s-d exchange interaction induced magnetoresistance in magnetic ZnO

The magnetoresistance (MR) effect in Co-doped ZnO films prepared by pulsed laser deposition on a-plane sapphire substrates with electron concentration at 5 K ranging from 8.3×1017 cm−3 to 9.9×1019 cm−3 has been studied experimentally and theoretically. A large positive MR of 124% has been observed in the film with the lowest electron concentration of 8.3×1017 cm−3, while only a negative MR of −1.9% was observed in the film with an electron concentration of 9.9×1019 cm−3 at 5 K. The positive MR is attributed to the quantum correction on the conductivity due to the s-d exchange interaction induced spin splitting of the conduction band. The negative MR is attributed to the magnetic field suppressed weak localization. The presented modelling of superimposed positive and negative MR well agrees with the experimentally observed MR and hints at the physical origin of MR in Co-doped ZnO.