Photon Factory Activity Report 2005 #23 Part B (2006)
Crystallography

10A, 3A/2005G1242+3+

Mixing of Co and Co ions in FexCo3-xO4 examined byXMCD and single-crystal X-ray diffraction
Norio SHIMIZU1, Maki OKUBE 1, Koichi OHKUBO1, Takayasu HANASHIMA1,1
2
2
1Kouji YAMAWAKI , Takeharu MORI , Masahiko TANAKA , Satoshi SASAKI*
1
Materials and Structures Lab., Tokyo Inst. Tech., Nagatsuta, Yokohama 226-0803, Japan
2KEK-PF, Tsukuba, Ibaraki 305-0801, JapanIntroduction
The spinel structure, which has the geometricalfrustration of a pyrochlore type, is attractive in thesuperexchange interaction between the A and B sites. Siteoccupancy between Fe and Co ions in ferrites andcobaltites is a key factor to control the physical properties.
The occupancy can be determined accurately with theanomalous scattering effects of synchrotron X-rays. As
well known, it is difficult to grow single crystals ofcobaltites. Fortunately, we have succeeded to synthesize
single crystals in the hydrothermal reaction. A
combination study of X-ray diffraction, X-ray absorption
near edge structure (XANES) and X-ray magnetic
circular dichroism (XMCD) makes it possible todetermine the valence and spin states of Fe and Co ions of
the mixed-valence compounds. Thus, in this study, thevalence and spin states of FexCo3-xO4 spinels [1] havebeen examined.
Experimental
Synchrotron diffraction experiments for FexCo3-xO4
were carried out in a four-circle diffractometry at BL-10A.
The horizontally polarized white X-rays weremonochromatized by the Si(111) monochromator. The
wavelengths used in this study are 1.6182 Å near Co Kabsorption edge. As a total, the intensity data with 349
reflections were measured with ω−2θ step-.
XANES and XMCD experiments were carried out on
the BL-3A. The X-ray beam from the Si(111)(H)(V)
2monochromator was limited to 3 x 2 mm at thesample position by a slit. The intensity after transmitting
through the sample was measured with 300 mm
ionization chamber which was filled with 75% N2 + 25%
Ar gas. The difference in the absorption coefficients forright- and left-circularly polarized X-rays was measured
with spins parallel and antiparallel to the direction of lighttravel. The incident beam was guided into a synthetic
single crystal of (001) diamond with a thickness of 0.492
mm in order to produce circularly polarized X-rays. A
standard transmission setup was used with the Faraday
arrangement, where X-rays irradiates the sample througha pair of pinholes in rare-earth magnets in a magnetic
field of 0.4 T.
Results and discussion
The anomalous scatterig method was used to determine
the cation distribution between Fe and Co ions with alarge difference in atomic scattering factors. Leastsquares refinements for an Fe1.2.Co1.8O4 sample gave a
final chemical formula of [Fe0.72Co0.28]A [Fe0.48Co1.52]BO4.
Co K-edge XANES and XMCD spectra of FexCo3-xO4(x = 0, 0.8, 1.0, 1.1, 1.4, 1.8, 2.0 and 2.1) gave thecompositional dependence on the valence state of Co ions.
At the energy of the main edge, the XANES spectra has achemical shift to move the higher region with the increaseof Co contents. This accords with the contents occupanied
3+by Co in FexCo3-xO4.
At the Co K edge, each of the XMCD peaks for thesamples with x < 1.4 splits into two peaks of E = 7707 eVand E = 7715 eV. The boundary composition is close to x
3+2+3+= 4/3, where Fe , Co and Co ions occupy the B sites inthe ratio of 1 : 1 : 1. The split of the XMCD peaks
suggests a change of the Co spin state among the triangle
B lattice of the spinel structure. The characteristics can bewell compared with the compositional dependence of BO interatomic distances.

Fig. 1: XMCD spectra of FexCo3-xO4 (1 ≤ x ≤ 2) at theCo K edge.
References[1] N. Kita et al., J. Synchrotron Rad., 8, 446 (2001).
*sasaki@n.cc.titech.ac.jp

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