Enhanced Magnetoresistance in Insulating Granular Systems: Evidence for Higher-Order Tunneling

We study the temperature and bias-voltage dependence of the magnetoresistance (MR) in insulating Co-Al-O granular films. The MR exhibits strong temperature dependence and is enhanced more than 20% at low temperatures, while it has no appreciable change in the bias-voltage dependence. The results provide clear evidence for the successive onset of higher-order processes of spin-dependent tunneling between large granules through intervening small ones with strong Coulomb blockade. The remarkable contrast between the temperature and bias-voltage dependence of the MR is consistently explained.