Communications

interaction between i) radical spins, Jp±p, and ii) radical andcobalt spins, Jp±Co, must be considered. None of these isstrong enough to give striking effects at room temperature,e.g., strong exchange narrowing or line broadening due tofast relaxation processes. However, as TC is lower in theradical-based compounds, it appears that the sandwichedradical layer is actually counteracting the driving inter-action for the bulk ferromagnetic ordering.

The similarities in the physicochemical characterizationof the three compounds allow any significant structuralmodification between the radical derivatives and the pre-cursor compound to be excluded. The dipolar interactionHdip is assumed to be the driving interaction for the settingup of bulk ferromagnetism within the three compounds, asdemonstrated for alkyl-carboxylate parent compounds.[20]

It involves the in-plane spin±spin correlation function,which diverges critically as the layers order, thus promotingbulk ferromagnetism for the three materials. The loweringof the critical temperature for Co-rad1 and Co-rad2 com-pared to Co-prec1 must be attributed to the presence ofthe radical spins, since the interlayer distance is much thesame in the various compounds. Among the possible mech-anisms, it can be stated that the radical spins do not simplyline up within the internal field created when the cobalt(II)layers order, otherwise the net magnetic moment would in-crease up to saturation more rapidly for the radical deriva-tives. Moreover, the lowering of TC cannot be understoodwithin this scheme. Therefore, the observed effects aremore likely due to the exchange interaction Jp±Co. The co-balt(II) layers including the radical spins must order at low-er temperature than the radical-free compound, due to theeffect of Jp±Co.

In this respect, this new class of hybrid magnets differsmarkedly from the classical intercalated layer compounds,mostly because both sub-networks (inorganic and organic)are in strong interaction. This is very promising for the fu-ture design of multifunctional materials in which the prop-erties of the organic and inorganic networks are in closesynergy.

Received: March 6, 1998Final version: April 27, 1998

±[1] Proc. of the Conf. on Molecule-Based Magnets (Eds: K. Itoh, J. Miller,

T. Takui), Mol. Cryst. Liq. Cryst. 1997, 305±306.[2] S. Descurtins, H. W. Schmalle, P. Schneuwly, J. Ensling, P. Gütlich, J.

Am. Chem. Soc. 1994, 116, 9521.[3] V. Gadet, T. Mallah, I. Castro, P. Veillet, M. Verdaguer, J. Am. Chem.

Soc. 1992, 114, 9213.[4] H. O. Stumpf, L. Ouahab, Y. Pei, D. Grandjean, O. Kahn, Science

1993, 261, 447.[5] K. Matsuda, N. Nakamura, K. Takahashi, K. Inoue, N. Koga, H. Iwa-

mura, J. Am. Chem. Soc. 1995, 117, 5550.[6] M. Kinoshita, P. Turek, M. Tamura, K. Nozawa, D. Shiomi, Y. Nakaza-

wa, M. Ishikawa, M. Takahashi, K. Awaga, T. Inaba, Y. Maruyama,Chem. Lett. 1991, 1225.

[7] F. Romero, R. Ziessel, M. Drillon, J. L. Tholence, C. Paulsen, N. Kyrit-sakas, J. Fisher, Adv. Mater. 1996, 8, 826.

[8] A. Caneschi, D. Gatteschi, J. P. Renard, P. Rey, R. Sessoli, Inorg.Chem. 1989, 28, 2940.

[9] A. Caneschi, D. Gatteschi, J. Laugier, P. Rey, R. Sessoli, J. Am. Chem.Soc. 1988, 110, 2795.

[10] K. Inoue, H. Iwamura, J. Am. Chem. Soc. 1994, 116, 3173.[11] P. Gütlich, Mol. Cryst. Liq. Cryst. 1997, 305, 17.[12] Y. Sano, M. Tanaka, N. Koga, K. Matsuda, H. Iwamura, P. Rabu, M.

Drillon, J. Am. Chem. Soc. 1997, 119, 8246.[13] M. Kurmoo, A. W. Graham, P. Day, S. Coles, M. B. Hursthouse, J. L.

