Using a new micro-SQUID setup, we investigate magnetic anisotropy in a single 1000-atom cobalt cluster. This system opens new fields in the characterization and understanding of the origin of magnetic anisotropy in such nanoparticles. For this purpose, we report three-dimensional switching field measurements performed on a 3 nm cobalt cluster embedded in a niobium matrix. We are able to separate the different magnetic anisotropy contributions and evidence the dominating role of the cluster surface.

Optical properties of mixed clusters Ni/Ag, Co/Ag, and Ni/Au of 2–5 nm in diameter, produced by laser vaporization and embedded in an alumina matrix, are investigated. The first part is devoted to $({\mathrm{Ni}}_x{\mathrm{Ag}}_{1-x}{)}_n$ clusters whose photoabsorption spectra reveal a surface plasmon resonance, damped, broadened, and blueshifted as compared to pure silver clusters. For a given mean size, a blueshift of the resonance band as well as a damping and broadening with increasing nickel proportion is observed, in good qualitative agreement with classical predictions, assuming a Ni-core/Ag-shell geometry. This core-shell structure is confirmed by low energy ion spectroscopy measurements showing that the cluster surface is essentially composed of silver. For a given composition, the size evolution of the plasmon band consists of a damping and broadening with decreasing size whereas no clear shift is noticed. Although the classical predictions, including the surface scattering limited mean free path contribution of the conduction electrons in the silver shell, account well for the size effects, inhomogeneous effects (size, shape, and local porosity) also contribute to the broadening and damping of the resonance in experiment. The second part concerns the optical properties of $({\mathrm{Co}}_{0.5}{\mathrm{Ag}}_{0.5}{)}_n$ and $({\mathrm{Ni}}_{0.5}{\mathrm{Au}}_{0.5}{)}_n$ clusters. The size evolution of the optical properties of $({\mathrm{Co}}_{0.5}{\mathrm{Ag}}_{0.5}{)}_n$ clusters is similar to that of $({\mathrm{Ni}}_{0.5}{\mathrm{Ag}}_{0.5}{)}_n$ clusters, with in addition, a weak blueshift of the resonance with decreasing size. As for the $({\mathrm{Ni}}_{0.5}{\mathrm{Au}}_{0.5}{)}_n$ clusters, their optical spectra do not display a marked resonance. The comparison with a core-shell model including a size dependent damping constant in the gold or silver shell gives a good understanding of these features for the two systems Co/Ag and Ni/Au.

The magnetic measurements on single cobalt and iron nanoclusters containing almost 1000 atoms are presented. Particles are directly buried within the superconducting film of a micro-SQUID (superconducting quantum interference device) which leads to the required sensitivity. The angular dependence of the switching field in three dimensions turns out to be in good agreement with a uniform rotation of cluster magnetization. The Stoner and Wohlfarth model yields therefore an estimation of magnetic anisotropy in a single cluster. In particular, uniaxial, biaxial, and cubic contributions can be separated. Results are interpreted on the basis of a simple atomic model in which clusters are assimilated to “giant spins.” We present an extension of the Néel model to clusters in order to estimate surface anisotropy. In the case of cobalt, this last contribution dominates and numerical simulations allow us to get the morphology of the investigated clusters.

We report on the synthesis of ${\mathrm{C}}_{2n-q}{\mathrm{Si}}_q$ clusters with $2n=32-100$ and $q<\mathrm{}4\mathrm{}$. The analysis of the abundance distribution and photofragmentation spectra provides clear evidences that such clusters remain in the fullerene geometry and that the Si atoms are located close to each other in the fullerene network. These results are corroborated by ab initio calculations within the density functional theory. The stability and electronic properties of these heterofullerenes are discussed.