Magnetic excitations in BaFe$_{2}$Se$_{3}$*

The ladder-like relative of the iron superconductors, BaFe2Se3, has recently been successfully synthesized and its thermo-magnetic and structural properties explored. Magnetic neutron diffraction and susceptibility data clearly reveal magnetic long-range order below $T_{N}=$256K. The proposed magnetic structure consists of antiferromagnetically aligned ferromagnetic blocks. To understand the origin of square plaquette formation, we carried out inelastic neutron scattering experiments on powder samples of BaFe$_{2}$Se$_{3}$. We identify spin-wave-like low energy excitations in the ordered state. The wave vector dependence of the low energy magnetic scattering is consistent with \textbf{\textit{k}}$=$(1/2,1/2,1/2) magnetic ordering. Comparison to a spin wave theory provides estimates for the dominant exchange interactions in BaFe$_{2}$Se$_{3}$.