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Effects of spin-orbit and s-d exchange interactions are probed by magnetoresistance ͑MR͒ measurements carried out down to 50 mK on ZnO and Zn 1−x Mn x O with x = 3 and 7% and electron concentration ϳ10 20 cm −3 . A description of the data for ZnO:Al in terms of weak-localization theory leads to the coupling constant so = ͑4.4± 0.4͒ ϫ 10 −11 eV cm of the kp hamiltonian for the wurzite structure, H so = so c͑s ϫ k͒. A complex and large MR of Zn 1−x Mn x O : Al is interpreted in terms of the influence of the s-d spin-splitting on the disordermodified electron-electron interactions, which explains positive MR. A large negative MR is tentatively assigned to a precursor effect of the magnetic polaron formation. It is suggested that the proposed model explains the origin of MR observed recently in many magnetic oxide systems.

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