Thermodynamic investigation on antiferromagnetic ordered state of the molecular π-d system λ-(BEDT-STF)₂FeCl₄

We performed heat capacity measurements on the molecular π–d system λ-(BEDT-STF)₂FeCl₄ to discuss the unconventional paramagnetic-like behavior. We observed a Schottky-type anomaly below T_N of 16 K which is inherent in the 3d spin degrees of freedom as is observed in λ-(BETS)₂FeCl₄. This result suggests that the paramagnetic-like behavior is not a unique behavior in λ-(BETS)₂FeCl₄ but a universal behavior in λ-type π–d systems. In the case of λ-(BETS)₂FeCl₄, a metal–insulator (M–I) transition occurs simultaneously with the antiferromagnetic transition at T_N of 8.3 K, and the paramagnetic-like behavior is observed below T_N. Because λ-(BEDT-STF)₂FeCl₄ is insulating over the entire temperature range, our results revealed that the M–I transition is not essential for the emergence of the paramagnetic-like behavior. By comparing the thermodynamic nature of λ-(BEDT-STF)₂FeCl₄ and λ-(BETS)₂FeCl₄, we discuss the possible origin of the paramagnetic-like behavior. (author)