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Local atomic order and element-specific magnetic moments of Fe3Si thin films on MgO(001) and GaAs(001) substrates

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We investigated the magnetic as well as the structural properties of Fe3Si films on GaAs(001)-(4×6), GaAs(001)-(2×2), and MgO(001) by x-ray magnetic circular dichroism (XMCD) and Mössbauer spectroscopy. From the XMCD spectra we determine averaged magnetic moments of 1.3–1.6μB per Fe atom on the different substrates by a standard sum-rule analysis. In addition, XMCD spectra have been calculated by using the multiple-scattering Korringa-Kohn-Rostoker method which allows the site-specific discussion of the x-ray spectra. The Mössbauer spectra show a highly ordered and stoichiometric growth of Fe3Si on MgO while the growth on both GaAs substrates is strongly perturbed, probably due to diffusion of substrate atoms into the Fe3Si film. Therefore, we have studied the influence of Ga or As impurities on the magnetic properties of Fe3Si by calculations using coherent-potential approximation within the Korringa-Kohn-Rostoker method. For selected impurity concentrations additional supercell calculations have been performed using a pseudopotential code (VASP).
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Local atomic order and element-specific magnetic moments of Fe3Si thin films on MgO(001)
and GaAs(001) substrates
B. Krumme,1,*C. Weis,1H. C. Herper,1F. Stromberg,1C. Antoniak,1A. Warland,1E. Schuster,1P. Srivastava,1,
M. Walterfang,1K. Fauth,2J. Minár,3H. Ebert,3P. Entel,1W. Keune,1,4 and H. Wende1
1Fachbereich Physik and Center for Nanointegration Duisburg-Essen (CeNIDE), Universität Duisburg-Essen,
Lotharstraße 1, D-47048 Duisburg, Germany
2Experimentelle Physik IV, Physikalisches Institut, Universität Würzburg, Am Hubland, D-97072 Würzburg, Germany
3Institut für Physikalische Chemie, Universität München, Butenandtstraße 5-13, D-81377 München, Germany
4Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D-06120 Halle, Germany
Received 21 May 2009; revised manuscript received 8 September 2009; published 9 October 2009
We investigated the magnetic as well as the structural properties of Fe3Si films on GaAs001-46,
GaAs001-22, and MgO001by x-ray magnetic circular dichroism XMCDand Mössbauer spectros-
copy. From the XMCD spectra we determine averaged magnetic moments of 1.3–1.6
Bper Fe atom on the
different substrates by a standard sum-rule analysis. In addition, XMCD spectra have been calculated by using
the multiple-scattering Korringa-Kohn-Rostoker method which allows the site-specific discussion of the x-ray
spectra. The Mössbauer spectra show a highly ordered and stoichiometric growth of Fe3Si on MgO while the
growth on both GaAs substrates is strongly perturbed, probably due to diffusion of substrate atoms into the
Fe3Si film. Therefore, we have studied the influence of Ga or As impurities on the magnetic properties of Fe3Si
by calculations using coherent-potential approximation within the Korringa-Kohn-Rostoker method. For se-
lected impurity concentrations additional supercell calculations have been performed using a pseudopotential
code VASP.
DOI: 10.1103/PhysRevB.80.144403 PACS numbers: 75.50.Bb, 78.70.Dm, 75.70.Ak, 76.80.y
I. INTRODUCTION
Since the idea of using the spin of the electrons in addi-
tion to their charge as a carrier of information, tremendous
effort has been made to create spin-polarized currents in
semiconducting materials. One approach is to make use of
spin injection, where an electric current is spin polarized by
a ferromagnetic electrode from which it is injected into a
semiconducting material.13Strong support for this approach
results from the possibility of combining such devices with
the existing semiconductor technology. However, there are
still many difficulties to overcome before such spintronic
devices may finally be established. For a high efficiency in
the spin-injection process the ferromagnetic electrode should
exhibit a high degree of spin polarization and a perfect inter-
face with the semiconductor.1In recent studies, Fe3Si on
GaAs turned out to be a promising ferromagnet-
semiconductor combination. Spin injection has successfully
been demonstrated with an efficiency of 3%.4As the quality
of the interface directly influences the spin injection, a better
understanding of the correlation between structure and mag-
netic properties at the interface is necessary to fulfill the task
of improving the spin injection.
