Magnetic ordering in ${\mathrm{EuAs}}_3$, Eu(${\mathrm{As}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{P}}_{\mathrm{x}}$${)}_3$, and β-${\mathrm{EuP}}_3$ has been determined by neutron diffraction on single crystals. Semimetallic ${\mathrm{EuAs}}_3$ and Eu(${\mathrm{As}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{P}}_{\mathrm{x}}$${)}_3$ with x≤0.98 order at $T_N$ (11.4–8.5 K), with a second-order phase transition to a sine-wave amplitude modulated phase with the magnetic moment m∥[010] or [01¯0] direction. As the temperature is lowered these compounds undergo a further first-order phase transition at $T_L$ (10.3–7.5 K), either to a commensurate antiferromagnetic phase with m∥[010] or [01¯0] for lower P concentrations, or to a helimagnetic phase with magnetic moments modulated in direction and amplitude in the (010) plane for higher P concentrations. The semiconducting β-${\mathrm{EuP}}_3$, on the other hand, is a simple antiferromagnet with a second-order phase transition at $T_N$=10 K, which shows no further temperature-induced phase transition down to T=2 K. The condition for the existence of the incommensurate ${\mathrm{EuAs}}_3$ has been investigated phenomenologically by varying the exchange interactions. A simple interpretation of the smooth temperature dependence of the modulation vector of the incommensurate ${\mathrm{EuAs}}_3$ is given in terms of the sine-Gordon soliton-lattice model.

• Received 4 May 1987

DOI:https://doi.org/10.1103/PhysRevB.37.269