Structural, optical and improved magnetic properties of nanostructured Co doped BiFeO3 powders prepared by sol–gel route

In this work, Co-doped BiFeO₃, specifically BiFe_0.90Co_0.10O₃ powder, was synthesized using a sol–gel derived route. The detailed investigation was carried out of the structural properties using X-ray diffraction with Rietveld analysis, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). The results demonstrate the successful substitution of Co for Fe in the R3c structure of the BiFe_0.90Co_0.10O₃. X-ray photoelectron spectroscopy (XPS) analysis revealed a larger contribution of Fe³⁺ and Co³⁺, along with a signature of Co²⁺ due to oxygen vacancies at the grain's surface. The temperature-dependence of magnetization (M) assessed through zero field cooling (ZFC) and field cooling (FC) curves indicates predominant ferromagnetic or ferrimagnetic behavior with a superparamagnetic contribution associated with the smallest nanoregions in the low-temperature region. Hysteresis M(μ₀H) curves confirm the magnetic interaction between both contributions like an exchange-spring effect. Measurements of the magnetization reversal and reversibility processes at 2 K illustrate a shift of the coercive field μ₀H_C values, attributed to the exchange bias effect between the blocked superparamagnetic state and the ferro- or ferrimagnetic phase. All such magnetic enhancements are ascribed to the influence of Co on the magnetic interactions within the BiFeO₃ lattice. UV-visible spectroscopy revealed a complex band structure characterized by three bandgap values: 1.95 eV and 2.98 eV attributed to direct transitions bandgaps for spin-down and spin-up states, respectively, and 1.21 eV associated with levels added by the Co ions or due to oxygen vacancies. The improved magnetic response and intriguing optical behavior suggest potential applications as voltage-controlled magnetic devices for spintronics or in optoelectronic devices for light-harvesting applications.