Effects of disorder on the exchange coupling in (Ga,Mn)As diluted magnetic semiconductors

Abstract

A theoretical study of the effects of disorder on the Mn-Mn exchange interactions for Ga 1−x Mn x As diluted magnetic semiconductors is presented. The disorder is intrinsically considered in the calculations, which are performed using an ab initio total energy density-functional approach, for a 128 atoms supercell, and by considering a variety of configurations with 2, 3 and 4 Mn atoms. Results are obtained for the effective J Mn−Mn n , from first (n = 1) all the way up to sixth (n = 6) neighbors via a Heisenberg Hamiltonian used to map the magnetic excitations from ab initio total energy calculations. One then obtains a clear dependence in the magnitudes of the J Mn−Mn n with the Mn concentration x. Moreover, we show that, in the case of fixed Mn concentration, configurational disorder and/or clustering effects lead to large dispersions in the Mn-Mn exchange interactions. Also, calculations for the ground-state total energies for several configurations suggest that a proper consideration of disorder is needed when one is interested in treating temperature and annealing effects.Keywords: Disorder; Exchange coupling; Diluted magnetic semiconductorsIn the past few years a lot of attention has been given to the study of diluted magnetic semiconductors (DMS), due to possible applications in new devices by manipulating both the spin and the charge of the carriers in semiconductor systems. This was even more so after the successful growth of ferromagnetic (Ga,Mn)As alloys [1], in which the critical temperature and hole concentration (cf. Fig. 1), as a function of Mn composition, are very much dependent on the details of growth conditions [2][3][4][5][6][7][8][9][10][11][12][13][14]. From the theoretical point of view, several calculations [15][16][17][18][19][20][21][22][23][24] have been performed to extract exchange couplings (J´s) for specific clusters containing Mn ions, with overall results suggesting a tendency of a decrease in |J| when more Mn atoms are added to nearby sites, and a considerable scatter in the values of the exchange couplings. Here we stress that the use of non-full potential muffin-tin-style approaches is not adequate to treat the electronic structure of covalent semiconductor systems such as (Ga,Mn)As DMS, and that disorder quite certainly is not realistically treated by simple effective-medium approaches such as the virtual crystal approximation (VCA) or CPA.We have performed total energy calculations within the density-functional theory (DFT) and the generalized-gradient approximation (GGA) for the exchange-correlation potential, with the electron-ion interactions described using ultrasoft pseudopotentials [25], and plane wave expansion up to 230 eV as implemented in the VASP code [26]. For a 128 atoms supercell, we consider a variety of configurations with 2, 3 and 4 Mn atoms, corresponding to concentrations of 3.1%, 4.7%, and 6.3%, respectively. We have considered typical configurations, as generated through the Special Quasi-random Structures (SQS) algorithm [27]. Results for the effectiv...