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

Abstract

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, ${J}^{(1)}=\ensuremath{-}25.6\ifmmode\pm\else\textpm\fi{}0.3$ K and ${J}^{(2)}=\ensuremath{-}8.5\ifmmode\pm\else\textpm\fi{}0.4$ 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.