Micro-SQUID technique for studying the temperature dependence of switching fields of single nanoparticles

An improved micro-SQUID technique is presented allowing us to measure the temperature dependence of the magnetisation switching fields of single nanoparticles well above the critical superconducting temperature of the SQUID. Our first measurements on 3 nm cobalt nanoparticle embedded in a niobium matrix are compared to the Neel Brown model describing the magnetisation reversal by thermal activation over a single anisotropy barrier.Comment: 3 pages, 4 figures; conference proceeding: 1st Joint European Magnetic Symposia (JEMS'01), Grenoble (France), 28th August - 1st September, 200

Structural and magnetic properties of CoPt mixed clusters

In this present work, we report a structural and magnetic study of mixed Co58Pt42 clusters. MgO, Nb and Si matrix can be used to embed clusters, avoiding any magnetic interactions between particles. Transmission Electron Microscopy (TEM) observations show that Co58Pt42 supported isolated clusters are about 2nm in diameter and crystallized in the A1 fcc chemically disordered phase. Grazing Incidence Small Angle X-ray Scattering (GISAXS) and Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) reveal that buried clusters conserve these properties, interaction with matrix atoms being limited to their first atomic layers. Considering that 60% of particle atoms are located at surface, this interactions leads to a drastic change in magnetic properties which were investigated with conventional magnetometry and X-Ray Magnetic Circular Dichro\"{i}sm (XMCD). Magnetization and blocking temperature are weaker for clusters embedded in Nb than in MgO, and totally vanish in silicon as silicides are formed. Magnetic volume of clusters embedded in MgO is close to the crystallized volume determined by GIWAXS experiments. Cluster can be seen as a pure ferromagnetic CoPt crystallized core surrounded by a cluster-matrix mixed shell. The outer shell plays a predominant role in magnetic properties, especially for clusters embedded in niobium which have a blocking temperature 3 times smaller than clusters embedded in MgO

Interface magnetic anisotropy in cobalt clusters embedded in a platinum or niobium matrix

A low concentration of cobalt clusters with a fcc structure and containing almost one thousand atoms are embedded in two different metallic matrices: platinum and niobium. Samples have been prepared using a co-deposition technique. Cobalt clusters preformed in the gas phase and matrix atoms are simultaneously deposited on a silicon substrate under Ultra High Vacuum conditions. This original technique allows to prepare nanostructured systems from miscible elements such as Co/Pt and Co/Nb in which clusters keep a pure cobalt core surrounded with an alloyed interface. Magnetic measurements performed using a Vibrating Sample Magnetometer (VSM) reveal large differences in the magnetic properties of cobalt clusters in Pt and Nb pointing out the key role of cluster/matrix interfaces.Comment: 7 pages (LaTeX), 12 PostScript figures, 1 PostScript tabl

Fluorescent oxide nanoparticles adapted to active tips for near-field optics

We present a new kind of fluorescent oxide nanoparticles with properties well suited to active-tip based near-field optics. These particles with an average diameter in the range 5-10 nm are produced by Low Energy Cluster Beam Deposition (LECBD) from a YAG:Ce3+ target. They are studied by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), classical photoluminescence, cathodoluminescence and near-field scanning optical microscopy (NSOM). Particles of extreme photo-stability as small as 10 nm in size are observed. These emitters are validated as building blocks of active NSOM tips by coating a standard optical tip with a 10 nm thick layer of YAG:Ce3+ particles directly in the LECBD reactor and by subsequently performing NSOM imaging of test surfaces.Comment: Changes made following Referee's comments; added references; one added figure. See story on this article at: http://nanotechweb.org/cws/article/tech/3606

Correlation between the Extraordinary Hall Effect and Resistivity

We study the contribution of different types of scattering sources to the extraordinary Hall effect. Scattering by magnetic nano-particles embedded in normal-metal matrix, insulating impurities in magnetic matrix, surface scattering and temperature dependent scattering are experimentally tested. Our new data, as well as previously published results on a variety of materials, are fairly interpreted by a simple modification of the skew scattering model

