Switching of magnetic ground states across the UIr_1-xRh_xGe alloy system

We investigated the evolution of magnetism in the UIr_1-xRh_xGe system by the systematic study of high-quality single crystals. Lattice parameters of both parent compounds are very similar resulting in almost identical nearest interatomic uranium distance close to the Hill limit. We established the x-T phase diagram of the UIr_1-xRh_xGe system and found a discontinuous antiferromagnetic/ferromagnetic boundary at x_crit = 0.56 where a local minimum in ordering temperature and maximum of the Sommerfeld coefficient γ ≈ 175 mJ/mol K² occurs in the UCoGe-URhGe-UIrGe system, signaling an increase in magnetic fluctuations. However, a quantum critical point is not realized because of the finite ordering temperature at x_crit. A magnon gap on the antiferromagnetic side abruptly suppresses magnetic fluctuations. We find a field-induced first order transition in the vicinity of the critical magnetic field along the b axis in the entire UIr_1-xRh_xGe system including the ferromagnetic region ~UCo_0.6Rh_0.4Ge - URhGe.