A-Site-Doping Enhanced B-Site Ordering and Correlated Magnetic Property in La2-xBixCoMnO6

A series of Bi-doped La2-xBixCoMnO6 double perovskite oxides are synthesized, and the impact of doping on crystal structures and magnetic properties is investigated comprehensively. Xray photoelectron spectroscopy and Raman spectrum analyses reveal that ordering of Co and Mn ions at B-site is gradually improved with the rise of Bi concentration. Meanwhile, magnetic disordering is suppressed greatly by showing larger magnetic moments. Structurally, the Rietveld refinement shows that the bonds are elongated, while the bond angles are shrunken after doping, giving rise to lowered Curie temperature. We also observe a large negative zero-field-cooling magnetization, which is attributed to the formation of spin antiparallel or canted ferromagnetic domains and clusters that are separated by the antiphase boundaries. First-principles calculations confirm the enhanced Co-Mn ordering upon Bi doping by taking into account both the ordering and disordering configurations of La2CoMnO6, LaBiCoMnO6, and Bi2CoMnO6. Moreover, we find a spin-state transition in the antisite Co ions from high-spin (Co2+-t(2g)(5)e(g)(2)) to low-spin state (Co3+-t(2g)(6)e(g)(0)), which is consistent with the increased total magnetic moments by the Bi doping.