Compensation, interstitial defects, and ferromagnetism in diluted ferromagnetic semiconductors

We present a quantitative theory for ferromagnetism in diluted III-V ferromagnetic semiconductors in the presence of the two types of defects commonly supposed to be responsible for compensation: As antisites and Mn interstitials. In each case we reduce the description to that of an effective random Heisenberg model with exchange integrals between active magnetic impurities provided by ab initio calculation. The effective magnetic Hamiltonian is then solved by a semianalytical method (locally self-consistent random-phase approximation), where disorder is treated exactly. Measured Curie temperatures are shown to be inconsistent with the hypothesis that As antisites provide the dominant mechanism for compensation. In contrast, if we assume that Mn interstitials are the main source for compensation, we obtain a very good agreement between the calculated Curie temperature and the measured values, in both as-grown and annealed samples.