Indication for an antiferromagnetically ordered state in the organic conductor kappa-(BETS)(2)FeCl4

In this paper, we report on the magnetic properties of the organic conducto r kappa-(BETS)(2)FeCl4, which were studied with a novel Faraday magnetomete r in the temperature range from 0.6 to 40 K and under magnetic fields up to 7 T. Above 1 K, kappa-(BETS)(2)FeCl4 exhibits a Curie-Weiss behavior with an average Curie-Weiss temperature of about -1.5 K. Slightly anisotropic Cu rie-Weiss laws have been obtained for the field directions parallel and per pendicular to the conducting ac-layer. Below 1 K, spin-flop transitions hav e been observed in low magnetic fields (0.7 T) for the field direction perp endicular to the conducting layer, which indicates that the system is in an antiferromagnetically ordered state with the easy axis normal to the condu cting layer. The Neel temperature is near 1 K. (C) 2000 Elsevier Science Lt d. All rights reserved.