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

Abstract

We have calculated the chemical trend of magnetic exchange parameters ($J_{dd}$, $N\alpha$, and $N\beta$) of Zn-based II-VI semiconductors $\text{Zn}A$ ($A=\text{O}$, S, Se, and Te) doped with Co or Mn. We show that a proper treatment of electron correlations by the local spin-density approximation $\left(\text{LSDA}\right)+U$ 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 $A=\text{Te}$ and Se) to strong coupling (for $A=\text{O}$) and a localized hole state in ZnO:Mn. This hole localization explains the apparent discrepancy between photoemission and magneto-optical data for ZnO:Mn.