Crystal Structure, Atomic Ordering And Magnetoresistance Of The Double-perovskite Oxides

The crystal structure, atomic ordering, electrical transport and magnetic properties of the double-perovskite Sr₂FeMoO₆ and its relative oxides Sr₂(Fe_1-xMnx)MoO₆(0≤x≤0.45), Sr₂(Fe_1-xCr_x)MoO₆(0≤x≤0.4) and (Sr_2-xBa_x)FeMoO₆(0≤x≤2) have been investigated systematically.Rietveld refinement of the x-ray powder diffraction profiles indicates that the compounds Sr₂(Fe_1-xMnx)MoO₆(0≤x≤0.45) and Sr₂(Fe_1-xCr_x)MoO₆(0≤x≤0.4) have a tetragonal structure with the space group I4/mmm, while the crystal structure of (Sr_2-xBa_x)FeMoO₆(0≤x≤2) changes from a tetragonal I4/mmm lattice to a cubic Fm3 m lattice with the substitution of Ba for Sr. The B-site degree of ordering in Sr₂(Fe_1-xMnx)MoO₆ and (Sr_2-xBa_x)FeMoO₆ exhibits an increasing tendency. However, the B-site degree of ordering in Sr₂(Fe_1-xCr_x)MoO₆ decreases with increasing x due to preferential occupancy of the Cr ions on the Mo sites.In the system Sr₂(Fe_1-xMn_x)MoO₆(0≤x≤0.45), an inhomogeneous model is proposed to elucidate the observations, which suggests a coexistence of Mn²⁺ and Mn³⁺ ions in the doped compounds and a saturated substitution of Mn³⁺ for Fe³⁺. When the doping level is greater than the percolation concentration, the valence state of Mn prefers the +2 valence. In the system Sr₂(Fe_1-xCr_x)MoO₆(0≤x≤0.4), there exists a linear relationship between the saturation magnetization and the anti-site concentration. The low-field magnetoresistance (LFMR) of the compounds shows a strong dependence on saturation magnetization, or equivalently the anti-site concentration. In the system (Sr_2-xBa_x)FeMoO₆(0≤x≤2), the optimum of LFMR and TC was found at (Sr_1.6Ba_0.4)FeMoO₆. The resistivity and magnetoresistance measurements clearly distinguish two composition regimes. In the first regime with x≦1.6, the compounds have low resistivities and exhibit metallic behavior, the LFMR shows an enhancement...