Instability of Ag nanoparticles in SiO2 at ambient conditions

The room temperature stability of nanometer-sized silver clusters in silica matrices has been investigated by following the temporal evolution of their surface plasmon absorption. Ag clusters in SiO2 were prepared by either annealing silica samples doped with atomic silver or by co-deposition of preformed clusters of defined size. Clusters were found to be unstable at ambient conditions on a time scale of days to weeks, independent of preparation conditions. The disappearance of the plasmonic resonance is explained by successive oxidation of the clusters even inside the matrix

Enhanced magnetic field sensitivity of spin-dependent transport in cluster-assembled metallic nanostructures

The emerging field of spintronics explores the many possibilities offered by the prospect of using the spin of the electrons for fast, nanosized electronic devices. The effect of magnetization acting on a current is the essence of giant or tunnel magnetoresistance. Although such spintronics effects already find technological applications, much of the underlying physics remains to be explored. The aim of this article is to demonstrate the importance of spin mixing in metallic nanostructures. Here we show that magnetic clusters embedded in a metallic matrix exhibit a giant magnetic response of more than 500% at low temperature, using a recently developed thermoelectric measurement. This method eliminates the dominating resistivity component of the magnetic response and thus reveals an intrinsic spin-dependent process: the conduction-electron spin precession about the exchange field as the electron crosses the clusters, giving rise to a spin-mixing mechanism with strong field dependence. This effect appears sensibly only in the smallest clusters, that is, at the level of less than 100 atoms per cluster

Interaction Effects in Dilute Cluster-Assembled Magnetic Nanostructures

We have prepared dilute nanostructured magnetic samples by co-deposition of preformed Cobalt clusters with a narrow size distribution around 40 atoms/cluster in Silver matrices. Magnetoresistance measurements are used to derive information about the magnetic structure of the samples. Effects of cluster size distribution or anisotropy can be neglected in our samples. Deviations from simple Langevin-type magnetization are observed as a function of temperature and identified as due to inter-cluster interactions. Pairwise magnetostatic and indirect exchange interactions as well as the model of interacting superparamagnets are found not to be adequate to explain the observed temperature dependences. We propose an interpretation as correlated spin glass, which shows that for small clusters spin glass behavior can be observed even at high dilutions

Combined fitting of alternative and direct susceptibility curves of assembled nanostructures

International audienceExperimental ac-susceptibility curves at different frequencies (0.1 Hz ≤ f ≤ 1 kHz) were performed on samples prepared by physical and chemical pathways. By combining the triple fit method and a careful analysis of ac-experimental curves, we demonstrate an unambiguous and consistent determination method of both the magnetic particle size distribution and anisotropy for diluted granular nanostructures of magnetic clusters. Specifically, we highlight the importance of the size distribution in the determination of the magnetic anisotropy constant as well as the low relevance of the deduced parameters by considering alternative measurements alone