Alexandre Tamion

Accurate determination of the magnetic anisotropy in cluster-assembled nanostructures

The simultaneous triple adjustment of experimental magnetization curves under different conditions is shown to allow the unambiguous and consistent determination of both the magnetic particle size distribution and anisotropy for granular nanostructures of Co clusters embedded in protective matrices. The importance of interface effects resulting in magnetically dead layers is demonstrated.

Functional nanostructures from clusters

Functional cluster-assembled nanostructures with original structures and properties are prepared using the Low Energy Cluster Beam Deposition method (LECBD). This technique consists in depositing supersonic clusters produced in the gas phase using a combined laser vapourisation-inert gas condensation source. Low energy clusters with typical sizes ranging from ∼1 to a few nm are not fragmented upon impact on the substrate (soft landing regime) leading to the formation of cluster-assembled nanostructures which retain the original structures and properties of the incident free clusters. Model nanostructured systems of any kind of materials (metallic, covalent, oxides) well suited for fundamental studies in various fields (nanoelectronics, nanomagnetism, nanophotonics, catalysis or nanobiology) and for applications to very high integration-density devices (∼Tbits/in/²) are prepared using this method. After a brief review of techniques to produce, analyse, mass select, and deposit clusters in the LECBD-regime, the specific aspects of the nucleation and growth process which govern the formation of cluster-assembled nanostructures on the substrate are presented, especially the preparation of 2D-organised arrays of cluster-assembled dots by depositing low energy clusters on FIB-functionalised substrates. Characteristic examples of cluster systems prepared by LECBD are also described: i) metallic (Au, Ag, Au-Ag, Ag-Ni, Ag-Pt)) and oxide (Gd2O3, ZnO) cluster-assembled nanostructures for applications to linear and non linear nano-optics; ii) magnetic nanostructures from Co-based nanoclusters (i.e., Co-Pt) exhibiting a high magnetic anisotropy which is well suited for applications to high density data storage devices; iii) gold or Pd-Pt or Au-Ti clusters for chemical reactivity and catalysis applications. In some specific cases, we were able to perform studies from an isolated individual nanocluster up to 2D or 3D-collections of non-interacting or interacting particles leading to a rather good understanding of the intrinsic as well as the collective properties at nanoscale.

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.

Demixing in cobalt clusters embedded in a carbon matrix evidenced by magnetic measurements

We report on the magnetic properties of cobalt clusters embedded in amorphous carbon using magnetic and structural investigations. From the analysis of the mFC, mZFC, and m(H) curves, we determine the Co magnetic diameter probability density function. An initially magnetically dead interface layer is attributed to a metastable carbide. We found that annealing at 750 K favors the graphitization of the matrix and removes the dead layer without deteriorating the nanoparticle size distribution or changing the magnetic anisotropy constant.

Coalescence-free L10 ordering of embedded CoPt nanoparticles

We have synthesized diluted samples of CoPt clusters and investigated by various techniques the effect of a 2 h anneal at 750 K. Transmission electron microscopy observations have put into evidence L10 chemical ordering without any detectable coalescence upon annealing. Magnetic measurements on CoPt clusters embedded in amorphous carbon have been used to provide additional and stronger evidence of the absence of coalescence. We show how the analysis of normalized ZFC curves can be used as a convenient and powerful technique to detect subtle variations in the cluster size distribution.

Microwave-Assisted Magnetization Reversal in Individual Isolated Clusters of Cobalt

At the nanometric range, the superparamagnetic state has become a crucial issue in fundamental research as well as practical applications. In both cases, technological improvements require understanding dynamical magnetization reversal processes at the nanosecond time scales. In this paper, we present the first magnetization reversal measurements performed on a single cobalt cluster (counting only 1000 spins) using the micro-SQUID technique by applying a constant magnetic field combined with a radio-frequency field pulse. First of all, we present the different technical steps necessary to detect the magnetic reversals at low temperature (<i>T</i>=35&nbsp;mK) of a well-defined nanoparticle prepared by low-energy clusters beam deposition. We previously showed that the 3-D switching Stoner-Wohlfarth astroid represents the magnetic anisotropy of the nanoparticle. Then, an improved device, coupled with a gold stripe line, allow us to reverse such a macrospin using an RF pulse. A qualitative understanding of the magnetization reversal by nonlinear resonance has been obtained with the Landau-Lifschitz-Gilbert equation.

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.

Ferritin surplus in mouse spleen 14 months after intravenous injection of iron oxide nanoparticles at clinical dose

In this study, we followed the biodegradation of ultra-small superparamagnetic iron oxide nanoparticles injected intravenously at clinical doses in mice. An advanced fitting procedure for magnetic susceptibility curves and low-temperature hysteresis loops was used to fully characterize the magnetic size distribution as well as the magnetic anisotropy energy of the injected P904 nanoparticles (Guerbet Laboratory). Additional magnetometry measurements and transmission electronic microscopy observations were systematically performed to examine dehydrated samples from the spleen and liver of healthy C57B16 mice after nanoparticle injection, with sacrifice of the mice for up to 14 months. At 3 months after injection, the magnetic properties of the spleen and liver were dramatically different. While the liver showed no magnetic signals other than those also present in the reference species, the spleen showed an increased magnetic signal attributed to ferritin. This surplus of ferritin remained constant up to 14 months after injection.

Signature of multimers on magnetic susceptibility curves for mass-selected Co particles

Even if efforts are currently made to produce nanoparticle samples by deposition of preformed clusters with a size dispersion as low as possible, the incident particle size distribution is necessarily degraded because of the statistical formation of multimers. Here we study diluted Co cluster samples synthesized by mass-selected low energy cluster beam deposition. Transmission electron microscopy is used to determine the cluster size distribution and, in particular, the proportion of dimers. We then show how multimers can have a strong impact on magnetic measurements even if they constitute only a small proportion of the total particles. However, a thorough analysis can be used to determine the respective proportion of dimers and trimers just from the magnetization curves. These proportions are found to be in excellent agreement with the model of random cluster deposition.