MAGNETIC-STRUCTURE OF UPD2SI2 IN A MAGNETIC-FIELD

Elastic neutron-scattering and magnetization measurements have been pe rformed 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 Neel temperature there is an incommensurate longitudinal spin-density-wave phase. For f ields above 1.9 T, a new phase appears that separates the two zero-fie ld phases. The new phase corresponds to a commensurate spin density wa ve of wave vector (0, 0, 2/3). In the (0, 0, 2/3) phase the neutron-sc attering data show that the amplitude of the spin-density-wave moment is (3.0 +/- 0.3)mu(B), while magnetization measurements show a net fer romagnetic moment of (0.30 +/- 0.05)mu(B) at 3 T.