Proton NMR study of the local magnetic field and fluctuations in a single crystal of the quasi-2D organic conductor $\lambda$-(BETS)$_{2}$FeCl$_{4}$

Measurements of the proton NMR spectrum and spin-lattice relaxation rate 1/T$_{1}$ in a single, 3 $\mu$g crystal of the quasi-2D organic conductor $\lambda$-(BETS)$_{2} $FeCl$_{4}$ at a magnetic field B$_{0}$ = 9 T $\parallel$ $a-c$ plane over the temperature ($T$) range 2-180 K are reported. They probe the static local field and its fluctuations in both the antiferromagneic insulator (AFI) and paramagnetic metal (PM) phases. As $T$ is decreased, there is an increase in the shift and the overall width of the NMR spectrum and a jump in these features at the PM-AFI transition ($\sim$4 K). A reasonable fit to these properties in the PM phase is obtained using the dipole field of non- interacting Fe$^{+3}$ ions. These features show that the proton spectrum and 1/T$_{1}$ are dominated by the Fe$^{+3}$ spins. The work at UCLA was supported by NSF Grants DMR-0334869 (WGC) and DMR-0203806 (SEB) and that at Indiana by the Petroleum Research Fund ACS-PRF-33912-ACI.