Fe3Si is a binary Heusler-type compound. Such Heusler
compounds are theoretically predicted to exhibit half-
metallic behavior involving a high degree of spin polariza-
tion up to 100%.5,6However, a spin polarization of 455%
is reported so far.7Ordered Fe3Si crystallizes in a D03struc-
ture which is described by four interpenetrating fcc lattices8
whereas randomly distributed Fe and Si atoms lead to a B2
structure. In perfectly ordered Fe3Si each fcc sublattice is
occupied by only one element, i.e., three sublattices are oc-
cupied by Fe atoms and one is occupied by Si atoms Fig. 1.
This crystal structure leads to two inequivalent Fe sites: Fe
atoms on sites Bare surrounded by eight nearest-neighbor Fe
atoms yielding a magnetic moment of 2.2
Bper atom.9Fe
atoms on Asites are surrounded by four nearest-neighbor Fe
atoms and four Si atoms exhibiting a magnetic moment of
1.35
Bper atom.9As a further consequence of the different
surroundings, Fe atoms on different sites have different hy-
perfine fields which can be resolved by conversion electron
Mössbauer spectroscopy CEMS.8The lattices of Fe3Si and
GaAs001match almost perfectly lattice mismatch 0.1%
with Fe3Si001
GaAs001兴兲. Epitaxial growth has been
carefully studied in detail.1012 In contrast, on MgO a lattice
mismatch of 5.2% occurs when Fe3Si grows rotated by 45°
FIG. 1. D03structure of bulk Fe3Si with Fe atoms on inequiva-
lent lattice sites Aand Band Si atoms on site C.
PHYSICAL REVIEW B 80, 144403 2009
1098-0121/2009/8014/1444038©2009 The American Physical Society144403-1
on MgO001, i.e., Fe3Si001
MgO110.13,14 With a Curie
temperature of 840 K Ref. 15Fe3Si allows for operation at
room temperature RT. In comparison to pure Fe/GaAs001
its interface is thermally more stable.16 Both are crucial facts
for the final application in devices.
In the present paper, we take a closer look at the correla-
tion between chemical ordering and magnetic properties in
thin films of Fe3Si. We present our combined x-ray absorp-
tion spectroscopy XASand Mössbauer studies for Fe3Si on
GaAs in comparison to MgO. We use Fe3Si on MgO001
for comparison as a quality standard for which we know that
Fe3Si films grow highly ordered on this substrate. CEMS
allows us to characterize the chemical ordering of the Fe3Si
films and to determine the hyperfine field distributions site
dependent. Complementary, the XMCD spectroscopy reveals
the averaged magnetic moment per Fe atom. In addition,
XAS and XMCD spectra have been calculated within
multiple-scattering theory. From the calculations we obtain
site-specific spectra, densities of states, and magnetic mo-
ments. Furthermore, we have studied the influence of Ga or
As impurities on the magnetic properties of Fe3Si since we
gained evidence of an interdiffusion of substrate atoms into
the Fe3Si film on GaAs from our CEMS measurements.
II. EXPERIMENTAL ASPECTS
Films of 80 Å Fe3Si 57 monolayer ML兲兴 共Ref. 17
were deposited in an UHV chamber at a base
pressure of 110−10 mbar on three different substrates:
MgO001, Ga-terminated GaAs001-46, and
As-terminated GaAs001-22. The Fe3Si films on MgO
and GaAs-46were prepared simultaneously so that the
growth conditions for both samples were identical. Before
introducing the substrates into the preparation chamber, we
cleaned them with propanol, the GaAs additionally with ac-
etone, and dried them in a stream of N2. To obtain the
GaAs-46surface reconstruction, the substrate was sput-
tered with 500 eV Ar+ions while being heated up to a tem-
perature of 870 K for 90 min. For the GaAs-22surface
reconstruction an As-capped GaAs substrate was heated up
to 820 K for 10 min. No contamination of the surface with O
or C was detectable via Auger spectroscopy after this proce-
dure.
