Bulk antiferromagnetism in $\mathbf{N}{\mathbf{a}}_{\mathbf{0.82}}\mathbf{C}\mathbf{o}{\mathbf{O}}_{\mathbf{2}}$ single crystals

Susceptibility, specific heat, and muon spin rotation measurements on high-quality single crystals of $\mathrm{N}{\mathrm{a}}_{0.82}\mathrm{C}\mathrm{o}{\mathrm{O}}_2$ have revealed bulk antiferromagnetism with N'{e}el temperature ${\mathrm{T}}_{\mathrm{N}}=19.8\pm 0.1$ K and an ordered moment perpendicular to the $\mathrm{C}\mathrm{o}{\mathrm{O}}_2$ layers. The magnetic order encompasses nearly 100% of the crystal volume. The susceptibility exhibits a broad peak around 30 K, characteristic of two-dimensional antiferromagnetic fluctuations. The in-plane resistivity is metallic at high temperatures and exhibits a minimum at ${\mathrm{T}}_{\mathrm{N}}$.