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

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

Titanium Evolution and Nickel Restoration Under Neutron Irradiation in Ni/Ti Multilayers

Ni/Ti multilayers are used as supermirrors for neutron guides. These multilayers are submitted to neutron irradiation. In order to determine the effect of irradiation on supermirrors performances we studied Ni/Ti multilayers irradiated with thermal neutrons. We present here the results obtained by neutron reflectivity, X-ray diffraction and EXAFS on one sample. It has been found that the supermirrors performances are not reduced. Nevertheless, Ti shows an evolution from the hcp structure to another crystalline state while Ni layers show a restoration of the fcc structure

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

Titanium Evolution Under Neutron Irradiation in NixC1-x/Ti Multilayers

Supermirrors, which are NixC1-x/Ti aperiodic multilayers, have become more and more important for use in neutron guides. The effects of neutron radiation on the structural and optical properties of such stacks have been studied to estimate the lifetime of the neutron guides. The results presented here have been obtained by neutron reflectivity, X-ray diffraction, Conversion-Electron Extended X-ray Absorption Fine Structure and High Resolution Transmission Electron Microscopy. It has been observed that the reflecting performances of the supermirrors were not altered by irradiation, but that a structural change did occur in the Ti layers