  Sr$_{2}$FeMoO$_6$ is a double perovskite compound, known for its high
temperature behavior. Combining different magnetic and spectroscopic tools, we
show that this compound can be driven to rare example of antiferromagnetic
metallic state through heavy electron doping. Considering synthesis of
Sr$_{2-x}$La$_x$FeMoO$_6$ (1.0 $\le{x}\le$ 1.5) compounds, we find compelling
evidences of antiferromagnetic metallic ground state for $x\ge$1.4. The local
structural study on these compounds reveal unusual atomic scale phase
distribution in terms of La, Fe and Sr, Mo-rich regions driven by strong La-O
covalency: a phenomenon hitherto undisclosed in double perovskites. The general
trend of our findings are in agreement with theoretical calculations carried
out on realistic structures with the above mentioned local chemical
fluctuations, which reconfirms the relevance of the kinetic energy driven
magnetic model.
