Magnetic properties of (BEDT-TTF)2X with localized spins

β′-(BEDT-TTF)2X(X=ICl2, AuCl2) have quasi-two dimensional structure consisting of ET dimers with a localized spin S=12, which are formed through the side-by-side and the face-to-face intermolecular interactions for c- and b-axes, respectively. The susceptibilities have a feature of two dimensional Heisenberg antiferromagnet with the exchange interaction J/kB∼59K. The antiferromagnetic transitions were observed at TN=22K (ICl2 salt), and 28K (AuCl2 salt). The magnetizations below TN showing spin flop transitions at Hsf=11kOe suggest that the easy spin axes are oriented parallel to c-axis for both compounds. From the spin flop transitions, we estimated the ratio of the anisotropy field to the exchange field HA/HE∼2×10−5.