Magnetic Phases of $\lambda $-(BETS)$_{2}$FeCl$_{4}$ investigated by proton NMR spectroscopy.

The organic conductor $\lambda $-(BETS)$_{2}$FeCl$_{4}$, is of interest because of its unusual phases, which include a paramagnetic metal (PM), an antiferromagnetic insulator (AFI), and a field-induced superconducting phase. Important drivers for these phases are the 3d Fe$^{3+}$ moments (spin $S_{d}$ = 5/2) from the FeCl$_{4}^{-}$ anions and the $\pi $ conduction electrons (spin $S_{\pi }$ = 1/2) in the BETS donor molecules, which generate a correlated $\pi $-d electron system. Here, we report a proton NMR spectroscopy study of these phases in a small ($\sim $3~$\mu $g) single crystal of $\lambda $-(BETS)$_{2}$FeCl$_{4}$ using an applied field of 9 T over the temperature ($T)$ range 2-180 K. The results show a complex spectrum that broadens and is shifted as $T$ is lowered in the PM phase, and additional changes associated with the PM-AFI transition. The main spectral features at all $T$ are attributed to the large dipolar field from the 3d Fe$^{3+}$ ions at the proton sites. A phenomenological model provides a reasonable fit to them. On lowering $T$ through the PM-AFI transition at 3.5 K, the spectrum smears and its second and first moments change discontinuously. These features indicate that the transition is first order and that the $\pi $-d interaction is important for its properties. The work at UCLA is supported by NSF Grants DMR-0334869 (WGC) and 0203806 (SEB).