Gadolinium oxide films, prepared by the sol-gel process, present waveguiding properties. Their structures were studied by waveguide Raman spectroscopy (WRS) and confirmed using transmission electron microscopy (TEM). The structural evolution of the layers with annealing temperatures from 650 up to 900 • C was investigated. The WRS results and TEM observations were correlated and revealed that crystallization started at 650 • C into the cubic phase. The mean diameter of the crystallites dispersed in the amorphous phase is around 5 nm at this temperature. The film is totally crystallized into the cubic phase at 700 • C. The monoclinic phase simultaneously appears at 800 • C and both phases still co-exist after heat treatment at 900 • C. The films present a heterogeneous structure at 900 • C with grain sizes varying from 5 up to 50 nm and as consequence their waveguiding properties strongly decrease.
Size effects, such as structure transition, have been reported in small clusters of alkali halide compounds. We extend the study to rare earth sesquioxide (Gd(2)O(3)) clusters which are as ionic as the alkali halide compounds, but have a more complicated structure. In a clean and controlled environment (ultra high vacuum), such particles are well crystallized, facetted and tend to adopt a rhombic dodecahedron shape. This indicates the major role of highly ionic bonds in preserving the crystal lattice even at small sizes (a few lattice parameter). Based on both cathodo-luminescence and transmission electron microscopy, we report the existence of a structural transition from bcc to monoclinic at small sizes.
In this present work, we report a structural and magnetic study of mixed Co 58 Pt 42 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 Co 58 Pt 42 supported isolated clusters are about 2 nm 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 Dichroïsm (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.
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