LAUSR

We present a model compound for the $S=2$ antiferromagnetic (AF) spin chain composed of the salt ($o$-MePy-$\mathrm{V}){\mathrm{FeCl}}_{4}$. Ab initio molecular-orbital calculations indicate the formation of a partially stacked two-dimensional (2D) spin model comprising five types of exchange interactions between $S=1/2$ and $S=5/2$ spins, which locate on verdazyl radical and Fe ion, respectively. The magnetic properties of the synthesized crystals indicate that the dominant interaction between the $S=1/2$ and $S=5/2$ spins stabilizes an $S=2$ spin in the low-temperature region, and an effective $S=2$ AF chain is formed for $T\ensuremath{\ll}10$ K and $Hl4$ T. We explain the magnetization curve and electron-spin-resonance modes quantitatively based on the $S=2$ AF chain. At higher fields above quantitatively 4 T, the magnetization curve assumes two-thirds of the full saturation value for fields between 4 and 20 T, and approaches saturation at $\ensuremath{\sim}40$ T. The spin model in the high-field region can be considered as a quasi-2D $S=1/2$ honeycomb lattice under an effective internal field caused by the fully polarized $S=5/2$ spin.