796 research outputs found
Étude de l'activité phosphatasique particulaire au sein d'un écosystème pollué : le port de Toulon
L'activité phosphatasique a été mesurée sur le matériel particulaire obtenu par filtration d'eau de mer sur des membranes de 90 µm, 5 µm et 0,25 µm de vide de maille, de décembre 1999 à mars 2000. Le substrat utilisé est du paranitrophénylphosphate (pNPP) dissous dans l'eau de mer. Dans ces conditions, deux types d'activités, à faible et à forte affinités, ont été caractérisés pour chaque classe de taille. La contribution de la classe de taille comprise entre 0,25 et 5 µm à l'ensemble de l'activité a été la plus faible des trois fractions, alors que celle de la classe de taille supérieure à 90 µm a souvent été la plus forte. Des activités associées à la présence de bactéries ont été mises en évidence sur les fractions zooplanctonique et phytoplanctonique. Toutefois, celles-ci n'ont pu rendre compte de la totalité des activités mesurées, en particulier pour le zooplancton.Phosphatase activity was estimated on particulate material resulting from filtration of sea water on 90, 5 and 0.25 µm membranes, from December 1999 to March 2000 in Toulon seaport. Para-nitrophenylphosphate (pNPP) dissolved in seawater was used as substrate. In these conditions, activities with low and high affinities were disclosed on each size class. The contribution of the 0.25-5 µm fraction was low, whereas the activity of the size class superior to 90 µm was elevated. Enzyme activities of fixed bacteria were characterized in the zooplanktonic and phytoplanktonic fractions. However, they cannot explain the totality of the measured activities in particular for zooplankton
Etude du milieu terrestre des atolls de la Polynésie française : caractéristiques et potentialités agricoles
Cet article présente les principales caractéristiques des sols des atolls et des récifs de la Polynésie française. Les auteurs discutent également les problèmes de la mise en valeur des sols sur ces îlots coralliens et illustrent les différentes techniques culturales utilisée
Optical control of the spin state of two Mn atoms in a quantum dot
We report on the optical spectroscopy of the spin of two magnetic atoms (Mn)
embedded in an individual quantum dot interacting with either a single
electron, a single exciton and single trion. As a result of their interaction
to a common entity, the Mn spins become correlated. The dynamics of this
process is probed by time resolved spectroscopy, that permits to determine the
optical orientation time in the range of a few tens of . In addition, we
show that the energy of the collective spin states of the two Mn atoms can be
tuned through the optical Stark effect induced by a resonant laser field
Dynamic binding of driven interfaces in coupled ultrathin ferromagnetic layers
We demonstrate experimentally dynamic interface binding in a system
consisting of two coupled ferromagnetic layers. While domain walls in each
layer have different velocity-field responses, for two broad ranges of the
driving field, H, walls in the two layers are bound and move at a common
velocity. The bound states have their own velocity-field response and arise
when the isolated wall velocities in each layer are close, a condition which
always occurs as H->0. Several features of the bound states are reproduced
using a one dimensional model, illustrating their general nature.Comment: 5 pages, 4 figures, to be published in Physical Review Letter
Magnetic domain structure and dynamics in interacting ferromagnetic stacks with perpendicular anisotropy
The time and field dependence of the magnetic domain structure at
magnetization reversal were investigated by Kerr microscopy in interacting
ferromagnetic Co/Pt multilayers with perpendicular anisotropy. Large local
inhomogeneous magnetostatic fields favor mirroring domain structures and domain
decoration by rings of opposite magnetization. The long range nature of these
magnetostatic interactions gives rise to ultra-slow dynamics even in zero
applied field, i.e. it affects the long time domain stability. Due to this
additionnal interaction field, the magnetization reversal under short magnetic
field pulses differs markedly from the well-known slow dynamic behavior.
Namely, in high field, the magnetization of the coupled harder layer has been
observed to reverse more rapidly by domain wall motion than the softer layer
alone.Comment: 42 pages including 17 figures. submitted to JA
Spin Pumping and Inverse Spin Hall Effect in Platinum: The Essential Role of Spin-Memory Loss at Metallic Interfaces
Through combined ferromagnetic resonance, spin-pumping and inverse spin Hall
effect experiments in Co|Pt bilayers and Co|Cu|Pt trilayers, we demonstrate
consistent values of spin diffusion length
nm and of spin Hall angle for Pt. Our
data and model emphasize on the partial depolarization of the spin current at
each interface due to spin-memory loss. Our model reconciles the previously
published spin Hall angle values and explains the different scaling lengths for
the ferromagnetic damping and the spin Hall effect induced voltage.Comment: 6 pages, 3 figures (main text) and 8 pages supplementary. Published
with small modifications in Phys. Rev. Let
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
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