Antiferromagnetic Ordering in the Conducting π-d System κ-(BEDT-TSF) 2FeCl 4 (where BEDT-TSF is Bis(ethylenedithio)tetraselenafulvalene, C 10S 4Se 4H 8)

To study the magnetic ground state in the title compound of a quasi two-dimensional charge transfer salt containing Fe 3+ ( S = 5/2) paramagnetic ions, we have measured the magnetic torque at 0.4 K ≤ T ≤4 K and H ≤14 T using a capacitive cantilever torque meter. The torque and magnetization data above 1 K are well scaled by the Brillouin plot with a Weiss temperature of -0.8 K. From theoretical analysis for a uniaxial antiferromagnet, anomalous torque curves below 1 K reveal that an antiferromagnetic ordering with its easy axis perpendicular to the conducting a c -plane occurs between 0.5 K and 1 K. This antiferromagnetic order coexists with the metallic state of π-electrons, indicating quite weak π- d interaction.