Size Effects on the Structural and Magnetic Properties of Ultrafine Fe-Pd Particles

The structural and magnetic properties of ultrafine Fe-Pd particles have been studied near the equiatomic composition. Particles with an average size in the range of 65–360 A were prepared by gas evaporation under argon atmosphere with the Ar pressure varied between 0.5–40 torr. All of the as-made particles had a face centered cubic (fcc) structure. A fcc to face centered tetragonal (fct) phase transformation was observed in particles with a size in the range 120–360 A after annealing the powders at 500°C; smaller particles with equiatomic composition did not show this transformation. X-ray diffraction and Mossbauer data showed the presence of a higher percentage of Fe-O in the surface of the smaller particles which leads to an inhomogeneous Fe-Pd composition distribution. An enhanced magnetic moment of about 3.0 μ B per atom was found for Fe, and this is attributed to the polarization of Pd atoms.