The evolution of magnetic properties of isostructural and isoelectronic solid solutions of the superconducting itinerant $5f$-electron ferromagnet (FM) UCoGe with antiferromagnet (AFM) UIrGe was studied by magnetization, AC susceptibility, specific heat, and electrical resistivity measurements of a series of $\mathrm{U}{\mathrm{Co}}_{1-x}{\mathrm{Ir}}_x\mathrm{Ge}$ compounds in polycrystalline and single-crystalline form at various temperatures and magnetic fields. Both the weak FM and superconductivity in UCoGe were found to have vanished already for very low Ir substitution for Co $(x=0.02)$. The AFM of UIrGe is gradually suppressed. This is documented by a rapid decrease in both Néel temperature and the critical field of the metamagnetic transition with decreasing Ir concentration, which both tend to vanish just above $x=0.8$. The section of the $T\text{−}x$ phase diagram in the range $0.02\le x\le 0.8$ is dominated by a correlated paramagnetic phase (CPM) exhibiting very broad bumps in temperature dependencies of $b$-axis magnetization and specific heat developing with increasing $x$. For $x\ge 0.24$, wide peaks appear in the $c$-axis thermomagnetic curves due to AFM correlations which may eventually lead to frozen incoherent spin configurations at low temperatures. The CPM is also accompanied by specific electrical resistivity anomalies. The $T\text{−}x$ phase diagram determined for the $\mathrm{U}{\mathrm{Co}}_{1-x}{\mathrm{Ir}}_x\mathrm{Ge}$ compounds contrasts with the behavior of the related $\mathrm{U}{\mathrm{Rh}}_{1-x}{\mathrm{Ir}}_x\mathrm{Ge}$ system, which was reported to exhibit an extended concentration range of stable FM in Rh-rich compounds and a discontinuous transformation between the FM and AFM phases at a critical Rh-Ir concentration. The striking difference is tentatively attributed to (a) the instability of tiny U moment in the weak itinerant FM UCoGe induced by substitutional disorder at already very low Ir doping, (b) the rather stable U moment in URhGe formed by much less delocalized $5f$ electrons assisted by weakly varying lattice parameters of $\mathrm{U}{\mathrm{Rh}}_{1-x}{\mathrm{Ir}}_x\mathrm{Ge}$ compounds.

• Received 18 June 2021
• Revised 8 January 2022
• Accepted 11 January 2022

DOI:https://doi.org/10.1103/PhysRevB.105.014436