Spin manipulation in Co-doped ZnO

The magnetoresistance of n-type conducting, paramagnetic Co-doped ZnO films prepared by pulsed laser deposition on sapphire substrates has been studied experimentally and theoretically. Positive magnetoresistance (MR) of 124% has been observed in the film with the lowest electron concentration, while only a negative MR of −1.9% was observed in the film with a large electron concentration 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. Voltage control of the electron concentration in Schottky diodes revealed a drastic change of the magnetoresistance and demonstrated the electrically controllable magnetotransport behavior in Co-doped ZnO. The magnetically controllable spin polarization in Co-doped ZnO has been demonstrated at 5 K in magnetic tunnel junctions with Co-doped ZnO as a bottom electrode and Co as a top electrode. There spin-polarized electrons were injected from Co-doped ZnO to a crystallized Al2O3 layer and tunnelled through an amorphous Al2O3 barrier. Our studies demonstrate the spin polarization and manipulation in Co-doped ZnO.