The two elements Fe and Si were coevaporated at a sub-
strate temperature of TS=520 K from a resistively heated
crucible and by electron-beam deposition, respectively. For
the CEMS measurements, 57Fe isotopes were deposited in-
stead of natural Fe. Figure 2shows the reflection high-energy
electron diffraction RHEEDwe used to monitor the Fe3Si
film growth on GaAs-46兲共right columnand MgO left
column. Both pattern sets consist of three pictures compar-
ing the crucial phases of the Fe3Si growth. The substrate
reflections of MgO and GaAs-46are shown at the top of
the columns and were measured along the 11
¯
0direction of
Fe3Si. On MgO the substrate reflections directly change into
Fe3Si001reflections within the first ML. This indicates a
layer-by-layer growth from the very beginning. In contrast,
on GaAs-46the substrate reflections totally vanish when
the deposition is initiated. The first Fe3Si reflections appear
only at a film thickness of about 6 ML indicating a coales-
cence of the initial islands at that thickness. Recent in situ
studies by real-time x-ray diffraction find that the growth of
Fe3Si on GaAs001begins with islands of 6 atomic layers
height and changes to a two-dimensional layer-by-layer
growth at about 14 atomic layers.10 Finally, the samples were
capped with 20 Å Au for transportation to the measuring
chamber.
We investigated XAS and XMCD spectra at the dipole
beamline PM3 at BESSY in Berlin, Germany. We detected
the XAS at the Fe L2,3 edges in total-electron yield mode by
measuring the drain current of the sample. For a simulta-
neous determination of the incoming photon flux I0we re-
corded the photocurrent of a Au mesh in the incident beam.
The dichroic spectra were obtained by changing the direction
of the magnetization while the helicity of the circularly po-
larized synchrotron radiation remained constant. The easy
axis of magnetization is in plane along the 100direction of
the Fe3Si.18 Thus, we measured at grazing incidence with an
angle of 20° between the photon wave vector and the surface
of the sample. The sum rules derived by Thole et al.19 and
Carra et al.20 were applied to determine spin- and orbital-
resolved magnetic moments from the dichroic signal of the
Fe atoms in the sample. This method reveals magnetic mo-
ments per Fe atom averaged over the two different Fe sites.
Before normalizing the XAS to unity, the measured spectra
have been normalized to the incoming photon flux and cor-
rected for a small linear background. Saturation effects have
been considered as described, e.g., in Refs. 2123.
CEMS was applied to characterize the chemical ordering
of the Fe3Si films. We used a 57Co source embedded in a Rh
FIG. 2. RHEED patterns of Fe3Si001film growth on
MgO001兲共left columnand GaAs-46兲共right columnmeasured
during growth at 520 K with an electron energy of 15 keV. The
growth was observed along the 100direction of MgO and along
the 11
¯
0direction on GaAs. Both correspond to the 11
¯
0direction
of Fe3Si details see text.
