Magnetic structure of UNi2Si2.

Abstract

The magnetic ordering and phase transitions have been studied in ${\mathrm{UNi}}_{2}$${\mathrm{Si}}_{2}$ single crystals by neutron diffraction. Three distinct ordered phases are observed, all with magnetic moments along the c axis. The high-temperature (103T124 K) phase is an incommensurate longitudinal spin-density wave with temperature-dependent wave vector. At 110 K, q=(0,0,0.745\ifmmode\pm\else\textpm\fi{}0.002). The intermediate temperature (53T103 K) phase is a simple body-centered antiferromagnet with a moment of (1.6\ifmmode\pm\else\textpm\fi{}0.3)${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$. 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 ${\mathrm{\ensuremath{\mu}}}_{\mathit{z}}$ of (1.0\ifmmode\pm\else\textpm\fi{}0.3)${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ per uranium atom. This pattern can be interpreted either as a longitudinal spin-density wave with maximum moment ${\mathrm{\ensuremath{\mu}}}_{0}$=(2.7\ifmmode\pm\else\textpm\fi{}0.3)${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ and a ferromagnetic component or as a squared-up wave with uranium moments of 2.22${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$, 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\ifmmode\pm\else\textpm\fi{}1 K there is a critical phase transition to the paramagnetic state with \ensuremath{\beta}=0.35\ifmmode\pm\else\textpm\fi{}0.03. Above 124 K conventional critical scattering is observed centered on the wave vector q.