Magnetocaloric Effect In Layered Organic Conductor Lambda-(Bets)(2)Fecl4

Magnetocaloric effect (MCE) and magnetic torque measurements have been carried out in the pi-d system lambda-(BETS)(2)FeCl4 [BETS = bis(ethylenedithio)tetraselenafulvalene], which shows an antiferromagnetic insulating (AFI) phase below similar to 8.5 K. In the magnetic torque curve, a sharp structure at similar to 1.2 T and a step at similar to 10 T are observed at low temperatures, which are caused by the spin-flop (SF) transition and the transition from the AFI to paramagnetic metallic (PM) phase, respectively. The MCE, directly related to the magnetic entropy, shows a small sharp peak at the SF transition and a sharp dip at the AFI-PM transition. The overall feature above 3K is qualitatively interpreted by a simple picture: antiferromagnetic (AF) pi spins and paramagnetic 3d spins at the Fe sites. However, a broad dip in the MCE is additionally found at similar to 5 T below similar to 3K, which is not explained by the above picture. The results are compared with those of kappa-(BETS)(2)FeBr4, which shows an AF order of the 3d spins at the Fe sites.