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

beta'-(BEDT-TTF)(2)X(X=ICl2, AuCl2) have quasi-two dimensional structure consisting of ET dimers with a localizd spin S=1/2, 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/k(B) similar to 59K. The antiferromagnetic transitions were observed at T-N=22K (ICl2 salt), and 28K (AuCl2 salt). The magnetizations below T-N showing spin flop transitions at H-sf=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 H-A/H-E similar to 2x10(-5).