Fe Substitution in URu_2Si_2: Singlet Magnetism in an Extended Doniach Phase Diagram

The application of pressure as well as the successive substitution of Ru with Fe in the hidden order (HO) compound URu_2Si_2 leads to the formation of the large moment antiferromagnetic phase (LMAFM). Here we have investigated the substitution series URu_2-xFe_xSi_2 from x = 0.0 to 2.0 by U 4f core-level photoelectron spectroscopy and have observed non-monotonic changes in the spectra. The initial increase and subsequent decrease of the spectral weight of the 4f core level satellite with increasing x stands for a non-monotonic 5f filling across the substitution series. The competition of chemical pressure and increase of the density of states at the Fermi energy, both due to substitution of Ru with Fe, can explain such a behavior. An extended Doniach phase diagram including the x dependence of the density of states is proposed. Also in URu_2-xFe_xSi_2 the ground state is a singlet or quasi-doublet state consisting of two singlets. Hence, the formation of magnetic order in the URu_2-xFe_xSi_2 substitution series must be explained within a singlet magnetism model.