Magnetic anisotropy of organic conductor λ-(BETS)2FeCl4

λ-(BETS)2FeCl4, where BETS is bis(ethylenedithio)tetraselenafulalene, is an organic molecular metal with magnetic anions. It undergoes a metal-insulator transition at about TM−I ∼ 8 K. This insulating state can be suppressed and a metallic or highly-conducting state can be restored by applying external magnetic field higher than 10 T. The nature of this phase transition and the ground state, whether a spin-Peierls or antiferromagnet, has not been clarified. s paper, we report the anisotropy of magnetic properties of λ-(BETS)2FeCl4 studied using SQUID magnetometer. The temperature dependence of the static magnetic susceptibility below TM−I was found to show a strong field dependence when the field is applied parallel to the crystalline long axis (H//c). The nonlinear behavior in magnetization (M-H), reminiscent of a spin-flop transition of antiferromagnet, is observed at magnetic fields around 1 T, well below the field necessary for restoration of highly-conducting state. This strong field dependence is absent for other magnetic field directions, suggesting that the ground state of λ-(BETS)2FeCl4 is antiferromagnet rather than spin-Peierls.