Block-type antiferromagnetic state in BaFe ₂ Se ₃ studied by ⁵⁷ Fe nuclear forward scattering under pressure

The iron-based spin-ladder compound BaFe₂Se₃exhibits block-type antiferromagnetic order, accompanied by a subtle structural distortion from the orthorhombic to the monoclinic symmetry. We investigated the block-type antiferromagnetic states of BaFe₂Se₃under pressure using⁵⁷Fe nuclear resonant forward scattering combined with interference polarimetry. This technique enables the evaluation of the polarization characteristics of photons scattered from the sample by analyzing their interference with reference photons scattered from magnetized α-Fe. As the result, it provides detailed insight into the symmetry of the hyperfine interactions at the Fe nuclei. Four nonequivalent Fe sites were identified, which reproduced the⁵⁷Fe NFS time spectra below the antiferromagnetic ordering temperature. The refined magnetic hyperfine fields above 220 kOe are tilted at significantly different angles from the<i>b</i>-axis approximately toward the [101] axis direction in orthorhombic notation while maintaining the magnetic propagation vector characteristic of the block-type antiferromagnetic order. The monoclinic distortion remained minimal up to 3.8 GPa at 10 K within the experimental resolution, with lattice parameters decreasing monotonically under pressure. Correspondingly, the exotic block-type antiferromagnetic structure persisted up to 3.4 GPa, with refined hyperfine interaction parameters at the four Fe sites exhibiting negligible changes under pressure. These findings demonstrate the robustness of block-type antiferromagnetic order in BaFe₂Se₃under hydrostatic pressure.