Neutron-diffraction study of the evolution of antiferromagnetic order in UPt3 doped with Pd

Neutron-diffraction experiments have been carried out on a series of heavy-electron pseudobinary U(Pt1-xPdx)(3) single crystals (x less than or equal to 0.05). The small-moment antiferromagnetic order reported for purl UPt3 is robust upon doping with Pd and persists till at least x=0.005. The ordered moment grows from 0.018 +/-0.002 mu(B)/U atom for pure UPt3 to 0.048+/-0.008 mu(B)/U atom for x=0.005. The Neel temperature T-N is approximately 6 K and, most remarkably, does not vary with Pd contents. The order parameter squared for the small-moment antiferromagnetism has an unusual quasilinear temperature variation. For x greater than or equal to 0.01 a second antiferromagnetic phase with much larger ordered moments is found. For this phase at optimum doping (x=0.05) T-N attains a maximum value of 5.8 K and the ordered moment equals 0.63+/-0.05 mu(B)/U atom. T-N(x) for the large-moment antiferromagnetic order follows a Doniach-type phase diagram. From this diagram we infer that the antiferromagnetic instability in U(Pt1-xPdx)(3) is located in the range 0.5-1 at. % Pd. [S0163-1829(99)07325-7].