LAUSR

We have successfully synthesized a hyperkagome antiferromagnet ${\mathrm{Zn}}_{2}{\mathrm{Mn}}_{3}{\mathrm{O}}_{8}$ by a topochemical method, where ${\mathrm{Mn}}^{4+}$ ions with $S=3/2$ spins form a three-dimensional (3D) corner-sharing triangle network. Magnetic susceptibility and heat capacity measurements revealed an antiferromagnetic transition at ${T}_{\mathrm{N}}=5.8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Although no evidence was found for any structural distortion accompanying the transition, the low-temperature magnetic heat capacity showed a nearly ${T}^{2}$ dependence, suggesting the realization of a 2D magnonlike excitation in the 3D hyperkagome lattice.