  Metals with large positive magnetoresistance are rare. We demonstrate that
antiferromagnetic metallic states, as have been predicted for the double
perovskites, are excellent candidates for huge positive magnetoresistance. An
applied field suppresses long range antiferromagnetic order leading to a state
with short range antiferromagnetic correlations that generate strong electronic
scattering. The field induced resistance ratio can be more than tenfold, at
moderate field, in a structurally ordered system, and continues to be almost
twofold even in systems with upto 25 % antisite disorder. Although our explicit
demonstration is in the context of a two dimensional spin-fermion model of the
double perovskites, the mechanism we uncover is far more general, complementary
to the colossal negative magnetoresistance process, and would operate in other
local moment metals that show a field driven suppression of non-ferromagnetic
order.
