Enhanced B-site cation ordering and improved magnetic properties: Role of A-site Bi substitution in La2NiMnO6 double perovskites

Abstract

The influence of A-site Bi substitution on B-site cation ordering, crystal structure, and magnetic properties of La2-xBixNiMnO6 double perovskites has been studied. Compounds with and without Bi substitution adopt monoclinic crystal structure with the P21/n space group as determined from the Rietveld refinement of x-ray diffraction data. X-ray photoelectron spectroscopy indicated an increase in Mn4+/Mn3+ and Ni2+/Ni3+ ratios in Bi substituted sample, and hence emphasized improved Ni and Mn ordering. This has been well supported by bond length analysis which suggested a compression in<Mn–O>and elongation in<Ni–O>bond lengths. The Bi substituted compound showed a relatively larger moment as compared to the parent sample, which was attributed to the suppression of antisite disorder i.e., enhanced Ni-Mn cation ordering due to Bi substitution. The temperature-dependent dc magnetization studies reveal a ferromagnetic to paramagnetic phase transition, and the Curie temperature has been observed to increases with Bi substitution. The critical exponent analysis around phase transition also confirmed the increase in the critical temperature for the Bi substituted compound. A detailed analysis of the critical behavior has been performed using different models, which suggested a long-range ferromagnetic ordering for both pure and Bi substituted samples.