Caulfield, J. Singleton, F. L. Pratt, W. Hayes, L. Ducasse, P. Guion-neau, J. Am. Chem. Soc. 1995, 117, 12 209.

[14] R. ClØment, P. G. Lacroix, D. O'Hare, J. Evans, Adv. Mater. 1995, 6,794.

[15] M. Drillon, C. Hornick, V. Laget, P. Rabu, F. M. Romero, S. Rouba, G.Ulrich, R. Ziessel, Mol. Cryst. Liq. Cryst. 1995, 273, 125.

[16] V. Laget, S. Rouba, P. Rabu, C. Hornick, M. Drillon, J. Magn. Magn.Mater. 1996, 154, L7.

[17] P. Rabu, S. Rouba, V. Laget, C. Hornick, M. Drillon, Chem. Soc.Chem. Commun. 1996, 1107.

[18] W. Fujita, K. Awaga, Inorg. Chem. 1996, 35, 1915.[19] V. Laget, P. Rabu, C. Hornick, F. RomØro, R. Ziessel, P. Turek, M.

Drillon, Mol. Cryst. Liq. Cryst. 1997, 305, 291.[20] M. Drillon, P. Panissod, J. Magn. Magn. Mater., in press.[21] L. Markov, K. Petrov, V. Petkov, Thermochim. Acta 1986, 106, 283.[22] W. Stählin, H. R. Ostwald, Acta Crystallogr. B 1970, 26, 860. R. All-

mann, Z. Kristallogr. 1968, 126, 417.[23] P. Rabu, S. Angelov, P. Legoll, M. Belaiche, M. Drillon, Inorg. Chem.

1993, 32, 2463.

Crystallization of Mesoscale Particles overLarge Areas**

By Sang Hyun Park, Dong Qin, and Younan Xia*

Crystalline assemblies of mesoscale particles are usefulin many areas. For example, two-dimensional assemblieshave been used as arrays of microlenses in imaging[1] andas physical masks in microlithography;[2] three-dimensionalassemblies have been used as precursors in producing high-strength ceramics,[3] as templates in forming porous silicamembranes,[4] and as diffractive elements in fabricatingnew types of sensors,[5] optical components,[6] and photonicbandgap structures.[7] Although a number of methods havebeen demonstrated for assembling mesoscale particles,[8±13]

only two of them are capable of producing crystalline as-semblies with domain sizes of 1000 mm3 that are useful infabricating optical devices. One of the methods is based onsedimentation[10] and the other on electrostatic interactionof highly charged particles in appropriate solvents free ofelectrolytes.[12] The method based on sedimentation is ex-tremely slow; it usually takes several days or weeks to pro-duce a crystalline assembly, and it has almost no control

±

[*] Prof. Y. Xia, Dr. S. H. ParkDepartment of Chemistry, University of WashingtonSeattle, WA 98195-1700 (USA)

Dr. D. QinDepartment of Bioengineering and Washington Technology CenterUniversity of WashingtonSeattle, WA 98195-2141 (USA)

[**] This work has been supported in part by a New Faculty Award fromthe Dreyfus Foundation, a subcontract from the AFOSR MURI Cen-ter at the University of Southern California, and start-up funds fromthe University of Washington. It used the Microfabrication Laboratoryat the Washington Technology Center (WTC).

1028 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1998 0935-9648/98/1309-1028 $ 17.50+.50/0 Adv. Mater. 1998, 10, No. 13

over the surface topology and the number of layers of thecrystalline assemblies.[10] The method based on electro-static interaction is also a relatively slow process; it has verystrict requirements on the experimental conditions such asthe density of charges on the surface of the particle, theconcentration of the particle, the concentration of freeelectrolytes in the dispersion solvent, and the tempera-ture.[12] Here we describe a simple and practical methodthat allows the rapid formation of crystalline lattices of me-soscale particles over areas as large as ~1 cm2 with well-controlled numbers of layers from one up to at least fifty.