KRUMME et al. PHYSICAL REVIEW B 80, 144403 2009
144403-2
matrix. The samples were mounted in a gas flow proportional
counter with a He-5% CH4mixture. The CEMS were all
measured in zero external field at room temperature. Since
the surroundings of Fe atoms of type A, FeA, differs from
that of type B atoms, FeB, their hyperfine magnetic fields
are different. Therefore, the individual Mössbauer spectra of
the two types of Fe atoms have a different splitting of the
lines in the corresponding sextet. Hence, the experimental
spectrum will mainly show the superposition of two such
sextets,8as is illustrated in Fig. 3for a bulklike Fe3Si film on
MgO001. The spectrum was least-squares fitted using the
computer code NORMOS.24 The four-fitted subspectra at the
bottom shifted downward for clarityrepresent the two sex-
tets of Fe atoms on the two inequivalent sites whereas the
two additional subspectra with smaller intensity originate
from a not perfectly ordered D03crystal structure. The solid
line above is the sum of the sextets, fitting well the experi-
mental data that are represented by the solid circles. It is thus
possible to clearly distinguish the two dominant inequivalent
Fe sites in a Mössbauer spectrum by fitting sextets with dif-
ferent hyperfine fields to the experimental data. The hyper-
fine field, Bhf, and isomer shift relative to bulk bcc Fe at
room temperature,
of the A-site and B-site subspectra in
Fig. 3have been determined as BhfB=30.780.02 T,
B=0.0900.002 mm/s, BhfA= 19.960.01 T, and
A=0.2500.001 mm/s. The errors given are the statisti-
cal errors. These values are in good agreement with those
reported in the literature for bulk Fe3Si alloy.25
For a more detailed analysis and a clearer inspection of
the interface properties, the Mössbauer spectra were least-
squares fitted in two steps: ispectra of highly chemically
ordered Fe3Si films can be fitted satisfactorily by a procedure
described by Arita et al.,26 subspectrum 1in Fig. 4.iiTo
account for disorder at the interfaces of the films, an addi-
tional subspectrum subspectrum 2in Fig. 4with a hyper-
fine field distribution PBhfis required in the case of
GaAs001substrates.
Within the first step of the fitting routine the sites are
randomly occupied by Fe and Si atoms. The fit to the experi-
mental data is obtained by varying this site occupancy and
calculating the corresponding CEM spectra. Long-range,
SD03,SB2, and short-range,
1,
2, order parameters
are computed from the occupancy of Fe sites with different
nearest and next-nearest Si atoms that leads to the best
matching simulated spectrum subspectrum 1兲兴. If after this
first step a considerable part of the CEM spectra cannot be
satisfactorily fitted, an additional subspectrum with a hyper-
fine field distribution subspectrum 2兲兴 is used to describe
the remaining part in the second step, accounting for an ad-
ditional, non-Fe3Si-like Fe phase.
III. COMPUTATIONAL ASPECTS
The theoretical XMCD data are obtained from ab initio
calculations employing the spin-polarized relativistic
Korringa-Kohn-Rostoker KKRmethod as implemented in
the SPR-KKR code.2729 For the exchange-correlation func-
tional the local-density approximation in the parametrization
of Vosko et al.30 has been used. For the calculations of the
corresponding XAS and XMCD spectra we used an expres-
sion based on Fermi’s golden rule which was implemented
within the SPR-KKR code.27 For the KKR calculations we
have used an angular momentum expansion up to lmax =2 and
ak-point mesh of 222222 which corresponds to 843
irreducible kpoints. In order to study the influence of Ga
and As impurities on the magnetic structure of Fe3Si KKR
calculations within the coherent-potential approximation
CPAas well as supercell calculations employing the VASP
code and the projector-augmented wave pseudopotentials31,32
have been performed. For the latter, orthorhombic supercells
with 32 atoms have been used in which one or two Fe atoms
of type B have been replaced by Ga or As, respectively. The
generalized gradient correction PW91兲共Ref. 33has been
used for the supercell calculations. A mesh of 1584k
points and an energy cutoff of 381.3 eV have been used. The
FIG. 3. Color onlineCEMS of bulklike Fe3Si001film 57
MLon MgO001least-squares fitted by four sextets.
FIG. 4. Color onlineMeasured CEMS solid circlesof 57 ML
Fe3Si on different substrates at RT together with fitting curves
linesas described in the text.
