Extended X-ray absorption fine structure (EXAFS) - Technische ...

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Journal of Magnetism and Magnetic Materials 320 (2008) e578–e582
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Extended X-ray absorption fine structure (EXAFS) and
X-ray absorption near-edge structure (XANES) study
of melt-spun Fe 85 Ga 15 ribbons
R. Sato Turtelli a, , S. Pascarelli b , M. Ruffoni b , R. Gro¨ssinger a , C. Eisenmenger-Sittner a
a Institut f. Festkörperphysik, Technische Universität Wien, Wiedner Haupts. 8-10, A-1040 Vienna, Austria
b European Synchroton Radiation Facility, BP 220, 38043 Grenoble, Cedex, France
Available online 10 April 2008
Abstract
The local structure in melt-spun Fe 85 Ga 15 ribbons with a width 3 mm and thickness 60 mm produced in argon atmosphere was
studied by analyzing EXAFS and XANES data. The following results were obtained: Ga–Ga bonds were not detected excluding the
tendency to form clusters of Ga atoms; Ga substitutes Fe creating a local strain of about +1% on the first shell Fe–Ga bond, whereas on
the second Fe–Ga shell strain quickly relaxes down to +0.3%; XANES spectra are compatible with a random substitution of Fe atoms
by Ga atoms in the A2 structure. From the AFM investigation, we observed that at the surface (free side) of the ribbon the particles are
elongated along the ribbon (2 mm 5 mm) and each particle is formed by small grains of average size of 200 nm.
r 2008 Elsevier B.V. All rights reserved.
PACS: 75.50.Bb; 75.80.+q; 61.10.Ht
Keywords: Fe–Ga alloy; Magnetostriction; EXAFS; XANES
1. Introduction
Fe–Ga alloys can provide low-cost magnetostricitve
materials with high mechanical strength, good ductility,
large magnetostrictive at low saturation fields with
negligible magnetostrictive hysteresis and attractive magnetic
properties to be used in acoustic sensors and
actuators. The large magnetostriction of Fe–Ga alloys
depends strongly on sample production methods and
thermal history, which result in different crystallographic
texture and grain morphology [1–5]. Single crystal [1 0 0]
oriented Fe 100 x Ga x alloys, where 13pxp23, can exhibit
magnetostriction in the l 100 direction of up to 270 ppm [3].
Ductility of Fe–Ga alloys has been largely improved by the
melt-spun method, producing ribbons. In addition, thin
ribbon sample minimizes ac-eddy current losses and be well
suited for working at high frequency. However, reports of
Corresponding author. Tel.: +43 1 58801 13150;
fax: +43 1 58801 13199.
E-mail address: reiko.sato@ifp.tuwien.ac.at (R. Sato Turtelli).
different magnetostriction values in the literature, for
similar compositions, are not uncommon. These include a
value of 130 ppm in strained ribbons of Fe 83 Ga 17 [6], a
giant magnetostriction up to 1300 ppm in stacked ribbons
of Fe 85 Ga 15 [7], and a value of 3000 ppm in annealed
Fe 81 Ga 19 ribbons [8]. Some reported values are larger than
that of Terfenol-D.
The enhancement of magnetostriction in melt-spun
samples has been attributed to a change in local structure
due to the melt spinning method itself, and also to the
short-range preferential ordering of Ga atoms in the [0 0 1]
direction, which supposedly form clusters. However,
different researchers have reported contradicting results
of crystallographic phases. For melt-spun Fe–Ga alloys
with Ga contents lower than 21 at%, Ref. [6] reports the
existence of only a disordered A2 phase, while Ress. [7,9]
report the appearance of a D0 3 structure. In order to
understand these different results in similar ribbons, more
microstructure and local structure investigations are
necessary. Therefore in this work, the local structure of a
melt-spun Fe 85 Ga 15 ribbon with a width 3 mm and
0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2008.04.036

thickness 60 mm was produced in an argon atmosphere
and was studied by analyzing extended X-ray absorption
fine structure (EXAFS) and X-ray absorption near-edge
structure (XANES) data. Additionally a Topometrix
‘‘Explorer’’ atomic force microscope (AFM) was used to
investigate the grain structure of the ribbon.
