Alloying-driven transition between ferromagnetism and antiferromagnetism in UTGe compounds: UCo1-xIrxGe
Abstrakt
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 UCo1-xIrxGe 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 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 0.02 = 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-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 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 5 f electrons assisted by weakly varying lattice parameters of URh1-xIrxGe compounds.