LOCAL ATOMIC ORDER AND ELEMENT-SPECIFICPHYSICAL REVIEW B 80, 144403 2009
144403-3
experimental lattice constant of Fe3Si a=5.65 Å has been
used for all calculations containing Fe3Si neglecting possible
tetragonal distortions which may occur if Fe3Si is grown on
a substrate. In order to ensure that small tetragonal distor-
tions have no significant influence on the magnetic moments
of the system we have performed KKR-CPA calculations in
which the c/aratio is varied by 5%keeping the volume
fixed. The maximum change in the orbital spinmoments is
less than 2% 4%. However, the average change in magnetic
moments is smaller being about 1.5%. This holds also in the
presence of impurities. Relaxation of the lattice constant due
to the Ga or As impurities has been neglected so far.
IV. RESULTS
The characterization of the chemical ordering of the Fe3Si
films with CEMS is shown in Fig. 4. Solid circles represent
the experimental data. The lines represent the subspectra 1
and 2used for the fitting as described above, and the total
spectrum. On MgO it was adequate to fit the experimental
data with a calculated spectrum for nearly perfectly ordered
Fe3Si and the fitting was completed after the first step as
described above. The spectra of Fe3Si films on GaAs-4
6and GaAs-22had to be fitted with two subspectra
in order to account for the nonperfect order in the film. Spec-
trum 1is calculated following the work of Arita et al.26 and
spectrum 2is the additional subspectrum with a distribu-
tion of magnetic hyperfine fields representing a non-Fe3Si
phase, which we attribute to an imperfect interface. This
could be related to an interdiffusion at the interface between
Fe3Si and the substrate. Hsu et al.34 were able to ascribe this
interdiffusion to Ga atoms. Such an interdiffusion of non-
magnetic impurity atoms would result in a lower magnetic
ordering and a reduced magnetic Fe moment. The chemical
order parameters that we obtained from our analysis are sum-
marized in Table. I. In the first row of the table, the expected
values for perfectly ordered Fe3Si are given. In real samples
the order can be perturbed by the interface to the substrate or
a capping layer, thus lowering the order parameters. From
the CEM spectra of the Fe3Si film on MgO we obtained
parameters SD03and SB2which are reduced by 17% and
22% compared to the ideal values. In the case of GaAs sub-
strates the change in the order parameters becomes even
more dramatic resulting in a deviation of up to 50%.
Comparison of the CEM spectra of the Fe3Si films on the
Ga-terminated GaAs-46and the As-terminated
GaAs-22surfaces reveals more pronounced peaks in the
case of GaAs-22indicating a better chemical ordering
on GaAs-22than on GaAs-46. In the case of
GaAs-46subspectrum 2has a spectral area of
20.80.2%whereas for GaAs-22this value becomes
29.60.2%. This leads us to the conclusion that the disorder
in the films on GaAs-46is due to Ga interdiffusion at the
interface and that on GaAs-22the interdiffusion is due
to Ga as well as As atoms.
57Fe CEMS of Fe3Si does not directly yield information
about the value of the magnetic moment of the Fe atoms.
Thus, we applied the XMCD spectroscopy to determine spin-
and orbital-resolved magnetic moments. Figure 5shows the
XAS as well as the XMCD spectra at the Fe L2,3 edges of the
Fe3Si films on MgO and GaAs-46together with a bulk-
like Fe film as a reference. The L2,3 edges of the XAS spectra
are broader for the Fe3Si films in comparison to the Fe ref-
erence. Concurrently, the maximum of the absorption signal
at the L3edge decreases by 8%for Fe3Si on MgO001
and by 17%on GaAs-46. Additionally, a shoulder oc-
curs 2 eV above the L3edge in the Fe3Si XAS. However, at
the L2edge the absorption intensity is nearly unchanged. The
broadening as well as the shoulder can be ascribed to a hy-
bridization of Fe and Si atoms in the lattice. Such a feature
has been observed in other Heusler systems by Kallmayer
et al.35 The white lines of Fe3Si on GaAs-46and
GaAs-22in Fig. 6exhibit no obvious difference indicat-
ing a very similar electronic structure of Fe3Si on various
surface reconstructions.
