Abstract
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 impurity- and host-induced effects on the exchange constants is carried out by comparing (i) Mn- and 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.
Highlights
A strong ‘environmental’ effect: along specific crystallographic directions, the values of the exchange constant Ji j strongly depends on the distribution of the Mn atoms around sites i and j
In order to have a deep understanding of the exchange constants in group-IV DMS, we focus on the dependence on (i) the concentration; (ii) the semiconducting host and (iii) the impurity
First-principles simulations have been carried out for TM-doped Ge, mainly focusing on the exchange interactions calculated by means of the frozen-magnon approach
Summary
We have carried out first-principles calculations within the density functional theory using the generalized gradient approximation (GGA) [18] to the exchange–correlation potential. The GGA + U approach is generally used for systems with strongly correlated electrons, it was successfully applied to SCs [23] to partially correct the deficiencies of a bare GGA treatment even in SCs. A value of U = 2 eV was found to reproduce the correct experimental band gap in pure Ge (Egap ∼ 0.65 eV) and was consistently applied in all the simulations (i.e. in CrGe and GaMnAs—on the As p-states). In order to simulate both concentration and environmental effects, we considered the following systems, each containing one Mn atom (located in the origin) per unit cell: (i)
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