Ferromagnetic metal-based materials display properties that make them of interest for microwave applications, namely higher working frequencies and a broader working frequency band than bulk ferrimagnetic oxides. As far as microwave absorbing properties are concerned, metals have to be used as fine particles dispersed in an insulating matrix. Such composite magnetic materials exhibit magnetic losses (characterized by a non-zero imaginary part of the permeability) in the microwave range due to a gyromagnetic resonance phenomenon, their microwave properties depending on both the intrinsic characteristics of the particles and their volume concentration. The influence of the latter can be quite well described by mixture laws derived from the Bruggeman effective medium theory. [1,2] Less studied is the control of microwave properties of composite materials by altering the intrinsic properties of the magnetic particles. Two main objectives can be defined: first, the design of high-permeability composite materials with, in particular, optimal control of the resonance width; secondly, a better understanding of the dynamic properties of fine particles and a tentative correlation with their static magnetic properties. In both cases, control of the morphology of the ferromagnetic particles is needed since the gyromagnetic resonance is highly dependent on the particle shape through the effect of the demagnetizing field. Therefore, materials made up of particles with poorly defined shapes present a very broad resonance band. Moreover, materials made up of too large particles present only a weak resonance. [3,4] The polyol process, [5,6] which is known for providing monodisperse fine metal particles, afforded us the opportunity to synthesize ferromagnetic metal particles smaller than 2 mm and to investigate their dynamic properties. Our first results provided evidence of the effect of particle size on microwave properties in the 2±0.2 mm range. [7,8] The scope of this paper is to show how it has been possible recently to reduce and to control the diameter of such monodisperse particles down to the nanometer size range for various compositions and therefore to study the influence of the particle size upon the microwave permeability of monodisperse powders made up of quasi-spherical particles with a size range varying over two orders of magnitude (2.5 mm±25 nm).Polymetallic fine particles Co x Ni (100±x) and Fe z [Co x -Ni (100±x) ] (1±z) were synthesized by precipitation from metallic precursors dissolved in 1,2-propanediol with an optimized amount of sodium hydroxide according to a previously published procedure [9±11] (see Experimental section). Upon heating, as both Co II and Ni II are quantitatively reduced by the polyol itself, the Co/Ni ratio in the metallic Co x Ni (100±x) powders depends only on their initial ratio. For iron-based particles of Fe z [Co x Ni (100±x) ] (1±z) composition, Fe is generated by disproportionation of Fe II whereas Co II and Ni II are quantitatively reduced. The disproportionation of Fe II allo...

On the influence of nanometer-thin antiferromagnetic surface layer on ferromagnetic CrO2 J. Appl. Phys. 112, 053921 (2012) Effect of microstructure on the electromagnetic properties of Al18B4O33w/Co and Al18B4O33w/FeCo composite particles J. Appl. Phys. 112, 053917 (2012) Ni80Fe20/Ni binary nanomagnets for logic applications Appl.Spherical, monodispersed, ferromagnetic, metallic particles of different compositions were obtained by the polyol process with a mean radius ranging from 30 nm to 1 m. The microwave permeability of metallic particles-dielectric matrix composites were studied in the range of 0.1-18 GHz. In the wide particle size range investigated, a size dependence of the dynamic permeability was observed. Whereas the permeability of micrometer-sized particles shows a single resonance band, the permeability of submicrometer-sized particles exhibits several narrow resonance bands which are shifted to high frequencies with decreasing particle size. This latter behavior was found to be in qualitative agreement with the exchange resonance modes calculated by Aharoni. That theory, however, gives an R Ϫ2 dependence on particle radius for the resonance frequency instead of the R Ϫ0.66 dependence observed experimentally.

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