Sur la preparation du difluorure de vanadium VF2

Two new preparation methods of vanadium (+II) fluoride are discussed:VF₂ is obtained by action of vanadium metal on VF₃ or substitution of chlorine by fluorine in VCl₂. The later method gives very pure VF₂ at relatively low temperature.     

Magnetism of the Fe/ZnSe(001) interface

The magnetism of epitaxial ultrathin films of Fe on ZnSe(001) has been investigated by x-ray magnetic circular dichroism down to the submonolayer regime. In contrast to other metallic ferromagnet/semiconductor interfaces, no reduction of the Fe magnetic moment was found at the Fe/ZnSe(001) interface. Furthermore, a significant enhancement of the Fe magnetic moment compared to the bulk value was observed for coverages up to one monolayer in agreement with theoretical predictions. We also demonstrate that the magnetic properties of the Fe/ZnSe(001) interface remain stable against thermal annealing up to 300 degrees C, a prerequisite for the future development of efficient spintronics devices.     

Catalytic action of gold atoms on the oxidation of Si(111) surfaces

Auger spectroscopy, electron energy loss spectroscopy and ion depth profiling techniques, under ultra high vacuum conditions, have been used in a comparative study of the oxidation of clean and gold precovered silicon (111) surfaces. Exposure of a Si surface covered by a few Au monolayers to an oxygen partial pressure induces the formation of SiO₄ tetrahedra even at room temperature. In contrast, oxidation under the same conditions of a clean Si(111) surface leads to the well known formation of a chemisorbed oxygen monolayer. In the case of the Au covered surfaces, the enhancement of the oxide growth is attributed to the presence of an AuSi alloy where the hybridization state of silicon atoms is modified as compared to bulk silicon. This Au catalytic action has been investigated with various parameters as the substrate temperature, oxygen partial pressure and Au coverage. The conclusions are two fold. At low temperature (T < 400°C), gold atoms enhance considerably the oxidation process. SiO₄ tetrahedra are readily formed even at room temperature. Nevertheless, the SiO₂ thickness saturates at about one monolayer, this effect being attributed to the lack of Si atoms alloyed with gold in the reaction area. By increasing the temperature (from 20°C to ～400°C), silicon diffusion towards the surface is promoted and a thicker SiO₂ layer can be grown on top of the substrate. In the case of the oxidation performed at temperature higher than 400°C, the results are similar to the one obtained on a clean surface. At these temperatures, the metallic film agglomerates into tridimensional crystallites on top of a very thin AuSi alloyed layer. The fact that the latter has no influence on the oxidation is attributed to the different local arrangement of atoms at the sample surface.     

Contribution a l'étude des propriétés magnétiques des phases AVX3 (A = Rb,Cs; X = Cl,Br, I)

The magnetic properties of RbVCl₃, CsVCl₃, CsVBr₃, and CsVI₃ have been investigated in the temperature range 4.2–800°K. In agreement with the 2H structure of these phases, the obtained results have been interpreted on the basis of a monodimensional Heisenberg model. The values of the exchange integrals have been determined by means of Smith and Friedberg [Phys. Rev.176, 660 (1968)] calculations. They involve strong antiferromagnetic interactions, which decrease from the chloride to the iodide. The nature of the coupling between vanadium atoms is discussed.