MAGNETIC-ANISOTROPY OF ORGANIC CONDUCTOR LAMBDA-(BETS)(2)FECL4

lambda-(BETS)(2)FeCl4, where BETS is bis(ethylenedithio)tetraselenaful valene, is an organic molecular metal with magnetic anions. It undergo es a metal-insulator transition at about TM-I similar to 8 K. This ins ulating state can be suppressed and a metallic or highly-conducting st ate 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. In this pap er, we report the anisotropy of magnetic properties of lambda-(BETS)(2 )FeCl4 studied using SQUID magnetometer. The temperature dependence of the static magnetic susceptibility below TM-I was found to show a str ong field dependence when the field is applied parallel to the crystal line 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 rest oration of highly-conducting state. This strong field dependence is ab sent for other magnetic field directions, suggesting that the groundst ate of lambda-(BETS)(2)FeCl4 is antiferromagnet rather than spin-Peier ls.