Direct determination of the zero-field splitting for the Fe 3 + ion in a synthetic polymorph of Na Mg Fe ( C 2 O 4 ) 3 · 9 H 2 O : A natural metal-organic framework

We employed inelastic neutron scattering (INS), specific heat, and magnetization analysis to study the magnetism in a synthetic polymorph of the quasi-two-dimensional natural metal-organic framework material, stepanovite $\mathrm{Na}\mathrm{Mg}\mathrm{Fe}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{3}\ifmmode\cdot\else\textperiodcentered\fi{}9{\mathrm{H}}_{2}\mathrm{O}$. No long-range magnetic order can be observed down to 0.5 K. The INS spectra show two dispersionless excitations at energy transfer 0.028(1) and 0.050(1) meV at base temperature, which are derived from the magnetic transitions between zero-field splitting (ZFS) of $S\phantom{\rule{4pt}{0ex}}=5/2$ ground state multiplets of ${\mathrm{Fe}}^{3+}$ ion. Further analysis of the INS results shows that the ${\mathrm{Fe}}^{3+}$ ion has an easy-axis anisotropy with axial ZFS parameter $D=\ensuremath{-}0.0128(5)$ meV and rhombic parameter $E=0.0014(5)$ meV. The upward behavior at zero field and Schottky-like peak under magnetic field of the low-temperature magnetic specific heat further support the INS results. Our results clearly reveal the magnetic ground and excited state of this stepanovite polymorph.