  Carbon-based molecular semiconductors are explored for application in
spintronics because their small spin-orbit coupling promises long spin life
times. We calculate the electronic transport from first principles through spin
valves comprising bi- and tri-layers of the fullerene molecules C60 and C70,
sandwiched between two Fe electrodes. The spin polarization of the current, and
the magnetoresistance depend sensitively on the interactions at the interfaces
between the molecules and the metal surfaces. They are much less affected by
the thickness of the molecular layers. A high current polarization (CP > 90%)
and magnetoresistance (MR > 100%) at small bias can be attained using C70
layers. In contrast, the current polarization and the magnetoresistance at
small bias are vanishingly small for C60 layers. Exploiting a generalized
Julli`ere model we can trace the differences in spin-dependent transport
between C60 and C70 layers to differences between the molecule-metal interface
states. These states also allow one to interpret the current polarization and
the magnetoresistance as a function of the applied bias voltage.
