Bias voltage dependence of tunneling magnetoresistance in granular C ₆₀Co films with current-perpendicular-to-plane geometry

Voltage-dependence of the tunneling magnetoresistance effect in the granular C ₆₀Co films has been investigated for the samples with the current-perpendicular-to-plane geometry. The transport measurements under this geometry demonstrate that the granular C ₆₀Co films show an unusual exponential bias voltage dependence of the magnetoresistance ratio down to zero voltage. Small characteristic energies of less than 10s meV are derived from the temperature dependences of the characteristic voltage in the exponential relationship. Considering the magnitudes of the voltage drop between Co nanoparticles and also the effect of cotunneling on the energy values, the characteristic energies for the voltage-induced degradation of the spin polarization are found to show a satisfactory agreement with that for the thermally-induced one. It can be reasonably expected that the onset of magnetic disorder to the localized d-electron spins at the interface region of the C ₆₀-based matrix (C ₆₀Co compound) with Co nanoparticles leading to the unusual voltage and temperature dependence of the magnetoresistance ratio and the spin polarization at low temperatures.