Magnetic and Transport Properties of $\pi$--$d$ System $\kappa$-(BDH-TTP)2FeCl4

Resistance, magnetic susceptibility, magnetic torque, and ESR measurements have been performed to investigate the electronic state of the two-dimensional organic conductor $\kappa$-(BDH-TTP) 2 FeCl 4 . The temperature dependence of the resistance shows monotonic metallic behavior down to 30 mK, and the magnetic susceptibility, arising mainly from the Fe $3d$ spins, follows the Curie--Weiss law down to 2 K. At low temperatures below 0.4 K, the magnetic torque steeply changes at low magnetic fields, associated with a sign change. The systematic measurements elucidate that the $3d$ spins undergo an antiferromagnetic order at about 0.4 K and that the torque sign change is caused by a spin-flop transition. A rapid decrease of the magnetoresistance at the spin-flop field provides clear evidence of a finite $\pi$--$d$ interaction. Characteristic temperature dependences of the magnetic susceptibility and ESR $g$-value are found in the paramagnetic phase, which are explained in terms of the single-ion anisotropy effect.