Development of an antiferromagnetic organic superconductor κ-(BETS)2FeBr4

The first antiferromagnetic organic superconductor, kappa-(BETS)(2)FeBr4 [BETS = bis(ethylenedithio)tetraselenafulvalene], was developed in the search for novel magnetic molecular conductors. This salt shows successive antiferromagnetic and superconducting transitions at T-N = 2.5 K and T-c = 1.1 K, respectively, at ambient pressure. Specific heat measurements and anisotropic behavior of intraplanar conducting properties under applied magnetic fields prove the coexistence of the antiferromagnetic ordering and superconductivity below T-c. The combination of metamagnetism of the magnetic layer and superconductivity of the conduction layer, which makes this salt a dual-functional system, yields two characteristic magnetic field-induced superconducting states around 1.6 and 12.5 T in terms of the Jaccarino-Peter compensation effect. Systematic studies on both the chemical pressure effect by a halogen exchange in the alloy system kappa-(BETS)(2)FeBCl4-x and the physical pressure effect by an application of real pressures in kappa-(BETS)(2)FeBr4 were performed to discuss the pi-d interaction in the kappa-(BETS)(2)FeBr4 system.