  We have synthesized the solid solution, Sr_2Fe_1+xMo_1-xO_6 with -1 <= x <=
0.25, the composition x = 0 corresponding to the well-known double perovskite
system Sr2FeMoO6. We report structural and magnetic properties of the above
system, exhibiting systematic variations across the series. These results
restrict the range of models that can explain magnetism in this family of
compounds, providing an understanding of the magnetic structure.
