Abstract:
We present a novel semi-analytical theory to study ferromagnetism in disordered ferromagnetic systems. The approach is based on finite temperature Green.s function formalism; it treats disorder effects exactly and spin fluctuations within a self-consistent local RPA (SC-LRPA). In order to illustrate the efficiency of this approach a detailed study of diluted magnetic semiconductors is provided. To allow a direct quantitative comparison with experimental measurements, the exchange couplings which enter the effective Heisenberg Hamiltonian are calculated from ab initio (TB-LMTO) calculations. The SC-LRPA theory gives very good agreement with published data for both, as grown and optimally annealed samples of Ga_1-xMn_xAs. In addition to show the validity of the SC-LRPA we also provide a direct comparison with Monte Carlo simulations. The theory is very general and allows to include effects of correlation, clustering in the disorder. It is also very easy to implement and appears to be 3 orders of magnitude faster than Monte Carlo simulations.