Anomalous behavior of temperature and bias-voltage dependence of tunnel-type giant magnetoresistance in insulating granular systems

We have investigated temperature and bias-voltage dependence of tunnel-type giant magnetoresistance (MR) in Co-Al-O insulating granular films. A remarkable enhancement of MR which is larger than the theoretical value by a simple model is observed for Co-Al-O granular films at low temperatures, and it steeply decreases with increasing temperature. On the other hand, the MR does not decrease with increasing bias voltage although the resistivity decreases by a few orders of magnitude. Furthermore, it has been found that the effect of temperature is definitely different from that of bias voltage. The anomalous behavior is in contrast to that of macroscopic tunnel junctions, suggesting the significant effect of Coulomb blockade in granular systems.