Figure 1 shows the procedure schematically.[14] After anaqueous dispersion of monodisperse polystyrene beads(~0.05 wt.-%, Polysciences, Warrington, PA) had been in-jected into the cell, a positive pressure was applied throughthe glass tube to force the solvent (water) to flow through

the channels. The beads accumulated at the bottom of thecell and were assembled into a close-packed lattice undercontinuous sonication.[15] When the dispersion of polysty-rene beads had almost disappeared from the glass tube, thecell was placed in an oven at ~65 �C for ~6 h to evaporatethe remaining solvent. After the top substrate was carefullyremoved, a crystalline assembly of polystyrene beads wasleft behind on the bottom substrate. Further details are giv-en in the Experimental section. Instead of generating as-

semblies of mesoscale particles on solid supports, we havealso produced free-standing films by injecting UV or ther-mally crosslinkable prepolymers into the cells.[16]

Figure 2 shows scanning electron micrographs (SEMs) ofa three-dimensional crystalline assembly of polystyrenebeads (diameter d = 0.48 mm) assembled in a 12 mm thickcell over an area of ~0.5 ´ 2 cm2. Figure 2a shows a topview of a portion of the crystal. The crack in the crystal wascreated by drying the sample at elevated temperatures anda similar observation was also made in previous studies ofmonolayers of polymer beads.[17] The point defects weregenerated in the uppermost surface when the top substratewas removed; the density of defects of this type could begreatly reduced by treating the surface of the top substratewith C6F12C2H4SiCl3 vapor for ~30 min to make this sur-face non-sticky to polystyrene beads.[18] Figure 2b shows anoblique view of the crystal along a crack. Figures 2c and 2dshow SEMs of the uppermost and the lowest layers, respec-tively, of this approximately 25-layer lattice of 0.48 mmbeads. These two layers have essentially the same hex-agonal structure with a lattice constant of ~0.48 mm (asmeasured from the SEMs).

The procedure demonstrated here is remarkable for itssimplicity, for its effectiveness in forming crystalline assem-blies of mesoscale particles over relatively large areas, andfor its superior control over the thickness of the multilayersit forms. We could easily control the number of layers bychanging the ratio between the thickness of the cell and thediameter of the particles. Figure 3 shows SEMs of crystal-line assemblies of polystyrene beads with different num-bers of layers. Figure 3a shows the SEM of a monolayerformed from 1.05 mm beads in a 1.2 mm thick cell; Fig-ure 3b shows the SEM of a bilayer formed from 0.48 mmbeads in a 1.2 mm thick cell; and Figure 3c shows the SEMof an approximately 25-layer assembly formed from0.48 mm beads in a 12 mm thick cell. When the number oflayers changed from one to 25, we did not observe anychange in the crystal structure: all assemblies have hex-agonal symmetry. The point defects in Figures 2b and 3bindicate that the adjacent layers are in the configurationfor a cubic close packing rather than a random stacking.Figures 2b and 3c also suggest that the polystyrene beadsformed a cubic-close-packed (ccp) structure that extendsmore than 25 layers along the direction perpendicular tothe surface of the substrate. The ccp structure can also bedescribed as a face-centered cubic (fcc) lattice with the(111) face parallel to the surfaces of the two substrates.[19]

The diameter d of particles that can be directly as-sembled in the cell is determined by the depth h of thechannels: d must be larger than h. The minimum depth hmin

of channels that we were able to obtain depended on thethickness H of the photoresist: for H = 12 mm, hmin »0.4 mm and for H = 1.2 mm, hmin » 0.2 mm. Although it wasdifficult to generate channels with depths less than 400 nmwhen the films of photoresist were thicker than 10 mm, wewere able to form multilayer assemblies from polymer

Communications

Fig. 1. Schematic outline of the experimental procedure. Aqueous disper-sions of polystyrene beads are injected into the cell through the rubber tubeusing a syringe. The rate of packing of polymer beads increases as the pres-sure of nitrogen increases.

