  The evolution of magnetic properties of isostructural and isoelectronic solid
solutions of the superconducting itinerant 5f-electron ferromagnet UCoGe with
antiferromagnet UIrGe was studied by magnetization, AC susceptibility, specific
heat, and electrical resistivity measurements of a series of UCo1-xIrxGe
compounds in polycrystalline and single crystalline form at various
temperatures and magnetic fields. Both the weak ferromagnetism and
superconductivity in UCoGe were found to have vanished already for very low Ir
substitution for Co x = 0.02. The antiferromagnetism of UIrGe is gradually
suppressed. This is documented by a rapid decrease in both Neel 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-x phase diagram in the range x between 0.02 and 0.8 is dominated by a
correlated paramagnetic phase exhibiting very broad bumps in temperature
dependencies of b-axis magnetization and specific heat developing with
increasing x. For x = 0.24, wide peaks appear in the c-axis thermomagnetic
curves due to antiferromagnetic correlations which may eventually lead to
frozen incoherent spin configurations at low temperatures. The correlated
paramagnetic phase is also accompanied by specific electrical resistivity
anomalies. The T-x phase diagram determined for the UCo1-xIrxGe compounds
contrasts with the behavior of the related URh1-xIrxGe system, which was
reported to exhibit an extended concentration range of stable ferromagnetism in
Rh rich compounds and a discontinuous transformation between the ferromagnetic
and antiferromagnetic phases at a critical Rh-Ir concentration.
