Metal-insulator transition in co-doped ZnO: Magnetotransport properties

The magnetotransport properties [magnetoresistance (MR) and Hall effect] of Co-doped ZnO films prepared by pulsed laser deposition have been investigated around the metal-insulator transition (MIT) as a function of temperature (from 5 to 290 K) under a maximum magnetic field strength of 6 T. From the MR behavior measured at 5 K we conclude that the MIT occurs at the critical electron concentration n(c)approximate to 4x10(19) cm(-3). At 5 K we observed positive MR in the insulating regime (n < n(c)) and negative MR in the metallic regime (n>n(c)). Furthermore, in the transition regime of the MIT (n similar to n(c)) negative MR at low magnetic field and positive MR at high field was observed. We consider the critical electron concentration n(c) as an important material parameter because n(c) does not depend on film thickness or Co content. The anomalous Hall effect being of importance for future spintronic materials was only clearly observed in Co-doped ZnO with n < n(c).