High-pressure synthesis of the layered iron oxyselenide BaFe2Se2O with strong magnetic anisotropy

Using a high-pressure reaction, we successfully synthesized $\mathrm{BaF}{\mathrm{e}}₂\mathrm{S}{\mathrm{e}}₂\mathrm{O}$ with a uniform stack of $[\mathrm{F}{\mathrm{e}}₂\mathrm{S}{\mathrm{e}}₂\mathrm{O}]$ layers. Magnetic susceptibility, heat capacity, M\"ossbauer spectroscopy, and neutron-diffraction measurements revealed that $\mathrm{BaF}{\mathrm{e}}₂\mathrm{S}{\mathrm{e}}₂\mathrm{O}$ undergoes antiferromagnetic order at 106 K with a 2-$k$ spin structure where each Fe moment points to a neighboring oxide anion. The same spin structure has been observed in the related iron oxyselenides but with a staggered stack of $[\mathrm{F}{\mathrm{e}}₂\mathrm{C}{\mathrm{h}}₂\mathrm{O}]$ (where Ch represents chalcogen) layers $(\mathrm{Ch}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}\mathrm{S},\mathrm{Se})$. We propose that the strong uniaxial anisotropy inferred from the 2-$k$ structure originates from spin-orbit coupling (SOC) induced by a $\mathit{trans}\ensuremath{-}\mathrm{Fe}{\mathrm{O}}₂\mathrm{S}{\mathrm{e}}₄$ octahedron, which provides a quasilinear coordination environment as often found in single molecule magnetic complexes. A first-principles calculation with inclusion of SOC supports the stabilization of the 2-$k$ spin structure, giving an unquenched orbital momentum of $0.1\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Fe}$. The present paper provides an idea of how to design magnetic lattices of uniaxial anisotropy using oxychalcogenides and more generally mixed-anion compounds.