  In this work we investigate Ising and classical Heisenberg models for two and
three dimensional lattices in presence of diluted ferromagnetic dimers. For
such models the Curie temperature as a function of ratio of intra-dimer
exchange coupling constant I_A and other inter-site coupling constants I_B is
calculated. In case dimer is treated exactly and environment within the
mean-field approach it was found that even for I_A/I_B\to\infty T_C remains
finite. Similar analysis is proposed for rhombohedral phase of intermetallic
compound Gd2Fe17 where so-called Fe1-Fe1 "dumbbell" forms the diluted
ferromagnetic dimer. It was shown that for such complex magnetic systems T_C is
determined by all variety of exchange interactions and for the interval 0 <
I_A/I_B < \infty T_C changes are not more than +/-10%.
