Magnetoresistance of 3d transition-metal-doped epitaxial ZnO thin films

Epitaxial ZnO thin films co-doped with 3d transition metal (TM) (TM = Cr, Mn, Fe, Co, Ni and Cu) and 1 mol% A1 were fabricated as a series of oxide-diluted magnetic semiconductors by pulsed-laser-deposition method. Magnetoresistance (MR) of the films was measured to investigate the s-d exchange interaction between the conducting s electron spins and the d electron spins localized at the magnetic TM impurities. A variety of MR behaviors were observed depending on the different TM impurities. It is deduced that the negative MR behavior in the vicinity of zero field is originated from an electron weak-localization effect. Caused by the s-d exchange interaction, the increase of Thomas-Fermi radius R_s and the decrease of spin-disorder scattering with increasingly aligned spins of the TM ion impurities are responsible respectively for the positive and negative MR in the higher magnetic filed.