Antiferromagnetic order inU1−xThxPt3

Neutron-diffraction studies show that the phase transition at 6.5 K in ${\mathrm{U}}_{0.95}$ ${\mathrm{Th}}_{0.05}$ ${\mathrm{Pt}}_3$ is to an antiferromagnetic ground state. The magnetic unit cell is obtained from doubling the nuclear unit cell along one direction in the basal plane. The ordered moment is $(0.65\mathrm{}\pm 0.1)\frac{{\mu }_B}{\mathrm{U}}$ atom, which is well below the moment of $2.6\frac{{\mu }_B}{\mathrm{U}}$ atom extracted from the high-temperature, Curie-Weiss susceptibility of U${\mathrm{Pt}}_3$.