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X-ray diffraction applied anomalous dispersion effect was used for characterizing the order-disorder transformation in Fe 50 Co 50 fine particles synthesized by polyol process. The long range order parameter was estimated from the superlattice peak intensities in the diffraction pattern of particles annealed in the temperature range between 20 and 800 C. The results showed that fine particles of as-synthesized Fe-Co have a disordered structure at room temperature, while the ordered structure is present, to a great extent, in Fe-Co bulk alloys. The fine particles of the disordered alloy were transformed to the ordered state by annealing up to about 450 C. The magnetic properties of the Fe-Co alloy fine particles annealed at different temperatures were also investigated using a vibrating sample magnetometer, in which magnetic fields of up to 15 kOe were applied to samples at room temperature. The saturation magnetization of the as-synthesized particles increased with annealing temperature. This may be attributed to transformation from disordered to ordered structure, along with sintering.

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X-ray photoelectron spectroscopy (XPS) has been used for analyzing the surface composition of polyol process-derived Fe30Co70, Fe50Co50, and Fe70Co30 alloy nanoparticles with diameters 50, 100 and 150 nm, respectively. These Fe-Co alloy particles have high oxidation resistance in the atmospheric environment even though their particle size is so small. The XPS results revealed that the concentration of iron at the surface of the as-synthesized Fe-Co alloy nanoparticles was lower than that in bulk and increased with increasing bulk cobalt composition, although the surface of nanoparticles was covered with native oxide layer formed during their exposure to atmosphere. This low concentration of iron and very thin oxide layer at the surface are considered to protect the particle from oxidation. The concentration of iron at the surface of Fe70Co30 nanoparticles increased when they were annealed at 573 K in N2 and H2 atmosphere. The results indicate that nonuniformity of the chemical composition between particle surface and core occurs during the formation of the same in polyol, and atomic diffusion at the surface of particle can occur even at relatively low temperature. The above is considered to arise from the difference in the chemical characteristics of iron and cobalt during co-precipitation in the polyol.