  Using Raman scattering, the quasi-two dimensional organic superconductor
k-(BEDT-TTF)2Cu[N(CN)2]Br (T_c=11.8 K) and the related antiferromagnet
k-(BEDT-TTF)2Cu[N(CN)2]Cl are studied. Raman scattering provides unique
spectroscopic information about magnetic degrees of freedom that has been
otherwise unavailable on such organic conductors. Below T=200 K a broad band at
about 500 cm-1 develops in both compounds. We identify this band with
two-magnon excitation. The position and the temperature dependence of the
spectral weight are similar in the antiferromagnet and in the metallic
Fermi-liquid. We conclude that antiferromagnetic correlations are similarly
present in the magnetic insulator and the Fermi-liquid state of the
superconductor.
