR-compounds) in spinels the CMR-effect can not
be explained in the same way.
But not only the magnetoresistance effect, also the valency of the spinel atoms
is still in question. Goodenough [25] developed a model in which the A-site
−2
is divalent [A2+ B3+
2 S4 ]. But in a different model Lotgering [23, 24] claims
the A-site to be monovalent and the B-site to be in a mixed valence state
[A1+ B3+ B4+ S−2
4 ]. For the Fe1−x Cux Cr2 S4 system the situation becomes even
more complicated, since there are several possibilities for the substitution
of Cu by Fe, depending on whether Cu is monovalent or divalent. Various
possibilities where discussed by several authors [26, 27, 28]. Recently it was
suggested that for low substitution of Cu (x=0.0, 0.1) Fe is in a ferrous (Fe2+ )
charge state whereas for higher substitution of Cu (x=0.3, 0.5) the charge
state of Fe is ferric (Fe3+ ) (x=0.0, 0.1) [29].
In the following XPS and XAS measurements of Fe1−x Cux Cr2 S4 (x=0.2, 0.6)
are presented and compared to other measurements and calculations. The
results are discussed with special attention to the valence state of Fe and
Cu. The single crystals where grown by chemical transport reaction. Dr. V.
Tsurkan is acknowledged for providing the spinels.
46
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 5. THE SPINEL FE1−X CUX CR2 S4
5.1
5.1.1
XPS
Cu 2p and 3s core level spectra
Figure 5.2 shows the Cu 2p XPS spectra of Fe1−x Cux Cr2 S4 (x=0.2, 0.6). The
spectra exhibit a doublet structure due to the spin orbit coupling (2p3/2 ,
2p1/2 ). The line at about 980 eV is the Cr L3 M23 M45 Auger peak. Compared
to other Cu 2p spectra [32] (Figure 5.3) the measurements closely resemble
spectra of Cu2 O and CuFeO2 , in which Cu ions are present as Cu1+ . This fact
is a strong indication for Cu being in a monovalent charge state, since satellite
structures like in CuO, containing Cu2+ ions, are not present. Nevertheless
there are Cu compounds like CuSe and CuS with Cu in a divalent charge
state that do not show shake-up satellites.
2p3/2
Intensity (arb. units)
Cu 2p-XPS
Fe08Cu02Cr2S4
2p1/2
Fe04Cu06Cr2S4
960
950
940
930
Binding energy (eV)
Figure 5.2: The XPS Cu 2p spectra of Fe0.8 Cu0.2 Cr2 S4 and Fe0.4 Cu0.6 Cr2 S4 .
47
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 5. THE SPINEL FE1−X CUX CR2 S4
Figure 5.3: XPS Cu 2p spectra adapted from [32].
To check the Cu valence in the investigated compounds in addition Cu 3s
spectra were measured. In Figure 5.4 the Cu 3s spectra are presented. It is
known that a spectral splitting of the 3s XPS core-level spectra can occur for
transition metals due to an exchange coupling between the 3s hole created
during the photoemission process and the 3d electron. Since the 3s splitting
is related to the total spin of the 3d electrons [35] from its value one can
obtain information about the valency of the Cu ions. There is no exchange
splitting of the Cu 3s level of the investigated compounds. This fact confirms
a 3d10 electronic configuration for the Cu1+ ions.
48
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 5. THE SPINEL FE1−X CUX CR2 S4
Intensity (arb. units)
Cu 3s-XPS
Fe08Cu02Cr2S4
Fe04Cu06Cr2S4
135
130
125
120
115
110
Binding energy (eV)
Figure 5.4: The XPS Cu 3s spectra of Fe0.8 Cu0.2 Cr2 S4 and Fe0.4 Cu0.6 Cr2 S4 .
5.1.2
Cr 2p core level spectra
In Figure 5.5 Cr 2p XPS core level spectra are presented. In these spectra,
additional to the spin orbit splitting (2p3/2 , 2p1/2 ), an exchange splitting of
the Cr 2p3/2 line occurs. The value of this splitting can be used to determine
the valence state of the Cr-ions [32]. The splitting of the Cr 2p3/2 line is
about 1 eV± 0.1 eV, which is an indication for trivalent Cr-ions.
49
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 5. THE SPINEL FE1−X CUX CR2 S4
2p3/2
Cr 2p-XPS
Intensity (arb. units)
2p1/2
Fe08Cu02Cr2S4
Fe04Cu06Cr2S4
595
590
585
580
575
570
Binding energy (eV)
Figure 5.5: The XPS Cr 2p spectra of Fe0.8 Cu0.2 Cr2 S4 and Fe0.4 Cu0.6 Cr2 S4 .
Since the splitting is not well resolved, neutron diffraction measurements
of the Cr magnetic moment could help to confirm the valence state of the
Cr-ions.
