In order to elucidate the nature of ferromagnetic signatures observed in (Zn,Co)O we have examined experimentally and theoretically magnetic properties and spin-dependent quantum localization effects that control low-temperature magnetoresistance. Our findings, together with a through structural characterization, substantiate the model assigning spontaneous magnetization of (Zn,Co)O to uncompensated spins at the surface of antiferromagnetic nanocrystal of Co-rich wurtzite (Zn,Co)O. The model explains a large anisotropy observed in both magnetization and magnetoresistance in terms of spin hamiltonian of Co ions in the crystal field of the wurtzite lattice.

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We report on a systematic study of structural, optical, electrical, and magnetic properties of Zn 1−x Co x O ͑x = 0.05-0.29͒ thin films codoped with Al ͑Ͻ0.1% ͒. Both codoped ͑in which Co is cosputtered with other elements͒ and ␦-doped ͑in which Co is doped digitally in the host matrix͒ samples have been prepared and studied. Prior to doping of Co, growth conditions were optimized to produce ZnO:Al films with a resistivity of about 1.3 m⍀ cm. Although all the films with x in the range of 0.05-0.29 showed clear hysteresis at room temperature in magnetometry measurement and absorption peaks associated with the d-d transitions of Co 2+ ions, only the most heavily doped samples have shown clear anomalous Hall effect. The latter also showed strong, but photon energy dependent, magnetic circular dichroism and negative magnetoresistance at room temperature. These results in combination with detailed structural analysis by transmission electron microscope and x-ray diffraction study revealed that the ferromagnetic properties of Zn 1−x Co x O were mostly originated from secondary phases and Co precipitates. The influence of inhomogeneity on the interpretation of various measurement results is also discussed.