Photon Factory Activity Report 2005 #23 Part B (2006)
Electronic Structure of Condensed Matter

11B, 2A/2005G097

The effective charge on the Si atom in FexCo1-xSi pseudobinary alloy
Kazuhiro EMI1, Kazutoshi KAWAKAMI1, Megumi OKI1, Nobuo NAKAJIMA1*,Hiroshi MARUYAMA1, Yoshinori KITAJIMA2, Kotaro ISHIJI2 and Toshiaki IWAZUMI2
1Grad. Sch. of Sci., Hiroshima Univ., 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan
2KEK-PF, 1-1 Oho, Tsukuba, Ibaraki 305-0801, JapanExperimental
X-ray absorption spectra were taken at BL-11B withtotal electron yield method. The experiments were carriedout under room temperature and high vacuum of the order-5of the order of 10 Torr.
Results and Discussion
Figure 1 shows the absorption spectra of FexCo1-xSi at
Si K edge for x=0.28, 0.42, 0.732 and 0.833 together withthe spectrum of Si for comparison. The vertical bars indicate inflection points at edges. The chemical shifts relative to Si versus nominal number of 3d electrons are plotted in Fig. 2. Quite similar correlation obtained from theSi KĮ fluorescence spectra is reported by Liu et al [3].
From their result, the effective charges on Si are increased with the chemical shift in Si KĮ lines, namely Si
is positively charged when the chemical shift is positive.
Then, our result directly indicates that the effectivecharges on the Si atom in FexCo1-xSi is positive and thatthe positive charge increases with increasing nominal

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Users' Report

FexCo1-xSi

0.28
0.42

XAS Intensity (arb. units)
1836

0.732
0.833
Si

1840

1844 1848 1852 1856
Photon Energy (eV)1860

number of 3d electrons. This result is also consistent with
our simple consideration described above.
References
2.0
Energy Shift (eV)Introduction
The pseudobinary alloys FexCo1-xSi exhibit weak itinerant ferromagnetism at low temperature and have attracted
much attention of many researchers in connection withverification of the spin fluctuation theory and with thecharacteristics of Kondo insulator. In these concerns, themain interest was concentrated on the host magnetic elements. Recently, we have performed soft X-ray fluorescence study at the Si L-edge [1] and have revealed thehybridization effect between Si and Fe or Co. The results
are summarized as follow: with increasing 3d electron
numbers by substituting Fe with Co, hybridization between Si 3p and transition-metal 3d orbitals is strongly
enhanced. Considering the crystal structure of this alloy,
B20 (a distorted rocksalt structure), the limbs of 3d orbitals should be elongated to the corners of a Si octahedron,e.g., t2g orbitals hybridized with Si 3p ones. From the result of band-calculation [2], Si 3p states locate at binding
energy (EB) of ~4eV, whereas the crystal-field-split 3dstates locate at EB= ~1eV and ~3.5eV. Therefore, t2g orbitals are more stable than eg ones. In order to achieve thisenergy diagram, the effective charge on the Si atomshould be positive.
In this report, we present the X-ray absorption spectraof FexCo1-xSi at the Si K edge and discuss about the hybridization effect based on a chemical shift of the edge.

1.5
1.0
0.5
0.0
6.0

6.2
6.4
6.6
6.8
Nominal Number of 3d Electrons

7.0

Fig. 2 Energy shift of Si K-edge versus the nominal
number of 3d electrons.
Fig. 1 X-ray absorption spectra of FexCo1-xSi at Si K
edge for x=0.28, 0.42, 0.732 and 0.833 together
with the spectrum of Si for comparison. The vertical bars indicate inflection points at edges..
[1] N. Nakajima et al., PF. Act. Rep. 22, 102 (2005).
[2] H. Yamada, private communication[3] Z. Liu et al., Phys. Rev. B69, 035106 (2004); H. Ishii
et al., Phys. Rev. B71, 205202 (2005).
*nobuo@hiroshima-u.ac.jp