First-principles Study On The Electronic Structures And Magnetism Of Double Perovskites Sr₂Fe_1-xCr_xMoO₆

The discovery of the tunnelling magnetoresistance at room temperature in polycrystalline Sr₂FeMo₆ has stimulated increasing interest in double perovskite compounds.These features make Sr₂FeMoO₆ to be a potential candidate of spintronic devices where the spin dependent carrier scattering processes are dominant.The crystal structures,magnetic properties,and Curie temperature of the double perovskites could be changed by doping appropriate elemental atom.Up to now,many experimental research groups have studied the structural and magnetic properties of the doped Sr₂FeMoO₆ in which the Fe site has been substituted by small atomic element.For instance,it is found that the saturation magnetization,Curie temperature,magnetoresistive effect,and cation ordering of the Sr₂Fe_1-xCr_xMoO₆ decrease with the increase of the Cr concentration.Considering that the Cr³⁺(3d³:t_2g³)ion has a similar electron configuration and ionic radius to the Fe³⁺ion,we expect the Sr₂Fe_1-xCr_xMoO₆ compounds would present particular physical behavior.In this thesis,electronic structures and magnetism of double perovskites Sr₂Fe_1-xCr_xMoO₆(x=0,1/3,0.5,1)have been investigated by using the plane wave pseudo-potential method based on density functional theory within the generalized-gradient approximation.Firstly,the geometrical structures of respective Cr-doping pencentration are optimized with the aim to obtain the ground states of Sr₂Fe_1-xCr_xMoO₆ compounds.The obtained lattice parameters,unit cell volume are decrease with the incrase of the Cr-doping concentration,and the tendency are consist with the experimental results.In addition,the crystal structures change from the tetragonal to triclinic,then back to tetragonal system.Secondly,the magnetic properties of Sr₂Fe_1-xCr_xMoO₆ have been further investiaged for the obatined ground state structures.The total magnetic moments of the Sr₂Fe_1-xCr_xMoO₆ and the average magnetic moments of the Fe ion decrease with doping pecentration,which are in good with the experimental predictions.Finally,the calculations of both the band structures and densities of states show that the system goes from the half-metallic to metallic and then to half-metallic.