Magnetism of BETS conductors kappa- and lambda-(BETS)(2)FeCl4

With a novel and sensitive Faraday type magnetometer, which is completely m ade out of quartz glass, we have measured the magnetisation of the organic conductors kappa- and lambda-(BETS)(2)FeCl4 (BETS = bis(ethylenedithio)tetr aselenafulvalene) in fields up to 7 Tesla and at temperatures down to 1.3 K . Nearly paramagnetic behaviour is observed for kappa-type salts with Curie -Weiss temperatures theta(ac) = -1.4 K and theta(b) = -1.6 K, the results i ndicating that Fe3+ is in a high spin state (S = 5/2). The results of lambd a-type salts shows an antiferromagnetic ordering at T < 8 K in the field di rection along c and a spin flop critical field of 1.2 T at 1.3 K, indicatin g that the easy axis is near the c-axis. The results indicate that there ar e indirect exchange interactions between Fe3+ ions via conduction electron spins.