On the evolution of the ground state in the system UxLa1-xS:: Polarized neutron diffraction and X-ray magnetic circular dichroism study

In the UxLa1-xS system there is an abrupt loss of the long-range ferromagnetic ordering found in pure US at a critical concentration x(c) similar to 0.57, which is far above the percolation limit. As the magnetic ground state in such a system can be strongly affected by small variations of the 5f localization, we have investigated a set of samples with different x by polarized neutron diffraction and X-ray magnetic circular dichroism (XMCD). The neutron results are consistent with early measurements performed on pure US. Even at the lowest U content (x = 0.15, below x) the shape of the induced form factor (f (Q)) is comparable with that found for x = 1 and is well reproduced by either a U4+ or a U3+ state. The ratio between the orbital and the effective spin moments in the XMCD measurements confirms this result, but the evolution of the shape at the M-5 edge suggests an abrupt change in the distribution of the electrons (holes) in the 5f density of states around x(c).