Nuclear spin-lattice relaxation in κ-(BETS)2FeBr4

We report results of 77Se NMR study for a newly found field-induced superconductor, κ-(BETS)2FeBr4. The nuclear spin lattice relaxation rate (1/T1) above 25 K has a linear relation with temperature, 1/T1 = aT+b, which can be analyzed in a framework of normal metal with dipolar fields from uncorrelated 3d local moments to the Se nuclei. However, 1/T1 has an evident kink at 25 K and shows a remarkable decrease within a narrow temperature range of 2.5 K. Below 22.5 K, we found that 1/T1 follows a power law 1/T1∼T1.43, although the material is still conductive. The temperature where 1/T1 shows a kink agrees with the temperature where the temperature devivative of the resistivity dρ/dT diverses. We discuss a possible reconstruction of the electronic state, which may attain magnetic correlation below this temperature.