5.1.3
Fe 2p core level spectra
Figure 5.6 shows Fe 2p XPS core level spectra of Fe0.8 Cu0.2 Cr2 S4 and
Fe0.5 Cu0.5 Cr2 S4 together with spectra of other Fe compounds adapted from
[32]. The spectra exhibit a doublet structure due to the spin orbit coupling
(2p3/2 , 2p1/2 ). An comparison of the Fe 2p lines reveal that the position
and satellite structure of Fe0.8 Cu0.2 Cr2 S4 resemble them of Fe0.5 Cu0.5 Cr2 S4
and FeCr2 S4 . In FeCr2 S4 the valence state of Fe is assumed to be divalent,
whereas in Fe0.5 Cu0.5 Cr2 S4 the Fe ions should be trivalent. The fact that
both spectra resemble each other was explained with charge transfer effects.
During the photoemission process a charge transfer from one S2− to a Fe3+
ion takes place and therefore most of the Fe ions are excited in Fe2+ (S− )
50
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 5. THE SPINEL FE1−X CUX CR2 S4
states [32]. Because of the preservation of charge neutrality and according to
the valence model of Lotgering in Fe0.8 Cu0.2 Cr2 S4 a mixed Fe valence state
should be present. The Fe2+ /Fe3+ ration is assumed to be 3/1 [24]. This
fact and the occurrence of charge transfer effects agree with the XPS Fe 2p
spectrum of Fe0.8 Cu0.2 Cr2 S4 showing divalent Fe ions.
Intensity (arb. units)
2p3/2
2p1/2
Fe08Cu02Cr2S4
Fe05Cu05Cr2S4
Fe 2p-XPS
740
735
730
725
720
715
710
705
Binding energy (eV)
Figure 5.6: The XPS Fe 2p spectra of Fe0.8 Cu0.2 Cr2 S4 and Fe0.5 Cu0.5 Cr2 S4 together with spectra of (a) Fe0.5 Cu0.5 Cr2 S4 , (b) FeCr2 S4 and (c) Fe2 O3 adapted
from [32].
51
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 5. THE SPINEL FE1−X CUX CR2 S4
In Figure 5.7 XPS Fe 2p spectra of Fe0.8 Cu0.2 Cr2 S4 , Fe0.5 Cu0.5 Cr2 S4 and
Fe0.4 Cu0.6 Cr2 S4 are presented. As one can see the latter one has significant
differences compared to the other two spectra. This can be due to a contaminated surface, a small amount of Fe in the compound or little differences in
the sample quality. But if one again considers the Lotgering model another
explanation reveals. According to this model for x > 0.5 holes in the S valence band are predicted. This can reduce the charge transfer process and
cause less Fe ions to be measured in a Fe2+ S− state, but in a Fe3+ charge
state. This would lead to lines measured at higher binding energies as seen
in the spectra of Fe0.4 Cu0.6 Cr2 S4 . The satellite structures at both Fe 2p lines
seem to broaden while the intensity of the main lines, especially the 2p3/2 ,
seem to decrease. In addition also in the other two spectra this special tendency occurs. Compared to the spectrum of Fe0.8 Cu0.2 Cr2 S4 the Fe 2p lines
of Fe0.5 Cu0.5 Cr2 S4 seem to broaden, the satellites increase and the 2p3/2 line
has less intensity than the 2p1/2 line. In contrast to this, the 2p3/2 line in
Fe0.8 Cu0.2 Cr2 S4 has more intensity than the 2p1/2 line. In Fe0.5 Cu0.5 Cr2 S4 all
Fe ions are predicted to be trivalent, whereas in Fe0.8 Cu0.2 Cr2 S4 only 20% of
the ions at the A-site are assumed to be Fe3+ . Therefore a bigger amount of
measured trivalent Fe ions in Fe0.5 Cu0.5 Cr2 S4 , despite charge transfer effects,
is likely.
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CHAPTER 5. THE SPINEL FE1−X CUX CR2 S4
Intensity (arb. units)
Fe08Cu02Cr2S4
2p3/2
2p1/2
Fe05Cu05Cr2S4
Fe04Cu06Cr2S4
740
Fe2p-XPS
735
730
725
720
715
710
705
Binding energy (eV)
Figure 5.7: XPS Fe 2p spectra of Fe0.8 Cu0.2 Cr2 S4 , Fe0.5 Cu0.5 Cr2 S4 and
Fe0.4 Cu0.6 Cr2 S4 .
In order to confirm the valence state of the Fe ions and for a better
understanding of the line shape of the spectra, further measurement of these
and other Fe1−x Cux Cr2 S4 compounds are needed.
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CHAPTER 5. THE SPINEL FE1−X CUX CR2 S4
5.1.4
S 2p core level spectra
Here XPS S 2p core level spectra of Fe0.8 Cu0.2 Cr2 S4 , Fe0.5 Cu0.5 Cr2 S4 and
Fe0.4 Cu0.6 Cr2 S4 are presented (Figure 5.8). Again spin orbit splitting occurs.
