Magnetic properties of ultrathin Fe3Si films on GaAs(001)

Abstract

For a detailed understanding of the interface between Fe3Si and GaAs, we investigate Fe3Si films in the ultrathin limit down to a few monolayers and compare the results to Fe3Si/MgO(001) which serves as a reference in the present study. From X-ray magnetic circular dichroism measurements we determine averaged spin and orbital magnetic Fe moments. Further insight follows from SPR-KKR calculations. Conversion electron Mössbauer spectroscopy (CEMS) yields information on the chemical ordering and is able to distinguish inequivalent Fe lattice sites. The CEMS results indicate structural disorder which we attribute to an interdiffusion at the Fe3Si/GaAs interface.

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Magnetic properties of ultrathin Fe3Si films on GaAs(001)
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Magnetic properties of ultrathin Fe3Si films on
GaAs(001)
C Weis, B Krumme, H C Herper, F Stromberg, C Antoniak, A
Warland, P Entel, W Keune and H Wende
Fakult¨at f¨ur Physik and Center for Nanointegration Duisburg-Essen (CeNIDE), Universit¨at
Duisburg-Essen, Lotharstraße 1, D-47048 Duisburg, Germany
E-mail: claudia.weis@uni-due.de
Abstract. For a detailed understanding of the interface between Fe3Si and GaAs, we
investigate Fe3Si films in the ultrathin limit down to a few monolayers and compare the
results to Fe3Si/MgO(001) which serves as a reference in the present study. From X-ray
magnetic circular dichroism measurements we determine averaged spin and orbital magnetic Fe
moments. Further insight follows from SPR-KKR calculations. Conversion electron M¨ossbauer
spectroscopy (CEMS) yields information on the chemical ordering and is able to distinguish
inequivalent Fe lattice sites. The CEMS results indicate structural disorder which we attribute
to an interdiffusion at the Fe3Si/GaAs interface.
1. Introduction
The Heusler-like binary compound Fe3Si has entered into the focus of current research as a
promising candidate for spintronic applications. Due to the low lattice mismatch (theoretically
0.1 %), GaAs(001) is a suitable substrate for which electrical spin injection from Fe3Si contacts
was observed [1]. Most important for improved spin injection efficiency are high quality and
control of structural and magnetic properties at the interface. Fe3Si crystallizes in the D03
structure and can be regarded as a quasi Heusler compound (FeA)2(FeB)Si with two inequivalent
Fe sites [2, 3]. Fe atoms (FeB) on sites B have 8 nearest neighbors (nn) Fe. Fe atoms (FeA)
on sites A have only 4 nn Fe while the other 4 nn are Si. Fe3Si can be grown epitaxially on
GaAs(001) and MgO(001) [4, 5].
In the present work, we investigated the magnetic moments in epitaxial Fe3Si films on
GaAs(001) and MgO(001) by X-ray magnetic circular dichroism (XMCD) spectroscopy and
correlate them with the chemical ordering as obtained from conversion electron M¨ossbauer
spectroscopy (CEMS). To complete our study, XAS and XMCD spectra have been calculated
within a multiple scattering theory. In the calculations, Ga impurities in Fe3Si have been
considered, as CEMS measurements yield evidence for perturbed growth on GaAs.
2. Methods
We deposited Fe3Si films on MgO(001) and GaAs(001)-(4 ×6) by co-evaporation of 57 Fe and
Si at a substrate temperature of TS= 520 K in ultrahigh vacuum conditions as described in
detail elsewhere [6]. The samples were covered by a 40 ˚
A gold layer to prevent oxidation during
transportation to the measuring chamber. We monitored the growth by reflection high energy
International Conference on Magnetism (ICM 2009) IOP Publishing
Journal of Physics: Conference Series 200 (2010) 072105 doi:10.1088/1742-6596/200/7/072105
c
2010 IOP Publishing Ltd
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0
2
4
6
Fe reference
on MgO
on GaAs
XMCD & XAS (arb. units)
-2
Energy (eV)
700 720
-5 0 5 10 15 20 25
Energy (eV)
-4
-2
0
2
!µFe
L2,3
(a.u.)
