MAGNETIC PHASE-TRANSITIONS IN UNI2SI2

We report neutron diffraction measurements on a single crystal of UNi2 Si2. Three magnetically ordering phases are observed below 123 K with the magnetic moment parallel to the c-axis in all three phases. The low temperature (T < 53 K) phase is a superposition of a commensurate spin density wave with wave vector q = (0 0 0.667 +/- 0.002) and a ferromagnetic ordered moment of 1.1 +/- 0.3-mu-B, the intermediate (53 K < T < 103 K) is a simple body-centered tetragonal antiferromagnet with an ordered magnetic moment of 1.8 +/- 0.2-mu-B and the high-temperature phase is an incommensurate spin density wave with wave vector q = (0 0 0.744 +/- 0.002) at 110 K. The phase transitions at 53 and 103 K are first order.