The intensity of the 2p3/2 line is normalised. As one can see compared to
the other two spectra for Fe0.4 Cu0.6 Cr2 S4 the intensity of the 2p1/2 line is
a bit increased and broadened. Like for the Fe spectra this can be due
to a contaminated surface of the sample or little differences in the sample
quality. But holes in the valence band of S lead to a bigger 2p1/2 peak [32]
and according to Lotgering in Fe0.4 Cu0.6 Cr2 S4 a small amount of ligand holes
(about 5%) should occur. This would be in good agreement with the XPS
Fe 2p core level discussion.
2p3/2
Intensity (arb. units)
S 2p-XPS
2p1/2
Fe04Cu06Cr2S4
Fe08Cu02Cr2S4
Fe05Cu05Cr2S4
166
164
162
160
Binding energy (eV)
158
Figure 5.8: The XPS S 2p spectra of Fe0.8 Cu0.2 Cr2 S4 , Fe0.5 Cu0.5 Cr2 S4 and
Fe0.4 Cu0.6 Cr2 S4 .
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CHAPTER 5. THE SPINEL FE1−X CUX CR2 S4
5.2
XAS
5.2.1
Cr 2p XAS spectra
The Cr 2p lines of Fe0.8 Cu0.2 Cr2 S4 and Fe0.4 Cu0.6 Cr2 S4 were measured at the
same binding energies and look very similar (Figure 5.9). Due to the spin orbit coupling a doublet structure occurs (L2 , L3 ). An exponential background
and a step-function were subtracted from the spectra as described in chapter
2.2. There is a small shoulder at the Cr L3 edge of Fe0.4 Cu0.6 Cr2 S4 and the L2
edge shows a higher intensity. This could be due to the low resolution, charge
transfer effects or a difference in the quality of the compounds. Recently Cr
2p XAS spectra of Fe0.5 Cu0.5 Cr2 S4 measured at 50K were presented. The
Cr-ions were determined to be trivalent as well [31]. There are significant
differences in the shape of these lines compared to our measurements. A
possible reason for this could be the low measuring temperature, which is
further discussed in the next section.
L3
Intensity (arb. units)
Cr 2p-XAS
L2
Fe08Cu02Cr2S4
Fe04Cu06Cr2S4
595
590
585
580
575
570
565
Photon energy (eV)
Figure 5.9: The XAS Cr 2p core level spectra of Fe0.8 Cu0.2 Cr2 S4 and
Fe0.4 Cu0.6 Cr2 S4 .
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CHAPTER 5. THE SPINEL FE1−X CUX CR2 S4
An atomic multiplet calculation of Cr3+ is shown in Figure 5.10. The calculations were performed using the TT-multiplet program [44] [45]. Core-hole
intrinsic lifetime broadening was set to 0.3 eV for both edges and a Gaussian broadening (0.4 eV) was applied to account for experimental resolution.
Octahedral sites with an crystal field of 1.5 eV were assumed according to
Aniruddha Dep et al. [31]. There are some significant differences between
the Cr 2p spectra and the calculation, especially at the L2 edge. These can be
due to the fact that many side effects, like charge transfer, are not included
in the calculation. Nevertheless the line shape of the Cr 2p spectra resemble
the calculation, which could be an indication for trivalent Cr ions.
Cr 2p-XAS & Calculation
L3
Intensity (arb. units)
Fe04Cu06Cr2S4
3+
Cr _calculation
(10Dq=1.5eV, [Oh])
L2
595
590
585
580
575
570
565
570
565
Photon energy (eV)
L3
Intensity (arb. units)
Cr 2p-XAS & Calculation
595
Fe08Cu02Cr2S4
3+
Cr _calculation
(10Dq=1.5eV, [Oh])
L2
590
585
580
575
Photon energy (eV)
Figure 5.10: The XAS Cr 2p core level measurements and an atomic multiplet
Cr3+ calculation.
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CHAPTER 5. THE SPINEL FE1−X CUX CR2 S4
5.2.2
Fe 2p XAS spectra
Figure 5.11 shows the measured Fe 2p spectra of Fe0.8 Cu0.2 Cr2 S4 and
Fe0.4 Cu0.6 Cr2 S4 . The spectra exhibit a doublet structure due to the spin
orbit coupling (L2 , L3 ). An exponential background and a step-function
were subtracted from the spectra as described in chapter 2.2. As one can see
the Fe 2p lines look very different which indicates that the two compounds
differ in the Fe valence state. In addition the shoulder at higher photon
energies of the L3 edge of Fe0.8 Cu0.2 Cr2 S4 corresponds to the main peak of
the Fe0.4 Cu0.6 Cr2 S4 L3 edge. Similarly the little peak at lower photon energies
beside the L3 edge of Fe0.4 Cu0.6 Cr2 S4 corresponds to the main peak of the
Fe0.8 Cu0.2 Cr2 S4 L3 edge. At the L2 edge a correspondence of the lines can
be seen as well. This could be an indication for charge transfer effects or a
mixed valence state of the Fe-ions.
Figure 5.11: The XAS Fe 2p core level spectra of Fe0.8 Cu0.2 Cr2 S4 and
Fe0.4 Cu0.6 Cr2 S4 .
