Evolution of Magnetism in UCo1−xRuxGe

UCoGe is an archetypal 5f-electron ferromagnet (TC ~ 3K) which becomes superconducting at ambient pressure (TSC ~ 0.6K). The low values of TC and the spontaneous magnetic moment of 0.03μB/f.u. indicate nearness of a ferromagnetic instability. We have prepared a series of UCo1-xRuxGe polycrystalline samples and studied development of the magnetism and superconductivity by measuring the magnetization, specific heat and resistivity as functions of temperature and magnetic field. We have observed that the Ru doping leads to a rapid increase of the TC up to 8.5K for x = 0.1 and simultaneous suppression of superconductivity. Further doping reduces TC towards a ferromagnetic quantum critical point at x ≈ 0.3. The expected quantum criticality is reflected in specific scaling in the temperature dependence of magnetization, specific heat and electrical resistivity. We have also grown an UCo0.88Ru0.12Ge single crystal. It exhibits strong magnetocrystalline anisotropy similar to UCoGe but the spontaneous magnetization in the easy magnetization direction is considerably higher. To shed more light on the microscopic background of these findings we have performed a polarized neutron diffraction experiment at D3 in ILL. In contrast to the antiparallel orientation of the Co and U moments in UCoGe we have observed parallel orientation in UCo0.88Ru0.12Ge. This result rises a question whether the magnetic state of Co plays a momentous role for the presence of superconductivity in the U(Co,Ru)Ge compounds.