Antiferromagnetic ordering and estimation of the exchange interaction in the π−d system λ−(BEDSe−TTF)2FeCl4 studied using C13 NMR

We performed $¹³\mathrm{C}$ NMR measurements on $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BEDSe}\text{\ensuremath{-}}\mathrm{TTF})}₂{\mathrm{FeCl}}₄$[BEDSe-TTF = bis(ethylenediseleno) tetrathiafulvalene] to investigate the antiferromagnetic transition in this material and evaluate the exchange interaction ${J}_{\ensuremath{\pi}\mathit{d}}$ between the donor molecules and ${\mathrm{FeCl}}₄^{\ensuremath{-}}$ anions. Based on the NMR spectrum and the nuclear spin-lattice relaxation rate, we determined that the antiferromagnetic transition in the donor layers occurs at 25 K. The structure of the NMR spectra in the antiferromagnetic phase resembles that of $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BEDSe}\text{\ensuremath{-}}\mathrm{TTF})}₂{\mathrm{GaCl}}₄$. We used the angle dependence of the NMR shift in the paramagnetic phase and the experimentally determined demagnetization factor to estimate ${J}_{\ensuremath{\pi}\mathit{d}}$ of $\ensuremath{\lambda}{\text{-(BEDSe-TTF)}}₂{\mathrm{FeCl}}₄$ as 3.4(7) $\mathrm{T}/\textmu{}{}_{\mathrm{B}}$ . This result shows that the $\ensuremath{\pi}\text{\ensuremath{-}}d$ interaction of $\ensuremath{\lambda}{\text{-(BEDSe-TTF)}}₂{\mathrm{FeCl}}₄$ is weaker than that of $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BETS})}₂{\mathrm{FeCl}}₄$. The different signs of ${J}_{\ensuremath{\pi}\mathit{d}}$ between $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BEDSe}\text{\ensuremath{-}}\mathrm{TTF})}₂{\mathrm{FeCl}}₄$ and $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BETS})}₂{\mathrm{FeCl}}₄$ could be attributed to the different paths for the $\ensuremath{\pi}\text{\ensuremath{-}}d$ interaction in the two systems.