The Preparation,Characterization And Coupling Of Sr₂FeWO_6-xN_x,Sr₂FeMoO₆

In recent years,the double perovskite oxides have a unique magnetoresistance effect and a great application prospect in the field of magnetoresistance materials,however there are still some problems to solve in the theory and application aspect.In this paper,double-perovskite Sr₂FeWO₆ and its relative oxides Sr₂FeWO_6-xN_x and Sr₂FeMoO₆（SFMO）have been successfully synthesized by the solid-state reaction method.Sr₂FeMoO₆ and Sr₂FeWO_6-xN_x were respectively used to form a two layer coupling system called A,B,C,D.Then the crystal structure,electrical transport and magnetic properties of the samples have been investigated systematically.The crystal structures of Sr₂FeWO_6-xN_x series samples had been successfully synthesized all belong to monoclinic system.The space groups of these samples was P2₁/n.Compared with the sample without doping,the Fe²⁺of doped samples oxidation number increases and crystal deformation is reduced,makes the cell volume decreases.Then with longer duration of ammonia,the number of N^3-average radius is larger increased,makes cell volume increases.The Fe/W order degree in the Sr₂FeWO_6-xN_x system increases substantially as the sample ammonia passing time increases.The XRD of SFMO has been refined and fitted to show that it belongs to the tetragonal system with the space group I4/mmm.The Sr₂FeWO_6-xN_x system is an antiferromagnetic material which exhibits paramagnetic properties at room temperature so the Neil temperature is below room temperature.The magnetization increases as the ammonia passage time of the sample increases.The number of single electron in the sample also increases with the increase of ammonia passing time,which is due to the increase of Fe²⁺oxidation caused by N doping.SFMO is a ferromagnetic substance with a Curie temperature of 410.6K and a saturation magnetization of 25.712 emu g^-11 at 297 K.The critical index of SFMO:β=0.474,γ=1.105,δ=2.986.The magnetic properties of the coupler are similar to SFMO and are also ferromagnetic,probably due to the magnetizationof SFMO is more stronger.The saturation magnetization of the coupled samples is higher than that of SFMO,and the Curie temperature is also higher than SFMO,and both increase with the extension of ammonia pass time of Sr₂FeWO_6-xN_x monomer.The critical index of A:β=0.426,γ=0.976,δ=2.968.The critical index of B:β=0.474,γ=1.123,δ=2.947.The critical index of C:β=0.590,γ=0.966,δ=2.914.The critical index of D:β=0.455,γ=0.981,δ=3.156.The state equation fitting confirmed that the data is highly reliable.The Sr₂FeWO_6-xN_x system samples are semiconductor conductivity（dρ/dT<0）,and its resistivity decreases with the extension of ammonia-passing time.Both of them exhibit large negative magnetoresistance effect,and the conduction mechanism conforms to the thermally activated range transition model.By using the negative Seebeck coefficient,it is known that the series of Sr₂FeWO_6-xN_x are all n-type semiconductors,and most of the carriers are electrons.The SFMO is metal conductivity（dρ/dT>0）,and it exhibit large negative magnetoresistance effect.The resistivity in the temperature range of 300-394 K is affected by the scattering of electron^electron and electron^magnon.In addition to the above influencing factors,the resistivity is affected by electron^phonon scattering in the range of 394-453 K.The carriers of SFMO are mostly electrons and belong to n-type metals.The coupling samples are semiconductor conductivity（dρ/dT<0）.Due to the coupling effect of the two monomers,the resistivity is smaller than that of the Sr₂FeWO_6-xN_x system,but much larger than the resistivity of SFMO.The electrical properties of coupling samples are similar to those of the Sr₂FeWO_6-xN_x system,probably due to the much higher resistivity of Sr₂FeWO_6-xN_x than SFMO.