Thermal stability, oxidation behavior and magnetic properties of Fe-Co ultrafine particles prepared by hydrogen plasma-metal reaction

Six kinds of Fe-Co ultrafine particles (UFPs) with an average particle size of about 45 nm were prepared by a hydrogen plasma-metal reaction method (HPMR). Particle feature, thermal stability, oxidation and magnetic properties of these UFPs were investigated. Fe-Co UFPs are spherical in shape and have the same crystal structure as bulk samples, but the Co content in Fe-Co UFPs is slightly richer than that in the master samples. Characteristics such as particle shape, mean particle size and specific surface area do not vary remarkably with the Co content. Fe-Co UFPs are stable in Ar and air below 473 K, but oxidize quickly in air above 473 K and simultaneously vary from a spherical shape into acicular shape during oxidation. Although the saturation magnetizations of Fe-Co UFPs both before and after oxidation are smaller than those of bulk samples, they exhibit a composition dependence similar to that of the master alloys. High residual magnetization and coercive force are obtained for Fe-Co UFPs before and after oxidation. (C) 1998 Elsevier Science B.V. All rights reserved.