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CHAPTER 5. THE SPINEL FE1−X CUX CR2 S4
For a better understanding of the Fe 2p line shape and the Fe valence state
we look at XAS core level Fe 2p measurements and configuration-interaction
calculations of FeO (Fe2+ ) and Fe2 O3 (Fe3+ ).
As shown in Figure 5.12 the XAS measurements of FeO (from [54]) and
Fe2 O3 (from [33]) look very similar to our measurements. Especially the
latter one is surprisingly equal to the Fe 2p spectra of Fe0.4 Cu0.6 Cr2 S4 . In
Figure 5.13 both spectra are plotted. The integrals of the two measurements
were normalised.
L3
Intensity (arb. units)
Fe 2p-XAS
L2
FeO
Fe2O3
730
725
720
715
710
705
700
Photon energy (eV)
Figure 5.12: XAS Fe 2p core level measurements of FeO (from [54]) and
Fe2 O3 (from [33]).
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CHAPTER 5. THE SPINEL FE1−X CUX CR2 S4
L3
Intensity (arb. units)
Fe 2p-XAS
Fe04Cu06Cr2S4
Fe2O3
L2
735
730
725
720
715
710
705
700
Photon energy (eV)
Figure 5.13: The measured XAS Fe 2p core level spectra of Fe0.4 Cu0.6 Cr2 S4
and Fe2 O3 (from [33]). The integrals of both measurements are normalised.
The fact that the Fe-ions in Fe0.4 Cu0.6 Cr2 S4 occupy tetrahedral sites
whereas they occupy octahedral sites in Fe2 O3 makes the agreement of both
measurements even more astonishing. But if one looks at atomic multiplet calculations of Fe3+ XAS spectra for different crystal fields, one can see
that for low crystal fields there is almost no change in the shape of the Fe
2p lines calculated for tetrahedral or octahedral sites (Figure 5.14). Since
for Fe2 O3 a crystal field of 10Dq=0.88eV has been determined [30] and for
Fe0.5 Cu0.5 Cr2 S4 10Dq=0.5eV was assumed [31], a low crystal field is very
likely for this compounds. The calculations were performed using the TTmultiplet program [44] [45]. Core-hole intrinsic lifetime broadening was set
to 0.1 eV for both edges and a Gaussian broadening (0.4 eV) was applied to
account for experimental resolution. The line shape of the calculation differs
from the measurements due to the fact that many side effects like charge
transfer effects were not included in the calculations.
59
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Intensity (arb. units)
CHAPTER 5. THE SPINEL FE1−X CUX CR2 S4
3+
Fe _calculation (10Dq=1,11eV, [Oh] )
3+
Fe _calculation (10Dq=1,11eV, [Td] )
730
720
710
700
Photon energy (eV)
Intensity (arb. units)
3+
Fe _calculation (10Dq=0.88eV, [Oh] )
3+
Fe _calculation (10Dq=0.88eV, [Td] )
730
720
710
700
Intensity (arb. units)
Photon energy (eV)
3+
Fe _calculation (10Dq=0.5eV, [Oh] )
3+
Fe _calculation (10Dq=0.5eV, [Td] )
730
720
710
700
Photon energy (eV)
Figure 5.14: Atomic multiplet calculations of an Fe3+ XAS spectrum for
different crystal fields, tetrahedral and octahedral sites.
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CHAPTER 5. THE SPINEL FE1−X CUX CR2 S4
If charge transfer effects are included in the Fe3+ XAS calculation, it is in
good agreement with the measurement. The Figure 5.15 shows the XAS Fe2p
measurements of Fe2 O3 and Fe0.4 Cu0.6 Cr2 S4 together with a configurationinteraction calculation done by Crocombette et al. [30]. The integrals of all
spectra were normalised.
L3
Intensity (arb. units)
Fe2p-XAS & Calculation
735
Fe04Cu06Cr2S4
Fe2O3
3+
Fe _Calculation
(J.P.Crocombette et al.)
L2
730
725
720
715
710
705
700
Photon energy (eV)
Figure 5.15: XAS Fe 2p core level measurements of Fe0.4 Cu0.6 Cr2 S4 and
Fe2 O3 (from [33]) together with a configuration-interaction calculation
(adapted from [30]). The integrals of all spectra are normalised.
All this indicates the Fe-ions in Fe0.4 Cu0.6 Cr2 S4 to be in a 3+ valence
state, which is in good agreement with the Lotgering valency model [24]. In
addition one can say that charge transfer effects act on the shape of the Fe2p
XAS spectra in the same way as they do in Fe2 O3 .
If we now compare the Fe0.8 Cu0.2 Cr2 S4 and FeO Fe 2p XAS spectra, we see
a general correlation as well. But there are some significant differences like
the shoulder at the L3 edge, which is narrower in the FeO spectrum. Fe2+
calculations, also done by Crocombette et al., show a narrow shoulder on the
L3 edge as well (Figure 5.16).