For a detailed study of the structure the electronic density
of states, DOS, and the XAS of bulk Fe3Si have been calcu-
lated, allowing a site-specific analysis of the spectra. From
the DOS it is obvious that the two types of Fe atoms have a
different electronic structure. Atoms of type B have nearly
the same DOS as bulk Fe whereas the A-type atoms show a
TABLE I. Chemical order parameters for 57 ML Fe3Si on dif-
ferent substrates, determined from a comparison of measured
CEMS and CEMS simulations as described in the text.
Substrate SD03SB2
1
2
Theoretical values 1 0.67 −0.33 −0.33
MgO 0.830.07 0.520.12 −0.23 −0.31
GaAs-460.680.002 0.320.1 −0.09 −0.38
GaAs-220.780.03 0.420.12 −0.38 −0.40
FIG. 5. Color onlineNormalized XAS topand XMCD bot-
tomspectra measured at the Fe L2,3 edges of 57 ML Fe3Si on MgO
and GaAs-46compared to the spectra of a bulklike Fe sample
as a reference. The inset shows an enlargement of the XMCD signal
between the L2,3 edges. The spectra were measured at RT.
KRUMME et al. PHYSICAL REVIEW B 80, 144403 2009
144403-4
completely different behavior, see Fig. 7, which is related to
the fact that in the latter case only half of the nearest neigh-
bors are Fe atoms. This is in agreement with previous
investigations.36,37 The different properties of the two Fe
types are also translated to the x-ray spectra, see Fig. 8.
Similar to the experimentally observed shoulder cf. Fig. 5a
small bump P between 2 and 6 eV is observed in the theo-
retical spectrum. However, this feature occurs only in the L3
peak of the FeAatoms and seems to be caused by a hy-
bridization of an Fe dorbital with a sstate of Si, see Fig. 8.
The spectrum of the FeBatoms looks very similar to that of
bulk Fe showing no P-like structure at the L3peak. Further-
more, from Fig. 8it is obvious that the spectra of the two
types of Fe are slightly shifted against each other, i.e., the
maxima of the FeAL3peak occurs about 0.25 eV above the
L3peak of the FeBatoms. Such a shift of the XAS spectra
could not be observed directly in the experiment because the
energy difference is smaller than the linewidth of the L3
peak. The different position of the two XAS is accompanied
by a change in the number of dholes. The atoms of type A
have less dholes compared to bulk Fe, namely, h=−0.24
whereas the number of holes for FeBremains unchanged
compared to bulk Fe. The difference between the A and B Fe
atoms becomes also obvious from the XMCD. If we disclaim
Gaussian broadening to simulate the broadening caused by
the experimental measurement, a tiny positive signal occurs
in case of FeB兲共see inset of Fig. 8which is known from
bulk bcc Fe cf. Fig. 5.
Comparing the XMCD signal of the Fe reference with
those of the Fe3Si films in Figs. 5and 6no features can be
found which allow a deconvolution of the magnetic contri-
butions of the different Fe sites. Only a decrease in the
XMCD intensity can be observed for Fe3Si films. For Fe3Si
on MgO the XMCD signal corresponding to the L3edge is
decreased by 15%. In the case of Fe3Si on GaAs-46
this difference becomes 31%. Even a change in the surface
reconstruction to GaAs-22results in a further reduction
in the XMCD signal to 38%. The diminution of the XMCD
goes along with diminishing magnetic moments obtained
from the sum-rule analysis shown in Table. II. The magnetic
moment on MgO calculated with the sum rules is 1.6
Band
is in very good agreement with results from the literature.9
On GaAs we obtained decreased moments. Whereas the
magnetic moment on GaAs-46is reduced by
6%1.5
B, which is within the estimated error bar of 10%
for the sum rules, on GaAs-22we obtained a moment
decreased by 19%1.3
B. Taking a closer look at the
XMCD signal between the L2,3 edges reveals a change in the
spectral trend. The inset of Fig. 5shows an enlargement of
this feature. Contributions to the XMCD signal in this energy
range are ascribed to sp-hybridized states or spd-wave
mixing.23 Pure Fe has a clear positive XMCD signal in this
FIG. 6. Color onlineNormalized XAS topand XMCD bot-
tommeasured at RT at the Fe L2,3 edges of 57 ML Fe3Si on
GaAs-22and GaAs-46.
