Magnetic structure of UNi2Si2

The magnetic ordering and phase transitions have been studied in UNi2Si2 single crystals by neutron diffraction. Three distinct ordered phases are observed, all with magnetic moments along the c axis. The high-temperature (103<T<124 K) phase is an incommensurate longitudinal spin-density wave with temperature-dependent wave vector. At 110 K, q=(0,0,0.745±0.002). The intermediate temperature (53<T<103 K) phase is a simple body-centered antiferromagnet with a moment of (1.6±0.3)μB. The low-temperature phase shows a similar scattering pattern to that of the high-temperature phase with q=(0,0,2/3), and with extra scattering at the nuclear Bragg peaks corresponding to a ferromagnetic moment μz of (1.0±0.3)μB per uranium atom. This pattern can be interpreted either as a longitudinal spin-density wave with maximum moment μ0=(2.7±0.3)μB and a ferromagnetic component or as a squared-up wave with uranium moments of 2.22μB, two thirds of which are along +z and one third along -z. Hysteresis is observed in the low-temperature phase transition. The phase transitions at 53 and 103 K are both first order. At 124±1 K there is a critical phase transition to the paramagnetic state with β=0.35±0.03. Above 124 K conventional critical scattering is observed centered on the wave vector q