Role of the d−d interaction in the antiferromagnetic phase of λ−(BEDT−STF)2FeCl4

Magnetic susceptibility and proton nuclear magnetic resonance $(¹\mathrm{H}\text{\ensuremath{-}}\mathrm{NMR})$ measurements were performed for the quasi-two-dimensional $\ensuremath{\pi}\text{\ensuremath{-}}d$ interacting system $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{STF})}₂{\mathrm{FeCl}}₄$ at ambient pressure. Magnetic susceptibility arising from the $3d$ spins of the ${\mathrm{FeCl}}₄$ anion show an anisotropy at low temperature and its temperature dependence for the external field parallel to the $c$ axis is described as a broad peak structure at 8 K. A sharp peak in the temperature dependence of ${T}₁^{\ensuremath{-}1}$ associated with the antiferromagnetic (AF) transition is observed at ${T}_{\mathrm{AF}}=16$ K, together with the drastic splitting of the NMR spectrum below ${T}_{\mathrm{AF}}$. The relation between the static susceptibility and the splitting of the NMR shift suggests the existence of the relatively strong $d\text{\ensuremath{-}}d$ AF interaction. These results can be explained by the model considering the AF-coupled $d$-spin system in the AF long-range-ordered $\ensuremath{\pi}$-spin system. We find that the AF phases in $\ensuremath{\lambda}$-type salts can be universally explained by this model.