Thesis presented March 06, 2002

Abstract:
We present the results of structural and morphological characterization of discontinuous metal oxide multilayers composed of equally spaced planes of metallic nanoparticles embedded in an oxide matrix. The amplitude of tunnel magnetoresistance depends on the organisation of the metal at different length scales. Several techniques were used to probe the local structural and magnetic order (XAFS, Mössbauer Spectroscopy), the medium range order of the particles (X-ray Reflectivity, GISAXS, TEM) and the long range atomic order (X-ray diffraction). The data obtained are compared to those of magnetic measurements. Three systems were investigated, Fe/ZrO₂, Co/ZrO₂ and Co/SiO₂, for different nominal deposited metal layer thicknesses eM, annealing temperatures TR and deposition rates. The paramagnetic ions that are revealed at the periphery of the nanoparticles do not belong to a known oxide of the deposited metal. These atoms are magnetically and electrically "dead". The fraction of "dead" atoms is greater for zirconia than for silica and can be reduced by increasing TR, eM and/or by decreasing the deposition rate. The value of eM corresponding to the percolation threshold in the layer planes increases with increasing TR. Wetting at the metal-oxide interface is significant for zirconia (compared to silica) and for iron (compared to cobalt). The resulting morphology of the granular layers, in particular the size of the particles, governs the superparamagnetic or ferromagnetic behaviour of the metallic particles. Schematic diagrams (TR, eM) summarize the magnetic, structural and morphological properties for each metal oxide system.

Keywords:
Tunnel Magnetoresistance, Mössbauer Spectroscopy, discontinuous multilayer, Structure and Morphology, Nanoparticles, size and shape, Superparamagnetism and ferromagnetism

On-line thesis.