$^{77}$Se NMR investigation of the paramagnetic metal phase of $\lambda $-(BETS)$_{2}$FeCl$_{4}$

We report $^{77}$Se NMR measurements of the spectrum and the spin-lattice relaxation rate (1/$T_{1})$ in a 7 $\mu$g single crystal of $\lambda $­(BETS)$_{2}$FeCl$_{4}$ over the temperature ($T)$ range 2.5-10 K in an applied field of 10.9 T parallel to the $a$-axis (paramagnetic metal phase). A behavior close to 1/$T_{1}T$ = constant is observed. It indicates that for these conditions, 1/$T_{1}$ is dominated by the hyperfine interaction between the $^{77}$Se spins and the conduction electrons, in contrast to 1/$T_{1}$ for the protons, which is driven by the magnetic fluctuations of the Fe$^{3+}$ spins [W.G. Clark et al., Appl. Mag. Res. \textbf{27}, 279 (2004)]. From these proton measurements, we estimate that the contribution of the Fe$^{3+}$ fluctuations to 1/$T_{1}$ of $^{77}$Se is negligible. Work at UCLA was supported by NSF Grants DMR-0334869 (WGC) and DMR-0520552 (SEB).