VOLUME 89, NUMBER 3 PHYSICAL REVIEW LETTERS 15 JULY 2002 Magnetic Enhancement in Nanoscale CoRh Particles David Zitoun, 1 Marc Respaud, 2,3, * Marie-Claire Fromen, 4 Marie José Casanove, 4 Pierre Lecante, 4 Catherine Amiens, 1 and Bruno Chaudret 1 1 LCC, 205 Route de Narbonne, 31077 Toulouse, France 2 LNCMP, 143 avenue de Rangueil, BP 4245, Toulouse, 31432, France 3 LPMC, INSA, 135 avenue de Rangueil, 31077 Toulouse, France 4 CEMES-CNRS, 29 rue Jeanne Marvig, BP 4347, 31077 Toulouse, France (Received 20 July 2001; published 27 June 2002) The influence of size reduction on the magnetism of CoRh has been studied on a system of isolated nanometric spherical bimetallic ultrafine particles embedded in a polymer matrix. Pulsed fields up to 30 T were used in order to approach the magnetic saturation (M S ). Particles with a mean diameter of 1.65 60.1 nm display a value of 2.38m B per CoRh unit strongly enhanced compared to values calculated or measured on a bulk alloy. These results were interpreted as the first evidence of the cooperative role of both alloying and size reduction to the enhancement of M S in this system associating a 3d FM metal with a 4d metal. DOI: 10.1103/PhysRevLett.89.037203 PACS numbers: 75.70.Rf, 61.46.+w, 71.20.Lp One of the most active research topics of the past few years is the investigation of finite size effects in magnetic materials. Technological developments require more and more magnetic nanomaterials of controlled properties tak- ing advantage of their small size. With decreasing the size of magnetic particles occurs a transition from poly domain to single domain systems, and the apparition of superpara- magnetism [1]. The modification of the electronic band structure of magnetic materials of nanometer size, at the border of the molecular and metallic states, induces un- usual magnetic properties. As a consequence, an enhanced magnetic moment is predicted for nanoparticles of 3d fer- romagnetic (FM) metals [2]. Such effects have been first demonstrated in the case of Fe, Co, and Ni metal clusters containing less than 1000 atoms, using molecular beam de- flection measurements in high vacuum [3]. Recently, the case of 4d metal has been addressed. Bulk 4d metals do not display any FM behavior. However, a spin polarization can be induced by a very small perturbation of the lattice parameter, by elaborating layered structures with a FM ma- terial [4], and more efficiently by alloying with a 3d FM metal [5,6]. Size reduction also induces the appearance of FM in species at the border of FM as demonstrated by molecular beam deflection measurements for Rh nanopar- ticles up to 34 atoms [7,8], in agreement with theoretical calculations. As a consequence, one can expect unusual magnetic behaviors in Rh based alloys with 3d FM species, where the size reduction should play an important role on the electronic spin polarization. As far as we know, the magnetism of bimetallic 3d-4d particles in the nanometer range (1– 3 nm) has not been re- ported. The synthesis of bimetallic nanoparticles or clus- ters is far from easy, since it necessitates the control of both the particle stoichiometry and the surface state. Classical chemical methods have been demonstrated to be very ef- ficient for the high yield synthesis of nanomaterials, but with a more or less good control of the surface chem- istry, which may alter or cancel many of the unconven- tional properties resulting from the outer shells. In the past few years, we have developed a new chemical elabo- ration process based on the decomposition in mild condi- tions of an organometallic precursor in the presence of a stabilizing polymer. This approach, applied for the elabo- ration of Co nanoparticles in polyvinylpyrrolidone (PVP), interestingly produces a nanomaterial which displays the same enhancement of magnetic moment per atom as that observed for gas phase aggregates, an enhancement of the anisotropy and an unusual polytetrahedral crystalline struc- ture [9–11]. These results illustrate the weak character of the interaction of the particle surface with this polymer. Furthermore, using a similar approach, bimetallic isolated magnetic particles of Co 12x Pt x of nanometric size and ad- justable composition have been obtained. They evidence a strong influence of the Pt concentration on the anisotropy, in connection with the formation of new crystalline phases [12,13]. In this Letter, we present the first synthesis and magnetic study of bimetallic CoRh nanomaterials and their magnetic characterization. Two systems of well isolated CoRh par- ticles with sizes near 1.6560.1 and 2.360.1 nm, em- bedded in the organic polymer PVP, have been synthesized using the organometallic approach mentioned here above. Their magnetic data clearly demonstrate the bimetallic na- ture of the CoRh nanoparticles, with reduced exchange energy with the nearest neighbors and larger magnetic anisotropy. The main result concerns the saturation mag- netization of this system. Spectacularly, high fields up to 30 T are not sufficient to fully saturate the magnetization of the smaller nanoparticles system which reaches a value twice as large as the predicted bulk value of the CoRh alloy, suggesting a huge influence of the particle size reduction. The particles were synthesized using a chemical pro- cedure based on the simultaneous decomposition of two organometallic precursors Coh 3 -C 8 H 13 h 4 -C 8 H 12  and 037203-1 0031-9007 02 89(3) 037203(4)$20.00 © 2002 The American Physical Society 037203-1