Magnetic structure of UPd2Si2 in a magnetic field

Elastic neutron-scattering and magnetization measurements have been performed on a single crystal of UPd2Si2 in fields along the c axis. In zero field there is a simple body-centered antiferromagnetic structure for temperatures up to 108 K. Between 108 K and the Néel temperature there is an incommensurate longitudinal spin-density-wave phase. For fields above 1.9 T, a new phase appears that separates the two zero-field phases. The new phase corresponds to a commensurate spin density wave of wave vector (0,0,2/3). In the (0,0,2/3) phase the neutron-scattering data show that the amplitude of the spin-density-wave moment is (3.0±0.3)μB, while magnetization measurements show a net ferromagnetic moment of (0.30±0.05)μB at 3 T.