Antiferromagnetic ordering in the conducting π-d system κ-(BEDT-TSF)₂FeCl₄ (where BEDT-TSF is bis(ethylenedithio)tetraselenafulvalene, C₁₀S₄Se₄H₈)

To study the magnetic ground state in the title compound of a quasi two-dimensional charge transfer salt containing Fe³⁺ (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 ac-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.