Magnetic Phase Diagram of the UAs-US System

The magnetic phase diagram of the $\mathrm{U}{\mathrm{As}}_{1-x}{\mathrm{S}}_x$ system ($0\le x\le 1$) has been investigated by neutron diffraction with powder samples. The solid solutions are face-centered cubic (NaCl crystal structure), and the anion atoms are distributed randomly over the anion sublattice. For $x=0\mathrm{}$ (UAs), the type-I antiferromagnetic ordering ($T_N=127\mathrm{}$ K), consisting of ferromagnetic sheets stacked + - + -, changes at $0.5T_N$ to the type-IA ordering, in which the sheets are stacked + + - -. This transition is accompanied by an abrupt increase in the magnetic moment from $1.9 \mathrm{to} 2.2{\mu }_B$ per uranium atom. As $x$ increases from zero, the stability of the type-IA structure increases until for $x=0.20\mathrm{}$ this phase is present at all temperatures below $T_N$. For $x$ between 0.25 and 0.34 the magnetic moments are not arranged in a simple commensurate structure, but their modulation from layer to layer follows a sinusoidal form. Near $T_N$, a single harmonic is present: The repeat distance in real space varies from ∼ 2.8 unit cells at $x=0.25\mathrm{} \mathrm{to} \sim 4.2$ unit cells at $x=0.34\mathrm{}$. A third harmonic, implying some squaring of the sine wave, appears at about $0.5T_N$ The periodicity is a function of concentration ($x$) only and does not vary with temperature for a given sample. Although at the ends of this intermediate range the stable phases at low temperature tend to be either type IA (low $x$) or ferromagnetic (high $x$), the longitudinal-wave structure is stable to at least 2.2 K over a narrow composition range.