0
5
10
15
20
25
µFe
L2,3
(a.u.)
Fe3Si
Fe2.9Ga0.1Si
Fe2.8Ga0.2Si
Fe2.7Ga0.3Si
Fe2.6Ga0.4Si
Figure 1. XAS (top) and XMCD (bottom) Left: of 57 ML Fe3Si on MgO and GaAs and a bulk-
like Fe reference measured at RT, Right: from calculations considering various concentrations
of Ga impurities in bulk Fe3Si.
electron diffraction (RHEED), thereby confirming the layer-by-layer growth of Fe3Si(001) on the
MgO(001) substrate from the beginning of deposition. On GaAs the first Fe3Si(001) reflections
appear at a film thickness of about 6 ML when the initial islands start coalescing [7].
We measured thickness-dependent XAS and XMCD at the helical undulator beamline
UE56/2-PGM 1 at BESSY by detecting the total electron yield at the Fe L2,3edges. The
easy axis of magnetization is in the film plane of Fe3Si [8], we recorded the XMCD spectra at an
angle of 20◦between the photon wave vector and the surface of the sample. The spectra were
divided by the incoming photon flux, i.e. the photocurrent of a Au mesh in the incident X-ray
beam, normalized to unity and corrected for saturation effects. We determined spin and orbital
magnetic moments per Fe atom averaging over the two different Fe sites by a standard sum rule
procedure [9, 10].
Theoretical spectra are obtained from ab initio calculations using the Munich SPR-KKR
code within the local density approximation in the parametrization of Vosko, Wilk, and Nussair
[11, 12, 13]. To study the influence of Ga impurities on the magnetic structure of Fe3Si KKR
calculations within the coherent potential approximation (CPA) were carried out. The XAS
and XMCD have been obtained from an expression based on Fermi’s Golden rule [12]. Further
details will be published [6].
CEMS measurements in zero external field at room temperature revealed the chemical
ordering of the Fe3Si films. Due to the different surroundings of Fe atoms of type A and B, they
have different hyperfine fields that can be distinguished by CEMS. Therefore it is possible to
discriminate contributions from the two inequivalent Fe lattice sites [14]. For the final analysis we
fitted the M¨ossbauer spectra in a two step procedure accounting (i) for highly ordered Fe3Si (FeA
and FeBsextets) and (ii) for an additional spectral contribution (hyperfine field distribution)
that we attribute to an additional, non-Fe3Si-like Fe phase due to disorder in the film [6].
3. Results
Fig. 1 shows the XAS and the XMCD spectra at the Fe L2,3edges as obtained from measurements
(left) and calculations (right). The experimental XAS lines of Fe3Si are broadened compared to
International Conference on Magnetism (ICM 2009) IOP Publishing
Journal of Physics: Conference Series 200 (2010) 072105 doi:10.1088/1742-6596/200/7/072105
2

Relative Emission
5%
GaAs
10%
MgO
084-8 -4
Velocity (mm/s)
700 720
-1
0
norm. XMCD (arb. units)
Photon Energy (eV)
57 ML
17 ML
9 ML
40 K
Figure 2. Left: Measured CEM spectra (solid circles) of 57 ML Fe3Si on MgO(001) and
GaAs(001) at RT together with fitting curves (lines) as described in the text. Right: Thickness-
dependent XMCD spectra of Fe3Si films on GaAs(001) measured at T= 40 K. The corresponding
XAS are normalized to unity.
bulk Fe with a somewhat lower maximum at the Fe L3edge (decrease by ∼8 % on MgO(001)
and ∼17 % on GaAs-(4 ×6)). We note that on GaAs the Fe-XMCD signal of Fe3Si is lower
than on MgO. In addition, a shoulder occurs as observed also in other Heusler systems [15]. A
standard sum rule analysis yields an averaged total magnetic moment per Fe atom of 1.6µBon
MgO (error bar 10 %) in agreement with literature values [2]. On GaAs the sum rules yield
1.5µBper Fe atom – a value that, although still within the error bar of the sum rule analysis,
follows the trend of reduced magnetization that the reduced XMCD spectrum indicates. The
orbital to spin moment ratio is 9 % and 6 % on MgO and GaAs, respectively.
