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

Studies of the quasi-two-dimensional organic conductors lambda-(D)(2)MCl4 [D = donor molecules, M = Ga, Fe] have shown that lambda-(BETS)(2)GaCl4 [BETS = bis(ethylenedithio)tetraselenafulvalene] undergoes an unconventional superconducting transition and lambda-(BETS)(2)FeCl4 undergoes a field-induced superconducting transition. In lambda-type salts, the interactions between donor molecules and FeCl4- (pi-d interactions) are important. To investigate pi-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 pi-d interaction paths. lambda-(BEST)(2)MCl4 [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 lambda-(BEST)(2)FeCl4. We find that a magnetic transition is observed at around 26 K. The lambda-(BEST)(2)MCl4 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 pi-d interaction path between chalcogens and the anion, and low-field magnetization suggests that the pi-d interaction in lambda-(BEST)(2)FeCl4 is smaller than that in lambda-(BETS)(2)FeCl4. Our results show that the inner chalcogen of donor molecules is important as the path of the interaction.