Adv. Mater. 1998, 10, No. 13 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1998 0935-9648/98/1309-1029 $ 17.50+.50/0 1029

Communications

beads with diameters <400 nm by consecutively injectingdispersions of polymer beads with decreasing sizes. The as-sembled structure of big polymer beads provides a barrierfor the accumulation and assembly of small polymer beadsinjected afterwards. At a given pressure of N2 gas, the rate

of packing along the direction of solvent flow decreaseddramatically as the dimension of polymer beads decreased:for a 12 mm thick and 2 cm wide cell, the rate was ~5, ~0.1,and ~0.05 mm/h for 2.8, 0.48, and 0.20 mm polystyrenebeads, respectively. Figure 3d shows the top view SEM of a

Fig. 2. SEMs of a crystalline assembly of 0.48 mmpolystyrene beads that was formed in a 12 mm-thickcell: a) top view; b) oblique view along a crack.c,d) SEMs of the uppermost and the lowest layersof this approximately 25-layer assembly. The sam-ple was sputtered with gold before imaging bySEM. The 1.2 mm-thick films of photoresist (AZ1512) were exposed through the two masks for 20 sand 1.5 s, respectively.

Fig. 3. SEMs of crystalline assemblies of polystyrene beadswith different diameters d formed in cells with differentthicknesses H: a) a monolayer, b) a bilayer, and c) an ap-proximately 25-layer multilayer. d) Top-view SEM of theboundary between two hcp structures assembled in a12 mm thick cell from 0.48 mm (top right) and 0.20 mm(bottom right) polystyrene beads.

1030 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1998 0935-9648/98/1309-1030 $ 17.50+.50/0 Adv. Mater. 1998, 10, No. 13

portion of the boundary between the assembled structuresof 0.48 mm and 0.20 mm polystyrene beads. In this case, theaqueous dispersion of 0.48 mm beads was injected first toform a crystalline lattice that subsequently acted as a bar-rier for the accumulation and assembly of 0.20 mm beadsinjected afterwards. The 0.20 mm beads also formed a sin-gle crystalline structure when they were a few particlesaway from the boundary. The smallest particles that wehave been able to assemble over relatively large areas usingthis procedure are polystyrene beads of ~60 nm diameter;the assembled structures of these nanoparticles still need tobe characterized.[20]

In summary, we have demonstrated a convenient andversatile route to crystalline assemblies of mesoscale parti-cles with domain sizes as large as 12 mm ´ 0.5 cm ´ 2 cm.Further study is under way, including characterization ofmultilayers of polystyrene beads by confocal microscopyand formation of crystalline assemblies from a binary mix-ture of polymer beads with two different sizes (as in ioniccrystals) and/or with different compositions. Although ourpresent demonstration only focused on monodisperse poly-styrene beads with diameters of 0.20 mm and more, we be-lieve that this procedure should be extendible to a varietyof other mesoscale particles (e.g., colloids of silica, TiO2,and metals; and quantum dots of semiconductors) with awide range of sizes.[21] By adjusting the thickness of the cell,we can obtain crystalline monolayers or multilayers (withspecific numbers of layers) of these materials that are po-tentially useful in fabricating photonic and electronic de-vices. Moreover, the top substrate of the cell can have pat-terned relief structures on its surface to give assemblies ofmesoscale particles with different surface topologies; thebottom substrate can also be patterned with appropriate ar-rays of holes to control the crystallographic structure of theassembled crystals.[22]

Experimental

A cell was constructed from two glass substrates and a square frame ofphotoresist, and tightened with binder clips. One side of the square framehad channels that could retain the particles while letting the solvent flowthrough. The thickness H of the cell is determined by the thickness of thephotoresist film, which can be easily changed from ~0.5 to ~50 mm by usingdifferent photoresist solutions and/or different speeds for spin-coating. Asmall hole (~3 mm in diameter) was generated in the top glass substrate byetching in an aqueous HF solution, and a glass tube (~6 mm in diameter)was attached to this hole using an epoxy adhesive. A 12 mm thick film ofphotoresist (Shipley 1075) was spin-coated on the bottom substrate and ex-posed to UV light for ~67 s through a mask with a test pattern of a squareframe (~2 ´ 2 cm2 in area, line width ~50 mm) on its surface. This photoresistfilm was then exposed to UV light for an additional ~5 s through anothermask covered with a test pattern of parallel lines (500 mm in width and sepa-rated by 100 mm). The two masks were aligned such that the parallel lineson the second mask overlapped with only one side of the square frame.After developing, a square frame of photoresist was formed on the surfaceof the bottom substrate; one side of the frame had an array of shallowtrenches in its surface that subsequently served as channels for the flow ofsolvent.

