Evolution of ferromagnetic and non-Fermi-liquid states with doping: The case of Ru-doped UCoGe

We have investigated the impact of Ru substitution for Co on the behavior of the ferromagnetic superconductor UCoGe by performing x-ray diffraction, magnetization, specific heat, and electrical resistivity measurements on polycrystalline samples of the UCo1-xRuxGe series (0 >= x <= 0.9). The initial Ru substitution up to x approximate to 0.1 leads to a simultaneous sharp increase of the Curie temperature and spontaneous magnetization up to maximum values of T-C = 8.6 K and MS = 0.1 mu(B) per formula unit, respectively, whereas superconductivity vanishes already for x approximate to 0.03. Further increase of the Ru content beyond x approximate to 0.1 leads to a precipitous decrease of both TC and MS towards a ferromagnetic quantum critical point (QCP) at x(cr) = 0.31. Consequently, the T - x magnetic phase diagram consists of a well-developed ferromagnetic dome. We discuss the evolution of ferromagnetism with x on the basis of band structure changes due to varying 5f-ligand hybridization. This scenario is supported by the results of electronic structure calculations and consideration of the simplified periodic Anderson model. The analysis of the temperature dependencies of the electrical resistivity and heat capacity at low temperatures of the samples in the vicinity of the QCP reveals a non-Fermi-liquid behavior and assigns the ferromagnetic quantum phase transition to be most likely of a continuous Hertz-Millis type.