We search for general patterns that explain the low field magnetoresistance at low temperatures in the system $A_{(2-x)}{A}_x^{\prime }{\mathrm{FeMoO}}_6$. The observed linear dependence of the low field magnetoresistance with the saturation magnetization for the series is related to the antisite disorder at the $\mathrm{Fe}$ and $\mathrm{Mo}$ sites. This is explained in terms of a spin dependent crossing of intragranular barriers originated from the presence of antiferromagnetic ${\mathrm{SrFeO}}_3$ patches that naturally develop when antisite disorder occurs in the double perovskite. The presence of a moderate level of antisite disorder is at the very root of low field magnetoresistance although effects such as disorder distribution, connectivity, or morphology add their contribution.

• Received 7 September 2000

DOI:https://doi.org/10.1103/PhysRevLett.86.2443