Unravelling the role of Bi-substitution on vibronic ferromagnetism in La2−xBixMnCoO6 (x = 0, 0.5 and 1)

Abstract The crucial role of high Bi-content on magnetic properties of ambient pressure synthesized double perovskite La2−xBixMnCoO6 (x = 0, 0.5 and 1) has been investigated. Structural characterization was carried out using X-ray and electron diffraction analyses as well as HAADF-STEM imaging, which confirm the monoclinic structure with P21/n space group for all compositions. A comprehensive dc-magnetization and ac-susceptibility studies unravel the complex magnetic features of parent La2MnCoO6 and its associated changes upon substitution of La by Bi. Both the ferromagnetic TC and reduced magnetization with increase in Bi-content, originated from weakening of ordered Mn4+—O—Co2+ superexchange interactions. Bi-substitution also leads to suppression in magnetic disordering, which should strengthen the ferromagnetic state. These antagonist effects i.e. suppression of disordering and lowering of TC can be related to the evolution of Mn3+—O—Co3+ (e1—O—e1) vibronic ferromagnetism of smaller exchange stabilization. The local lattice distortion due to stereoactive 6s2-electron pairs of Bi3+ possibly results in cooperative lattice distortion favouring the stabilization of Jahn-Teller active intermediate-spin state Co3+ and high spin Mn3+. It is suggested that Bi-substitution favours the reaction Co2+ + Mn4+ = Co3+ + Mn3+ to the right side, which is otherwise biased to left by 0.2 eV.