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Abstract
Crystals of BETS conductors containing FeX 4 − (X=Cl, Br) anions are polymorphic. λ-BETS 2 FeCl 4 undergoes a π-d coupled antiferromagnetic insulating transition at about 8.5 K. But it exhibits a paramagnetic metal - antiferromagnetic metal transition and superconducting transition at high pressure. κ-BETS 2 FeX 4 (X=Cl, Br) are antiferromagnetic organic superconductors with the transition temperatures of T N =2.4 K (Br), 0.45 K (Cl) and T c =1.1 K (Br) and 0.1 K (Cl). Resistivity step observed at T N is the first direct evidence for π-d interaction in organic metals. No distinct anomaly in the specific heat at T c and the anisotropy of the recovery of resistivity under magnetic field at T<T c suggest the coexistence of superconductivity and magnetic order in λ-BETS 2 FeBr 4 . In contrast to the case of κ-BETS 2 FeBr 4 exhibiting three-dimensional magnetic transition, small entropy of the transition of κ-BETS 2 FeCl 4 indicates the low-dimensionality of the spin system.