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...

Ruthenium nanoparticles were prepared by reduction of RuCl 3 in a liquid polyol. The mean particle size was restricted to the 1-6 nm range by appropriate choice of the reduction temperature and the acetate ion concentration in the solution. Very narrow particle diameter distributions were obtained. In some samples, among nearly isotropic particles, platelets with aspect ratios as low as 1/4 were detected. Colloidal solutions in toluene were obtained by coating the metal particles with dodecane thiol. Self-assemblies of 4-nm-sized coated particles were studied on a transmission electron microscope grid. The dodecane thiol concentration in the colloidal solution was found to determine, within the particle monolayer, the formation of either columnar units made up of edgewise stacked platelets, or a hexagonal network with a mean distance between the particles of 2 nm. The stacking of hexagonal arrays of particles was also studied, and both closed-packed and noncompact stackings were found. In the noncompact stacking, moire ´images resulted from the twisting of the two hexagonal layers with respect to each other. Reconstructions of moire ´patterns were observed to favor the 6-fold and 2-fold sites.

Fine equiaxial a-Fe particles were obtained by disproportionation of iron@) hydroxide in liquid polyols with yields in the range 5-8%. The influence of the polyols as the reactive medium upon the solid phase formation during this reaction and its effect upon the Fe formation yield are discussed. Fine polymetallic powders Fe,M~loo-x, (M =Ni, Co; 0 b x b 25) and Fe,[Ni,Co(, -y~](loo-x) (0 < x < 25; 0 < y < 1 ), were prepared from mixed hydroxides in liquid polyols, with metallic iron being generated in solution by disproportionation, and metallic nickel and/or cobalt by reduction of Ni" and Co" hydroxides by the polyols. Characterization (using X-ray diffraction, energy dispersive spectroscopy and electron microscopy) showed FeNi powders, made up of spherical particles with a mean diameter (d,) in the sub-pm size range, a narrow size distribution (standard deviation c < 10% d,) and a fairly good homogeneity of composition; FeCo powders were polyphasic and made up of polydisperse agglomerates of Fe and Co particles while FeNi powders appeared as a single phase; FeCoNi powders appeared as monodisperse and polyphasic. The growth mechanism of the particles is discussed in relation to their characteristics: for FeNi monodisperse particles a growth mode by aggregation of nm-sized primary particles is proposed.

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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Cobalt and Co 80 Ni 20 metal particles were prepared by reduction of acetate salts in a liquid polyol. The relative amount of the hcp phase and the probability of stacking faults in this phase vary with the experimental conditions of synthesis. The formation of either isotropic or anisotropic particles appears to be closely related to the stacking faults. Several kinds of anisotropic particles were detected by TEM: platelets, rods or diabolo-like particles. For isolated diabolos and rods, HRTEM and electron diffraction showed that the crystallographic c axis is parallel to the rotation axis of the particles. Magnetic property measurements showed high values of coercivity and remanence for the anisotropic particles. The anisotropic shapes differ markedly from those previously observed for cobalt-based particles prepared by the polyol process. A mechanism that involves a solid intermediate phase as either precursor or template is proposed.

Fine, equiaxed copper particles have been obtained by reduction of CuO in ethylene glycol. Cu, O always exists as an intermediate solid phase. Copper particles with a narrow size distribution within the micrometre range can be obtained if the nucleation and growth steps are completely separated and if agglomeration is avoided.Addition of D-sorbitol, which acts as a protective agent, prevents particle sintering. Addition of a strong base, e.g. NaOH, enhances the solubility of the precursor CuO and of the intermediate Cu,O. Under these conditions the overall reaction appears to be controlled by the nucleation and growth steps of the metallic particles. Their mean size can be largely controlled by varying the NaOH concentration. The synthesis of copper particles in liquid polyols, which act as both solvent and reducing agent, is a simple method for producing highly pure, equiaxed, non-agglomerated monodisperse particles.