Towards shape-oriented Bi-doped CoCr2O4 nanoparticles from theoretical and experimental perspectives: structural, morphological, optical, electrical and magnetic properties

Abstract

In the present work, Co_(1−x)Bi_xCr₂O₄ (where x = 0.00, 0.05, and 0.10) nanoparticles were investigated in detail by combining theoretical and experimental efforts. The Co_(1−x)Bi_xCr₂O₄ (where x = 0.00, 0.05, and 0.10) nanoparticles were synthesized via a solution combustion method and studied through structural, microstructural, electrical, and magnetic analysis complemented by DFT calculations. The structural analysis confirmed a single-phase spinel cubic structure with singular structural distortions associated with Bi-doping and the generation of metal vacancies. The nanoparticles mainly enclosed the (100), (110), and (311) surfaces of the cubic spinel chromite structure, resulting in under-coordinated Co, Cr and Bi centers along their exposed surfaces. The dielectric parameters were explained using Koop's phenomenological theory, Maxwell-Wanger theory, and thermal energy of the vibrations, highlighting the role of Bi-doping in controlling the ferroic orders for the Co_(1−x)Bi_xCr₂O₄ nanoparticles. Moreover, the magnetic analysis indicated a paramagnetic to long-range collinear ferrimagnetism transition at the Curie temperature and non-collinear ferrimagnetism at the spiral transition temperature. These transitions are associated with the exchange coupling constants, as evaluated by DFT calculations before and after Bi-doping. Indeed, the effect of Bi-doping on the magnetic properties was determined to be a reduction in superficial magnetism, resulting in a diminished magnetization value for the doped samples, which is attributed to the major exposure of the (110) and (311) surfaces. Furthermore, the investigation of the morphological modulation of Co_(1−x)Bi_xCr₂O₄ nanoparticles through this innovative and unified procedure can guide other experimental studies in the rational design of superior multiferroic nanoparticles.