Magnetic Anisotropy of a Single Cobalt Nanoparticle

Using a new microSQUID set-up, we investigate magnetic anisotropy in a single 1000-atoms cobalt cluster. This system opens new fields in the characterization and the 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.Comment: 4 pages, 8 figure

Structure and Magnetism of well-defined cobalt nanoparticles embedded in a niobium matrix

Our recent studies on Co-clusters embedded in various matrices reveal that the co-deposition technique (simultaneous deposition of two beams : one for the pre-formed clusters and one for the matrix atoms) is a powerful tool to prepare magnetic nanostructures with any couple of materials even though they are miscible. We study, both sharply related, structure and magnetism of the Co/Nb system. Because such a heterogeneous system needs to be described at different scales, we used microscopic and macroscopic techniques but also local selective absorption ones. We conclude that our clusters are 3 nm diameter f.c.c truncated octahedrons with a pure cobalt core and a solid solution between Co and Nb located at the interface which could be responsible for the magnetically inactive monolayers we found. The use of a very diluted Co/Nb film, further lithographed, would allow us to achieve a pattern of microsquid devices in view to study the magnetic dynamics of a single-Co cluster.Comment: 7 TeX pages, 9 Postscript figures, detailed heading adde

Experimental evidence of a new crystallographic structure of samarium deposited by cluster beam

A new fcc phase of Sm has been obtained by deposition of Sm clusters with the low-energy cluster beam deposition technique. The formation of this new phase corresponds to a commensurable phase which accomodates the large lattice parameter of divalent surface atoms and trivalent core atoms of the supported Sm clusters.Une nouvelle structure de samarium obtenue par dépôt d'agrégats a été observée. Cette structure est du type cubique faces centrées. Dans le cas d'agrégats, les formes particulières de la structure cfc (particules multimaclées, icosaèdre, ...) permettent d'avoir une phase commensurable qui accomode l'importante différence de paramètre de maille entre les atomes de surface divalents et les atomes de coeur trivalents des agrégats déposés

ZnO cluster assembled film with low defect concentration: towards ultra-low power sensors

23-25 mars 2015International audienceSmall, low power sensors and actuators are vital for systems of all kinds to interact with their environment. Interest in gas sensing has been prompted from the need of monitoring our environment and particularly hazardous substances having negative effects on the environment and human welfare. Chemical sensors have been used extensively for the detection of hazardous pollutant gases, combustible gases and organic vapors. In this way, past few decades have found widespread applications for semiconducting metal nano oxides as solid-state gas sensors. The operating principle is based from the dependence of the conductivity through physisorption with the composition and the concentration of the surrounded atmosphere. Among them ZnO is very popular due to its superior reactivity, its non-toxicity, its chemical robustness, a large bandgap and a low synthesis price. However, the natural and often uncontrolled n doping, is a strong barrier for reproducibility results. We report a state of the art ZnO cluster assembled film nearly defect free which exhibit a huge and reproducible reactivity in a limited range of temperature. This perfect control of the defects is the key point for gas sensor upgrading with respect to the sensitivity, the selectivity, the reproducibility, the linearity and the life time. It takes an innovative look for reducing both size and power consumption allowing the integration in complex chips

A comparative theoretical study of the Peierls distortion in type-II alkaline-doped clathrates

We show that a Peierls distortion may occur in expanded metals. When the sodium is expanded it undergoes a symmetry breaking and diatomic molecules are formed. This phenomenon is a result of an electronic instability mechanism. As shown by recent EXAFS experiments, sodium atoms included in chlathrate cages do not stay at the center of the cages. We describe this distortion by two very different methods: an ab initio study based on the local density approximation of the density-functional theory and a tight-binding description. This study is also extended to the other alkaline-doped silicon clathrates. This dimerization process is related to hydrogen dimer existence