Spiral magnetic ordering of the Eu moments in EuNi$_{2}$As$_{2}$

The ground-state magnetic structure of EuNi$_{2}$As$_{2}$ was investigated by single-crystal neutron diffraction. At base temperature, the Eu$^{2+}$ moments are found to form an incommensurate antiferromagnetic spiral-like structure with a magnetic propagation vector of $\mathit{k}$ = (0, 0, 0.92). They align ferromagnetically in the $\mathit{ab}$ plane with the moment size of 6.75(6) $\mu_{B}$, but rotate spirally by 165.6(1){\deg} around the $\mathit{c}$ axis from layer to layer. The magnetic order parameter in the critical region close to the ordering temperature, $\mathit{T_{N}}$ = 15 K, shows critical behavior with a critical exponent of $\beta_{Eu}$ = 0.34(1), consistent with the three-dimensional Heisenberg model. Moreover, within the experimental uncertainty, our neutron data is consistent with a model in which the Ni sublattice is not magnetically ordered.