The S-D Exchange Model As The Underlying Mechanism Of Magnetoresistance In Zno Doped With Alkali Metals

High field magnetoresistance has been studied in epitaxial n-type ZnO:Na and ZnO:Li thin films in a temperature range between 4 K and 150 K. The resulting negative magnetoresistance can be well fitted using a semiempirical model of Khosla and Fischer based on third order contributions to the s-d exchange Hamiltonian. The parameters obtained from this model were carefully analyzed. One of these parameters is related to a ratio between electron mobilities at zero field (a non-exchange scattering mobility mu(0) and an exchange or spin dependent one mu(J)). From Hall effect measurements mu(0) was obtained, displaying a weak temperature dependence in accordance with highly n-doped ZnO while the extracted mu(J) exhibits an anomalous T-dependence. On the other hand, our magnetoresistance data cannot be properly fitted using Kawabata's expression based on a weak-localization model.