Received: March 19, 1998Final version: May 3, 1998

±[1] S. Hayashi, Y. Kumamoto, T. Suzuki, T. Hirai, J. Colloid Interface Sci.

1991, 144, 538.[2] C. B. Roxlo, H. W. Deckman, J. Gland, S. D. Cameron, R. R. Chianel-

li, Science 1987, 235, 1629. M. C. Buncick, R. J. Warmack, T. L. Fer-rell, J. Opt. Soc. Am. B 1987, 4, 927. F. Lenzmann, K. Li, A. H. Kitai,H. D. H. Stöver, Chem. Mater. 1994, 6, 156. F. Burmeister, C. Schäfle,T. Matthes, M. Böhmisch, Langmuir 1997, 13, 2983. J. Boneberg, F.Burmeister, C. Schäfle, P. Leiderer, D. Reim, A. Fery, S. Herminghaus,Langmuir 1997, 13, 7080. C. Padeste, S. Kossek, H. W. Lehmann, C. R.Musil, J. Gobrecht, L. Tiefenaur, J. Electrochem. Soc. 1997, 143, 3890.

[3] M. D. Sacks, T.-Y. Tseng, J. Am. Ceram. Soc. 1984, 67, 526. P. Calvert,Nature 1985, 317, 201.

[4] O. D. Velev, T. A. Jede, R. F. Lobo, A. M. Lenhoff, Nature 1997, 389,447.

[5] J. H. Holtz, S. A. Asher, Nature 1997, 389, 829.[6] P. L. Flaugh, S. E. O'Donnell, S. A. Asher, Appl. Spectrosc. 1984, 38,

847. S.-Y. Chang, L. Liu, S. A. Asher, J. Am. Chem. Soc. 1994, 116,6739. J. M. Weissman, H. B. Sunkara, A. S. Tse, S. A. Asher, Science1996, 274, 959.

[7] I. I. Tarhan, G. H. Watson, Phys. Rev. Lett. 1996, 76, 315.[8] The method based on solvent evaporation: N. D. Denkov, O. D. Velev,

P. A. Kralchevsky, I. B. Ivanov, H. Yoshimura, K. Nagayama, Lang-muir 1992, 8, 3183; C. D. Dushkin, K. Nagayama, T. Miwa, P. A. Kral-chevsky Langmuir 1993, 9, 3695; G. S. Lazarov, N. D. Denkov, O. D.Velev, P. A. Kralchevsky, K. Nagayama, J. Chem. Soc., Faraday Trans.1994, 90, 2077; A. S. Dimitrov, K. Nagayama, Langmuir 1996, 12,1303; S. Rakers, L. F. Chi, H. Fuchs, Langmuir 1997, 13, 7121.

[9] The method based on the Langmuir film technique: K.-U. Fulda, B.Tieke, Adv. Mater. 1994, 6, 288; M. Kondo, K. Shinozaki, L. Bergström,N. Mizutani, Langmuir 1995, 11, 394.

[10] The method based on sedimentation: R. Mayoral, J. Requena, J. S.Moya, C. López, A. Cintas, H. Miguez, F. Meseguer, L. Vµzquez, M.Holgado, A. Blanco, Adv. Mater. 1997, 9, 257; L. N. Donselaar, A. P.Philipse, J. Suurmond, Langmuir 1997, 13, 6018; H. Miguez, F. Mese-guer, C. López, A. Mifsud, J. S. Moya, L. Vµzquez, Langmuir 1997, 13,6009.

[11] The method based on electrophoretic deposition: M. Giersig, P. Mul-vaney, Langmuir 1993, 9, 3408; M. Trau, D. A. Saville, I. A. Aksay, Sci-ence 1996, 272, 706; S.-R. Yeh, M. Seul, B. I. Shraiman, Nature 1997,386, 57; Y. Solomentsev, M. Böhmer, J. L. Anderson, Langmuir 1997,13, 6058.

