Gradual Localization of 5f States in Orthorhombic UTX Ferromagnets Polarized Neutron Diffraction Study of Ru Substituted UCoGe

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 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 amp; 956;spont 0.11 amp; 956;B f.u. and amp; 956;spont 0.21 amp; 956;B f.u., respectively in comparison to the tiny spontaneous moment of UCoGe about 0.07 amp; 956;B 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 amp; 956;UL, 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 hybridizatio