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 URu2Si2 leads to the formation of the large-moment antiferromagnetic phase. Here, we investigate the substitution series URu2-xFexSi2 from x = 0.0 to 2.0 by U 4 f core-level photoelectron spectroscopy and observe nonmonotonic changes in the spectra. The initial increase and subsequent decrease in the spectral weight of the 4 f core-level satellite with increasing x stands for a nonmonotonic 5 f filling across the substitution series. The competition of chemical pressure and increase in the density of states at the Fermi energy, both due to substitution of Ru with Fe, can explain such behavior. An extended Doniach phase diagram including the x dependence of the density of states is proposed. Also in URu2-xFexSi2 the ground state is a singlet or quasidoublet state consisting of two singlets. Hence, the formation of magnetic order in the URu2-xFexSi2 substitution series must be explained within a singlet magnetism model.