[12] The method based on electrostatic interaction: N. Ise, Angew. Chem.Int. Ed. Engl. 1986, 25, 323; H. B. Sunkara, J. M. Jethmalani, W. T.Ford, Chem. Mater. 1994, 6, 362; C. A. Murray, D. G. Grier, Am. Sci.1995, 83, 238; A. E. Larsen, D. G. Grier, Nature 1997, 385, 230.

[13] Other methods: M. M. Burns, J.-M. Fournier, J. A. Golovchenko, Sci-ence 1990, 249, 749; E. Kim, Y. Xia, G. M. Whitesides, Adv. Mater.1996, 8, 245; S. Neser, C. Bechinger, P. Leiderer, T. Palberg, Phys. Rev.Lett. 1997, 79, 2348.

[14] The methodology described here seems to be a general one for produc-ing crystalline assemblies of mesoscale particles; this procedure can beapplied to aqueous dispersions of a wide range of particles regardless oftheir compositions and surface properties (charge densities and chemi-cal groups). We have successfully applied this method to aqueousdispersions of polystyrene beads with different densities of carboxylicgroups on their surfaces, polystyrene beads coated with silver, silicacolloids, and polycarbonate beads. S. H. Park, Y. Xia, unpublished.

[15] The particles were assembled into the crystalline structure by a mecha-nism similar to sedimentation. In our case, the particles were concen-trated at the bottom of the cell by a hydrodynamic flow of the solvent.The continuous sonication used here provided a kind of ªmechanicalshakingº that forced the particles to settle down in the most stablethermodynamic stateÐa close-packed structure.

[16] S. H. Park, Y. Xia, Chem. Mater., in press.[17] A. T. Skjeltorp, P. Meakin, Nature 1988, 335, 424.[18] M. K. Chaudhury, G. M. Whitesides, Science 1992, 255, 1230.[19] We also characterized the 25-layer crystals by optical diffraction. The

observed positions of the first-order Bragg diffraction peaks could beprecisely calculated using a model based on a fcc lattice with the (111)face parallel to the surfaces of the two glass substrates. S. H. Park, Y.Xia, unpublished.

[20] The crystalline assemblies made from polystyrene beads as small as~150 nm still show iridescent colors in the visible region, but we havenot been able to obtain good SEM images from the assemblies madefrom particles smaller than ~150 nm due to limitations arising fromthe resolving power of our scanning electron microscope.

Communications

Adv. Mater. 1998, 10, No. 13 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1998 0935-9648/98/1309-1031 $ 17.50+.50/0 1031

Communications

[21] See, for example, E. Matijevic, Chem. Mater. 1993, 5, 412; N. A. M.Verhaegh, A. van Blaaderen, Langmuir 1994, 10, 1427; C. B. Murray,C. R. Kagan, M. G. Bawendi, Science 1995, 270, 1335.

[22] A. van Blaaderen, R. Ruel, P. Wiltzius, Nature 1997, 385, 321.

Monodisperse Ferromagnetic Particles forMicrowave Applications

By Philippe Toneguzzo, Guillaume Viau, Olivier Acher,Françoise FiØvet-Vincent, and Fernand FiØvet*

Ferromagnetic metal-based materials display propertiesthat make them of interest for microwave applications,namely higher working frequencies and a broader workingfrequency band than bulk ferrimagnetic oxides. As far asmicrowave absorbing properties are concerned, metalshave to be used as fine particles dispersed in an insulatingmatrix. Such composite magnetic materials exhibit mag-netic losses (characterized by a non-zero imaginary part ofthe permeability) in the microwave range due to a gyro-magnetic resonance phenomenon, their microwave proper-ties depending on both the intrinsic characteristics of theparticles and their volume concentration. The influence ofthe latter can be quite well described by mixture laws de-rived from the Bruggeman effective medium theory.[1,2]

Less studied is the control of microwave properties ofcomposite materials by altering the intrinsic properties ofthe magnetic particles. Two main objectives can be defined:first, the design of high-permeability composite materialswith, in particular, optimal control of the resonance width;secondly, a better understanding of the dynamic propertiesof fine particles and a tentative correlation with their staticmagnetic properties. In both cases, control of the morphol-ogy of the ferromagnetic particles is needed since the gyro-magnetic resonance is highly dependent on the particleshape through the effect of the demagnetizing field. There-fore, materials made up of particles with poorly definedshapes present a very broad resonance band. Moreover,materials made up of too large particles present only aweak resonance.[3,4]

