  Shubnikov-De Haas effect (SdH) effect and angular dependent magnetoresistance
oscillations (AMRO) were observed in the organic superconductor:
$\kappa$-(BETS)$_2$FeBr$_4$. In contrast to its isostructural compound
$\kappa-(BETS)$_2$FeCl$_4$, SdH oscillations, for fields perpendicular to the
conducting planes, reveal three Fermi Surface (FS) closed orbits $\alpha$,
$\beta$, and $\gamma$ whose cross sectional areas are 19.8 %, 99.9 %, and 2.4 %
of the first Brillouin zone, respectively. The conduction electron effective
masses were found to be: $\mu_{\alpha} = (4.7 \pm 0.2)$ m$_e$, $\mu_{\beta} =
(8.0 \pm 1.0)$ m$_e$, and $\mu_{\gamma} = (2.0 \pm 0.2)$ m$_e$. The observation
of a $\gamma$ orbit is not expected from band structure calculations,
$\mu_{\beta}$ is among the heaviest masses ever reported for an organic
conductor. The observed Yamaji-like AMRO indicates a 2-D closed FS, warped
along the $k_z$ direction.

Strongly Correlated Electrons;Superconductivity