Bimetallic CoRh and CoRu nanoparticles: size induced enhanced magnetisation D. Zitoun a , M. Respaud b , * M.-C. Fromen c , P. Lecante c , M.-J. Casanove c , C. Amiens a , and B. Chaudret a* a Laboratoire de Chimie de Coordination- CNRS, 205 Route de Narbonne, 31077 Toulouse Cedex 04, France b Laboratoire de la Physique de la Matière Condensée, INSA, 135 avenue de Rangueil, 31077 Toulouse, France c Centre d’Elaboration des Matériaux et d’Etudes Structurales- CNRS, BP 4347, 29 rue Jeanne Marvig, 31055 Toulouse Cedex, France Elsevier use only: Received date here; revised date here; accepted date here Abstract The magnetism of bimetallic Co x M 1-x (M : Ru and Rh) clusters has been investigated on systems of nanoparticles with diameters smaller than 2nm embedded in a polymer obtained by means of organometallic synthesis. Whatever the composition (up to x = 0.75), the investigation of magnetism evidence a ferromagnetic behavior. As a consequence of size reduction, both CoRh and CoRu systems display enhanced magnetic moments and anisotropy compared to bulk values. © 2001 Elsevier Science. All rights reserved. Keywords: nanocrystals; nanoparticles; clusters; size effect; polarisation; anisotropy. PACS numbers: 78.67.Bf; 71.20.Be; 75.30.Gw; 75.75.+a * e-mail: respaud@insa-tlse.fr, chaudret@lcc-toulouse.fr. 1. Introduction The modification of the electronic band structure of magnetic materials of nanometer size, at the border of the molecular and metallic states, induces unusual magnetic properties. As a consequence, an enhanced magnetic moment is predicted for nanoparticles of 3d ferromagnetic (FM) metals as a result of surface and structure effects. Such effects have been first demonstrated in the case of Fe, Co and Ni metal clusters containing less than 1000 atoms.[1] Size reduction also induces the appearance of FM as demonstrated by molecular beam deflection measurements for Rh clusters up to 34 atoms although Ru remains non magnetic.[2] These results demonstrate that size reduction promotes in some cases an electronic polarization in species at the border of FM. Moreover, a spin polarization can be induced by a very small perturbation of the lattice parameter, by elaborating layered structures with a FM material and more efficiently by alloying with a 3d FM metal. As a consequence, one can expect unusual magnetic behaviors in 3d FM/4d clusters, where size reduction should play an important role on spin and orbital polarization. The synthesis of bimetallic nanoparticles requires size, composition and surface state controls. In the past few years, we have developed a new chemical elaboration process based on the decomposition in mild conditions of an organometallic precursor in the presence of a stabilizing polymer.[3,4] This route leads to nanoparticles of narrow size distribution that fulfill the requirements stated above, where the by-products of the synthesis do not alter the surface magnetism. This approach has been applied to the case of bimetallic CoRh and CoRu for different leading to ultrafine particles with diameters below 2nm. A FM behavior has been observed in all the cases with a strong enhancement of the magnetic moment.