  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
NaMgFe(C$_2$O$_4$)$_3\cdot$9H$_2$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 $S$
= 5/2 ground state multiplets of Fe$^{3+}$ ion. Further analysis of the INS
results shows that the Fe$^{3+}$ ion has an easy-axis anisotropy with axial ZFS
parameter $D$ = $-$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.
