SIZE EFFECTS ON THE MAGNETIC-PROPERTIES OF FINE FE-CR PARTICLES

The magnetic and structural properties of Fe100-xCrx ultrafine particles with x = 5-20 have been studied as a function of particle size. Particles with a size in the range of 80-360 angstrom were prepared by gas evaporation under argon atmosphere. The particles with smaller diameter had a high coercivity at low temperatures and showed a stronger temperature dependence of coercivity. The x = 20 particles with a size 80 angstrom had a coercivity about 2100 Oe at 10 K with a superparamagnetic blocking temperature about 150 K. Mossbauer spectra showed the presence of Fe-Cr, alpha-Fe and Fe-oxide components in the bigger particles, and Fe-Cr and Fe-oxides in the smaller particles. The coercivity at low temperatures increased with decreasing particle size and this was attributed to the higher percentage of Fe-oxide on the surface of the smaller particles. This interpretation was further supported by the temperature dependence of coercivity of Fe-Cr particles sandwiched between two Ag films.