Antiferromagnetic Ordering and Spin Structure in the Organic Conductor, $\mathit{\kappa }$-(BEDT-TTF${)}_2$Cu[N(CN${)}_2$]Cl

The magnetism of an insulator, $\kappa$-(BEDT-TTF${)}_2$Cu[N(CN${)}_2$]Cl, situated near the 10 K superconducting phase in $\kappa$-type BEDT-TTF family, has been studied by ${}^1$H NMR for the first time. A commensurate antiferromagnetic ordering with a moment of (0.4–1.0) ${\mu }_B$/dimer was found below 26–27 K. The features, quite different from the conventional spin density wave, suggest that the magnetic order is driven by strong electron correlation rather than by nesting of the Fermi surface, and that the present salt is a Mott insulator. Magnetization measurements show spin canting parallel to the conducting layer below 23 K at low fields.