To investigate the reduction of the XMCD signal on GaAs in further detail, we calculated
XAS and XMCD considering various various concentrations of Ga impurities in bulk Fe3Si. The
resulting spectra are shown on the right-hand side of fig. 1. The larger the number of impurities in
the samples, the smaller the XAS intensity becomes. In the same manner, the XMCD decreases
with increasing Ga concentration. These results indicate, that our experimental finding of
reduced XMCD in Fe3Si on GaAs may be related to an increased interdiffusion of substrate Ga
atoms into the ferromagnetic film. According to the calculations, these impurities preferably
tend to occupy FeAsites.
Fig. 2 (left) shows the CEM spectra of 57 ML Fe3Si films on MgO and GaAs. The two
samples were grown simultaneously so that the growth conditions for both samples were identical.
Already from the bare experimental data (solid circles in the figure) it becomes obvious that
the chemical ordering of Fe3Si is higher on MgO than on GaAs. The M¨ossbauer lines are
sharper and more pronounced. On MgO a satisfactory fit can be obtained from a calculated
spectrum for nearly perfectly ordered Fe3Si. No additional spectral contribution is required.
In contrast, on GaAs a second sub-spectrum is needed to account for the non-perfect order in
the film. From our fitting routine, that is based on the work by Arita and coworkers [16], we
obtain long range order parameters S(D03) and S(B2) of 0.83 and 0.52, respectively, on MgO
and 0.68 and 0.32, respectively, on GaAs. In the ideal case they should be S(D03) = 1 and
S(B2) = 2/3 for Fe3Si with perfect D03structure. This larger disorder in Fe3Si on GaAs may
explain the experimentally observed reduction of XMCD, possibly as a result of diffusion of
substrate atoms into the Fe3Si film. As discussed above, our calculations with Ga impurities
support this explanation. In contrast, on MgO the Fe3Si films appear quite well ordered and
International Conference on Magnetism (ICM 2009) IOP Publishing
Journal of Physics: Conference Series 200 (2010) 072105 doi:10.1088/1742-6596/200/7/072105
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may thus serve as a reference standard.
On the right-hand side of fig. 2 we plot the thickness dependence of the XMCD spectra of
Fe3Si on GaAs measured at T= 40 K. The corresponding XAS (not shown) are normalized
to unitiy. Within the experimental error they are identical to the one of 57 ML Fe3Si. These
thickness-dependent measurements show a clear decrease of the XMCD intensity and thus of
the magnetization with decreasing thickness. Two possible reasons may lead to this reduction:
(i) the well know finite size effect of thin film, and (ii) interdiffusion of substrate atoms into the
film. Up to now, our investigations do not allow for discrimination between these two origins. In
the future, CEMS with 57Fe3Si tracer layers at the interface will contribute to the clarification.
4. Conclusion
In conclusion, XMCD and CEMS together with DFT calculations yield a deeper insight into the
relation between structural and magnetic properties of Fe3Si on GaAs and MgO. The case
of Fe3Si on GaAs is especially interesting with respect to future spintronic devices, where
ferromagnet/semiconductor interfaces play an important role. Our investigations yield evidence
of disorder and/or interdiffusion at the interface of Fe3Si/GaAs, while the magnetic moments of
Fe are not much influenced. Thickness-dependent XMCD measurements show a decrease of the
magnetization at low temperature with decreasing thickness. Further investigations will follow,
e.g. by CEMS with tracer layers at the interface to clarify the origin of this reduction.
Acknowledgments
We gratefully acknowledge U. v. H¨orsten for help with sample preparation and CEMS
measurements, M. Walterfang for contributing the first implementation of the CEMS fitting
routine, and the BESSY staff for support during our beamtimes. Financially supported by DFG
(SFB 491 and SFB 445) and BMBF (05 ES3XBA/5).
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Journal of Physics: Conference Series 200 (2010) 072105 doi:10.1088/1742-6596/200/7/072105
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