Bulk antiferromagnetism in $bf Na_{0.82}CoO_2$ single crystals

Susceptibility, specific heat, and muon spin rotation measurements on high-quality single crystals of $rm Na_{0.82}CoO_2$ have revealed bulk antiferromagnetism with N{e}el temperature $rm T_N = 19.8 pm 0.1$ K and an ordered moment perpendicular to the $rm CoO_2$ 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 $rm T_N$.