Organic Antiferromagnetic Metals Exhibiting Superconducting Transitions #kappa#-(BETS)_2FeX_4 (X = Cl, Br): Drastic Effect of Halogen Substitution on the Successive Phase Transitions

摘要

The magnetic and thermal properties of an organic conductor incorporating localized magnetic moments, #kappa#-(BETS)2FeCl_4, were investigated down to 60-70 mK. Similar to the Br analogue, #kappa#-(BETS)_2FeBr_4, #kappa#-(BETS)2FeCl_4 exhibited successive antiferromagnetic and superconducting transitions with lowering temperature (T_N = 0.45 K,T_C = 0.1 K). That is #kappa#-(BETS)2FeCl_4 is the second antiferromagnetic organic metal pressure. It became clear that the halogen exchange (Br->Cl) in the anions results in the strong reduction of both magnetic and superconducting transition temperatures. Resistivities showed a small drop at 0.45 K ( = T_N), which gave a electrons and localized magnetic moments of Fe atoms. In contrast to #kappa#-(BETS)_2FeBr_4 exhibiting a three-dimensional nature of the magnetic transition, the specific heat of kappa#-(BETS)2FeCl_4 indicates the low dimensionality of spin system.