FIG. 7. Color onlineCalculated spin-resolved density of states
of Fe3Si. The inset shows the total not spin-resolvedDOS of the
region above the Fermi energy EF=0being relevant for the cal-
culation of XMCD spectra.
FIG. 8. Color onlineCalculated XAS topand XMCD bot-
tomspectra of Fe in bulk Fe3Si EF=0. The inset is an enhance-
ment of the energy region between the L3and L2peak. In order to
make these tiny features visible we disclaim smoothing of the data.
LOCAL ATOMIC ORDER AND ELEMENT-SPECIFICPHYSICAL REVIEW B 80, 144403 2009
144403-5
range, in contrast to Fe3Si, for which the signal almost com-
pletely vanishes. So one can conclude that the sp hybridiza-
tion in Fe3Si is different to that of pure Fe. We attribute this
difference to the effect of the Si bonding with Fe which
happens by 4selectrons which can be also seen in the inset
of Fig. 7at an energy of 4 eV.
A possible explanation for the reduced magnetic moments
in Fe3Si/GaAs001is the diffusion of As or Ga atoms from
the substrate in the ferromagnet. The influence of alike dif-
fusion is studied by placing Ga and As impurities in bulk
Fe3Si. Although the lattice mismatch between Fe3Si and the
GaAs substrate is rather small this should be a reasonable
model. We have used two different methods to investigate
the influence of impurities on the magnetic properties of
Fe3Si, namely, CPA method where Fe and impurity atoms
partially share the same lattice site and supercell calculations
in which single Fe atoms are replaced by Ga or As atoms.
Impurity concentrations up to 10% have been taken into ac-
count, whereby it turned out from our calculations that Ga
and As preferably occupy FeBsites. Impurities on the low
moment FeAsites are less important because one has to
effort more energy to place the impurities on FeAinstead
of FeBsites. In order to figure out the size of possible
impurity concentrations in Fe3Si the mixing energy
Emix =EFe3−xYxSi 3−xEFe +xEY+ESi 1
has been calculated, which determines whether the alloy
Fe3−xYxSi with Ybeing Ga or As is stable against decompo-
sition. In Eq. 1the energies on the right side correspond to
the calculated ground-state energies of the elements, which is
a reasonable choice in case of Ga. For As the situation is
more complex because there exist a huge number of Fe-As
alloys such that the alloy may decompose in one or more of
these alloys instead of its elements. However, Eq. 1gives
evidence of the amount of the impurity concentration. In
case of Ga the mixing energies are negative if the impurity
concentration does not exceed 3.3%. The As-containing alloy
seems to be stable for all investigated concentrations, which
may be related to the above-mentioned problem of the proper
choice of the decomposition components.
In case of 3.33% Ga or As the energy difference between
the impurities sitting on FeBor FeAsites amounts to
138.5 and 182.7 meV/f.u., respectively. The same trend is
observed for the supercell calculations. An impurity concen-
tration of 4.17% i.e., one Fe atom is replaced by an impu-
rityof Ga Ason FeAsites leads to a 157.4 meV/f.u.
