Influence of 5d-metal on magnetic properties of UNi_1-xT_xAl compounds

We have studied structure and magnetic properties of UNi_1-xT_xAl (T = Ir, Pt) alloys. UNiAl does not form continuous solid solutions with either UPtAl or UIrAl. The solubility ranges extend only to 20% from each side in both systems. The alloys with 0.20<x <0.80 contain two isostructural phases with the ZrNiAl-type structure. In both systems, the compounds with x≥0.80 order ferromagnetically, whereas those with x≤0.20 exhibit metamagnetic behaviour characteristic for antiferromagnets like UNiAl. The initial Pt (Ir) substitutions lead to an increase (decrease) of the Neel temperature whereas the critical field for metamagnetism becomes gradually reduced in both cases. The electronic specific-heat coefficient γ is strongly reduced in UNi_1-xPt_xAl (from 164 to 113 mJ mol^-1 K^-2 for x increasing from 0 to 0.2) and enhanced in UNi_1-xIr_xAl (up to 198 mJ mol^-1 K^-2 for x = 0.2). The obtained results are discussed in the context of the effects induced by substitutions of other transition metals in UNiAl considering the evolution of the 5f-d hybridization.