Spiral magnetic ordering of the Eu moments in EuNi${}_2$As${}_2$

Abstract

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 $k$ = (0, 0, 0.92). They align ferromagnetically in the $ab$ plane with the moment size of 6.75(6) ${\mu }_B$, but rotate spirally by 165.6(1)° around the $c$ axis from layer to layer. The magnetic order parameter in the critical region close to the ordering temperature, $T_N$ = 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.

6 pages, 5 figures