  We report on a microscopic study of the evolution of ferromagnetism in the Ru
substituted ferromagnetic superconductor (FM SC) UCoGe crystallizing in the
orthorhombic TiNiSi-type structure. For that purpose, two single crystals with
composition UCo0.97Ru0.03Ge and UCo0.88Ru0.12Ge have been prepared and
characterized by magnetization, AC susceptibility, specific heat and electrical
resistivity measurements. Both compounds have been found to order
ferromagnetically below TC = 6.5 K and 7.5 K, respectively, which is
considerably higher than the TC = 3 K of the parent compound UCoGe. The higher
values of TC are accompanied by enhanced values of the spontaneous moment
mspont. = 0.11 mB/f.u. and mspont. = 0.21 mB/f.u., respectively in comparison
to the tiny spontaneous moment of UCoGe (about 0.07mB/f.u.). No sign of
superconductivity was detected in either compound. The magnetic moments of the
samples were investigated on the microscopic scale using polarized neutron
diffraction (PND) and for UCo0.88Ru0.12Ge also by soft X-ray magnetic circular
dichroism (XMCD). The analysis of the PND results indicates that the observed
enhancement of ferromagnetism is mainly due to the growth of the orbital part
of the uranium 5f moment mL(U), reflecting a gradual localization of the 5f
electrons with Ru substitution. In addition, the parallel orientation of the U
and Co moments has been established in both substituted compounds. The results
are discussed and compared with related isostructural ferromagnetic UTX
compounds (T - transition metals, X - Si, Ge) in the context of a varying
degree of the 5f-ligand hybridization.
