Magnetic state in the quasi-two-dimensional organic conductor λ-(BEST)2FeCl4 and the path of π-d interaction

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Studies of the quasi-two-dimensional organic conductors λ-(D)2MCl4 [D = donor molecules, M = Ga, Fe] have shown that λ-(BETS)2GaCl4 [BETS = bis(ethylenedithio)tetraselenafulvalene] undergoes an unconventional superconducting transition and λ-(BETS)2FeCl4 undergoes a field-induced superconducting transition. In λ-type salts, the interactions between donor molecules and FeCl4- (π-d interactions) are important. To investigate π-d interaction, a pair of magnetic M = Fe and nonmagnetic M = Ga salts which have the same ground state in the donor layer is desired. However, no such pair has been found, and few experimental studies have considered π-d interaction paths. λ-(BEST)2MCl4 [BEST = bis(ethylenediseleno)tetrathiafulvalene] are obtained for both anions, and M = Ga salt shows an antiferromagnetic transition, but the ground state has not been analyzed in M = Fe salt. We perform x-ray diffraction, magnetic susceptibility measurement, and Mossbauer spectroscopy in λ-(BEST)2FeCl4. We find that a magnetic transition is observed at around 26 K. The λ-(BEST)2MCl4 system is a system in which both FeCl4- and GaCl4- salts show antiferromagnetic transitions. In addition, the ethylene motions observed at room temperature are ordered around 108 K, resulting in the establishment of the π-d interaction path between chalcogens and the anion, and low-field magnetization suggests that the π-d interaction in λ-(BEST)2FeCl4 is smaller than that in λ-(BETS)2FeCl4. Our results show that the inner chalcogen of donor molecules is important as the path of the interaction.

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