ARTICLE IN PRESS
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2. Experimental procedure
An ingot of Fe 80 Ga 15 alloy with high purity (499.9%)
was prepared by arc melting. Ejecting the melt onto a
rotating copper wheel produced a rapidly quenched ribbon
with a thickness of 50 mm and width of 3 mm. From the
energy dispersive X-ray analysis, the as-quenched ribbon
exhibits a composition similar to that of the ingot.
The grain structure of the ribbon was investigated by
means of a Topometrix ‘‘Explorer’’ AFM with a Si3N4 tip
mounted on a cantilever of spring constant 0.3 N/m. Local
structural investigations were performed on a thin sample,
7 mm thick, obtained by polishing both the wheel and free
sides of the ribbon. EXAFS spectra were recorded at the Fe
and Ga K-edges at beam line BM29 of the European
synchrotron radiation facility (ESRF). A 4 mm thick highpurity
Fe foil (from Goodfellow) was used as a standard.
3. Results and discussions
Fig. 1 shows AFM pictures obtained on the wheel side of
the Fe 85 Ga 15 ribbon. As can be seen in the upper picture,
the grains are elongated along the direction of the ribbon
with an average grain size of 2–3 by 5–10 mm. It is
interesting to note that each grain is composed of fine
elongated particles around 200 nm in the direction of the
thickness of the ribbon. This kind of small particle also
exists in melt-spun Fe 80 Ga 20 samples [10]. The grain size
determined by X-ray diffraction is of same order of
magnitude and presents a texture [10]. As magnetostriction
depends strongly of structural properties of grains (such as
shape and texture [1–5]), magnetostriction measurements in
each direction may result in different values.
X-ray absorption fine structure (XAFS) spectroscopy is
particularly suited to probe the local environment of the
absorber atom. We have independently analyzed the
extended fine structure region of the spectra (or EXAFS),
and the near-edge (or XANES) region. The former,
characterized by relatively large photoelectron energy
(450 eV) with a relatively low mean free path, can be
interpreted in a simplified manner using a single-scattering
formalism, whereby the emitted photoelectron wave is
scattered once by the electronic potentials of the neighboring
atoms. The latter, where the photoelectron energy
is low with a large mean free path, arises from complex
multiple scattering processes and its interpretation
is difficult because details in the description of the
potentials cannot be neglected so easily as can be done in
EXAFS region.
Fig. 1. AFM pictures obtained on the wheal side of the Fe 85 Ga 15 ribbon.
Up: sensor picture; bottom: topography picture with high amplification.
Consequently, EXAFS is very sensitive to the first
neighbor shells and can yield information on the chemical
nature of the nearest neighbors, their number, distance
from the absorber atom and thermal and static disorder
relative to the absorber atom. XANES, on the other hand,
is very sensitive to the local bonding geometry and
electronic structure.
We based our EXAFS analysis on a structural model
obtained from crystallographic data [4], which found that
Fe 85 Ga 15 ribbons crystallize in the A2 phase [bcc phase]
with lattice parameter a 0 ¼ 2.900 A˚ . Based on these input
structural parameters, we built atomic clusters (one
centered on Fe and one centered on Ga) for the EXAFS
simulations. Due to the short-range sensitivity of the
EXAFS probe, we considered only the region of R-space
that extends up to about 3 A˚ from the absorber, i.e. the first
two coordination shells of the bcc lattice. The software
package FEFF8.2 [11] was used to calculate ab initio
scattering phases and amplitudes for a Fe and a Ga
absorber using the atomic clusters defined above, and

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simulated the EXAFS signal using a high-order path
expansion.
