H-NMR study of magnetic anomaly in (BEDT-TTF)/sub 3/CuBr/sub 4/

Summary form only given. (BEDT-TTF)/sub 3/CuBr/sub 4/ is a charge transfer complex with large /spl pi/-d interaction. This salt is semiconductive at ambient pressure, while a metallic state is induced above 100 K by applying pressure higher than 5 kbar. At ambient pressure the static magnetic susceptibility, /spl chi//sub stat/, which obeys a Curie-Weiss law at high temperatures, is reduced by about a half below 59 K. The reduction of /spl chi//sub stat/ is explained as due to the dimerization of BEDT-TTF molecules. Below 7.5 K an antiferromagnetic long range order is observed. We have performed /sup 1/H-NMR measurementss at ambient pressure in order to investigate the changes of the electronic states at these transitions. The /sup 1/H spin-lattice relaxation rate, T/sub 1//sup -1/, was almost constant from room temperature to 60 K, below which it decreased gradually toward about a half at 10 K. This reflects the disappearance of the spins at BEDT-TTF molecules. T/sub 1//sup -1/ formed a conspicuous peak at 8 K and an enormous broadening of the absorption line was observed below this temperature. These are clear indications of the antiferromagnetic ordering. The change in the exchange interaction at 59 K was estimated from T/sub 1//sup -1/. The absolute values are consistent with the model mentioned above.