Anomalous localization effects associated with excess volume of cobalt catalyst in multiwalled carbon nanotubes

We report on the anomalous localization effects strongly associated with excess volume of a cobalt catalyst in multiwalled carbon nanotubes (MWNTs) synthesized in nanoporous alumina membranes. These effects bring about the following anomalies in bulk MWNTs: (a) A slight increase in the volume of excess cobalt changes antilocalization (AL) to weak localization (WL), (b) a further increase in excess changes this WL back to the AL, but only in magnetoresistance (MR) oscillation, and (c) even under this AL in MR, AL can not be observed in the conductance versus logarithmic temperature relation. Mechanisms for these anomalies were discussed based on the unique MWNT structures.