Switching of magnetic ground states across the UIr1-xRhxGe alloy system

We investigated the evolution ofmagnetism in the UIr1-xRhx Ge 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 distances close to the Hill limit. We established the x-T phase diagram of the UIr1-xRhx Ge system and found a discontinuous antiferromagnetic/ferromagnetic boundary at x(crit) = 0.56, where a local minimum in ordering temperature andmaximum of the Sommerfeld coefficient gamma approximate to 175 mJ/molK(2) occurs in the UCoGe-URhGe-UIrGe system, signaling an increase in magnetic fluctuation. 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 (H-b, crit) in the entire UIr1-xRhx Ge system, including the ferromagnetic region similar to UCo0.6Rh0.4Ge-URhGe.