Development of an Antiferromagnetic Organic Superconductor κ-(BETS)2FeBr4

The first antiferromagnetic organic superconductor, κ-(BETS)2FeBr4 [BETS = bis(ethylenedithio)tetraselenafulvalene], was developed in the search for novel magnetic molecular conductors. This salt shows successive antiferromagnetic and superconducting transitions at TN = 2.5 K and Tc = 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 Tc. 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 κ-(BETS)2FeBrxCl4−x and the physical pressure effect by an application of real pressures in κ-(BETS)2FeBr4 were performed to discuss the π–d interaction in the κ-(BETS)2FeBr4 system.