Competing exchange interactions in Co-doped ZnO: Departure from the superexchange picture

International audience; We report the results of a comprehensive study of the exchange interactions in Co-doped ZnO using inelastic neutron scattering, electron paramagnetic resonance, and magnetic property measurements. In particular, we observe an unprecedentedly strong spatial anisotropy of the two nearest-neighbor exchanges, J (1) = −25.6 ± 0.3 K and J (2) = −8.5 ± 0.4 K, along with the distant-neighbor J values of ferromagnetic sign. We argue that the superexchange mechanism alone cannot account for the obtained data and we suggest that an additional mechanism leading to a strong ferromagnetic spin coupling is responsible for these findings. We also discuss the origin of this ferromagnetic mechanism.