21 research outputs found
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
EXAFS IN CuZn BRASS ALLOYS DURING MARTENSITIC TRANSFORMATION
No abstract availabl
In situ X-ray absorption spectroscopy study of the thermal behaviour of giant magnetoresistance Co
Heterogeneous CoxAg1−x and NixAg1−x alloys have giant
magnetoresistance properties. Such alloys, with atomic concentrations x = 0.20 and 0.37,
were studied by transmission X-ray Absorption Spectroscopy at the Co or Ni K-edge after
in situ thermal annealing. For alloys as-deposited and annealed at 200 °C,
Extended X-ray Absorption Fine Structure analysis displays both Co-Co (Ni-Ni) bonds related
to Co (Ni) atoms agglomerated in magnetic particles and Co-Ag (Ni-Ag) bonds related to Co
(Ni) atoms in substitutional sites in the Ag matrix. At the same alloy concentration, the
miscibility in the Ag matrix is found larger for Ni than for Co. After annealing around
250 °C, the marked decrease of the Ag neighbour peak corresponds to a diffusion of
magnetic atoms outside the Ag matrix. The Co-Co coordination number increases regularly with
annealing temperatures up to 450 °C reflecting a progressive expansion of Co
particles. On the contrary, for Ni alloys, no further particle expansion has been observed
in the same annealing range. This different behaviour may be linked both to the difference
between the as-deposited structural states and to the Co/Ag and Ni/Ag interfaces energies.
Using a simple model, evolution of the mean particle size has been estimated as a function
of annealing
Total Electron Yield Detector Working at Low Temperature for Linear Dichroïsm Studies on Monocrystalline Samples
Electron detection in EXAFS is commonly used at room temperature for thick samples (where transmission
measurements are not possible) or nanostructures with high element concentrations (where fluorescence detection is not
appropriate). Recently, a Total Electron Yield detector with He gas flow at atmospheric pressure working at liquid nitrogen
temperature has been developed and successfully tested on the French CRG/TF beamline at ESRF. At 80 K, the substantial
decrease of the dynamic part of the Debye-Waller (DW) factor enables to record EXAFS signals on a larger k range and hence
to obtain better signal-to-noise ratio of the corresponding Fourier Transform. A variation of the temperature of the sample from
80 to 300 K can be performed to evaluate the vibrational and the structural part of the DW factors. Moreover, in order to get rid
of Bragg peaks in the EXAFS signal in case of monocrystalline samples, a special 360° rotating sample holder has been
designed. Finally, polarization studies for anisotropic systems may be realized by simply rotating the sample holder by 90° from
the in-plane to the out-of-plane polarization sample geometry
Nanostructure of Giant Magnetoresistance Heterogeneous Alloys Ni0.20Ag0.80 After Annealing
Heterogeneous alloys Ni0.20Ag0.80, presenting giant magnetoresistance properties, have been studied by Total Electron Yield X-ray absorption spectroscopy at liquid nitrogen temperature at the Ni K edge from as-deposited to annealed stages. Up to 150°C annealing, the Ni atoms are mainly in small poorly ordered agglomerates and the local Ni atomic environment is very stable and disordered. Up to 250°C annealing, some Ni atoms occupy substitutional sites in the Ag matrix and 40 to 50 % of the Ni nearest neighbors are Ag atoms. After annealing at 400°C during 10 mn, the nanostructures evolve towards larger, more compact and well ordered granules. The Ni-Ni distance varies from 0.246 to 0.248 nm as the annealing temperature increases indicating that the small agglomerates are under strain in the Ag matrix