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CHAPTER 5. THE SPINEL FE1−X CUX CR2 S4
Intensity (arb. units)
Fe2p-XAS & Calculation
L3
L2
Fe08Cu02Cr2S4
FeO
2+
Fe _Calculation
730
725
720
715
710
705
700
Photon energy (eV)
Figure 5.16: The measured XAS Fe 2p core level spectra of Fe0.8 Cu0.2 Cr2 S4
and FeO (adapted from [54]) together with a configuration-interaction calculation (adapted from [30]). The integrals of the spectra are normalised.
The similar line shape of the spectra could indicate, that the Fe-ions in
Fe0.8 Cu0.2 Cr2 S4 are in a 2+ valence state. But the broad shoulder at the L3
edge can not be explained by Fe2+ ions alone, since only a narrow shoulder
appears in the spectrum of FeO and the calculation of Crocombette et al.
[30].
The valency model of Lotgering [24] can give a possible interpretation of the
big shoulder to higher photon energies at the Fe 2p L3 peak of Fe0.8 Cu0.2 Cr2 S4 .
According to the model for x<0.5 the Fe-ions are predicted to be in a mixed
valence state. In Fe0.8 Cu0.2 Cr2 S4 20% of the ions at the A-site are assumed
to be Fe3+ whereas 60% should be Fe2+ . Figure 5.17 shows the XAS spectra
together with an atomic multiplet calculation mixing Fe2+ and Fe3+ spectra
in a ratio of 3 to 1. Additional the Fe3+ calculation was increased by a factor 5/4 due to the higher amount of unoccupied states in the d band. The
calculations were done with the TT-multiplet program [44] [45]. Core-hole
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CHAPTER 5. THE SPINEL FE1−X CUX CR2 S4
intrinsic lifetime broadening was set to 0.1 eV for the L3 edges and 0.5 eV
for the L2 edges. Additional an Gaussian broadening (0.4 eV) was applied to
account for experimental resolution. The Fano-parameter was set to 25 for
the L3 and to 999 for the L2 edge to simulate a Doniach Sunjic line shape.
The calculation resembles the spectra, but there are some significant differences, especially at the L2 edge. These can be due to the fact that many side
effects, like charge transfer, are not included in the calculations.
Intensity (arb. units)
Fe 2p & Calculation
Fe08Cu02Cr2S4
Calcultion
730
725
720
715
710
705
700
695
Photon energy (eV)
Figure 5.17: The measured XAS Fe 2p core level spectrum of Fe0.8 Cu0.2 Cr2 S4
and an atomic multiplet calculation of a Fe2+ /Fe3+ mixed valence state in
the ratio of 3/1. The integrals of the spectra are normalised.
Nevertheless the calculation indicates that it is possible to have a mixed
Fe valence state in the spinel according to the Lodgering model. Further
investigations of this compound and differently doped Fe1−x Cux Cr2 S4 spinels
are needed to reveal the valency of Fe.
Recently an XAS Fe 2p spectrum of Fe0.5 Cu0.5 Cr2 S4 that was measured at
50K by using circularly polarized synchrotron radiation was presented [31].
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CHAPTER 5. THE SPINEL FE1−X CUX CR2 S4
In this work the charge state of Fe-ions was determined to be trivalent.
Figure 5.18 shows the XAS Fe 2p spectra of Fe0.5 Cu0.5 Cr2 S4 (sum of both
spin directions), Fe0.4 Cu0.6 Cr2 S4 and Fe0.8 Cu0.2 Cr2 S4 . The position of the
Fe0.5 Cu0.5 Cr2 S4 XAS lines were adjusted to the lines of Fe0.4 Cu0.6 Cr2 S4 . All
the integrals were normalised. As one can see the first two spectra, both assumed to show trivalent Fe-ions, resemble each other, but there are peaks at
lower binding energies beside the L3 and L2 edges of Fe0.4 Cu0.6 Cr2 S4 that are
not measured for Fe0.5 Cu0.5 Cr2 S4 . As mentioned before also the Cr-peaks of
these two compounds show differences. This can be due to the low measuring
temperature, which has an significant influence on the chalcogenide spinels.
For example Fe1−x Cux Cr2 S4 with x=0.0 and 0.5 showed semiconducting behaviors at T>Tc and TTc , whereas metallic features were observed in the
finite temperature range below Tc [28]. For Fe0.5 Cu0.5 Cr2 S4 the Curie temperature is Tc =348K [32]. If we assume a similar Curie temperature for
Fe0.4 Cu0.6 Cr2 S4 it is possible that our XAS measurements, done at room
temperature, show the metallic state of the spinels. Whereas the measurements of Fe0.5 Cu0.5 Cr2 S4 , done at 50K, show the semiconducting state of the
spinels. Further investigations at different temperatures could clarify this
fact.
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CHAPTER 5. THE SPINEL FE1−X CUX CR2 S4
L3
Intensity (arb. units)
Fe2p-XAS
L2
Fe05Cu05Cr2S4
Fe04Cu06Cr2S4
Fe08Cu02Cr2S4
735
730
725
720
715
710
705
Photon energy (eV)
Figure 5.18: Measured XAS Fe 2p core level spectra of Fe0.5 Cu0.5 Cr2 S4
(adapted from [31]), Fe0.4 Cu0.6 Cr2 S4 and Fe0.8 Cu0.2 Cr2 S4 . The integrals of
the spectra are normalised.
