$J_{\mathrm{e}\mathrm{f}\mathrm{f}}$ = 1/2 Hyperoctagon Lattice in Cobalt Oxalate Metal-Organic-Framework

We report the magnetic properties of a cobalt oxalate metal-organic-framework featuring the hyperoctagon lattice. Our thermodynamic measurements reveal the $J_{\mathrm{e}\mathrm{f}\mathrm{f}}$ = 1/2 state of the high-spin Co${}^{2+}$ (3${\mathit{\text{d}}}^7$) ion and the two successive magnetic transitions at zero field with two-stage entropy release. ${}^{13}$C-NMR measurements reveal the absence of an internal magnetic field in the intermediate temperature phase. Multiple field-induced phases are observed before full saturation at around 40 T. We argue the unique cobalt oxalate network gives rise to the Kitaev interaction and/or a bond frustration effect, providing an unconventional platform for frustrated magnetism on the hyperoctagon lattice.