Structural stability of (Ga,Mn)As from first principles: Random alloys, ordered compounds, and superlattices

We calculate by a combination of density functional theory and mixed-basis cluster expansion the structural stability of ordered and disordered zincblende GaAs-MnAs systems. We find that the ground state of this system is phase separating into GaAs+MnAs, even though the strain energy is negligible. The study of short-period superlattices shows that the least-unstable superlattices are along the (111) orientation whereas the most-unstable orientation is the (201). The formation enthalpy of the random alloy has been calculated; combining it with a mean-field approximation, we obtain the temperature-composition phase diagram showing the miscibility-gap temperature below which the alloy phase separates. The stabilization energy for (100) (Ga_1−xMn_xAs)₁∕(GaAs)_n superlattices shows that these superlattices prefer ferromagnetic order over a nonferromagnetic arrangement. Remarkably, the decay of the exchange interactions with superlattice period n is slower for the Mn dilute x=0.5 case than for x=1. This shows that as the system becomes more Mn dilute the range of the exchange interactions increase. This reveals an exceptional property of dilute magnetic semiconductors, namely that the system counter balances dilution of the magnetic ions by extending the range of exchange interactions, hence maintaining ferromagnetism down to small concentrations of a magnetic ion.