If one compares the Fe0.5 Cu0.5 Cr2 S4 XAS spectrum with the Fe0.8 Cu0.2 Cr2 S4
spectrum, a second possible interpretation becomes apparent. The
Fe0.5 Cu0.5 Cr2 S4 spectrum shows a shoulder to higher photon energy that does
not occur in the Fe0.4 Cu0.6 Cr2 S4 Fe 2p spectrum but in the Fe0.8 Cu0.2 Cr2 S4
spectrum. In addition the distances between the L3 edges of the Fe 2p and Cr
2p spectra reveal that the Fe 2p L-edges of Fe0.5 Cu0.5 Cr2 S4 are more likely at
photon energies corresponding to Fe0.8 Cu0.2 Cr2 S4 (Figure 5.19). This indicates that most of the Fe-ions in Fe0.5 Cu0.5 Cr2 S4 were measured in a divalent
charge state. As mentioned before XPS measurements of this compound also
showed Fe2+ -ions [32].
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CHAPTER 5. THE SPINEL FE1−X CUX CR2 S4
L3
Intensity (arb. units)
Fe 2p-XAS
L2
Fe05Cu05Cr2S4
Fe04Cu06Cr2S4
Fe08Cu02Cr2S4
735
730
725
720
715
710
705
Photon energy (eV)
Figure 5.19: Measured XAS Fe 2p core level spectra of Fe0.5 Cu0.5 Cr2 S4
(adapted from [31]), Fe0.4 Cu0.6 Cr2 S4 and Fe0.8 Cu0.2 Cr2 S4 . Their relative positions are determined by their distances from the Cr 2p spectra. The integrals
of the spectra are normalised.
Nevertheless the Fe-ions were assumed to be trivalent giving large charge
transfer effects as an explanation for the measuring of divalent Fe-ions [32].
The preservation of charge neutrality, magnetic measurements [50] and investigations with Mößbauer spectroscopy [24][29] confirm this. In
Fe0.4 Cu0.6 Cr2 S4 most of the Fe-ions are measured in a trivalent charge state,
which could be due to an decrease in the charge transfer. The higher temperature, at which the sample was measured, or ligand holes, that occur in
Fe0.4 Cu0.6 Cr2 S4 according to Lotgering, can be responsible for this.
66
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Chapter 6
Conclusion and outlook
The aim of the present work was to investigate the influence of the substitution of different atoms in selected CMR compounds. These materials have
attracted much interest due to their magnetic behavior and applicability in
spintronic devices, magnetic sensors or other magneto electronic applications.
Two complementary x-ray spectroscopic methods and computer calculations
have been applied. The results of the spectroscopic study led to the following
conclusions:
The double perovskite Sr2FeReO6
During the present work four samples of the double perovskite Sr2 FeReO6
were produced in the crystal growth facility of the University of Osnabrück.
The O 1s and Re 4f XPS core level spectra of the samples were investigated
with attention to their quality. The H2 flow, under which the samples are
reduced, is found to have a big influence on the sample quality. While the
reduction with a big gas flow leads to a lot of additional valences of O and Re,
a small gas flow ≤ 1.5 l/h does not show many different valences indicating a
better structure. The sample that was reduced with 1.5 l/h is very interesting
since the Re shows clearly two valences. Since in Sr2 FeMoO6 the Mo ion is
present also in two valences, which makes double exchange possible, this
could indicate a well ordered double perovskite structure.
67
CHAPTER 6. CONCLUSION AND OUTLOOK
The spinel chalcogenide Fe1−xCuxCr2S4
The spinels Fe1−x Cux Cr2 S4 (x=0.2, 0.6) were investigated by x-ray photoelectron spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) and
compared to other measurements and calculations. The results seem to confirm the Lotgering valency model [24].
For both samples the XPS data indicate Cu to have a 3d10 electronic configuration and Cr to be trivalent. The XPS Fe 2p core level spectra of the two
samples reveal most of the Fe ions in Fe0.8 Cu0.2 Cr2 S4 to be divalent, while
in Fe0.4 Cu0.6 Cr2 S4 predominantly Fe3+ ions seem to occur. In addition the
investigation of the Fe and S XPS spectra reveals the possibility of ligand
holes in Fe0.4 Cu0.6 Cr2 S4 .
The XAS measurements are in good agreement with the XPS data. The
XAS Cr 2p core level spectra indicate Cr to be trivalent in both samples.
The Fe spectra show Fe3+ ions in Fe0.4 Cu0.6 Cr2 S4 , while in Fe0.8 Cu0.2 Cr2 S4
Fe seems to be in a mixed valence state according to Lotgering.
In addition recently presented XAS measurements of Fe0.5 Cu0.5 Cr2 S4 [31]
were compared to our measurements. Significant differences were found in
the line shapes. A reason for this could be the fact, that our measurements
were done at room temperature, while Fe0.5 Cu0.5 Cr2 S4 was measured at 50K.
