The determination of accurate magnetic structure of magnetoelectric materials is a mandatory key step to understand their properties. In order to couple to the lattice and induce an electric polarization, this structure is unfortunately complex in most cases, and results from magnetic frustration, either geometrical or exchange in nature. The compound $\mathrm{Ba}{\mathrm{Fe}}_2{\mathrm{Se}}_3$ is one of the rare quasi-one-dimensional compounds that exhibits such magnetoelectric properties. In this paper, thanks to a single crystal neutron diffraction study and symmetry analysis, an umbrellalike magnetic structure has been revealed, with tilted moments in the $(a,c)$ and $(b,c)$ planes, confirmed by polarized neutron diffraction. It corresponds to a magnetic order with antiferromagnetic blocks and a propagation vector $k=(\frac{1}{2},\frac{1}{2},\frac{1}{2})$, belonging to the $C_{a}m$ magnetic space group. This work should allow further theoretical studies of the coupling of this magnetic order with the lattice and the ferroelectric order. It also provides key information on the magnetism, of which fluctuations may be at the origin of the pressure-induced superconductivity in this family.

• Received 6 July 2022
• Accepted 26 September 2022

DOI:https://doi.org/10.1103/PhysRevB.106.134429