Cation size variance effects in magnetoresistive Sr2FeMoO6 double perovskites

A series of Sr2-x(Ca0.55Ba0.45)(x)FeMoO6 (0 less than or equal to x less than or equal to 0.8) double perovskite oxides has been prepared to assess the effects of cation size variance, sigma(2), at constant mean A site cation radius. All of the samples are ferromagnetic with Curie temperatures T-C = 390-435 K and show metal-to-insulator transitions at T-MI = 10-115 K. Both transition temperatures show a linear decrease with sigma(2). The degree of Fe/Mo order increases with sigma(2) from 81% to 90% under the synthesis conditions used, which increases the saturated magnetization and electronic conductivity of the ceramic samples. A decreasing structural anisotropy across the series accounts for a decrease in the magnetic coercivity. Negative colossal magnetoresistances (CMRs) are observed, up to 30% for the x = 0.2 sample at 5 K, but these show no trend with sigma(2), which reflects the microstructural origin of the CMR in Sr2FeMoO6 materials.