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 $\lambda \text{−}{\left(\mathrm{BEDSe}\text{−}\mathrm{TTF}\right)}_2{\mathrm{FeCl}}_4$[BEDSe-TTF = bis(ethylenediseleno) tetrathiafulvalene] to investigate the antiferromagnetic transition in this material and evaluate the exchange interaction $J_{\pi \mathit{d}}$ between the donor molecules and ${\mathrm{FeCl}}_4^{-}$ 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 $\lambda \text{−}{\left(\mathrm{BEDSe}\text{−}\mathrm{TTF}\right)}_2{\mathrm{GaCl}}_4$. We used the angle dependence of the NMR shift in the paramagnetic phase and the experimentally determined demagnetization factor to estimate $J_{\pi \mathit{d}}$ of $\lambda {\text{-(BEDSe-TTF)}}_2{\mathrm{FeCl}}_4$ as 3.4(7) $\mathrm{T}/\mu {}_{\mathrm{B}}$ . This result shows that the $\pi \text{−}d$ interaction of $\lambda {\text{-(BEDSe-TTF)}}_2{\mathrm{FeCl}}_4$ is weaker than that of $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{FeCl}}_4$. The different signs of $J_{\pi \mathit{d}}$ between $\lambda \text{−}{\left(\mathrm{BEDSe}\text{−}\mathrm{TTF}\right)}_2{\mathrm{FeCl}}_4$ and $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{FeCl}}_4$ could be attributed to the different paths for the $\pi \text{−}d$ interaction in the two systems.