Therefore it is possible that our measurements show the metallic state of the
spinels, while the investigations of Fe0.5 Cu0.5 Cr2 S4 show the semiconducting
state. This could lead to differences in the spectra. It is also possible that,
maybe because of the low measuring temperature, in Fe0.5 Cu0.5 Cr2 S4 charge
transfer effects increased. Thus the Fe ions were not measured completely in
a trivalent charge state, as assumed by Aniruddha et al. [31], but most of
them in a divalent state. The similar line shape and position of the XAS Fe
2p spectra of Fe0.5 Cu0.5 Cr2 S4 and Fe0.8 Cu0.2 Cr2 S4 confirm this. The higher
measuring temperature or the possible presence of ligand holes could inhibit
charge transfer in Fe0.4 Cu0.6 Cr2 S4 . Therefore these measurements show Fe
to be trivalent. The charge transfer effects could also lead to the differences
in the Cr spectra.
68
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 6. CONCLUSION AND OUTLOOK
Outlook
Further productions of Sr2 FeReO6 samples under different conditions combined with XRD measurements are highly desirable, in order to get a Sr2 FeReO6
double perovskite with a well ordered structure.
To clarify the valence state of the ions in the spinel chalcogenide Fe1−x Cux Cr2 S4
additional investigations of compounds with different Cu concentrations are
necessary. Besides XPS and XAS also magentic measurements and Mößbauer
spectroscopic studies can help to reveal the ion valences.
69
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CHAPTER 6. CONCLUSION AND OUTLOOK
70
(p) 2006, Christian Taubitz, University of Osnabrück
Chapter 7
Zusammenfassung und Ausblick
Das Ziel dieser Arbeit war die Untersuchung des Einflusses der Substitution verschiedener Atome in ausgewählten CMR-Materialien. Diese Materialien sind aufgrund ihres magnetischen Verhaltens und möglicher industrieller Anwendung von hohem wissenschaftlichen Interesse. Zwei komplementäre röntgenspektroskopische Verfahren und Computerrechnungen wurden zur Untersuchung verwendet.
Das Doppel-Perowskit Sr2FeReO6
Im Laufe der vorliegenden Arbeit wurden 4 Proben des Doppel-Perowskits
Sr2 FeReO6 in der Kristallogie der Universität Osnabrück hergestellt. Die O
1s und Re 4f XPS Linien wurden gemessen und im Hinblick auf die Qualität
der Proben untersucht. Dabei stellte sich heraus, dass der H2 -Fluss, unter
dem die Proben reduziert wurden, großen Einfluss auf die Probenqualität
hat. Während ein großer Gasfluss zu vielen zusätzlichen O und Re Valenzen
führte, geschah dies nicht bei einem geringen Fluss ≤ 1.5 l/h. Die Probe, die
unter einem H2 -Fluss von 1.5 l/h reduziert wurde, ist besonders interessant,
da das Re deutlich zwei Valenzen zu zeigen scheint. Dies kann auf eine gut
geordnete Doppel-Perowskit Struktur hindeuten, da in Sr2 FeMoO6 das Mo
auch in zwei Valenzen vorliegt, was den Doppelaustausch möglich macht.
71
CHAPTER 7. ZUSAMMENFASSUNG UND AUSBLICK
Die Spinelle Fe1−xCuxCr2S4
Die Spinelle Fe1−x Cux Cr2 S4 (x=0.2, 0.6) wurden mit Hilfe der Röntgenphotoelektronenspektroskopie (XPS) und der Röntgenabsorptionsspektroskopie
(XAS) untersucht. Vergleiche mit anderen Messungen und Rechnungen scheinen das Valenzmodel von Lotgering [24] zu bestätigen.
Bei beiden Proben deuten die XPS Daten darauf hin, dass Cu in der elektronische Konfiguration 3d10 vorliegt und Cr dreiwertig ist. Die XPS Fe 2p
Linien zeigen hauptsächlich zweiwertiges Fe in Fe0.8 Cu0.2 Cr2 S4 , während in
Fe0.4 Cu0.6 Cr2 S4 überwiegend Fe3+ vorhanden zu sein scheint. Untersuchungen der Fe und S XPS Spektren deuten außerdem auf mögliche Löcher im S
Valenzband von Fe0.4 Cu0.6 Cr2 S4 hin.
Die XAS Messungen stimmen gut mit den XPS Daten überein. In beiden
Proben deuten die Cr 2p XAS Linien auf dreiwertiges Cr hin. Die Fe Spektren zeigen Fe3+ in Fe0.4 Cu0.6 Cr2 S4 und einen gemischten Valenzzustand in
Fe0.8 Cu0.2 Cr2 S4 übereinstimmend mit dem Lotgering-model.
