1 Se p 20 06 Search for the ferromagnetic quantum critical points in URhGe by chemical substitution

In the electrically correlated metal URhGe ferromagnetic order (T C = 9.5 K) and superconductivity (T s = 0.25 K) coexist at ambient pressure. Here we report on chemical substitution of Rh by Ru, which enables one to tune the Curie temperature to T = 0. URuGe has a paramagnetic ground state and is isostructural to URhGe. We have prepared a series of polycrystalline URh 1−x Ru x Ge samples over a wide range of x values. Magnetization and electrical resistivity data show, after an initial increase, a linear suppression of T C with increasing x. The critical Ru concentration for the suppression of ferromagnetic order is x cr ≈ 0.38. The U(Rh,Ru)Ge samples are further investigated by X-ray powder diffraction, specific heat, and magnetization under pressure. For the critical Ru concentration x cr ≈ 0.38 non-Fermi liquid (NFL) behavior is observed in the specific heat at low temperatures (C/T ∼ − ln(T /T 0)). In addition, we have studied the evolution of ferromagnetism in polycrystalline URh 1−x Co x Ge samples with 0 ≤ x ≤ 1. Magnetization measurements show that ferromagnetic order is suppressed for x close to 1 only so that UCoGe is in fact close to a quantum phase transition. Finally, magnetization data on URhGe 1−x Si x samples (0.0 ≤ x ≤ 0.20) show no change in T C up to x = 0.20.