First-principles Study Of Electron Doping In Double Perovskite Sr_2-xLa_xCrReO₆

The double perovskite family has attracted more and more attentions, since Kobayashi discovered the Colossal Magnetoresistance (CMR) in the compound Sr₂FeMoO₆. Band structure calculations have shown that this kind of compound is half-metallic, where the majority spin channel is metallic, while the minority is insulated. Moreover, a lot of new physical phenomena have also been observed in these compounds, recently, such as the large magnetostriction, structural phase transition driven by an external field, and anomalousthermal expansion, etc. Because of the above novel characters, this kind of compounds has received lots of attraction.Among the double perovskites, Sr₂CrReO₆ is known as having the highest Curie temperature (T_c) till now, of up to 635 K, which has broadened the application of this kind of compound in the spintronic fields. The band structure calculations have shown that there exits a mixed valence character between Cr and Re ions, which couples antiferromagnetically with each other, thus forming the half-metallic conduction band. On the other hand, a series of work have been done to study the electron doping effects on the A or B site of some of the double perovskites. It is found that it would have great effect on the Curie temperature and the magnetic properties of this kind of compound. Considering the importance of the effect of electron doping in furthering the understanding of Sr₂CrReO₆, a partial substitution of Sr²⁺ by La³⁺ in this compound has been performed and the doping effects has been investigated on the structural, electronic and magnetic properties in the double perovskite Sr_2-xLa_xCrReO₆ on the basis of first-principles calculations in our paper.In this thesis, we firstly give a general review of the history and the present research situation in the perovskite family. Secondly, we mainly introduce the density functional theory as well as the ab initio software package, which are our theoretical foundation and computational implementation to deal with the issues discussed in this thesis. Then, we give the details of our work, where we use the results of first-principles band structure calculations to study the electronic structure and magnetic properties induced by La³⁺ substitution in Sr_2-xLa_xCrReO₆ (x=0, 0.25, 0.5,1). We find that La³⁺ substitution not only promotes a structural distortion owning to the different ionic radii of Sr²⁺ and La³⁺ but also provides carriers to the conduction band. As a result, the band gap around the Fermi level in the down-spin channel is enlarged (x<1), and the magnetic moment decreases in accordance with the variation of x. When x=1, it changes from ferromagnetic with half-metallicity to metallicity with strong ferromagnetism, which has good agreement with the experimental results.