LSDA+U versus LSDA:: Towards a better description of the magnetic nearest-neighbor exchange coupling in Co- and Mn-doped ZnO -: art. no. 134418

We calculate the magnetic interactions between two nearest-neighbor substitutional magnetic ions (Co or Mn) in ZnO by means of density functional theory and compare it with the available experimental data. Using the local spin density approximation, we find a coexistence of ferro- and antiferromagnetic couplings for ZnO:Co, in contrast to experiment. For ZnO:Mn both couplings are antiferromagnetic, but deviate quantitatively from measurement. That points to the necessity of treating the strong electron correlation at the transition ion site more accurately by applying LSDA+U. We show that we have to distinguish two different nearest-neighbor exchange integrals for the two systems in question that are all antiferromagnetic with values between -0.8 and -1.8 meV, in reasonable agreement with experiment.