Short-range and long-range magnetic order in Fe(Te1.5Se0.5) O5Cl

Considerable attention has been paid recently to $\mathrm{Fe}{\mathrm{Te}}_{2}{\mathrm{O}}_{5}\mathrm{Cl}$ due to reduced dimensionality and frustration in the magnetic subsystem, succession of phase transitions, and multiferroicity. The efforts to grow its selenite sibling resulted in the mixed halide compound $\mathrm{Fe}({\mathrm{Te}}_{1.5}{\mathrm{Se}}_{0.5}){\mathrm{O}}_{5}\mathrm{Cl}$, which was found crystallizing in a different structural type and possessing properties drastically different from those of a parent system. Its magnetic subsystem features weakly coupled ${\mathrm{Fe}}^{3+}\ensuremath{-}{\mathrm{Fe}}^{3+}$ dimers showing the regime of short-range correlations at ${T}_{\mathrm{M}}\ensuremath{\sim}70\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and long-range order at ${T}_{\mathrm{N}}=22\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. In a magnetically ordered state, sizable spin-orbital interactions lead to a small canting of ${\mathrm{Fe}}^{3+}$ moments. Magnetic dipole-dipole interactions contribute significantly to the experimentally observed orientation of magnetization easy axis in the $ac$ plane. The first principles calculations of leading exchange interactions were found in agreement with measurements of thermodynamic properties and Raman spectroscopy.