Simultaneous suppression of ferromagnetism and superconductivity in UCoGe by Si substitution

We investigate the effect of substituting Si for Ge in the ferromagnetic superconductor UCoGe. dc-magnetization, ac-susceptibility, and electrical resistivity measurements on polycrystalline UCoGe1-xSix samples show that ferromagnetic order and superconductivity are progressively depressed with increasing Si content and simultaneously vanish at a critical concentration xcr~=0.12. The non-Fermi-liquid temperature variation in the electrical resistivity near xcr and the smooth depression of the ordered moment point to a continuous ferromagnetic quantum phase transition. Superconductivity is confined to the ferromagnetic phase, which provides further evidence for magnetically mediated superconductivity.