Direct determination of the zero-field splitting for Fe³⁺ ion in the oxalate mineral stepanovite

We employed inelastic neutron scattering (INS), specific heat, and magnetization analysis to study the magnetism in a synthetic sample of a two-dimensional natural metal-organic framework material, stepanovite NaMgFe(C₂O₄)₃(9H₂O). No long-range magnetic order can be observed down to 0.5 K. The INS spectra show two dispersionless excitations at energy transfer, 0.03 and 0.05 meV, which are derived from the magnetic transitions between zero-field splitting (ZFS) of S = 5/2 ground state multiplets of Fe³⁺ ion. Further analysis of the INS results shows that the Fe³⁺ ion has an easy-axis anisotropy with axial ZFS parameter D = −0.0128(5) meV and rhombic parameter E = 0.0014(5) meV.