Electronic structure and exchange constants in magnetic semi - conductors digital alloys : chemical and band - gap effects

First-principles simulations have been performed for [001]-ordered Mn/Ge and Mn/GaAs " digital alloys " , focusing on the effects of i) a larger band-gap and ii) a different semiconducting host on the electronic structure of the magnetic semiconductors of interest. Our results for the exchange constants in Mn/Ge, evaluated using a frozen-magnon scheme, show that a larger band-gap tends to give a stronger nearest-neighbor ferromagnetic coupling and an overall enhanced in-plane ferromagnetic coupling even for longer-ranged coupling constants. As for the chemical effects on the exchange constants, we show that Mn/GaAs shows a smaller nearest-neighbor ferromagnetic coupling than Mn/Ge, but exchange constants for higher Mn-Mn distance show an overall increased ferromagnetic behavior in Mn/GaAs. As a result, from the magnetic-coupling point of view, the two systems behave on average rather similarly. 1 Introduction Diluted magnetic semiconductors (DMS), i.e. mainstream semiconductors (SC) doped with small amounts of transition metal impurities, represent a class of materials with potentially relevant applications in modern spintronics. The discovery of ferromagnetism (FM) persisting to temperatures up to ≤170 K in the DMS prototype[1], i.e. GaMnAs, motivated a wealth of studies focused on Mn-doped semiconductors[2]. However, crucial issues are still fully open to debate, such as i) the origin of ferromagnetism in DMS, from the theoretical point of view, ii) a careful control over the material quality during growth, from the experimental point of view and iii) the choice of a material that might show a Curie temperature higher than room temperature, from the technological point of view. One of the key problems in DMS is the low solubility of Mn in semiconductors and severely out-of-equilibrium techniques (such as Molecular Beam Epitaxy, MBE) have to be used to successfully achieve a DMS growth without precipitates or large concentration of defects. However, many groups succeeded in doping semiconductors with large Mn concentrations by means of " digital alloys " (DA), artificial heterostructures that typically consists of a Mn plane with a nominal 50% concentration alternated with 10-15 layers of SC[4, 5]. In addition, group-IV-based spintronics is extremely appealing, due to high compatibility with mainstream Si-based technology and this has motivated the recent interests on Mn-doped Ge DMS[3, 6, 7, 8]. In particular, several studies ascertained the ferromagnetism of MBE-grown homogeneously-doped MnGe alloys up to temperatures of ∼ 110 K.[9] In addition, an extensive characterization was performed on MnGe DMS grown by means of ion-implantation, which revealed the coexistence …