FeCo alloy particles synthesized by liquid phase reduction are disorder distribution of the alloying elements in bcc crystalline lattice. The particle size dependence of thermally ordering behavior during heating was investigated. Superlattice lines such indices as 100, 111 etc., which appear in the x-ray diffraction pattern when the atomic arrangement is ordered, are difficult to detect due to quite small difference between Fe and Co in the atomic x-ray scattering factor. In this study, the energy near to Fe K absorption edge for incident x-ray was used in order to apply the anomalous x-ray dispersion effect. From results of the in situ heated x-ray diffraction experiments using the Synchrotron radiation source, different ordering-onset temperature was observed for the two samples with different particle size. This is considered due to a difference on the lattice distortion and its elimination during heating.
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