Transport and magnetic properties of Sr<SUB>2</SUB>FeMo<SUB>x</SUB>W<SUB>1−x</SUB>O<SUB>6</SUB>

We have investigated transport and magnetic properties of a series of double-perovskite alloys in the system Sr2FeMoxW1&#8722;xO6. These compounds exhibit a metal-insulator transition as a function of doping, x. The compounds with x in the range 1.0&#8805;x&#8805;0.3 show metallic behaviour in the resistivity while compounds with x in the range 0.2&#8805;x&#8805;0 are insulating. All compounds with 1.0&#8805;x&#8805;0.2 are ferrimagnetic, while Sr2FeWO6 (x=0) is antiferromagnetic. The magnetization (M) is shown to increase slowly with decreasing x in the range 1.0&#8805;x&#8805;0.3 due to an enhanced crystallographic ordering; however, M decreases rapidly with decreasing x thereafter (x&lt;0.3), probably due to composition fluctuations near the critical concentration, xc. Our data suggest primarily Fe3+-(Mo, W)5+ ordering for x&#8805;0.3, while compositions with smaller x possibly contain both Fe3+ and Fe2+ species inhomogeneously due to the presence of the W6+ state. All samples with x&#8805;0.3 show a significant amount of negative magnetoresistance, as has been observed earlier for Sr2FeMoO6 (x=1.0)