Magnetism and fermiology ofκ-(BEDT-TSF)2FeCl4

Magnetization studies reveal that the ground state of $\kappa$-(BEDT-TSF)${}_2$FeCl${}_4$ is $J$ = 1/2 with weak antiferromagnetic correlations. Quantum oscillation studies indicate that the effective masses of the quasiparticles are considerably enhanced with respect to those of isostructural $\kappa$-(BEDT-TSF)${}_2$GaCl${}_4$. We attribute the mass enhancement to spin fluctuation effects involving the Fe magnetic moments within the inorganic layer.