Elastic-neutron-scattering and magnetization measurements have been performed on a single crystal of ${\mathrm{UPd}}_2$${\mathrm{Si}}_2$ in the 12–160-K temperature range. Two magnetically ordered phases are observed with the magnetic moment parallel to the c axis. At low temperatures (T<108 K), there is a simple body-centered-tetragonal antiferromagnet with an ordered moment of 2.3±0.3${\mathrm{\mu }}_{\mathit{B}}$. At higher temperatures (108 K<T<136 K), there is an incommensurate spin-density wave with wave vector q=(0,0,0.732) and magnetic moment 2.3±0.3${\mathrm{\mu }}_{\mathit{B}}$ at 120 K. The phase transition at 108 K is first order while there is a critical phase transition at 136 K to the paramagnetic phase with critical index β=0.30±0.03. The low-temperature phase contains a small amount of a (0,0,2/3) phase; the maximum intensity from this phase is about 0.8% of that from the simple antiferromagnetic phases at 90 K. The magnetization in low field shows an analogous peak at 90 K.

• Received 30 October 1992

DOI:https://doi.org/10.1103/PhysRevB.47.8672