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 behaviour measured at 5 K we conclude that the MIT occurs at the critical electron concentration nc~4×1019 cm−3. At 5 K we observed positive MR in the insulating regime (n>nc) and negative MR in the metallic regime (n>nc). Furthermore, in the transition regime of the MIT (n~nc) negative MR at low magnetic field and positive MR at high field was observed. We consider the critical electron concentration nc as an important material parameter because nc 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<nc.