  We suggest an approach to account for spatial (composition) and thermal
fluctuations in "disordered" magnetic models (e.g. Heisenberg, Ising) with
given spatial dependence of magnetic spin-spin interaction. Our approach is
based on introduction of fluctuating molecular field (rather than mean field)
acting between the spins. The distribution function of the above field is
derived self-consistently. In general case this function is not Gaussian,
latter asymptotics occurs only at sufficiently large spins (magnetic ions)
concentrations $n_i$. Our approach permits to derive the equation for a
critical temperature $T_c$ of ferromagnetic phase transition with respect to
the above fluctuations. We apply our theory to the analysis of influence of
composition fluctuations on $T_c$ in diluted magnetic semiconductors (DMS) with
RKKY indirect spin-spin interaction.

Disordered Systems and Neural Networks;Materials Science