Electronic location and magnetism in uranium and neptunium mono-chalcogenides and mono-pnictides, study of the systems: U_xLa_1-x(S,Se), U_0.2(La_0.15Y_0.85)_0.8Te and NpAs_1-xSe_x

This thesis concerns the evolution of the magnetic properties in some solid solutions (U_xLa_1-x(S,Se), U_0.2(La_0.15Y_0.85)_0.8Te, NpAs_1-XSe_x) based on Uranium and Neptunium. This experimental study is an attempt to improve the comprehension of the behavior of the 5f electrons, which are generally considered as responsibles for the physical properties observed in these systems, when a modification 'under control' of their chemical environment occurs. The first part of this thesis is devoted to the study of the effect of the reduction of the density of the magnetic centers (substitution U → La) on some physical properties, mainly magnetic, of the U mono-chalcogenide systems. The ferromagnetic long-range ordering observed in US and USe abruptly collapses at a critical U concentration far above the percolation limit, whereas short-range ferromagnetic correlations are measured well below this critical concentration. Magnetic form factor and X-ray magnetic circular dichroism measurements were performed to relate experimentally the change observed in the macroscopic properties of the materials to the electronic structure. The second part is devoted to the study of the evolution of the magnetic structures in the NpAs_1-xSe_x system. In this case a p electron is added, presumably to the conduction band, thus modifying the chemical potential. The magnetic phase diagram up to a Se concentration of 20%, as determined by neutron diffraction, magnetization measurements, and Moessbauer spectroscopy is reported. (author)