Neutron-diffraction experiments on phosphorus-rich $\mathrm{Eu}{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_3$ have established that the ordered magnetic phase just below $T_N\left(8.5\mathrm{} \mathrm{K}\right)$ is a sine-wave phase which undergoes a first-order phase transition at about 7.5 K to a helimagnetic phase. This is the first observation of such a sine-wave—to—helical phase transition in real magnetic system.

• Received 21 July 1987

DOI:https://doi.org/10.1103/PhysRevB.36.7300