Exchange interactions in III-V and group-IV diluted magnetic semiconductors

Abstract

Effective pair exchange interactions between Mn atoms in III-V and group-IV diluted magnetic semiconductors are determined from a two-step first-principles procedure. In the first step, the self-consistent electronic structure of a system is calculated for a collinear spin structure at zero temperature with the substitutional disorder treated within the framework of the coherent-potential approximation. The effective exchange pair interactions are then obtained in a second step by mapping the total energies associated with rotations of magnetic moments onto an effective classical Heisenberg Hamiltonian using the magnetic force theorem and one-electron Green functions. The formalism is applied to Ga 1Ϫx Mn x As alloys with and without As antisites, and to Ge 1Ϫx Mn x alloys recently studied experimentally. A detailed study of the behavior of pair exchange interactions as a function of the distance between magnetic atoms as well as a function of the concentrations of the magnetic atoms and compensating defects is presented. We have found that due to disorder and the half-metallic character of the system the pair exchange interactions are exponentially damped with increasing distance between the Mn atoms. The exchange interactions between Mn atoms are ferromagnetic for distances larger than the ones corresponding to the averaged nearest-neighbor Mn-Mn distance. The pair exchange interactions are also reduced with increasing concentrations of the Mn atoms and As antisites. As a simple application of the calculated exchange interactions we present mean-field estimates of Curie temperatures.

Prospects for high temperature ferromagnetism in (Ga,Mn)As semiconductors

“…The former point can be expected to affect T c at large hole densities, i.e., when the hole Fermi wavelength approaches inter-atomic distances. In the opposite limit of strongly compensated systems, where the overall magnitude of the holemediated exchange is weaker, antiferromagnetic superexchange can dominate the near-neighbor Mn Ga u Mn Ga coupling, 41 leading to a reduced Curie temperature. 18,20 This type of magnetic interaction was ignored in the previous section.…”

“…II B we evaluate the Stoner enhancement of the Curie temperature due to hole-hole exchange interaction. Suppression of T c due to antiferromagnetic superexchange contribution to the near-neighbor Mn Ga u Mn Ga coupling in highly compensated samples 41 is illustrated in Sec. II C. In this section we discuss also effects on T c arising from the discreteness of random Mn Ga positions in the lattice that becomes important in the opposite regime, i.e., in systems with low charge compensation or co-doped with additional nonmagnetic acceptors.…”

“…In weakly compensated ͑Ga,Mn͒As the ferromagnetic contribution takes over. 41,46 Besides the above effects of random Mn distribution, Mn positional disorder can directly modify the p-d interaction when the coherence of Bloch states becomes significantly disturbed. Microscopic theories, such as the TBA calculations 20 presented in this section or approaches based on ab initio LDA band structure, 18 capture all these effects on an equal footing and can be used to estimate trends in mean-field T c beyond the virtual-crystal approximation.…”

Disorder and the effectiveMn−Mnexchange interaction inGa1−xSilva1, Fazzio2, Santos3 et al. 2005Phys. Rev. B15150View full textAdd to dashboardCite

“…18,19,20,21,22,23,24,25,26 Some predict an alternating sign for the exchange coupling, but these predictions should be taken with extreme care, since these theoretical calculations are based on non-fullpotential muffin-tin-type potentials which are not reliable to treat the electronic structure of covalent semiconductor systems such as (Ga,Mn)As DMS. Also, disorder quite certainly is not adequately taken into account within simple effective-medium approaches such as VCA or CPA, as fluctuations in the Mn positions essentially lead to variations in the Mn-Mn exchange-coupling parameters, as apparent from Fig.…”

Compensation, interstitial defects, and ferromagnetism in diluted ferromagnetic semiconductors

“…In the first step of this method, we derive the exchange integrals between magnetic impurities using the Local Density Approximation (TB-LMTO) and magnetic force theorem [10] providing an effective classical random Heisenberg Hamiltonian. Note that the calculated exchange integrals, include the effect of disorder within a Coherent Potential Approximation (CPA) for electronic motion.…”