Coexistence of antiferromagnetic order and superconductivity in organic conductors

We have recently found various magnetic organic conductors exhibiting novel electric and magnetic properties, where the interplay between magnetism an d conductivity play an essential role. There are mainly two types of crysta l modification (lambda and kappa) in (BETS)(2)FeX4 [BETS = bis(ethylenedith io)tetraselenafulvalene]. In the triclinic modification, lambda-(BETS)(2)Fe Cl4, conduction electrons in the BETS layers interact with spins localized on the FeCl4- anions to give an antiferromagnetic insulating ground state. But the system becomes a superconductor at high pressure. In addition, lamb da-(BETS)(2)FexGa1-xCl4 (0.35 < x < 0.5) undergoes an unprecedented superco nductor-to-insulator transition. The orthorhombic modification, kappa-(BETS )(2)FeBr4, has been revealed to be the first antiferromagnetic organic supe rconductor (T-N = 2.4 K and T-c= 1.1 K). The specific heat measurements sho wed evidence for the coexistence of magnetic order and superconductivity be low T-c. The easy axis of the antiferromagnetic metal phase below T-N is pa rallel to the a axis and the field dependence of the magnetization showed a metamagnetic behavior around 2 T. Recently, we have found that isostructur al lambda-(BETS)(2)FeCl4 under-goes similar magnetic and superconducting tr ansitions at lower temperatures. (C) 2001 Elsevier Science Ltd. All rights reserved.