  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 $\mathrm{UCo}_{1-x}\mathrm{Ru}_{x}\mathrm{Ge}$
series ($0\geq x\leq0.9$). The initial Ru substitution up to $x\approx0.1$
leads to a simultaneous sharp increase of the Curie temperature and spontaneous
magnetization up to maximum values of $T_{\mathrm{C}}=8.6 K$ and
$M_{\mathrm{S}}=0.1 \mu_{\mathrm{B}}$ per formula unit, respectively, whereas
superconductivity vanishes already for $x\approx0.03$. Further increase of the
Ru content beyond $x\approx0.1$ leads to a precipitous decrease of both,
$T_{\mathrm{C}}$ and $M_{\mathrm{S}}$ towards a ferromagnetic quantum critical
point (QCP) at $x_{\mathrm{cr}}=0.31$. 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 5$f$-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.
