Effect of cotunneling and spin polarization on the large tunneling magnetoresistance effect in granular C60-Co films

The tunneling magnetoresistance (TMR) effect of granular C-60-Co films at low temperatures and also at small applied voltages is studied in the current-perpendicular-to-plane (CPP) geometry to elucidate the spin-dependent tunneling process, bringing about a remarkably high magnetoresistance (MR) as compared to the expectations from the conventional theory on sequential tunneling. The current-voltage characteristics showed ohmic and power-law dependences in the Coulomb blockade regime, which are interpreted as the occurrence of cooperative tunneling (so-called cotunneling) through a few to several Co nanoparticles. The zero-bias MR ratios are in the range of 50%-90% at a few degrees Kelvin and show strong and unconventional temperature dependence depending on the temperature range. Furthermore, the spin polarization of tunneling electrons evaluated based on the cotunneling model is in the range of 50%-80%, suggesting that the enhanced spin polarization of tunneling electrons at the interface between Co nanoparticles and a C-60-based matrix (C-60-Co compound) is crucial for large TMR effects.