Antiferromagnetic ordering and estimation of the exchange interaction in the -d system -(BEDSe-TTF)2FeCl4 studied using 13C NMR

We performed 13C NMR measurements on lambda-(BEDSe-TTF)2FeCl4[BEDSe-TTF = bis(ethylenediseleno) tetrathiafulvalene] to investigate the antiferromagnetic transition in this material and evaluate the exchange interaction J pi d between the donor molecules and 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-(BEDSe-TTF)2GaCl4. We used the angle dependence of the NMR shift in the paramagnetic phase and the experimentally determined demagnetization factor to estimate J pi d of lambda-(BEDSe-TTF)2FeCl4 as 3.4(7) T/mu B . This result shows that the pi-d interaction of lambda-(BEDSe-TTF)2FeCl4 is weaker than that of lambda-(BETS)2FeCl4. The different signs of J pi d between lambda-(BEDSe-TTF)2FeCl4 and lambda-(BETS)2FeCl4 could be attributed to the different paths for the pi-d interaction in the two systems.