Transport and magnetic properties of Sr₂FeMo_xW_1−xO₆

Abstract

We have investigated transport and magnetic properties of a series of double-perovskite alloys in the system Sr₂FeMo_xW_1−xO₆. These compounds exhibit a metal-insulator transition as a function of doping, x. The compounds with x in the range 1.0≥x≥0.3 show metallic behaviour in the resistivity while compounds with x in the range 0.2≥x≥0 are insulating. All compounds with 1.0≥x≥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≥x≥0.3 due to an enhanced crystallographic ordering; however, M decreases rapidly with decreasing x thereafter (x<0.3), probably due to composition fluctuations near the critical concentration, x_c. Our data suggest primarily Fe³⁺-(Mo, W)⁵⁺ ordering for x≥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≥0.3 show a significant amount of negative magnetoresistance, as has been observed earlier for Sr₂FeMoO₆ (x=1.0).