Crystal Structures And Properties Of Double Perovskites Sr₂FeMoO₆ And Its Doping System

In recent years,scientists have done a lot of research on the structure and magnetic and electrical transport properties of the double perovskite oxide materials,and made a great progress on materials function.The double perovskite oxides have a great application prospect in the manufacture of magnetoresistance materials,it is worth noting that the double perovskite oxide has a significant effect of tunnelling magnetoresistance effect,making this material most likely to be used at room temperature.However,there are still some problems which attract us to do further study.In this paper,double-perovskite Sr₂FeMoO₆ and its relative oxides Sr₂FeMoO_6-yN_y and Sr_2-xLa_xFeMoO₆ have been successfully synthesized by the solid-state reaction method.Then the crystal structure,electrical transport and magnetic properties of the samples have been investigated systematically.Rietveld refinement of the x-ray powder diffraction profiles indicates that the compounds Sr₂FeMoO_6-yN_y and Sr_2-xLa_xFeMoO₆ have a tetragonal structure with the space group I4/mmm.The cell volume of the samples decreases with the increase of dopant concentration,and the volume is larger with respect to that of undoped Sr₂FeMoO₆.The Fe/Mo degree of ordering in Sr₂FeMoO_6-yN_y exhibits an increasing tendency.However,the degree of ordering in Sr_2-xLa_xFeMoO₆ decreases with increasing x due to electronic doping.All samples exhibits ferromagnetic characteristics by detection of the hysteresis loops of Sr₂FeMoO_6-yN_y and Sr_2-xLa_xFeMoO₆ at 291 K.The saturation magnetization of the samples increases with the increase of order degree,it is shown that order degree is an important factor that affects magnetization intensity.Magnetic measurements show that all samples underwent ferromagnetic（FM）to paramagnetic（PM）phase transition near the Curie temperature（Tc）.At the same time,the ferromagnetic interaction between Fe atoms is enhanced due to the doping of N and La,leading to that the Curie temperature rises with the increase of the amount of doping.All samples have an obvious negative magnetoresistance effect through the changing trend of resistivity with magnetic field.Within the temperature range 290 to 450 k,the resistivity of samples increases with the increase of temperature,the behavior which they exhibited belong to typical metal conduction behavior.By piecewise fitting the curve of the sample’sρ-T,its conduction mechanism conforms to the formula:ρ（T）=ρ₀+ρ₁T²+ρ₂T^4.5,it shows that the electron transport process is mainly related to the electron^electron scattering and electron^magnon scattering,while the temperature is is near Curie temperature,the coefficient of T^4.5 becomes negative,which shows that the transport of electrons is also affected by the electron^phonon scattering.