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.
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Mitani et al. (1998) studied this question.