Organic Antiferromagnetic Metals Exhibiting Superconducting Transitions -(BETS)2FeX4 (X=Cl, Br): Drastic Effect of Halogen Substitution on the Successive Phase Transitions

The magnetic and thermal properties of an organic conductor incorporating localized magnetic moments, -(BETS)2FeCl4, were investigated down to 60–70 mK. Similar to the Br analogue, -(BETS)2FeBr4, -(BETS)2FeCl4 exhibited successive antiferromagnetic and superconducting transitions with lowering temperature (TN=0.45 K, TC=0.1 K). That is, -(BETS) 2FeCl4 is the second antiferromagnetic organic metal that exhibits a superconducting transition at ambient pressure. It became clear that the halogen exchange (BrCl) in the anions results in the strong reduction of both magnetic and superconducting transition temperatures. Resistivities showed a small drop at 0.45 K (=TN), which gave a direct evidence of the existence of –d interaction between metal electrons and localized magnetic moments of Fe atoms. In contrast to -(BETS)2FeBr4 exhibiting a three-dimensional nature of the magnetic transition, the specific heat of -(BETS)2FeCl4 indicates the low dimensionality of the spin system.