Ferromagnetic nanocomposites

A survey of magnetic nanocomposites applicable in high-frequency signal and power electronics is given. First, the preparation and properties of ribbon and powder cores from the nanocrystalline bulk alloys (Finemet and Nanoperm) is reviewed. A technology is presented to apply continuously a large stress during the annealing and winding of the rapidly quenched ribbons in order to induce uniaxial anisotropy in it. The obtained toroidal cores with flat hysteresis curve are applicable up to 1 MHz with considerable permeability (similar to 250). The powder cores prepared from ground Finemet with powder size of 30-400 mu m are applicable up to 1 MHz and in some cases up to 10 MHz for smaller powder sizes with low permeability (similar to 10). Finally, the most common methods used for the preparation of metallic nanoparticles are presented. Presently, the compacts prepared from nano-size (40-80 nm) iron powders do not show the expected behavior. It is anticipated that the iron-based ferromagnetic nanocomposites should replace partly the ferrite-type materials in the forthcoming years. (C) 2000 Elsevier Science B.V. All rights reserved.