Induction of Colossal Magnetoresistance in the Double Perovskite Sr₂CoMoO₆

Sr 2 CoMoO 6 perovskite has been prepared in polycrystalline form by thermal treatment, in air, of previously decomposed citrate precursors. This material has been studied by X-ray (XRD) and neutron powder diffraction (NPD), thermal analysis, and magnetic, magnetotransport, and Hall effect measurements. At room temperature, the crystal structure is tetragonal with a space group I 4/ m, with a = 5.565 03(5) and c = 7.948 10(8) Å. The crystal contains alternating CoO 6 and MoO 6 octahedra, tilted by 6.8° in the basal ab plane. Magnetic measurements indicate an antiferromagnetic ordering below T N = 37 K. As prepared, the sample is an electrical insulator. The topotactic reduction of the stoichiometric sample, in H 2 /N 2 flows, leads to oxygen-deficient double perovskites, Sr 2 CoMoO 6 - δ, δ = 0.03 and 0.14. The magnetic behavior of the reduced samples suggests the presence of ferromagnetic domains characterized by Curie temperatures of T C = 350−370 K. The conductivity dramatically increases upon H 2 reduction; the number of charge carriers, as determined by Hall effect measurements, are 1.7 × 10 - 7 and 6 × 10 - 6 electrons per unit cell for δ = 0.03 and 0.14 samples, respectively. Moreover, the reduced samples exhibit colossal magnetoresistance, as high as 30% at 12 K for H = 9 T . Magnetic and magnetotransport results are consistent with a large component of itinerancy for down-spin Mo t 2g electrons, injected through hydrogen reduction.