77Se NMR evidence for the development of antiferromagnetic spin fluctuations of π-electrons in λ-(BETS)2GaCl4

The microscopic magnetic properties of superconducting (T-C congruent to 6 K) lambda-(BETS)(2)GaCl4 (BETS = bis(ethylenedithio)tetraselenafulvalene) have been investigated by Se-77 NMR in the normal state between 7.1 and 100 K on randomly oriented single crystals with kappa-(BETS)(2)GaCl4 as a reference compound. From the T-dependences of the Knight shift and the linewidth. the intrinsic uniform susceptibility chi(q=0)(T) has been found to decrease with decreasing T below 30 K for lambda-(BETS)(2)GaCl4. Nevertheless. 1/(T1T) does not show any corresponding decrease but is constant down to the lowest T, which indicates increasing contributions with decreasing T from antiferromagnetic spin fluctuations. The Knight shift and 1/(T1T) have been analyzed quantitatively and the enhancement factor K(alpha) > 1 is deduced in the low-T limit. On the basis of these results, the development of anti ferromagnetic spin fluctuations of pi-electrons at low T is concluded for lambda-(BETS)(2)GaCl4.