We investigated the evolution of magnetism in the UIr1-xRhxGe 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-xRhxGe system and found a discontinuous antiferromagnetic/ferromagnetic boundary at xcrit=0.56, where a local minimum in ordering temperature and maximum of the Sommerfeld coefficient γ≈175mJ/molK2 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 xcrit. 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 (Hb,crit) in the entire UIr1-xRhxGe system, including the ferromagnetic region ∼UCo0.6Rh0.4Ge-URhGe.
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