Journal of Magnetism and Magnetic Materials 215}216 (2000) 253}259 Invited paper Ferromagnetic nanocomposites F. Mazaleyrat*, L.K. Varga LESIR-URA-CNRS-CNAM-ENSC, 61, av. Du Pdt Wilson 94235 Cachan, Cedex, France Research Institute for Solid State Physics and Optics, HAS, P.O.B. 49, 1525 Budapest, Hungary Abstract 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 `bulka 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 #at hysteresis curve are applicable up to 1 MHz with considerable permeability (&250). The powder cores prepared from ground Finemet with powder size of 30}400 m are applicable up to 1 MHz and in some cases up to 10 MHz for smaller powder sizes with low permeability (&10). Finally, the most common methods used for the preparation of metallic nano- particles 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.  2000 Elsevier Science B.V. All rights reserved. Keywords: Magnetic composites; Nanoparticles; High-frequency applications 0. Introduction In the last decade, much work has been done with the aim of increasing the operating frequency in electronic devices. The tendency is to reduce the volume (or weight) of systems using inductive elements. This is observed in all types of electronics such as radio-transmission, tele- communications, micro devices, power electronics and so on. The demand for reduction of size is obvious in the micro-systems and portable devices like telephones. In contrast, the motivation for this is less simple in the case of power converters for domestic and industrial appar- atus in which the weight is not so important. Actually, in these kinds of applications the price is the most impor- tant property of the products. Let us consider, for example, the plug-in AC/DC converters, which are * Corresponding author. Tel.: #33-147-402-108; fax: #33- 147-402-199. E-mail address: mazaleyrat@lesir.ens-cachan.fr (F. Mazaleyrat). spread over the world as much as billions. This kind of converter supplies about 10}50 W for a weight of 200}400 g and a volume of 100}200 cm. The widespread procedure applies a 50}60 Hz transformer lowering the network voltage down to about 10 V, and then recti"es it. Taking into account the reduction in price of semicon- ductor-based units while the price of magnetic materials is stable, it is more e$cient to rectify directly the network voltage, and then to transform it after chopping at high frequency. By using this method, a 12 W supplier can be made as small as a normal plug. In the high-frequency range, ferrite is commonly used. Beside the low price, they present the advantage of high resistivity and the possibility of making cores of various forms. However, they exhibit a negative thermal coe$c- ient of resistivity and have relatively low Curie temper- ature. Moreover, the high-permeability MnZn ferrites have a high relative permittivity (up to 10) so the wave velocity in the medium is low and a dimensional re- sonance is observed at about 1 MHz when the core dimensions are within several mm [1]. Due to their low magneto-crystalline anisotropy they are limited in fre- quency by the ferromagnetic resonance (FMR), the so- called Snoek limit [2]. 0304-8853/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 1 2 8 - 1