  The influence of substitutional disorder on the magnetic properties of
diluted Heisenberg spin systems is studied with regard to the magnetic
stability of ferromagnetic diluted semiconductors (DMS). The equation of motion
for the magnon Green's function within Tyablikov approximation is solved
numerically for finite systems. The resulting spectral density is then used to
estimate the magnetization and Curie temperature of an infinite system. This
method is suitable for any form of a ferromagnetic exchange interaction.
Besides different lattices and spin magnitude $S$, exchange interactions of
different range are examined. The results show that, for short-range
interaction, no magnetic order exists below the critical percolation
concentration, whereas a linear dependence of the Curie temperature on the
concentration of spins is found for ferromagnetic long-range interaction.
