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 $\ensuremath{\lambda}\text{\ensuremath{-}}{(D)}_{2}M{\mathrm{Cl}}_{4}$ [$D=\text{donor molecules}$, $M=\mathrm{Ga},\mathrm{Fe}$] have shown that $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BETS})}_{2}{\mathrm{GaCl}}_{4}$ [BETS = bis(ethylenedithio)tetraselenafulvalene] undergoes an unconventional superconducting transition and $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BETS})}_{2}{\mathrm{FeCl}}_{4}$ undergoes a field-induced superconducting transition. In $\ensuremath{\lambda}$-type salts, the interactions between donor molecules and ${\mathrm{FeCl}}_{4}^{\ensuremath{-}}$ ($\ensuremath{\pi}\text{\ensuremath{-}}d$ interactions) are important. To investigate $\ensuremath{\pi}\text{\ensuremath{-}}d$ interaction, a pair of magnetic $M=\mathrm{Fe}$ and nonmagnetic $M=\mathrm{Ga}$ 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 $\ensuremath{\pi}\text{\ensuremath{-}}d$ interaction paths. $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BEST})}_{2}M{\mathrm{Cl}}_{4}$ [BEST = bis(ethylenediseleno)tetrathiafulvalene] are obtained for both anions, and $M=\mathrm{Ga}$ salt shows an antiferromagnetic transition, but the ground state has not been analyzed in $M=\mathrm{Fe}$ salt. We perform x-ray diffraction, magnetic susceptibility measurement, and M\"ossbauer spectroscopy in $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BEST})}_{2}{\mathrm{FeCl}}_{4}$. We find that a magnetic transition is observed at around 26 K. The $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BEST})}_{2}M{\mathrm{Cl}}_{4}$ system is a system in which both ${\mathrm{FeCl}}_{4}^{\ensuremath{-}}$ and ${\mathrm{GaCl}}_{4}^{\ensuremath{-}}$ salts show antiferromagnetic transitions. In addition, the ethylene motions observed at room temperature are ordered around 108 K, resulting in the establishment of the $\ensuremath{\pi}\text{\ensuremath{-}}d$ interaction path between chalcogens and the anion, and low-field magnetization suggests that the $\ensuremath{\pi}\text{\ensuremath{-}}d$ interaction in $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BEST})}_{2}{\mathrm{FeCl}}_{4}$ is smaller than that in $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BETS})}_{2}{\mathrm{FeCl}}_{4}$. Our results show that the inner chalcogen of donor molecules is important as the path of the interaction.

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