A novel antiferromagnetic organic superconductor kappa-(BETS)(2)FeBr4 [where BETS = bis(ethylenedithio)tetraselenafulvalene]

The electrical and magnetic properties of kappa-(BETS)(2)FeBr4 salt [where BETS = bis(ethylenedithio)tetraselenafulvalene] showed that this system is the first antiferromagnetic organic metal at ambient pressure (T-N = 2.5 K) . The characteristic field dependence of the magnetization at 2.0 K indicat es a clear metamagnetic behavior. The small resistivity drop observed at T- N clearly shows the existence of the interaction between pi metal electrons and localized magnetic moments of Fe3+ ions. In addition, this system unde rwent a superconducting transition at 1.1 K. That is, kappa-(BETS)(2)FeBr4 is the first antiferromagnetic organic metal exhibiting a superconducting t ransition below Neel temperature. The magnetic field dependence of the supe rconducting critical temperature indicated that the superconductivity in th is system is strongly anisotropic also in the conduction plane because of t he existence of the metamagnetically induced internal field based on the an tiferromagnetic ordering of the Fe3+ 3d spins in contrast to the cases of t he other conventional organic superconductors. Furthermore, the specific he at measurement exhibited a lambda -type large peak of zero-field specific h eat corresponding to the three-dimensional antiferromagnetic ordering of hi gh-spin Fe3+ ions. The lack of distinct anomaly in the C-p vs T curve at T- c suggests the coexistence of the superconductivity and the antiferromagnet ic order below T-c.