Among various parent compounds of iron pnictide superconductors, ${\text{EuFe}}_2{\text{As}}_2$ stands out due to the presence of both spin density wave of Fe and antiferromagnetic (AFM) ordering of the localized ${\text{Eu}}^{2+}$ moment. Single-crystal neutron-diffraction studies have been carried out to determine the magnetic structure of this compound and to investigate the coupling of two magnetic sublattices. Long-range AFM ordering of Fe and Eu spins was observed below 190 and 19 K, respectively. The ordering of ${\text{Fe}}^{2+}$ moments is associated with the wave vector $\mathbf{k}=\left(1,0,1\right)$ and it takes place at the same temperature as the tetragonal to orthorhombic structural phase transition, which indicates the strong coupling between structural and magnetic components. The ordering of the Eu moments is associated with the wave vector $\mathbf{k}=\left(0,0,1\right)$. While both Fe and Eu spins are aligned along the long $a$ axis as experimentally determined, our studies suggest a weak coupling between the Fe and Eu magnetism.

• Received 28 August 2009

DOI:https://doi.org/10.1103/PhysRevB.80.174424