Organic antiferromagnetic metals exhibiting superconducting transitions kappa-(BETS)(2)FeX4 (X= Cl, Br): Drastic effect of halogen substitution on the successive phase transitions

The magnetic and thermal properties of an organic conductor incorporating l ocalized magnetic moments, kappa-(BETS)(2)FeCl4, were investigated down to 60-70 mK, Similar to the Br analogue, kappa-(BETS)(2)FeBr4, kappa-(BETS)(2) FeCl4 exhibited successive antiferromagnetic and superconducting transition s with lowering temperature (T-N = 0.45 K, T-C= 0.1 K). That is, kappa-(BET S)(2)FeCl4 is the second antiferromagnetic organic metal that exhibits a su perconducting transition at ambient pressure. It became clear that the halo gen exchange (Br --> Cl) in the anions results in the strong reduction of b oth magnetic and superconducting transition temperatures. Resistivities sho wed a small drop at 0.45 K (= T-N), which gave a direct evidence of the exi stence of pi -d interaction between pi metal electrons and localized magnet ic moments of Fe atoms. In contrast to kappa-(BETS)(2)FeBr4 exhibiting a th ree-dimensional nature of the magnetic transition, the specific heat of kap pa-(BETS)(2)FeCl4 indicates the low dimensionality of the spin system. (C) 2001 academic Press.