Magnetism of BETS conductors κ- and λ-(BETS)2FeCl4

With a novel and sensitive Faraday type magnetometer, which is completely made out of quartz glass, we have measured the magnetisation of the organic conductors κ- and λ-(BETS)2FeCl4 (BETS = bis(ethylenedithio)tetraselenafulvalene) in fields up to 7 Tesla and at temperatures down to 1.3 K, Nearly paramagnetic behaviour is observed for κ-type salts with Curie-Weiss temperatures θac = −1.4 K and θb = −1.6 K, the results indicating that Fe3+ is in a high spin state (S = 5/2). The results of λ- type salts shows an antiferromagnetic ordering at T < 8 K in the field direction along c and a spin flop critical field of 1.2 T at 1.3 K, indicating that the easy axis is near the c-axis. The results indicate that there are indirect exchange interactions between Fe3+ ions via conduction electron spins.