237.8 meV/f.u.higher energy compared to impurities on
FeBsites. Independent which method—CPA or
supercells—has been used the magnetic moments decrease in
equal measure with increasing impurity concentration, see
Fig. 9. Assuming an impurity concentration of 3.33% the
average spin moment is reduced by 0.12
B0.15
Bin case
of Ga As. The absolute values of the orbital moments ob-
tained from plain spin density-functional calculations are al-
ways too small compared to experiment. However, we ob-
serve the same trend as for the spin moments, i.e., the orbital
moments are slightly smaller compared to the value obtained
for Fe3Si. However, the above discussed results suggest that
the magnetic moments of Fe3Si/GaAs-46and −22
are related to alloy formation at the interface. An impurity
concentration of 3.3% seems to be sufficient to decrease the
average magnetic moment by about 0.1
B, which is of the
same magnitude as the change in the measured magnetic
moments, see Table II. The even smaller magnetic moment
of the 22-reconstructed surface may be related to addi-
tional As diffusion, which can be larger as compared to Ga
diffusion. Although the absolute value of the magnetic mo-
ments is decreased by Ga and As impurities this has only
minor influence on the spin polarization. The spin polariza-
tion is mainly determined by the difference of the majority
NEFand minority density of states NEFat the Fermi
level EF,
P=NEFNEF
NEF+NEF.2
From Eq. 2we obtained a spin polarization of −42.3%for
pure Fe3Si which is close to the value known from the
literature.7For Fe2.9Ga0.1Si impurity concentration=3.33%
TABLE II. Total averaged magnetic moment per Fe atom and
ratio of orbital moment mlto spin moment msas obtained from
sum-rule analysis for 57 ML Fe3Si. We estimate the error bar in the
order of 10%.
Substrate mtot in
Bml/ms
Fe reference 2.2 0.04
MgO 1.6 0.09
GaAs-461.5 0.06
GaAs-221.3 0.11
FIG. 9. Color online兲共aSpin and borbital magnetic mo-
ments of Fe3−xYxSi Y=As,Gadepending on the impurity concen-
tration xin percent. Results marked by circles and squares obtained
from KKR-CPA calculations. Stars denote the magnetic moments
derived from supercell calculations VASPfor Fe24−nYnSi8
Y=As,Gawith n=1,2.
KRUMME et al. PHYSICAL REVIEW B 80, 144403 2009
144403-6
the spin polarization still amounts to −36.8%which is close
to the value obtained for bulk Fe3Si. The high degree of spin
polarization as well as the weak dependence between spin
polarization and interface quality in the system
Fe3Si/GaAs001make this system interesting for further
studies.
V. SUMMARY
In summary, we have investigated structural and magnetic
properties of Fe3Si on GaAs and MgO by combining XMCD
and Mössbauer measurements with calculations within
multiple-scattering theory. From CEMS we gain evidence for
chemical disorder of Fe3Si on GaAs substrates. Measured
and calculated XMCD spectra match well even in details of
the fine structure between the L3and L2edges, thus yielding
well agreeing averaged Fe moments. As one result of the
calculations we obtain the different contributions to the
XMCD from Fe on the two inequivalent sites, which are not
distinguishable in the experimental spectra. To investigate
the influence on the magnetic properties by diffusion of sub-
strate atoms, we have carried out KKR calculations consid-
ering Ga and As impurities with various concentrations. Con-
cluding, our results indicate an interdiffusion at the interface
of Ga atoms from the substrate, although the spin polariza-
tion of Fe3Si is not dramatically affected.
ACKNOWLEDGMENTS
We thank U. v. Hörsten for help with sample preparation
and CEMS measurements. We gratefully acknowledge sup-
port during our beamtimes by BESSY staff. Financially sup-
ported by DFG SFB 491 and SFB 445and BMBF Grant
No. 05 ES3XBA/5.
*Corresponding author; bernhard.krumme@stud.uni-due.de
Permanent address: Nanostech Laboratory, Indian Institute of
Technology Delhi, New Delhi 110 016, India.
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... The former parameter has been quantitatively found as (1.107 ± 0.014) µ B /atom at RT, close to the bulk value of 1.175 µ B / atom at 6.5 K [94,95], while the interface roughness was about 2.3 ± 0.1 nm. On the other hand, via XMCD, Krumme et al. directly compared the magnetic moments of Fe 3 Si films on different surfaces including MgO(0 0 1), Ga-rich (4 × 6) and As-rich (2 × 2) of GaAs(0 0 1) [124]. Fig. 18 shows the X-ray absorption (XAS) and XMCD spectra at the Fe L 2,3 edges of those grown on MgO and Garich (4 × 6) surface. ...
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