Magnetic and Transport Properties of π–d System κ-(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 κ-(BDH-TTP)2FeCl4. 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 π–\(d\) 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.