For the Fe6¼K-edge we considered the following: (i) a
first coordination shell composed of a total of N Fe +
N Ga ¼ 8 atoms, with an unknown x ¼ N Ga /(N Fe +N Ga ).
The bond distances and mean square relative displacements
(msrd) of Fe–Fe and Fe–Ga are unknown; (ii) a second
shell composed, in a first approximation, of 6 Fe atoms at
an unknown distance (linked through crystallographic
constrains to the first shell Fe–Fe distance) and msrd. As
a second step, the presence of some Ga was also considered
in the second shell, but we did not have sufficient sensitivity
to confirm the number of Ga atoms in this shell.
For the Ga K-edge both the first and second shell are
composed of only Fe atoms (8+6) at an unknown
distance. The presence of Ga in the first shell was tested
and was excluded in this analysis.
The only paths contributing to the EXAFS signal in the
R-range up to 3 A˚ are the two single scattering (SS) paths
corresponding to the first (total path length 2R 1 ) and
second shell (total path length 2R 2 ). Fig. 2 illustrates the
results of ab initio calculations for the first shell SS paths,
plotted as a function of photoelectron wavevector k. These
signals show that, besides a small and almost constant
phase shift, the scattering amplitudes of Ga and Fe differ
mainly in the low k region.
Fig. 3 shows the normalized EXAFS spectra obtained on
the Fe 85 Ga 15 ribbon at the Fe and Ga K-edges, respectively.
The Fe K-edge EXAFS on a pure Fe foil standard is
also shown.
By comparing the Fe K-edge EXAFS oscillations of the
Ga-containing sample to that of pure Fe foil, besides a
reduction in amplitude, there is also a small change in the
shape of the first oscillations, at k3–4 A˚ . The difference in
calculated signals
0.5
0.4
0.3
0.2
0.1
0
-0.1
-0.2
4
k (A -1 )
Ga-Fe
Ga-Ga
Fe-Fe
Fe-Ga
6 8 10 12 14
Fig. 2. Ab initio calculations for the first shell SS paths: Blue curve: Fe
(absorber)–Fe (scatterer). Red curve: Ga(absorber)–Ga (Scatterer).
EXAFS χ(k)
0.4
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
4
Fe 85 Ga 15 Fe K-edge
k (A -1 )
the shape of the Fe K-edge EXAFS between the sample
and pure Fe at low k values, seen in Fig. 3, is likely to be
attributed to the detection of Ga in the first coordination
shell.
In order to extract the local structural parameters (bond
distance, R, msrd, Ds 2 , and ratio between Ga/Fe scatters,
x, we performed a least-square fitting of the first peak of
the Fourier transform (FT) of the EXAFS signals, where
the FT was performed in the range 2.6pkp15.0 A˚ 1 .
Using the calculated phases and amplitudes, we constructed
a model EXAFS signal as the sum of two SS paths
(first and second shell) with variable fitting parameters for
energy offsets, bond distances and msrds for two shells,
and chemical composition of the first shell. The Fe and Ga
K-edges data were fitted simultaneously. A more detailed
description of the fitting is given elsewhere [10].
Fig. 4 shows the results of the best fits at the Fe and Ga
edges for the Fe 85 Ga 15 ribbon and the standard Fe foil.
The structural parameters obtained from the best fits are
consistent with a picture whereby Ga substitutes Fe atoms
randomly, creating local strain. The obtained value of the
Fe–Ga bond is 2.52 A˚ , whereas the Fe–Fe bond is
2.50 A˚ , implying a +1% expansion. On the second shell,
strain quickly relaxes down to +0.3%, since we find
Fe–Ga bonds of 2.89–2.90 A˚ and Fe–Fe bonds of
2.88–2.89 A˚ . From Ga edge data we can conclude that
no clustering occurs since we do not detect any Ga–Ga first
shell bonds. We also performed fits at the Fe K-edge with a
second shell Fe–Ga contribution of R Fe Ga and Ds 2 Fe Ga,
equal to Ga K-edge values. However, these additional
parameters did not improve the quality of the fits.
