Chemical trend of exchange coupling in diluted magnetic II-VI semiconductors: Ab initio calculations

We have calculated the chemical trend of magnetic exchange parameters (J(dd), N alpha, and N beta) of Zn-based II-VI semiconductors ZnA (A=O, S, Se, and Te) doped with Co or Mn. We show that a proper treatment of electron correlations by the local spin-density approximation (LSDA)+U method leads to good agreement between experimental and theoretical values of the nearest-neighbor exchange coupling J(dd) between localized 3d spins in contrast to the LSDA method. The exchange couplings between localized spins and doped electrons in the conduction band N alpha are in good agreement with experiment as well. But the values for N beta (coupling to doped holes in the valence band) indicate a crossover from weak coupling (for A=Te and Se) to strong coupling (for A=O) and a localized hole state in ZnO:Mn. This hole localization explains the apparent discrepancy between photoemission and magneto-optical data for ZnO:Mn.