Vergleiche dieser Messungen mit kürzlich veröffentlichten XAS Messungen
des Spinells Fe0.5 Cu0.5 Cr2 S4 [31] zeigen deutliche Unterschiede im Kurvenverlauf. Die Tatsache, dass die Spinelle Fe0.8 Cu0.2 Cr2 S4 und Fe0.4 Cu0.6 Cr2 S4 bei
Raumtemperatur gemessen wurden, während die Messung von Fe0.5 Cu0.5 Cr2 S4
bei 50 K stattfand, könnte der Grund dafür sein. Es ist möglich, dass
aufgrund der Temperaturunterschiede bei ersteren beiden Proben ein metallischer Zustand, bei der letzten Probe jedoch ein halbleitender Zustand
vorlag. Dies könnte zu den Unterschieden in den Spektren geführt haben.
Es ist auch denkbar, dass, vielleicht aufgrund der niedrigen Temperatur, in
Fe0.5 Cu0.5 Cr2 S4 die Ladungsaustauscheffekte zugenommen haben. Demzufolge wären die Fe Ione nicht, wie von Aniruddha et al. [31] vermutet, im
dreiwertigen Ladungszustand gemessen worden, sondern die meisten von ihnen als Fe2+ . Die Ähnlichkeit der Form und Position der Fe 2p XAS Linien von Fe0.5 Cu0.5 Cr2 S4 und Fe0.8 Cu0.2 Cr2 S4 scheint dies zu bestätigen. Die
höhere Messtemperatur oder das mögliche Vorhandensein von Löchern im
S Valenzband könnten Ladungsaustausch im Fe0.4 Cu0.6 Cr2 S4 unterbunden
haben. Dies würde erklären warum die Fe 2p Linien bei dieser Probe dreiw-
72
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 7. ZUSAMMENFASSUNG UND AUSBLICK
ertiges Fe zeigen. Auch die Unterschiede in den Cr Spektren könnten damit
erklärt werden.
Ausblick
Die Produktion weiterer Sr2 FeReO6 Proben unter unterschiedlichen Bedingungen kombiniert mit XRD Messungen ist wünschenswert, um einen DoppelPerowskit mit gut geordneter Struktur herzustellen.
Um die Valenzzustände der Ionen in den Spinellen Fe1−x Cux Cr2 S4 aufzudecken,
sind weitere Untersuchungen von Verbindungen mit anderen Cu Konzentrationen nötig. Neben XPS und XAS könnten auch magnetische Messungen
und Mößbauer spektroskopische Untersuchungen helfen, die Valenzen der Ionen zu bestimmen.
73
(p) 2006, Christian Taubitz, University of Osnabrück
CHAPTER 7. ZUSAMMENFASSUNG UND AUSBLICK
74
(p) 2006, Christian Taubitz, University of Osnabrück
Chapter 8
Acknowledgement /
Danksagung
Diese Masterarbeit entstand in der Arbeitsgruppe Elektronenspektroskopie
der Universität Osnabrück.
Als erstes möchte ich apl. Prof. Dr. Manfred Neumann danken. Dafür,
dass er mir diese Arbeit ermöglicht hat und sich immer für Fragen und wissenschaftliche Diskussionen Zeit genommen hat. Seine Tür stand mir bei
Problemen immer offen.
Ich will auch Michael Räckers ganz herzlich danken. Er hat meine Arbeit
hervorragend betreut. Ohne ihn wäre diese Arbeit zu großen Teilen nicht
möglich gewesen.
Dr. V. Tsurkan will ich für die Bereitstellung der Spinelle danken.
Dr. R. Pankrath bin ich für die Herstellung der Sr2 FeReO6 Proben zu dank
verpflichtet.
Darüber hinaus will ich mich bei der gesamten Arbeitsgruppe Elektronenspektroskopie bedanken. Ich habe die familiäre Arbeitsatmosphäre sehr
genossen und mich bei den gelegentlichen Rollenspiel Abenteuern köstlich
amüsiert. Gedankt sei: Werner Dudas, Karsten Küpper, Stefan Bartkowski,
Michael Räkers, Georg Hofmann und Manuel Prinz. Viele von ihnen zähle
ich längst zu meinen besten Freunden.
Meinen restlichen Kommilitonen und dem Sekretariat will ich für die gute
75
CHAPTER 8. ACKNOWLEDGEMENT / DANKSAGUNG
und freundliche Stimmung im Fachbereich und die problemlose Verwaltung
danken.
Schließlich geht großer Dank an meine Familie und meine Freunde. Die Unterstützung meiner Eltern und Brüder war mir ein ständiger Begleiter auf
meinem Werdegang, und die Zeit mit meinen Freunden hat mir immer neue
Kraft gegeben.
76
(p) 2006, Christian Taubitz, University of Osnabrück
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BIBLIOGRAPHY
Eidesstattliche Erklärung
Hiermit erkläre ich an Eides Statt, dass ich die vorliegende Abhandlung
selbständig und ohne unerlaubte Hilfe verfasst, die benutzten Hilfsmittel
vollständig angegeben und noch keinen Versuch eine Masterarbeit zu schreiben
unternommen habe.
Osnabrück, August 4, 2006
Christian Taubitz
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(p) 2006, Christian Taubitz, University of Osnabrück