Atomic Organization In Magnetic Bimetallic Nanoparticles: An Experimental And Theoretical Approach
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Atomic Organization In Magnetic Bimetallic Nanoparticles : An Experimental And Theoretical Approach
Marie-Claire Fromen, Samuel Dennler, Marie-José Casanove, Pierre Lecante, Joseph Morillo and Pascale Bayle–Guillemaud1 CEMES, CNRS, 29 rue J. Marvig, B.P.4347, 31055 Toulouse cedex, France 1 CEA, DRFMC, 17 rue des Martyrs, 38054 Grenoble, France ABSTRACT Ultrafine bimetallic CoRh nanoparticles synthesized by a soft chemical route with compositions ranging from pure cobalt to pure rhodium are investigated using high-resolution and energy filtering transmission electron microscopy techniques as well as wide angle x-ray scattering. In parallel, they are simulated with the use of an n-body semi-empirical interaction model: quenched molecular dynamics and Monte-Carlo Metropolis simulated annealing are performed on these nanoparticles in order to find their most stable isomers as a function of composition and size. A progressive evolution from an original polytetrahedral structure to the face-centered cubic structure with increasing Rh content is observed in these particles. Strong tendency to Co surface segregation is both experimentally evidenced and confirmed by the simulations.
INTRODUCTION Nanometer-sized metallic particles are well known to exhibit unique strong size-related magnetic effects [1]. Moreover, alloying with a 3d metal is an effective way to induce spin polarization in 4d metals and to obtain at the same time a large magnetic moment with a high anisotropy. Magnetic 3d-4d bimetallic nanoparticles then present a particular interest. Indeed, a strong enhancement of the saturation magnetization in ultrafine CoRh particles (in the 1.6 to 2 nm range) compared to the predicted value in the bulk alloy was recently demonstrated [2] and later confirmed by density functional theory calculations on much smaller particles [3]. This motivates a systematic study of the atomic organization in these CoRh alloyed particles, about 2 nm large, synthesized by co-decomposition of two organometallic precursors under dihydrogen in the presence of a polymer (the polyvinylpyrrolidone, or PVP polymer). In the bulk, the CoRh alloy forms a continuous series of solid solutions and crystallizes in the hexagonal close-packed (hcp) structure below 50.5 at % Rh and in the face-centered cubic structure (fcc) for higher Rh contents. However, size reduction is likely to modify the phase diagram of this bimetallic alloy. Indeed, pure cobalt particles synthesized by the same chemical route, were seen to exhibit a very original polytetrahedral structure [4]. Besides, in such fine particles, the very high ratio of surface atoms against volume atoms considerably increases the surface effects. Preferential surface segregation of one of the chemical species, usually neglected in macroscopic samples, can here drastically modify the chemical distribution inside the particles. In order to investigate both the structural and chemical organization in these ultrafine materials, a wide range of experimental and modeling techniques have been imple
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