Ab-initio study of exchange constants and electronic structure in diluted magnetic group-IV semiconductors

Exchange-constants have been evaluated from first-principles in Mn-doped Ge, mainly focusing on the effects of the impurity concentration and of the arrangement of Mn-atoms in the semiconducting matrix. As expected, the Mn-concentration strongly affects the magnitude of the exchange constants (especially between Mn as first-nearest-neighbors (NNs) in the cation position). Interestingly, the arrangement of Mn atoms is found to be relevant for the behavior of the second-NN exchange-constant along the [110]bonding direction, which shows either a strongly ferromagnetic or a marked antiferromagnetic behavior, depending on whether the interaction is mediated by a Ge or by an Mn atom, respectively. This suggests that, at least for rather high values of the doping concentration (∼10%), a detailed knowledge of the Mn positions in the host is required for a careful prediction of the exchange constants and, therefore, of the ordering Curie temperature in Mn-doped Ge. The analysis of the impurityand host-induced effects on the exchange constants is carried out by comparing (i) Mnand Cr-doped Ge and (ii) Mn-doped Ge and GaAs. Our findings regarding the global weak antiferromagnetism for CrGe confirm that ferromagnetism in diluted magnetic semiconductors requires the presence of holes, whereas Ge and GaAs appear rather similar as for the exchange constants, both showing strong environmental effects. 3 Author to whom any correspondence should be addressed. New Journal of Physics 10 (2008) 055017 1367-2630/08/055017+12$30.00 © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft