Matthias Hillenkamp

Instability of Ag nanoparticles in SiO2 at ambient conditions

The room-temperature stability of nanometre-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 timescale 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.

A nanoparticle replica of the spin-glass state

A simple single-phase material, a random close-packed (volume fraction 67%) ensemble of highly monodisperse bare maghemite (γ-Fe2O3) nanoparticles, is shown to exhibit ideal superspin-glass behavior (mimicking that of model spin-glasses), namely, an unprecedentedly sharp onset of the absorption component of the ac susceptibility, narrow memory dips in the zero-field-cooled magnetization and a spin-glass characteristic field-dependence of the magnetic susceptibility. This ideal behavior is attributed to the remarkably narrow dispersion in particle size and to the highly dense and spatially homogeneous configuration ensured by the random close-packed arrangement. This material is argued to constitute the closest nanoparticle analogue to a conventional (atomic) magnetic state found to date.

Size effects in the magnetic anisotropy of embedded cobalt nanoparticles: from shape to surface

Strong size-dependent variations of the magnetic anisotropy of embedded cobalt clusters are evidenced quantitatively by combining magnetic experiments and advanced data treatment. The obtained values are discussed in the frame of two theoretical models that demonstrate the decisive role of the shape in larger nanoparticles and the predominant role of the surface anisotropy in clusters below 3 nm diameter.

Monodispersed metal clusters in solid matrices: A new experimental setup

We describe a new experimental setup for the production of samples of metal clusters embedded in matrices stable at ambient conditions. The cluster ions are generated in the gas phase and codeposited fragmentation-free together with the evaporated matrix. Mean cluster sizes range from a few to many thousands of atoms. For small clusters (n<20) mass selection is possible; larger clusters are deposited in narrow size distributions. Matrix materials include metals (Cu and Ag) and oxides such as quartz. The performance of the apparatus as well as sample characterization procedures are described. We show first results on the magnetic properties of different cobalt cluster size distributions (⟨n⟩=15,600,2300,6500) embedded in copper matrices, demonstrating intercluster as well as cluster-lattice interactions.

Magnetic Interactions Effects on Magnetic Measurements for Nanoparticle Assemblies

We report on the magnetic properties of 3 nm cobalt clusters embedded in gold matrix using magnetic investigations. From the analysis of the mFC, mZFC and m(H) curves, we determine precisely the Co magnetic diameter Probability Density Function (PDF) in highly diluted samples. We show that even with a 3% vol. concentrations of magnetic particles, the interactions between particles must necessarily be taken into account. For this purpose, we first determine the individual properties of particles in highly diluted samples (1%vol.). In a second step, we increase the concentration in order to retrieve the magnetic interactions signature. Exchange like interactions can be modeled by the formation of magnetic dimers, thus modifying the PDF.

Spin mixing processes in magnetic nanostructures detected by thermoelectric measurements

Spin-dependent transport properties of magnetic nanostructures have been investigated by means of magneto-thermogalvanic voltage measurements: the ac voltage response to an ac temperature oscillation is measured for various magnetic nanostructures under dc current. The samples studied include Co/Cu multilayered nanowires, homogeneous Ni nanowires and cobalt clusters embedded in copper films. The magnetic field dependence of this signal is always larger than the magneto-resistance (MR) and may be anisotropic even when the MR is not. A thermodynamic argument introduces spin mixing as the main process measured by this novel thermoelectric measurement technique. This effect is not observed in magnetite as can be justified by the absence of an accessible second spin channel.