The polyol process,[5,6] which is known for providingmonodisperse fine metal particles, afforded us the opportu-nity to synthesize ferromagnetic metal particles smallerthan 2 mm and to investigate their dynamic properties. Ourfirst results provided evidence of the effect of particle sizeon microwave properties in the 2±0.2 mm range.[7,8] The

scope of this paper is to show how it has been possible re-cently to reduce and to control the diameter of such mono-disperse particles down to the nanometer size range forvarious compositions and therefore to study the influenceof the particle size upon the microwave permeability ofmonodisperse powders made up of quasi-spherical particleswith a size range varying over two orders of magnitude(2.5 mm±25 nm).

Polymetallic fine particles CoxNi(100±x) and Fez[Cox-Ni(100±x)](1±z) were synthesized by precipitation from metal-lic precursors dissolved in 1,2-propanediol with an opti-mized amount of sodium hydroxide according to apreviously published procedure [9±11] (see Experimentalsection). Upon heating, as both CoII and NiII are quantita-tively reduced by the polyol itself, the Co/Ni ratio in themetallic CoxNi(100±x) powders depends only on their initialratio. For iron-based particles of Fez[CoxNi(100±x)](1±z) com-position, Fe is generated by disproportionation of FeII

whereas CoII and NiII are quantitatively reduced. The dis-proportionation of FeII allows polymetallic powders to besynthesized with z varying in the range 0±0.25. The accu-rate and reproducible control of the mean diameter of theparticles can be achieved in the submicrometer and nano-meter size range by heterogeneous nucleation for bothCoxNi(100±x) and Fez[CoxNi(100±x)](1±z) systems. The seedingof the reaction medium is achieved by using K2PtCl4 orAgNO3 as a nucleating agent. The precious-metal cationsare easily reduced and generate in situ numerous tiny metalparticles, which then act as suitable sites for the furthergrowth of the ferromagnetic metals. The final particleshave remarkable morphological characteristics: an almostspherical shape, a narrow size distribution (the standard de-viation being less than 15 % of the mean diameter), and alimited agglomeration, as exemplified in Figure 1. Theaverage diameter of the particles decreases as the molar ra-tio [MP]/[Co + Ni + Fe] increases (MP = Pt or Ag) in therange 10±6±10±4.

When Pt is used as a nucleating agent, a simple modelcan be used to account for this accurate control of the sizeof the final particles through the concentration of the nu-cleating agent.[6] This model is based on two hypotheses.First, one considers that the mean size of the precious met-al nuclei does not depend on the cationic concentration ofthe seeding solution. Second, it is supposed that the subse-quent growth of the ferromagnetic metals occurs for thewhole reaction time by a stepwise addition mechanismatom-by-atom rather than by coalescence of primary parti-cles; in other words, each precious metal nucleus generatesone final particle. From these starting hypotheses, a linearrelationship between the mean diameter dm and the inverseof the cubic root of the molar ratio [PM]/[Co + Ni + Fe]can be inferred according to Equation 1, where MFeCoNi,rFeCoNi, MPM, and rPM are the molecular weight and den-sity of the polymetallic powder FeCoNi and of the preciousmetal, respectively, VPM being the volume of a nucleus ofthe precious metal.

±

[*] Prof. F. FiØvet, Dr. G. Viau, F. FiØvet-VincentLaboratoire de Chimie des MatØriaux DivisØs et CatalyseUniversitØ Paris 7-Denis Diderot2 Place Jussieu, F-75251 Paris Cedex 05 (France)

Dr. P. Toneguzzo, Dr. O. AcherCEA, Centre d'Etudes du Ripault, DMAT/CF/MMHBP 16, F-37260 Monts (France)

1032 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1998 0935-9648/98/1309-1032 $ 17.50+.50/0 Adv. Mater. 1998, 10, No. 13