Magnetoresistance in ordered and disordered double perovskite oxide, Sr<SUB>2</SUB>FeMoO<SUB>6</SUB>

We have prepared crystallographically ordered and disordered specimens of the double perovskite, Sr2FeMoO6, 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 Mossbauer 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