  Ferromagnetism in Cr-based double perovskites is analyzed using effective
model as well as simulation approach.
  Starting from a microscopic model proposed recently for this class of double
perovskites, an effective spin-only model is derived in the limit of large
exchange coupling at the B-site. Analytic expressions for the resultant
exchange coupling is derived in certain limiting cases.
  The behaviour of this exchange is used to provide a plausible explanation for
the enhanced ferromagnetic tendency as also the enigmatic increase in Curie
temperature observed in Cr-based DP-s, in the series
Sr$_{2}$CrWO$_{6}$,Sr$_{2}$CrReO$_{6}$ to Sr$_{2}$CrOsO$_{6}$. The
superexchange between neighbouring B and B' sites is found to play a crucial
role both in stabilizing ferromagnetism, especially in the latter two
compounds, as well as increasing the T$_{c}$.
