Magnetoresistance in ordered and disordered double perovskite oxide, $Sr_2FeMoO_6$

We have prepared crystallographically ordered and disordered specimens of the double perovskite, $Sr_2FeMoO_6$, and investigated their magnetoresistance (MR) behaviour. The extent of ordering between the Fe and Mo sites in the two samples is determined by Rietveld analysis of powder X-ray diffraction patterns and reconfirmed by Mössbauer studies. While the ordered sample exhibits the sharp low-field response, followed by moderate changes in the MR at higher fields, the disordered sample is characterised by the absence of the spectacular low-field response. We argue that the low field response depends crucially on the half-metallic ferromagnetism, while the high-field response follows from the overall magnetic nature of the sample, even in absence of the half-metallic state