To simulate the X-ray absorption near-edge structure
region of the spectra at the Fe and Ga K-edges, we again
Fe
Fe 85 Ga 15 Ga K-edge
6 8 10 12 14
Fig. 3. EXAFS oscillations plotted as function of photoelectron
wavevector k.

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kχ(k)
1.5
1
0.5
0
-0.5
4
k (A -1 )
Ga K-edge
Fe K-edge
Fe 85 Ga 15
Fe 85 Ga 15
Fe
6 8 10 12 14
Fig. 4. Best fits for the Ga K-edge data on Fe 80 Ga 15 (top) and at the Fe
K-edges on Fe 80 Ga 15 and pure Fe foil (bottom).
All scattering paths within the same atomic cluster
(approximately 60 atoms) were summed to infinite order.
The imaginary part of the potential was not optimized
and our simulations underestimate this damping at the
absorption edge. Moreover, no disorder factor was
introduced in the calculations, so that the high-energy fine
structure is also under-damped in the simulations, compared
to the experimental data.
The Fe K-edge simulations were done for (i) the pure Fe
bcc phase (a ¼ 2.87 A˚ ); (ii) the A2 phase (a ¼ 2.90 A˚ )
where 9 Fe atoms out of 59 (about 15%) have been
substituted by Ga and (iii) the DO3 Fe 3 Ga phase
(a ¼ 2.90 A˚ ); and Ga K-edge simulations (see Fig. 5) for
(i) A2 phase where Ga is surrounded only by Fe atoms
(‘‘Ga impurity x-40’’) (ii) the A2 phase where Ga is
surrounded by Fe and 15% Ga randomly distributed (‘‘Ga
impurity x0.15’’) (iii) the A2 phase where Ga is
surrounded only by Ga in the first shell (‘‘Ga cluster’’),
and by Fe in further shell and (iv) the DO3 Fe 3 Ga phase.
A qualitative comparison of our XANES data with ab
initio simulations indicates that all the observed features
are reproduced by the A2 model, therefore the DO3 phase
and clustering of Ga can be excluded.
2
4. Conclusions
absorption
1.5
1
0.5
0
-0.5
-1
-1.5
-2
10.36
D03
A2 (Ga cluster)
A2 (x ~ 0.15)
A2 (x-> 0)
experimental data
10.38 10.4 10.42 10.44
Energy (keV)
(i) EXAFS: (a) No Ga–Ga bonds are detected, excluding
the tendency to form clusters of Ga atoms; (b) Ga
substitutes Fe, creating a local strain of about +1%
on the first shell Fe–Ga bond. On the second Fe–Ga
shell, strain quickly relaxes down to +0.3%.
(ii) XANES: (a) the spectra are compatible with a random
substitution of Fe atoms by Ga atoms in the A2
structure; (b) the presence of Ga clusters or of a D0 3
local environment around Ga has been tested and
excluded.
(iii) AFM: (a) the particles observed at the surface (wheel
side) of the ribbon are elongated (width 3 mm, length
5–10 mm) along the ribbon; (b) these particles are
formed by fine grains elongated perpendicular to the
surface of the ribbon with average width size of
200 nm.
Fig. 5. Ab initio full multiple scattering simulations of Ga K-edge data.
The following models are used for the local environment of Ga, from top
to bottom: the D03 phase, the A2 phase with only Ga surrounding Ga in
the first shell (Ga cluster), the A2 phase where the first shell around Ga is
composed of Fe atoms, and some Ga (x0.15) distributed randomly, the
A2 phase where Ga is surrounded only by Fe atoms (x-40). The bottom
curve is the experimental data.
used FEFF8.2, but this time to perform full multiple
scattering calculations. Ab initio scattering amplitudes and
phases were calculated using complex Hedin-Lundvquist
potentials, with the electronic density calculated selfconsistently
within a radius of 5.5 A˚ from the absorber.
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